chemistry of the biradicals produced in the norrish type...

Chemistry of the biradicals produced in the Norrish Type II reaction

J . C. Sca lano Radiation Laboratory, 1 University o f Notre Dame, Notre Dame, Indiana 46556

and

E. A. Liss i and M . V. Encina Departamento de Quimica, Universidad T~cnica del Estado, Santiago, Chile

C o n ~ n ~

I. Introduction and scope . . . . . . . . . . . . . . . . . . . . . . . . . 139 IL Generation of biradieals . . . . . . . . . . . . . . . . . . . . . . . . 141

Ill . Thermochemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . 145 IV. Mechanism of reaction . . . . . . . . . . . . . . . . . . . . . . . . . 146

A. The triplet state reaction . . . . . . . . . . . . . . . . . . . . . . . 146 B. The singlet state reaction . . . . . . . . . . . . . . . . . . . . . . 148

V. Intramolccular processes . . . . . . . . . . . . . . . . . . . . . . . . 151 A. Spin inversion . . . . . . . . . . . . . . . . . . . . . . . . . . . 151 B. Biradical isomerizations . . . . . . . . . . . . . . . . . . . . . . . 153 C. Formation of molecular products ................... 156

I. Gas phase studies . . . . . . . . . . . . . . . . . . . . . . . . 156 2. Studies in solutions . . . . . . . . . . . . . . . . . . . . . . . . 158 3. Studies in the solid state . . . . . . . . . . . . . . . . . . . . . 173

D. Assisted intramolecular processes . . . . . . . . . . . . . . . . . . . 174 VI. Intermolectflar reactions . . . . . . . . . . . . . . . . . . . . . . . . 179

A. Hydrogen abstraction . . . . . . . . . . . . . . . . . . . . . . . . 179 B. Addition to double bonds . . . . . . . . . . . . . . . . . . . . . . 180 C. Electron transfer reactions . . . . . . . . . . . . . . . . . . . . . . 183 D. Interaction with oxygen . . . . . . . . . . . . . . . . . . . . . . . 186 E. Trapping by SO2 . . . . . . . . . . . . . . . . . . . . . . . . . . 188

VII. Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . 188 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190

I. Introduction and scope

W h e n a d ia lkyl o r aryl ke tone bear ing 7 hydrogens is i r r a d i a t e d wi th uv l ight i t undergoes a p h o t o f r a g m e n t a t i o n and photocyc l iza t ion process , usual ly refer red to as the Nor r i sh Type I I react ion . 2.a (See Equa t ion (1).) The in ter - med iacy o f b i rad ica ls has been clearly es tabl ished in the t r ip le t s ta te react ion , 2,a as well as by the fact tha t the ke tone p roduced in the f r agmen ta - t ion is in i t ia l ly genera ted in the enol form. ~ s The qua n tum yields o f pho to - f r agmen ta t ion and pho tocyc l i za t ion qui te f requent ly do not a d d up to one. The reversal to the g round state o f the pa ren t ke tone k - r is in many cases respons ib le for this d i f ference? ,~-~

~) 1978, by Verlag Chemie International, Inc.

139

140

R

J. C. Scaiano, E. A. Lissi, and M. V. Encina

R

R _ k - , [ ~ _ ~ R

kr ~ OH ~ OH

•

OH ~ R + oletin

1 °

(1)

Since the original report by Norrish in 1937, x° hundreds of papers have been published on the subject. The Norrish Type II reaction is probably one of the best understood photochemical processes. The nature and behavior of the intermediate biradieal have received considerable attention during the last few years but its properties are still less well understood than those of the primary abstraction process.

This review is primarily concerned with the mechanistic and kinetic behavior of 1,4-ketyl-alkyl biradicals. Part of the discussion is inevitably centered on the Norrish Type II reaction. We have tried to limit the discussion of these aspects to only those which are directly relevant to the behavior of the intermediate biradical.

A process which is closely related to the Norrish Type II reaction is the photoenolization of ortha-alkyl substituted acetophenones, benzophenones and related compounds./1 The following reaction illustrates the case of 2-methyl- acetophenone.

cH \c:O CH \ oH

CHa.,,.c/OH

~ C H 2

(2)

Chemistry of the biradicals produced in the Norrish Type H reaction 141

The transient 1 has been written as a biradical to emphasize the similarity with reaction (1), although the argument as to whether I should be regarded as a biradical or the triplet state of the enol 2 is still open. la-x~ Reaction (2) is in general considerably less well understood than reaction (1) and will not be considered in any detail in this review. The reader is referred to the abundant literature available on the problem, e.g., references 11-23.

A number of photochemical processes frequently compete with the Type II reaction. Typical examples are fluorescence, 24 Type I cleavage, a'25-a2 intermolecular abstraction, a's'aa photoenolization 11,17 and quenching by products at high conversions. 7 In order to keep this review to a reasonable length, we have avoided detailed discussions of these competing processes and have tried to limit ourselves to mention briefly their importance and occasionally their quantum yields.

This review is not intended to be exhaustive, though we have tried to include all those references containing relevant kinetic or mechanistic information.

H. Generation of biradicals The most common source of Type II biradicals is of course the Norrish

Type II reaction, which has been covered in several reviews. 2"3"3~ The same biradicals can also be generated by a variety of processes, and quite frequently these alternative methods of generation have been used to support the intermediacy of biradicals in the Type II process.

Felt ss has examined the thermal decomposition of c/s-l,2,-dimethylcyclobutanol in the gas phase and observed the formation of 2-hexanone as well as the fragmentation products acetone and propene. The results were explained on the basis of the intermediacy of the biradical 3,

OH ~ ~ . . , j R 2 3a:R1 = H, R2 = CHs 3 b : R I = C H a , R 2 = H

R1 3

Feit's results are compared with the photochemistry of 2-hexanone (which leads to 3a) in Table 1. The thermolyses of other aliphatic cyclobutanols have been examined by Stephenson and Gibson. as

Table 1. Comparison of the pyrolysis of cis-l,2-dimethylcyclobutanol with the photolysis of 2- hexanone

Source of Original Yield 2-hexanone biradical T, °C Multiplicity Phase Yield acetone Reference

1,2-Dimethyl- 381 Singlet Gas 0.08 35 cyclobutanol 2-Hexanone 25 Singlet Solution 5.0" 36,37 2-Hexanone 25 Triplet Solution 1.1 ~ 36,37

"Yield of 2-hexanone based on the quantum yield of reversal (see k_, in reaction (1)), using data from reference 36 and the quantum yield of intersystem crossing reported by Encina and Lissi) 7

142 J. C. Scaiano, E. A. Lissi, and M. V. Encina

Padwa and Alexander as have examined the decomposition of 2-hydroxy-2- phenylbicyclo[1.1.1]pentane 4 which yields the biradical 5. The thermal decomposition

Ph ~ OH

4 5

and photochemistry of a number of related molecules were also examined, at~a Hornback *~ has been able to show that the biradical 8 can be generated by

photolysis of 1,4-diphenyl-4-penten-l-ol 6 or 1,4-diphenybl-pentanone 7, where the biradical is produced via abstraction by the excited alkene. 4s

p h ~ Ph

OH 6

p h J ~ - ~ ~ Ph

0 7

Ph ' J ~ " ~ Ph

8 OH

Reaction (a) occurs from the triplet state and shows a significant isotope effect. 1,4-Diphenyl-4-penten-l-ol also reacts from the excited singlet state producing 2-methyb2,5-diphenyltetrahydrofuran.

Cantrel140 has reported an example of a reverse Type II reaction and this is shown below.

C02H

ph/~"OH P h ~ O I J H

Pitts et a/. 47 have examined the y-radiolysis of ring substituted butyro- phenones. They observed that the Gn (yields of fragmentation) values are closely related to (I)u (quantum yield of photofragmentation). They suggested that the energy is initially absorbed by the benzene solvent, ultimately leading to the production of butyrophenone triplets via energy transfer. The G~ value was 1.57 for butyrophenone and found to be lower for all its derivatives.


Another example of molecular rearrangement analogous to the Norrish Type II reaction is the McLafferty rearrangement, a well known process in mass spectrometry. *s-52 Nicholson 53 was the first to recognize the correlation between the Norrish Type II process and the mass spectrometric cracking pattern. In a recent publication Gooden and Brauman 54 have reported a study of the photodissociation of butyrophenone cation which at 440 nm resembles the Type II reaction.

As pointed out above, the photolysis of ketones remains the most common source of Type II biradicals (reaction (1)). The process can occur from both the triplet and singlet state; the latter is relatively uncommon in the case of aromatic ketones.

The rate of intramolecular hydrogen abstraction, kr, (reaction (1)) is not directly related to the behavior of the resulting biradical and is not expected to determine its reactivity; however, quite frequently it does determine the viability of studies of biradical reactions (vide infra). In this sense, we feel it would be useful to provide a summary of representative values of kr, the importance of which will become apparent in the sections to follow. Tables 2 and 3 show these values for aromatic and aliphatic ketones respectively.

While abstraction at the y-position is favored by entropic factors el as well as the absence of ring strain, s5 abstraction at other positions is also possible. A number of a-methylene ketones will undergo abstraction at the ~ position. 62-6~ For example 9 undergoes both/3 and 3' hydrogen abstraction, ultimately leading to 10 and 11 respectively. 83

O O

9 10 11

Wagner et aL e5 have shown that 8-methoxyvalerophenone 12 undergoes approximately 50:50 y- and g-hydrogen abstraction.

Ph O

0 12 13

The photochemistry ofp-benzophenonecarboxylate esters 13 involves hydrogen abstraction at remote positions. 66-69

The current knowledge on abstractions at sites other than the r-position is so limited that it does not provide sufficient information on the resulting biradicals to justify a detailed discussion of their behavior.

144 J. C. Scaiano, E. A. Lissi, and 34. V. Encina

Table 2. Rates o f intramolecular hydrogen abstraction for some alkyl aryl ketone triplets

Arrhenius Ketone Conditions kr a parameters ~ Reference

Ph

Ph

Benzene, 25 ° 8.9 x I0 e

Benzene, 25 ° 1.3 × 10 e

E , = 7.0; log A -- 12.1 55,56

E , -- 5.7; log A -- 12.3 55,56

Ph

Benzene, 25 ° 5.5 x 10 e EL = 4.7; Iog A = 12.2 55

Ph

Benzene, 25 ° 9.0 x 10 e - - 31

O ~ ~ P h

Cyclooctane, 25 ° 3.9 x 10 ~ F_., -- 3.7; log A -- 12.3 57

Benzene, 25 ° 2.0 x 10 e - - 58

a In units of s - z b F_~ in kcal tool-Z; log A with A in s °x.

Chemistry o f the biradicals produced in the Norrish Type H reaction 143

Table 3. Rate constants for the Type H reactions o f some aliphatic ketones in n-hexane at room temperature

Ketone kr (sing let) " kr (triplet) a Reference

2-Pentanone 1.6 x 108 3.9 x l0 g 24,59,60 4-Methyl-2-pentanone 1.1 x 10 e 8.4 x l0 g 37,60 2-Hexanone 8.1 x l0 g 1.6 x 108 24,37,59,60 5-Methyl-2-hexanone 2.3 x l0 g 1.1 x l0 g 24,59,60,73

" In units of s- 1.

HI. Thermochendstry

The thermochemical properties of Type II biradicals and related species can be estimated rather easily. A typical assumption in order to estimate the heat of format ion of the biradical is to est imate i t f rom the cor responding carb inol

Table 4. Energetics o f the processes involved in the photochemistry o f 2-pentanone

Species H°~oe-(H° aos)2-pentanone8 Reference

eK 78 70 1K 85 70 8(/])n ~ 86 29 3(t/)x b 91 71 l(~)nb 89 29 l(~hb 85 72 SB 73 28 1B 75 28 Enol + ethylene 29.4 28 Acetone + ethylene 22.4 28 Cyclobutanol 18A 3Ethylene + acetone 99 28 CH~(~O + Pr" 77 28 Ethylene + Sacetone 100.4 28,70 Ethylene + aenol 106 28

"In kcal tool-1. The symbol # refers to the transition state configuration for the

reaction. For example l(~g)n refers to point A in Figure 1. The subscripts I and II refer to the Type I and Type II reactions respectively.

Based on an estimation using group additivity rules (S. W. Benson, "Thermochemical Kinetics," J. Wiley, New York, 1968).

Table 5. Energetics for the photochemistry o f some aromatic ketones a'b

Ketone (H°2,s)3s- (H°2,o)3x (H°2,e)s,- (HO=oe)E (H°29o)~ ¢ - (H°29e)~

Butyrophenone - 6.5 67.5 81.0 Valerophenone - 10.5 63.5 79.7 y-Methylvaleropbenone - 13.5 60.5 78.7

"From reference 55. b All values in kcal tool -1.

Refers to the transition state for the intramolecular hydrogen abstraction.

146 J. C. Scaiano, E. A. Lissi, and M. IF. Encina

assuming that the bond energies for the H-atoms in the 1,4-positions are the same as in a stable molecule, at e.g., in the case of butyrophenone the heat of formation of 14 can be estimated from 15:

Ph Ph

OH ~J~OH 1 4 15

AH~(14) = DH°([PhCOHRI--H) + DH°(PhCOCH2CHaCH2--H) + AH~(15~ - 2AH~(H.)

---- 74 + 98 -- 32.9 + (--104.2) ---- 34.9 kcal mo1-1 (from reference 31).

The heats of formation can also be estimated from the ketone and by using AH ° for hydrogen abstraction. 55

Table 4 shows a summary of thermodynamic data for 2-pentanone. The errors should be small for most of the values and differences in Table 4, with the exception perhaps of the energy gap between aB and tB.

Table 5 shows some values for phenyl alkyl ketones. Some additional data on y-methylvalerophenone 7a and other examples 31 have also been published.

IV. Mechanism of reaction

In this section we introduce the mechanism of the Norrish Type II process. We will not try to prove at this point that the scheme proposed is correct but rather, try to show its significance and what type of kinetic behavior should be expected. The arguments to be presented in Sections V and VI will show the reasons why we prefer this particular mechanism.

Biradicals have a "memory" as to the multiplicity of the parent excited state, and usually the decay to molecular products occurs exclusively from the singlet biradical. This section is divided into two parts depending on the multiplicity in which the biradical is initially generated.

A. The triplet state reaction A considerable amount of evidence has been accumulated during the last

few years, showing not only that biradicals are involved in the Norrish Type II reaction from the triplet state, but that they account for 100% of the fragmentation and cyclization products. Our current state of understanding is that a concerted reaction does not occur from the triplet manifold.

The products of the reaction are produced in the ground state. For example, at least 98.6% of the stilbene produced in the photofragmentation of 16 is generated as the trans isomer, showing that triplet stilbene (or at most 1%) is not involved in the reaction. 7~


Ph

p h / ~ O

Ph

O Ph h, , PhCCHs +

Ph

In the ease of simple aryl ketones, e.g., ~,-methylcaprophenone, the energy is not sufficient to produce an excited olefin. Ts Scheme 1 shows the basic set of reactions necessary to account for the behavior of triplet generated biradieals in the absence of triplet quenchers or biradieal traps. K is a ketone beating y-hydrogen atoms, B a biradical (see reaction (I)) and the superscripts 1 and 3 are the spin multiplicity. The main point (and perhaps the most controversial one) is that the triplet-singlet intersystem crossing in the biradical (k4) is an irreversible process. There is considerable evidence that this is so in solution: 8 It is possible that some singlet-triplet crossing occurs in the gas phase. 28

hll K > 1K

kt 1K > aK ka

aK > aB aK k, > other processes

k4 aB > XB ks XB > fragmentation ke XB > cyclization

IB k, > K

Scheme 1.

While discussing biradical reactions it is more useful to use reaction probabilities than quantum yields, i.e.,

ks (3) aPxx = ks + k6 + k7

k8 aPc~' - k5 + ko + k7 (4)

k~ (5) 3Px = k s + k e + k 7

Zpp = ,p~ + apo" = 1 - zp~ (6) where the subscripts II, cy, K and P stand for fragmentation, cyclization, reverse hydrogen transfer and products respectively. The requirement for these probabilities to be equal to the quantum yields of these processes is that the quantum


yield for biradical generation be unity, i.e., a quantum yield of unity for intersystem crossing and intramolecular hydrogen abstraction as the only mode of decay of the triplet state. This is frequently the situation encountered with simple phenyl alkyl ketones, a'8'g'~5

Aliphatic ketones typically undergo the Type II reaction from both singlet and triplet states and usually the probabilities (eqs. (3)--(6)) will be considerably higher than the quantum yields.

For most simple ketones in non-polar media

3p,~ > 3pn > 3pcy

The aK---> aB conversion in Scheme 1 correlates with the triplet biradical only for an n, ~r* triplet and not for a am ~r* state. 7v In terms of Dougherty's nomenclature it corresponds to an N-type reaction. 7a

B. The singlet state reaction

The singlet state reaction is considerably more complex than the triplet state process. While there is sufficient evidence that biradicals are involved in the process, all the indications are that part of the reaction occurs by a concerted mechanism or via biradicals having "memory" of the geometry in which they were initially produced.

The in, ~r* state correlates with the biradical (1D,, rr), while the x~r, ~r* state correlates with one of the zwitterions (Z2). 77"7~

Figure 1 shows a schematic representation of the surfaces involved in an intramolecular hydrogen abstractionY "77'79-sl These potential energy profiles include the possibility of a molecule having reached X going either to C (bi-

A

Figure 1. Energy surfaces involved in the singlet state reaction.

Chemistry of the birath'cals produced in the Norrish Type II reaction 149

radical) or to D (formally a radiationless decay). An important characteristic of the mechanism (which otherwise resembles that proposed by HeUer 82) is that the electronic-to-vibrational conversion has no effect on the kinetics of the process because the point X is located after the critical configuration A. The probability of reaction leading to 1B is largely determined by the movement of the system along the reaction coordinate at the time when the crossing occurred, sa No surface crossing occurs in the case of the triplet state reaction; the process leading from 8K to 3B is adiabatic, a7.77

Scheme 2 shows the proposed reaction mechanism/° where (tB)* is the initially produced singlet state biradical before there is any rotation around a C-C bond. We have not included in the mechanism the possibility of a concerted fragmentation because there is no correlation between ~K and the fragmentation ground state products, e°'77

The ringlet state reaction typically shows high (but not complete) stereospecificity and a very efficient reversal to the parent ketone (vide infra). The latter can be explained on the basis of a radiationless decay (ks) or an unfavored fragmentation compared to the reversion to the ground state in the case of (1B)*. There are no experimental data for or against an internal conversion (ks), although if it does occur it seems reasonable to assume that it would involve the mechanism illustrated in Figure 1 (1K--)-A---> X---> D) rather than a direct conversion (XK -+ K). a3

As in the case of the triplet state reactions it is best to use probabilities rather than quantum yields. If we define kx and ~ , in terms of all the molecules which cross the potential energy maximum of Figure 1, then

zPor = $o,~/¢~ (8)

tPK = ~reversa.l/~A (9)

1pF = 1pa + xp,~ (10)

These probabilities can of course be expressed in terms of the rate constants in Scheme 2 and kA, but the resulting equations are rather more complex than in the case of the triplet state reaction.

The Norrish Type II reaction from the singlet state is rather characteristic of aliphatic compounds and does not usually occur in simple aromatic ketones, e.g., none of the ketones in Table 2 undergoes reaction from the singlet state. This is largely due to a very fast intersystem crossing, s~'as For example in the case of valerophenone in benzene kl > 5 x 10 TM s-1. s~

Some examples of reaction of aromatic compounds from the singlet state include non-conjugated phenyl ketones s6 and phenyl acetates. 87-s9

A rather interesting difference between reactions (5) and (10) in Scheme 2 is that while they give the same product the most probable geometries of the transition state are different. The reaction from (1B)* must occur prior to bond rotation; therefore it must occur from a cisoid type of structure. On the other hand,

150 J. C. Scaiano, E. A. Lissi, and M. It. Encina

h~ K ~ 1K

1K k~ > 8 K

1K k. • K

1K k. ~. (1B) ,

(1B) , k~o • fragmentation

(1B) , k~ > cyclization

(1B) , k,.~. K

(1B) , kl0 ). 1B

ZB k. ~. fragmentation

~B k. ~ cyclization

1B k, > K

1K kl, ~. other processes

Scheme 2. Reaction numbers are consistent with Scheme 1.

fragmentation of the rotationally equilibrated biradical, 1B, is likely to occur from the less hindered transoid configuration. The two conformations are illustrated in Figure 2.

OH OH

• I R oct , R(~OH

¢ I SOlD TRANSOIO Figure 2. Cisoid and transoid structures of ketyl biradicals.

Chemistry o f the biradlcals produced in the Norrish Type H reaction 151

V. Intramolecnlar processes

This section is divided into four subsections according to the nature of the processes involved. The first one, spin inversion, is centered on the reaction from the triplet manifold where spin-flip is the factor determining the biradical lifetimes. It is followed by a brief section on biradical isomerization, a third on intramolecular reactions, which refers mainly to the calculations of the probabilities defined in equations (3) to (1(3) and their significance, and finally, assisted intramolecular reactions where some of the rather unusual complexation mechanisms of 1,4-biradicals are discussed. Whenever possible comparisons between solid, liquid and gas phases are made.

A. Spin inversion

As pointed out in Section IV A the reaction from the triplet manifold must involve spin inversion because the final products are generated in the ground state. 7~'~s The lifetimes of the biradicals from aromatic ketones in solution are in the 30--100 ns range. 76'~°-g2 They are essentially independent of the 9'- substituents, of the stability of the olefm produced and of the temperature. 78

Table 6. Triplet and biradical lifetimes in solution

Ketone Solvent ~T ~ (ref ) ~n ~ (ref .)

y-Methylvalerophenone Cyclohexane 2 ¢ 37(91) Benzene 1.9(55) 42(73),35(91) Methanol 4.7(76) 97(90),98(91) Wet Acetonitrile a 2.5(76) 70(76),76(91) t-Butanol-glycol" 28(76) 103(76)

Valerophenone Cyclohexane 7.2(93) f 38(91) Benzene 4(92) 34(91),60(92) Methanol 16(76) 102(76),93(91) Wet Acetonitrile a 8.7(76) 71(76),70(91)

Hexanophenone Methanol 17(76) 107(76) Butyrophenone Benzene 65(92),122(55), 100(94) 150(92) g

Wet Acetonitrilea 150(76) 110(76) n 1,4-Diphenyl-4-hydroxy butan-l-one Methanol 1.5(76) 84(76) Poly(phenyl vinyl ketone) Benzene 7(84) 63(92)

Film 80

In nanoseconds; most values based on quenching studies. b In nanoseconds at room temperature; all values obtained using laser flash photolysis

techniques, in some cases combined with quantum yield measurements. c Assuming roughly the same value as in benzene. At least in the case of valerophenone

the triplet lifetime seems to be the same in a number of nonpolar solventsY 3 " 10% water (v/v). "A mixture of t-butanol and ethylene glycol, 3:2 by volume, ,7 -- 13 cp at 24°C. t Assumed to be the same as in n-hexane) 8 ' Probably an overestimate? 1

Based on a comparison of quantum yield and time resolved studies. J. C. Scaiano, unpublished work.

152 Y. C. Scaiano, E. ,4. Lissi, and M. V. Encina

We have suggested that this behavior can only be accounted for if spin inversion is assumed to be the main, and probably the only factor determining the biradical lifetimes. In other words, reaction (4) in Scheme 1 is an irreversible process. Table 6 gives a summary of triplet and biradical lifetimes in solution. Only those examples where the biradical lifetimes 0-a) are based on time-resolved techniques have been included.

The suggestion that the lifetimes are determined by the rate of intersystem crossing in the biradical is consistent with similar observations in somewhat longer lived biradicals using CINDP techniques) 5

The biradical ends seem to interact very weakly. This conclusion is based on the following considerations: (a) the temperature independence of the lifetime (examined in the case of y-methylvalerophenone v6) suggests that the triplet biradical cannot lie more than 0.5 kcal tool -~ below the singlet biradical level, and (b) the biradical absorption spectrum, 91 Figure 3, resembles in shape, ~=~

800

T E

!

--- 400

I I I 400 450

), (nm) Figure 3. Absorption spectrum of the biradical from y-methylvalerophenone in

methanol. (Reproduced with permission from North-Holland Publishing Company, Amsterdam) z )

and extinction coefficients that of typical ketyl radicals (e.g., from acetophenone). 96,97

O'Neal et aL =s have estimated the energy gap between the singlet and triplet level for the biradical from 2-pentanone in the gas phase to be 2.3 kcal mo1-1. The calculation involved the assumption that the 1B--> OB process has no activation energy. For this reaction they estimated a rate constant of 4.6 x 107


s- 1, which is considerably slower than the rate of chemical decay of the singlet biradicals (reactions (5)--(7) in Schemes 1 and 2).

Direct measurements of biradical lifetimes (see Table 6) have only been achieved very recently. 7a'76'a°-a2 All the lifetimes estimated before 1977 are based on chemical competition studies under conditions of steady irradiation. This type of study can only lead to the evaluation of kr~'B, the product of the rate constant for the trapping reaction (kT) and the lifetime of the biradical. This approach has been used by Wagner 7s,a8 to estimate the lifetime of the biradicals from valerophenone and y-methoxybutyrophenone using thiols as biradical scavengers. The estimated lifetime of 1/zs in polar solvents bears the uncertain- ties involved in the estimation of kr.

Using a similar approach but with HBr as scavenger, O'Neal et aL 28 estimated the lifetime of the biradical from 2-pentanone in the gas phase to be 10 -~-9 s at their mean reaction temperature (115°C).

The approach discussed above involves the critical assumption that the reactivity of radical sites in biradicals is the same as in normal free radicals having the same substituents at the reactive center. Whether this assumption is correct or not is still an open question.

The assumption that the lifetime of the biradicals produced from the triplet manifold is controlled by spin inversion has not been unanimously accepted. Stephenson and Brauman 99 have attempted an alternative explanation for the differences in the singlet and triplet state reactions based on the different initial energy contents of the biradicals generated from both states. The same approach was used to interpret the differences between the thermal and photochemical generation of the biradicals. 38 Several authors have argued that the results are in fact best interpreted using the Bartlett-Porter model loo where the differences in behavior are simply attributed to the role of spin inversion in the biradical. 28.76,a°,1°1 This problem is further examined in Section V C.

B. Biradical isomerizations

Only a few examples of isomerizations involving Type II biradicals have been reported. Yang et al. l°a have shown that the photolysis of 6-hepten-2-one

17

1 1

18

(11)

154 J. C. Scaiano, E. ,4. Lissi, and M. V. Encina

17 leads to the formation of 1-methyl-3-cyclohexenol 18. The proposed mechanism is given below and supports the formation of cyclobutanols via the intermediacy of biradicals as originally proposed by Yang and Yang. TM

Wagner and Liu x°* have examined the photochemistry of a-allylbutyro- phenone 19 and found 2-phenyl-2-norbornanol 20 among the products (reaction (12)). The reaction accounts for less than 1% of the triplets initially produced, x°6 The main reaction path is intramolecular quenching by the fl-vinyl group.

Ph OH Ph OH

21 22 23

/ 0 OH

19 20

\

o

ph ~ + 24 25

On the assumption that the rate of intramolecular cyclization of 20 to give 21 and 22 and ultimately 23 can be taken to be equal to the rate of cyclization of the 5-hexenyl radical 26, the independent rates of fragmentation and ~cliza- tion of 20 to give 24 and 25 were estimated to be 6 x lO s s-1.1°7

© 2.6

In a recent communication Perkins et al. 112 proposed a rather unusual isomerization, reaction (13), in order to explain the results of the photolysis of 27:


O H.,. Ph

ph 'Y H 27

0

h , )

OH

l OH

1 OH

(13)

Padwa et al. ~xs have examined the photochemistry of trans-l,4-diphenyl-3,4- epoxybutan-l-one 28. The reaction, which occurs from the triplet state, yields as major products 29 and 30 along with minor amounts of acetophenone and

O H O

Ph " Ph + H 0 H 0 h

28 29 30

phenylacetic acid. The authors proposed that dibenzoylethane 29 results from the isomerization of the biradical 31 to give 32, reaction (14).

OH

OH / / / / " p h ~ . . ~ . Ph

p h / ~ " ~ l Ph , 29

O 32 (14)

30 A number of processes involving 1,4 and 1,3-biradicals closely related to those produced in the Type II reaction have also been examined. ~°'62,63,114.1xs

156 J. C. Scaiano, E. A. Lissi, and M. l/. Encina

C. Formation o f molecular products

With a few exceptions, some of which have been indicated in the previous section, Type II biradicals decay by the three processes shown in reaction (1). In this section and in the following, we will be discussing these processes. In Section V D we will concentrate on those examples in which the biradical decay is affected by relatively strong interactions with other molecules, while in this section we will discuss only gas phase, solid state conditions and those experiments in solution carried out in "inert" solventsY xe

We begin this section by emphasizing that there is no correlation between quantum yields and triplet state reactivity. 47.xxT'~x8 This is also true in the case of isotope labelling; 86 for example, the Type II reaction of 2-hexanone-5,5-d2 is slower than that of 2-hexanone; however, the photoreactions of the deuterium derivative occur with enhanced quantum yields. In other words, deuterium substitution reduces kz and kT, and the change in the latter results in an increase of ~Pp (eq. (6)).

The Norrish Type II reaction shows a number of complex features, many of which have been recognized for almost four decades, x°'119'x2° We now know that the biradicals of reaction (I) are largely responsible for these complexities. In this section we concentrate on one particular aspect of the reaction, i.e., the nature and quantum yield of the products of reaction (1). This type of study has led to the gradual understanding of the role of biradicals in the Type II reaction, a role which was later confirmed in trapping and laser photolysis studies.

1. Gas phase studies

A large number of gas phase studies on the photochemistry of aliphatic ketones bearing 7-hydrogens have been carded out. In spite of this, the information on biradical behavior is quite limited because in most cases the quantum yield of intersystem crossing has not been measured, therefore making it im- possible to correlate quantum yields with biradical behavior.

The best studied example is 2-pentanone. From the results of Ausloos and Rebbert t22 and Lissi et al. 12a we obtain the data shown in Table 7.

The trends of the product probabilities are very similar in the gas and solution phases (vide infra). The ratios NPKffPn and ~rPn/SPcy are larger for the singlet state, suggesting the contribution of processes which precede the geometrical randomization. We note that the difference in behavior between the two states cannot be attributed to energy differences, since one would expect a marked effect when the phase is changed (see for example Table 7).

The value of tPK in the gas phase is smaller than in solution (see Table 7; compare also with aPx). If we assume that the large yield of reversal in the singlet state reaction occurs to a large extent prior to geometrical randomization, then we have to conclude that the importance of this process is considerably smaller in the gas phase as a result of faster internal rotations. This effect could be partially due to the participation of "ho t" biradicals in the gas phase, since tB is

Chemistry of the biradicals produced in the Norrish Type H reaction

Table 7. Photochemistry of 2.pentanone 1~2.~2~

157

n-hexane ~ gasphase ~ XB 3B 1B aB

Px 0.82 0.60 0.70 0.48 Pn 0.14 0.25 0.26 0.34 Po, 0.04 0.15 0.04 0.18

Taking @me = 0.84 in solution and Cmo = 0.55 in the gas phase, t~3

initially produced with ~ 14 kcal mol-1 excess energy (see Table 4). This energy is larger than the rotational barriers. ~e.9°

The values of 1p~ and 3P a are similar in the gas phase; this is reflected experimentally in the fact that under different experimental conditions ¢)~ for a series of ketones is almost constant, even when the fraction of the reaction which occurs from the singlet and triplet state changes. The quantum yield of intersystem crossing ¢)mo usually decreases when the temperature increases or when the pressure and wavelength decrease; 2e.37't2* however, (1)xt remains almost independent of the experimental conditions. 126-129

O'Neal e t al. 2e have suggested that a considerable fraction of the 1B species generated from 2-pentanone in the gas phase lives long enough to undergo intersystem crossing to the aB state which can be trapped. This is not consistent with the behavior in solution, since the singlet reaction is not affected when the behavior of the ~B biradical is modified by solvent effects (unless k_,, Scheme 1, is very solvent dependenO. The effects of the experimental conditions on the yields of the Type II reaction in the gas phase have been examined for 2-pentanone, 12~.123,127.12s,13° 2-hexanone, 125'128 4-methyl-2-hexanone 131 and a series of n-propylalkyl ketones.t 2s Table 8 shows a summary of the results for 2-pentanone. The ratio Po,/Pa (or rather (I)oy/*a) decreases with decreasing pressure or wavelength and increasing temperature; this probably reflects the higher proportion

Table 8. Photolysis of 2-pentanone in the gas phase

PcT A, nm T °, C P, tor t ~ a ~x PLt Reference

313 58 40 0.25 0.20 - - 129 313 150 40 0.25 0.43 - - 129 313 28 1.5 0.30 0.25 122 313 150 1.5 0.31 0.14 122 313 28 0.13 0.32 0.09 122 313 28 3.2 0.32 0.33 122 313 25 ~20 0.27 127 313 129 ~20 0.27 0.39 127 254 25 ~20 0.28 127 313 28 1.5 0.25 122 254 28 1.5 0.11 122

Hg/photosens. 28 1.5 0.09 122


of singlet state reaction under these conditions 28'123 and the smaller value of 1P~y compared with 3p~y. However, if mercury photosensitization leads directly to the triplet manifold then the changes in Pcy/Pxx would have to reflect a dependence on energy rather than on multiplicity, since the value of 0.09 under these conditions could not be attributed to a relatively large xP~ value.

We could not find any reports on gas phase studies involving aromatic ketones. 2. Studies in solution

This aspect of the Type II reaction has been studied more extensively. The section has been subdivided according to the nature of the parent ketone. a. Aromatic compounds

Aromatic ketones usually undergo the Type II reaction solely from the triplet state. 2.~ The biradicals are initially produced in the triplet state in a reaction which is mostly independent of the solvent (see Table 6 and references 90 and 93)xa2 but largely dependent on the C-H bond strength at the y-posi- fion.2,9,31,55.56,65,x18,13a,135

The eyclobutanol/fragrnentation ratios have received considerable attention. While in a variety of cases the cyclobutanols amount to 10-257, of the products, 8 examples have been reported where no eyclobutanols are formed (e.g., 1,4- diphenyl-4-hydroxybutan-l-one x36) and where they account for 100% of the products: e.g., a,~-difluorovalerophenone, x3~ Table 9 shows a compilation of quantum yield data for the eases of aromatic ketones in benzene. Cycloalkyl phenyl ketones, of the type examined by Lewis et al., a2 have not been included. The values of ~m,x correspond to the maximum quantum yield of product formation, usually obtained in benzene-t-butyl alcohol or in "wet" acetonitrile; 7 they represent essentially the quantum yield of biradical generation in the system. Values lower than unity usually indicate that (a) ~tso < 1, or (b) the triplet state decays partially by routes other than intramolecular hydrogen abstraction. Values of Zpp (see eq. (6)) have normally been determined using equation (15).

app = ¢I)n + ~c~, (15) (I~][IIB, X

The roles of t-butyl alcohol and "wet" acetonitr/le will be discussed in Section VD.

The temperature dependence of the quantum yields of photofragmentation has been examined in a few cases, xa6'xaS,la~ We could not find any study of the effect of temperature on the cyclobutanol/fragmentation ratios for aryl alkyl ketones.

For most simple ketones the values of app are in the 0.3-0.5 range; y- hydrogen substitution tends to increase aPr. Ring substitution has a varied effect and since in many cases O~x has not been measured, it is difficult to decide whether aP e is low or simply whether the abstraction is inefficient. In the case of photoenolizable ketones, the low yields reflect the competition with abstraction from the ortho-substituent (see eq. (2)). xv

Chemistry o f the biradicals produced in the Norrish Type H reaction

Table 9. Quantum yield data for aromatic ketones in benzene at room temperature

159

Related Ketone a ~ n b ~7oCB* a p t " " ~ , ~ ! References References

B u t y r o p h e n o n e 0.36 12 0.40 1.0 8 31,47,65, 117,118,134, 138,139

Va le rophenone 0.30 18 0 .3 ( : 1.0 8 31,65,117, 118,134,138

a - M e t h y l b u t y r o p h e n o n e 0.28 29 - - - - 31 - - ~ - M e t h y l b u t y r o p h e n o n e 0.26 h 15 0.31 1.0 31 8,65,118 H e x a n o p h e n o n e 0.33 17 0.40 1.0 134 65,118 y -Me thy lva l e rophenone 0.22 11 0.25 1.0 8 31,117,118,

134,138 a.Methylvalerophenone 0.17 43 - - - - 31 - -

0.04 68 0.16 ~ 8 a , a - D i m e t h y l b u t y r o p h e n o n e 0.004 89 0.013 ~ - - 31 ~ , ~ - D i m e t h y l b u t y r o p h e n o n e 0.17 3 - - > 0 . 7 31,65 7 y -Viny lbu ty rophenone 0 .24 13 ,,,0.35 - - 8,134 41 H e p t a n o p h c n o n e 0.25 23 0.37 1.0 134 I 18 & M e t h y l h e x a n o p h e n o n e 0.33 - - - - 0.94 65 - - a , ~ - D i m e t h y l v a l e r o p h e n o n e 0.03 71 0.11 ~ - - 31 25,65 O c t a n o p h e n o n e 0.30 - - - - ~ 118 - - 8, 8 - D i m e t h y l h e x a n o p h e n o n e 0.22 28 0.30 (1.0) 8 65 e - M e t h y l h e p t a n o p h e n o n e 0.25 25 0.35 0.94 134 N o n a n o p h e n o n e 0.24 20 0.31 (1.0) 8 118,65 Nonanophenone -y ,y -d2 0.25 - - 0.37 0.82 8 a,/~,/3,y-Tetramethylvalero-

p h e n o n e 0.014 7 - - - - 31 e , e -D ime thy lhep t anophenone 0.24 28 0.33 1.0 134 - - D e c a n o p h e n o n e 0.24 20 0.31 0.98 134 - - H e x a d e c a n o p h e n o n e 0.26 10 - - - - 134 - - y - P h e n y l b u t y r o p h e n o n e 0.50 11 0.56 1.0 134 8,I 17,139 y - F l u o r o b u t y r o p h e n o n e 0.41 16 ~ - - 137 - - a -F l uo rova l e rophenone 0.34 51 - - ~ 137 a , a -Di f luo rova le rophenone < 0.01 > 98 - - ~ 137 y - C h l o r o b u t y r o p h e n o n e 0.11 3 0.63 0.18 134 - - & C h l o r o v a l e r o p h e n o n e 0.55 12 0.80 0.80 8 134 e - C h l o r o h e x a n o p h e n o n e 0.44 - - ~ 0.52 - - 8 134 a - M e t h o x y a c e t o p h e n o n e 0.51 - - - - - - 117 - - ~ , -Methoxybu ty rophenone 0.23 28 0.32 1.0 134 8 y - M e t h o x y v a l e r o p h e n o n e 0.20 20 0.25 1.0 134 8,118 ~ , -Methoxyva le rophenone 0.33 23 0.86 0.47 8 134 ~ , -Phenoxybu ty rophenone 0.29 - - ~ 0.35 - - 8 134 y - H y d r o x y b u t y r o p h e n o n e 0.31 - - ~ 0 . 3 4 1.0 8,134 - - 1 ,4 -Dipheny l -4 -hydroxybu tan -

1-one 0.49 0 - - J - - 136 y - C y a n o b u t y r o p h e n o n e 0.30 - - ~ 0.36 ~ 8 134 ~ -C yanova l e rophenone 0.48 9 0.53 1.0 134 8 [ - C y a n o h e x a n o p h e n o n e 0.45 - - ,, ,0.53 - - 8 134 ~ - C y a n o h e p t a n o p h e n o n e 0.37 - - ~ 0.44 - - 8 134 ~-Carboxyl icva le rophenone 0.55 - - ~ 0.70 - - 8 117,134

cont£nued


Table 9--continued

Related Ketone a O n e 7 o C B e Spp,Z., O f f i u l References References

y - C a r b o x y m e t h y l b u t y r o - p h e n o n e 0 . 4 7 - - ~ 0 . 5 5 - - 8 1 1 7 , 1 3 4

& C a r b o x y m e t h y l v a l e r o - p h e n o n e 0 . 6 1 2 3 0 . 7 9 1 . 0 1 3 4 8 , 1 1 7

B u t y r o p h e n o n e - y - a c e t a t e 0 . 4 8 - - ~ - - 1 3 4 p - M e t h y l b u t y r o p h e n o n e 0 . 3 9 - - - - ~ 4 7 1 3 9 p - P h e n y l b u t y r o p h e n o n e 0 ~ -- -- 1 3 9 - - p - M e t h y l v a l e r o p h e n o n e 0 . 4 1 18 0 . 5 0 ( 1 . 0 ) 8 m - M e t h y l v a l e r o p h e n o n e 0 . 3 4 - - ~ 0 . 4 2 - - 8 o - T r i f l u o r o m e t h y l v a l e r o -

p h e n o n e 0 . 2 0 - - ,,, 0 . 2 5 - - 8 m - T r i f l u o r o m e t h y l v a l e r o -

p h e n o n e 0 . 2 3 2 4 ~ 0 . 2 8 m 8 p - T r i f i u o r o m e t h y l v a l e r o -

p h e n o n e 0 . 2 6 - - ~ 0 . 3 2 - - 8 o - F l u o r o b u t y r o p h e n o n e 0 . 3 2 - - - - - - 47

m - F l u o r o b u t y r o p h e n o n e 0 . 3 8 - - - - - - 4 7 p - F l u o r o b u t y r o p h e n o n e 0 . 2 9 ~ ~ ~ 1 3 9 4 7 m - F l u o r o v a l e r o p h e n o n e 0 . 2 7 ~ ~ 0 . 3 3 ~ 8 - - p - F l u o r o v a l e x o p h e n o n e 0 . 3 6 ~ ~ 0 . 4 4 m 8 o - C ' h l o r o b u t y r o p h e n o n e 0 . 2 7 - - - - - - 4 7 m-Chlorobutyrophenone 0 . 3 4 - - - - ~ 4 7 p-Chlorobutyrophenone 0.41 7 -- -- 47,138 139

o - C h l o r o v a l e r o p h e n o n e 0 . 4 5 ~ ,,, 0 . 5 5 ~ 8 m - C h l o r o v a l e r o p h e n o n e 0 . 3 3 m ~ 0 . 4 0 - - 8 p - C h l o r o v a l e r o p h e n o n e 0 . 2 9 16 0 . 3 7 0 . 9 4 8 p-Bromobutyrophenone 0 -- -- -- 139 --

o - M e t h o x y b u t y r o p h e n o n e 0 . 1 2 - - - - - - 1 3 9 4 7 m - M e t h o x y b u t y r o p h e n o n e 0 . 0 0 5 - - ~ ~ 4 7 p - M e t h o x y b u t y r o p h e n o n e 0 . 1 0 2 0 ~ - - 8 , 4 7 1 3 9 o - M e t h o x y v a l e r o p h e n o n e 0 . 1 8 ~ - - 0 . 7 2 0 . 2 5 8 p - M e t h o x y v a l e r o p h e n o n e 0 . 1 4 2 0 0 . 6 9 0 . 2 6 8 p - M e t h o x y - , / - m e t h y l v a l e r o -

p h e n o n e ~ 13 - - - - 8 o - H y d r o x y b u t y r o p h e n o n e 0 - - - - - - 1 3 9 p - H y d r o x y b u t y r o p h e n o n e 0 - - - - ~ 1 3 9 - - p-Aminobutyrophenone 0 - - m ~ 1 3 9 p - A c e t o x y b u t y r o p h e n o n e 0 . 4 2 - - - - - - 1 3 9 - - p - A c e t a m i d o b u t y r o p h e n o n e 0 - - - - - - 1 3 9 m - ( N ) B u t y r o p h e n o n e z 0 . 2 8 - - ~ - - 1 4 0 p - ( N ) B u t y r o p h e n o n e z 0 . 2 3 ~ ~ ~ 1 4 0 o ( N ) - V a l e r o p h e n o n e ~ 0 . 1 6 - - 0 . t 8 1 . 0 8 , 1 4 0 m ( N ) - V a l e r o p h e n o n e z 0 . 2 3 - - 0 . 2 9 1 .0 8 , 1 4 0 p ( N ) - V a l e r o p h e n o n e z 0 . 2 5 - - 0 . 3 1 1 . 0 8 , 1 4 0 - -

o - M e t h y l b u t y r o p h e n o n e " o - M e t h y l v a l e r o p h e n o n e "

P h o t o e n o l i z a b l e K e t o n e s

0 . 0 0 1 4 m m - - 17

0 . 0 1 6 ~ - - - - 17

m


Table 9---continued

161

K e t o n e ~

Related On~ %CBO 3pd,, 0 I References References

o-Methyl-y-methyivaiero- phenone m 0.033 ~ ~ ~ 17

o-Ethylvalerophenone'* 0.021 ~ ~ ~ 17 2,4-Dimethyi-y-methylvalero-

p h e n o n , ~ 0.0024 ~ ~ - - 17 2,4-Di(perdeuteromethyl)-y-

methylvalerophenone"* 0.039 ~ - - - - 17

m

m

• The nomenclature used might seem rather unorthodox to some readers; to these we apologize. We have tried to name the ketones in such a way as to emphasize structure- reactivity relationships.

b Quantum yield of fragmentation. c Calculated as I00 x cyclobutanols/(cyclohutanols + ketone from fragmentation).

From eq. (15) unless otherwise indicated. • Approximate values are usually based on the assumption of 18~ cyclobutanols, see

reference 8. • Usually obtained in "wet" acetonitrile or benzene-t-butyl alcohol. See Section V D.

Values o f 0.36 and 0.4 are given in reference 8. h Wagneres reports O n ---- 0.40. Corrected to take ,,-cleavage into account, ax

s The usual method of obtaining Ore,= using alcohols or "'wet" acetonitrile is not applicable. 13e

k Includes cyclobutanols, s Pyridyl ketones.

~* The yield of cyclobutanols is negligible.

Electron withdrawing groups on the benzene ring decrease app, and electron withdrawing groups at the 8 and • positions increase app. Wagner et al. s have suggested that the transition state for disproportionation is stabilized by charge transfer f rom the alkyl end to the ketyl site of the biradical.

An interesting and well documented effect is the enhancement of the cydiza- t ion/fragmentation ratios induced by =-alkyl substitution. For example, the extent o f cyclization for butyrophenone, ~-methylbutyrophenone and =,=- dimethylbutyrophenone (see Table 9) is 12, 29 and 8 9 3 respectively. A careful examination of the yields at suggests that the result should be attributed to a decrease in ks, relative to k6 and k7 (Scheme 1). The effect can be explained in terms of the conformation required for the occurrence of reactions 5 and 6 (Scheme 1) (see Figure 2). The changes in ks would largely affect the possibility of reaching a conformation where there is continuous overlap between the p orbitals at the radical sites and the ~-fl bond. 2"a'a'at'llv This idea is also supported by the high yields of cyclization observed in the photochemistry of several exocyclic and endocyclic ketones. ~°'141-t46

A number of studies have been concerned with the photodegradation of polymers and copolymers of phenyl vinyl ketone, reaction (16). ~47

162 J. C. Scaiano, E. A. Lissi, and M. E Encina

7o 7o ?o , ?o ?o+ Ph Ph Ph Ph Ph Ph Ph Ph

06)

The homopolymer, poly(phenyl vinyl ketone), PPVK, cleaves in benzene with • = 0.24 and a triplet lifetime of 17 ns according to Guillet t~T and 7 ns according to Fanre et al. 8' Singlet-triplet intersystem crossing (kt in Scheme 1) occurs with kx > 3 x 108 s -1 in benzene, and is somewhat slower in chlorin- ated solvents. Quantum yields of chain scission in the range of 0.4-0.6 have also been reported. ~48 The techniques used to study the reaction have included con- ventional quantum yield and quenching studies, x~7 light scattering techniques, ~48't~ nanosecond laser photolysis techniques s*'92'9~'xs° and picosecond spectroscopy) 2 The biradical involved in the reaction has been detected directly; ~2 in benzene it has a lifetime of 63 ns and ~m,~ = 410 nm. Typical triplet quenchers, e.g., naphthalene, quench the reaction in much the same way as they would quench the Type II reaction in simple ketones. ~*'8~'~7-~s2 Some rather unusual oxygen effects (or, more frequently, a surprising lack of effect) have been reported. These will be discussed in Section V D.

Several of the studies mentioned above have also included an examination of copolymers of PVK with several monomers. Guillet ~7 has shown that the scission of a PVK-styrene copolymer occurs with lower quantum yields than in the case of PPVK and that quenching by 1,3-cyclooctadiene leads to non- linear Stern-Volmer plots. The effect has been attributed to differences in the triplet lifetimes of isolated and sequential carbonyl groups.

The photochemistry of a number of oligomers of PVK has also been examined. 14°'tS~-tSe For example, 1,5-diphenyl-2-methyl-l,5-pentadione, 33, undergoes Type II

O O

p h ~ / J ~ p h

33

cleavage giving acetophenone and propiophenone, depending on whether the biradical 34 or 35 is produced: lsa.ls4

OH 0

P h ~ ~ . Ph

34

O OH

P h @ @ P h

35

, propiophenone, 0 -- 0.037 ts3, 0.02 T M

acetophenone, 0 = O. 13 xs3, O. I 0 TM

(17)

Chemistry of the biradicals produced in the Norrish ~pe H reaction 163

Photoracemization studies have been carried out by Wagner et al. 75 in the case of (4S)-(+)-4-methyl-l-phenyl-l-hexanone, 36.

36

The sum of the quantum yields of products (acetophenone and unresolved cyclobutanols) and the yield of racemization is 0.95 + 0.05. Although the value is, within experimental error, unity, Wagner 75 has pointed out that the small rotation which is observed in the cyclobutanol products indicates that the biradical intermediate is not sufficiently long lived to completely lose its chirality by rotation of the ~ C-C bond. Combining this data with lifetime measurements we have estimated the barrier to rotation to be 4-5 kcal tool-1, g° The photoracemization of 36 is a strong argument in favor of the intermediacy of biradicals in the Type II process. Another example where internal rotations do not lead to the total randomization of the biradical structure has been reported by Lewis T M in the case of 1,4-diphenyl-4-hydroxybutan-l-one. 157 Since the biradical lifetime is not very different from those of simpler ketones (Table 6) the small ratio of the rates of rotation and decay has to be attributed to a rather high barrier to rotation 76 which we estimated to be 7.2 kcal mo1-1.

b. Alkyl ketones

As we have already pointed out, the situation in the case of aliphatic ketones is considerably more complicated, largely due to the fact that the reaction occurs from both singlet and triplet states, as well as to the uncertainty in ~sc values. Table 10 shows a summary of results for the three simplest ketones. As in the case of aromatic ketones we find that reversal is usually the main reaction path, while the cyclobutanols account only for a rather minor fraction of the biradicals. We also observe that (~Pn/1Poy) > (sPn/SPcr) and (1PnJlPK) < (3PnJ3PK). Since the values have been obtained in different solvents and by different research groups the comparison is only semiquantitative.

The percentage of cyclobutanols from the triplet state reaction is considerably higher in the aliphatic ketones as compared with the aromatic compounds

Table 10. Reaction~ of the biradicals derived from aliphatic ketones 8e'aT'l=s'ls8

Ketone 1P z 1Pn 1Pc T (:Pn/IPo,) 3PR 8Pn aPe, (aPn/aPc,)

2-Pentanone a 0.82 0.14 0.04 3.5 0.60 0.25 0.15 1.7 2-Hexanone b 0.83 0.16 0.01 16 0.43 0.40 0.17 2.3 5-Methyl-2-heptanone c 0.91 0.08 0.007 11 0.46 0.41 0.13 3.1

"Oxsc = 0.84. b Oxsc = 0.39. c q~xsc = 0.14.

164 J. C. Scaiano, E. ,4. Lissi, and M. It. Encina

(see Table 9). No cyclobutanols have been detected in the photochemistry of polymers, le°

Table 11 shows a summary of aP n and xPzx values for aliphatic ketones. We note that the errors involved are rather high, largely as a result of the un-

Table 11. Values ofaPn and IPn for aliphatic ketones in n-hexane at 20oc eo

Ketone 1Pn aPu

PRIMARY y-hydrogens 2-Pentanone 0.14 0.24 4-Heptanone 0.07 0.15 4-Methyl-2-pentanone 0.14 0.23 2,6,-Dimethyl-4-heptanone 0.08 0.13

SECONDARY y-hydrogens 2-Hexanone 0.15 0.28 2-Heptanone 0.13 0.36 2-Octanone 0.11 0.39 2-Nonadecanone 0.08 0.36 (40°C, ref. 161) 3-Heptanone 0.11 0.21 4-Octanone 0.07 0.33 (ref. 162) 5-Methyl-3-heptanone 0.08 0.20 6-Undecanone 0.07 0.24 (refs. 163, 164)

~R~AaY y-hydrogen 5-Methyl-2-hexanone 0.13 0.38 5-Methyl-2-heptanone 0.08 0.40 (ref. 159)

certainty in ~sc. For example in the case of 2-hexanone the values reported are: 0.6, le5 0.39, a7 0.37, 2 0.32,166 and 0.23. 5a

The data do however indicate that:

(i) a-substitution decreases ap~t; (ii) ]3-substitution has little effect on 3p,~;

(iii) y-substitution slightly increases ap,,; and (iv) apn is only slightly dependent on the chain length.

The trend with y-substitution is different than in the case of alkyl aryl ketones (compare with Table 9).

The quantum yields of photofragmentation of aliphatic ketones change slightly with the chain length; in particular, they are more sensitive to the length of the group which does not provide the hydrogen atom. Ou decreases with increasing chain length and the effect is more marked in ketones of the type RCOR than in the case of CHaCOR. Iel'16v For ketones of the RCOR type ~rt decreases from 0.11 to 0.059 when the number of carbon atoms in R increases from 4 to 21,16v and reaches a value of 0.035 for polyethylene containing a small fraction of carbonyl groups. 16a.18a Unfortunately these are overall quantum yields, i.e., for singlet and triplet state reactions. From a comparison with aromatic ketones, it would seem that the changes in ~ with chain length reflect


the behavior of the singlet-derived biradical; in other words, 1P n rather than 3P n would be responsible for the changes in On.

For the copolymer of methyl vinyl ketone with methyl acrylate ~ ~ 0.2,1~8 probably reflecting the fact that 3pn is of the same order of magnitude as in small molecules.

Ooy also seems to decrease with chain length ~79 and cyclobutanols have not been detected in polymer photodegradation3 6°

Guillet t61'~63.1e7 has attributed the dependence of On on chain length to a change in the rates of rotation in the biradical. The dependence on chain length has no effect when the length exceeds 20 carbon atoms; 1~°'t8° under these conditions, the internal rotations do not involve the whole chain but only segments.

The values of ~P~ and 3P~z are temperature dependent. If we arbitrarily define an apparent activation energy according to equation (18), we obtain the values given in Table 12. 8°

,(sPa) = - R8 [ln(NPa)]/8(1/T) (18)

We note that the values in Table 12 are not "true" activation energies, since ~rPn corresponds to an expression of the form NPn -- (kt/Ytkt). The values of e should simply be regarded as a convenient way of expressing the temperature dependence of these parameters.

Table 12. Temperature dependence of ~Pn for aliphatic ketones in n-hexane e° (according to eq. 18)

Ketone ~(IPn) ~ e(sPn) ~

4-Methyl-2-pentanone 0.0 0.6 3-Heptanone 1.0 0.6 2-Hexanone 0.8 - 0.1 2-Heptanone 0.7 0.4 5-Methyl-2-hexanone 1.5 - 0 . 6

° in kcal tool-i.

A large dependence of ~Jz on temperature has been observed in the photochemistry of the copolymer of styrene with methyl isopropenyl ketone. 18° Quite probably this dependence reflects changes in the quantum yields of biradical generation due to the competition between hydrogen abstraction and Type I cleavage.

All the values of r(sPa) are smaller than 1.5 kcal tool- 1. This is in agreement with the fact that similar activation energies for the different modes of decay of the biradicals are expected. 2a'l"t The results in Table 12 show trends similar to those observed for aromatic ketones la6aaa but the values of 8(3Pn) are smaller in the case of the aliphatic compounds, la2 This difference could be due to the


differences in the solvents used since the reactions of biradicals, as well as their apparent activation energies, are dependent on solvent polarity. 1aS'in9

If we ignore cyclization (this being a rather minor mode of decay) it appears that in those cases where e(sPa) < 0 (see Table 12 and refs. 60 and 138) and k7 > k5 (rate constants refer to Scheme 1), A~ _> As. This does not seem to be compatible with the transition state structures associated with fragmentation and reverse hydrogen transfer. In the reverse hydrogen transfer case four internal rotations are lost 18a while in the fragmentation (see Figure 2) only two rotations are lost, if we accept the maximum overlap restriction; a.117 we would therefore expect As > AT. T M O'Nears calculations 2" also suggest (AJAT) ~ 10. All these observations suggest that the decay of 1B (Scheme 1) is not controlled by chemical factors but that rotational control is more likely responsible for the product ratios and their temperature dependence. Furthermore, the differences in ~rPn, ~P~, and sP~ values would be largely derived from the fact that when the biradicals are initially generated in the singlet state (i.e., derived from the singlet excited state) a significant fraction decays prior to rotation.

A similar situation is found in the case of deuterated ketones where the product distributions can be explained on the basis of rotational control, a° For example, the values of 1 ~ and aO,~ obtained for 2-hexanone and 2-hexanone-5,5-d2 allow us to estimate the sP~ values from knowledge of tbmc. If we take Omo(2-hexanone) = 0.39, a7 the value for the deuterated derivative can be estimated as 0.573 8s For both ketones we obtain spa ~ 0.4, while if the process were chemically controlled one would expect a larger apa value for the deuterated ketone (due to the isotope effect on the reabstraction). For the example considered above rotational control seems to be a very important factor; however, the case could be different for those examples where a given decay path is largely unfavorable and chemical control can take over. 187

A number of reports i01,1s9.188 have been concerned with the photochemistry of molecules where the loss or retention of stereospecificity can be used as a monitor for the behavior of the intermediate biradical, either by examining the yield of photoracemization of the starting material ls8 or the cis-trans ratios of the olefins produced in the photofragmentation.l°l'189 The reactions of the triplet state show little selectivity v~.~89 while those of the singlet state are highly stereospecific. ~°~'~5g'~8~ In a study of the photochemistry of threo- and erythro- 4-methyl-2-hexanone-5-dl, 37, Casey and Boggs lol observed that the reactions from the singlet state are only 90 and 95~7o stereospecific for the erythro (37a) and threo (371)) isomers respectively. They concluded that the "partial loss of stereo- chemistry in the olefins formed from the singlet state...provides convincing evidence for a singlet 1,4-biradical intermediate." 1oi

H H H a C ~ x , ~ / H a C ~ - . , . ~

37a 371)


The most common and probably correct explanation for the results given above is that the triplet biradical lives long enough to undergo bond rotations as a result of the requirement of spin inversion, which is considered the rate determining process. 1°°,1°1,~s9 The singlet state reaction must involve a biradical; 1°1 however, this route does not necessarily account for 10070 of the singlet state reaction (see Scheme 2). The question as to whether a reaction which leads to reversal by the route XK --~ A --+ X ~ D of Figure 1 should be regarded as a biradical process is largely a matter of definition. Similar questions can be raised in the ease of singlet generated biradicals which decay to molecular products prior to bond rotation (reactions (10), (11), and (12) in Scheme 2); while the species (XB) is probably a biradical from an electronic point of view its chemical behavior is largely determined by the possibility and rate of bond rotation. Stephenson and Brauman 9~ have offered an alternative explanation for the differences in behavior between singlet and triplet generated biradicals, based on the postulate of the generation of "ho t" singlet 1,4-biradicals. As pointed out before (Section V ,4,) most authors regard the Bartlett modeP °° as the only one consistent with experiment. 28'Te'9°'~°~n59

We have pointed out that in the case of aromatic ketones the cyclization/ fragmentation ratios are very sensitive to a-substitution. In the case of aliphatie ketones this type of information is rather scarce, but there are some studies which also show the dependence of the product ratios on the possibility of attaining the required orbital orientation. For example the photolysis of l- adamanthylacetone, 38, yields only cyclobutanols 39 and 4o.14~.~90 CH3 OH ~ H ~

38 39 4O

Table 13 shows a summary of the results. An analysis of the initial conformation of the biradical shows that the formation of 39 requires less motion than that of 40. This would be reflected in the larger value of ~39[ff~o for the singlet state,

Table 13. Photochemistry of l-adamantylacetone xs°

Excited Solvent State ¢D~9 ¢~,o ~08]~o

Benzene Both 0.010 0.0033 3.3 Benzene Singlet 0.0083 0.0017 4.9 Benzene Triplet 0.0017 0.0016 1.1 Methanol Both 0.027 0.012 2.3 Methanol Singlet 0.0084 0.0016 5.3 Methanol Tr ip le t 0.0186 0.0104 1.8


i.e., product formation from the singlet state to a great extent occurs prior to geometrical randomization. Solvents effects are essentially normal (see also Section V D).

The Type II reaction has also been examined in aldehydes, TM carboxylic acids, taa S-alkyl thiocarbonates, taa esters and carbonates; TM these will not be discussed in this review. c. Cycloalkanones

Cycloalkanones having more than eleven carbon atoms in the ring give products which result from transannular hydrogen abstraction 1as'2°° as illustrated in reaction (19). The fraction of fragmentation products tends to increase

.o,) L. I I " ' l I +l 0,,

I ) with ring size, probably reflecting the ability of the larger ring ketones to adopt the correct conformation for fragmentation.

Small ring cycloalkanones can undergo intramolecular abstraction when they are substituted at the a-position, 3 e.g., reaction (20): 2ol.2o2

,J.

OH

1 0

+ C2H~

(2o)

The information on ~Pn, ~rpo~, and ~rPx is quite limited, largely due to the fact that ~-substitution makes the Norrish Type I reaction an efficient competitive process. This process also becomes more efficient as the ring size decreases, as a result of the increase in ring strain, t42,2°t'2°" The biradical generated in the Type I cleavage can proceed to photoproducts or regenerate the parent ketone. The latter reaction path cannot be distinguished from the reabstraction process in the Type II biradical to generate the same ketone. Detailed studies of the photochemistry of several substituted cyclohexanones and cyclopentanones have been reported by Turro et al. t~2'2°t'2°a


The cyclization/fragmentation ratios, which can be determined more easily than the values of a~p, show some interesting characteristics. 2°~2°6 These values are given in Table 14 together with those for some of the dialkyl ketones. We note that 3Pc~/3Pn values for the cyclic ketones are quite high while xPor/xP a values can be regarded as normal, particularly if we consider that ~-substitution usually results in an increase of the fraction of cyclobutanols. This is probably due to the fact that in the singlet state reaction the products are derived from conformations close to the one in which the biradical 1B is initially produced, while in the case of the triplet state of alicyclic ketones most of the products arise from transoid configurations which cannot lead to cyclobutanols, a°8 Compound 45 in Table 14 shows some interesting characteristics, discussion of which can be found in the original publications. 2°5"2°7

A number of other systems have also been published, 2°~-2~2 including some examples of cyclic carbonyl compounds of biological interest. 21"~,2~4

d. Unsaturated ketones

The processes involved in the photochemistry of unsaturated ketones are frequently complex as a result of the number of reaction paths available. For example 46 does not give any products that could be attributed to a Type II biradical. 9 Others, like 47, undergo reactions characteristic of abstraction by the /3-carbon atom, ~a,21~-2x~ while 48 and 49 undergo y-hydrogen abstraction leading mainly to cyclobutanols. 21s'219

The acetylenic ketone 50 undergoes cyclization (ff = 0.12) and fragmentation

46

48

0

47

0

49 50

(~ = 0.32) in benzene in a reaction which occurs from the triplet manifold. 22° The fraction of the biradicals which proceeds to products is somewhat greater than in the case of 2-hexanone, showing that this type of unsaturation does not appreciably modify the behavior of the biradical.

The photochemistry of/3,y-unsaturated ketones has been recently reviewed by Houk. 221 The photochemistry is rather complex and the Type 1I reaction (leading only to cyclobutanols) occurs only in a few cases (usually from the singlet state), 221,222 and in systems containing y-allylic hydrogens. 222-228


Table 14. Cyclization[fragmentation ratios

Ketone Solvent 'Pa,/1Pxx 3P~,/3Pu Reference

2-Hexanone Pentane 7:93 30:70 36 2-Octanone Benzene 2: 98 13: 87 207 4-Octanone n-Hexane 10:90 16:84 165

t-Butanol 13: 87 73: 27 205

41 OAc

H ~ t-Butanol 24: 76 42: 58 205 U Y . v

H

42

CoHl7

H

43 OAc

H

O H / H

t-Butanol 11: 89 30: 70 205

t-Butanol 46:54 70: 30 205

t-Butanol 76: 24 80: 20 205


Dalton and Chan 22a have examined the photochemistry of 51, 52, and 53. The last two can yield four different cyclobutanols (54-57);

51 52 53

CH3

CH3 OH CHa CHa H CH3 H ..OH 54 55 56 57

of these, three were detected and were produced in essentially the same ratio from either 52 or 53. This indicates that the biradieal produced in the intramolecular abstraction lives long enough for bond rotation to compete with biradieal decay. 228 These results seem to indicate that in this case the reactions of 1B are chemically controlled.

In the case of ~ unsaturated ketones the main reaction path features the interaction of the excited carbonyl groups with the double bond in a process taking place in the singlet stateY °3'229 Only in the case of y-tertiary hydrogens can some photofragmentation be detected. For example, in the case of 58, 9~x =

O

58

0.0008. 22~ Ketones with ~ unsaturation produce biradicals quite efficiently, as a result of the low bond energy of the y-hydrogens. Yang et al.a°a have examined the photochemistry of 6-hepten-2-one, 17, which has already been discussed (Section V B).

a-Diketones bearing y-hydrogen atoms undergo intramolecular y-hydrogen abstraction, 2a°-2aa ultimately leading to cyclobutanols. Wagner et aL la5 have examined the photochemistry of a series of phenyl ~-diketones; y-hydrogen abstraction always involves the 1-keto group, ultimately leading to ~-hydroxy- cyclobutanones. The reaction competes with photoenolization, a process which is not yet totally understood. 2a4

172 J. C. Scaiano, E. ,4. Lissi, and M. V. Encina

e. Isotope effects The magnitude of isotope effects can be used to decide the degree to which

a reaction is chemically controlled. If the reverse hydrogen transfer has a significant activation energy one could expect an important inverse isotope effect on the quantum yields of product formation whenever chemical control is a domi- nant factor. In the case of rotational control no isotope effect is expected. A very small isotope effect can be interpreted in terms of a very low activation energy or as the result of mixed control.

In the case of aromatic ketones the isotope effects on the quantum yields are

Table 15. Kinetic and quantum yield data for ketones 59--62 e7

Ketone ~n k~ x 10-° (s-l)"

~ph 59

0.11 3.4

0.11 2.7

60

H \D

h 61

0.16 1.1

D D

D D

h 62

0.16 0.86

"Rate of hydrogen or deuterium abstraction.


generally small 1°5 and sometimes negligible as is the case of 1,4-diphenyl-4- hydroxybutan-l-one, la6 although this example cannot be regarded as a typical biradical due to the possible importance of intramolecular hydrogen bonding. An important study has been carried out by Lewis et al. 57 who examined the ketones 59-62. The results are shown in Table 15.

The results show that there is no secondary isotope effect on ~rz, but that there is an inverse primary isotope effect of 1.5. This effect, though smaller than the isotope effect on the abstraction reaction, suggests a slower rate of reabstraction associated with the deuterium atom and implies a certain degree of chemical control.

In the case of aliphatic ketones analysis of the results is somewhat more difficult. We have already mentioned the results of Coulson and Yang, 36 which can be interpreted in terms of an isotope effect on the reverse hydrogen transfer 23s or in terms of a decrease in the quantum yield of intersystem crossing. 5g'x°5'xS~ The possibility of an isotope effect on ~se (as a result of changes in kg, see Scheme 2) makes the analysis of isotope effects on biradical behavior very difficult in all those cases where ~so has not been measured for the deuterium derivative. For example, the change in ~(C2D3H)/~(C2D2H2) with temperature, observed in the photochemistry of 2-pentanone-4,5,5-d3 reflects a change in the quantum yields of biradical production from the singlet and triplet state, in addition to changes in kalkD, NP~x(H) and NP~(D).~3°

Sauers et aL 23~ have examined the photochemistry of a series of bridgehead acetone derivatives. 2s7 In the case of adamantylacetone, 63, irradiation in

O=~ CH3

63

t-butyl aleohol-0-d showed the incorporation of some deuterium at the ring carbons (2%, where 50% of the ketone had been consumed). Surprisingly enough, when nonanophenone-7,7-d2 is irradiated in the presence of 0.3 M t-butyl alcohol no D--H exchange occurs, a The reason for this difference remains unexplained, though one obvious possibility is that the lifetime of the aliphatic biradieals could be considerably longer than that of aromatic biradieals.

3. Studies in the solid state Studies of the Norrish Type II reaction in the solid phase are not extensive.

Slivinskas and Guillet 2a8 showed that crystals of 8-pentadecanone did not yield any Type I or Type II products when photolyzed or subjected to y-radiolysis at a temperature slightly below the melting point of the crystals; however,


the reaction occurred when the sample was melted and treated under identical conditions. These results suggest that chain mobility is a very important parameter in determining the values of spp.leo The same conclusion can be reached upon examining the changes in 4~a with temperature in polymer films. In the cases of the copolymers of styrene and methylmethacrylate with phenyl vinyl ketone and in PPVK a sharp decrease in quantum yields has been observed when the temperature is brought below the glass transition temperature (T~). For example, in the copolymer of styrene and phenyl vinyl ketone (Tg = 104°C), the value of Sa decreases from 0.28 to 0.087 when the temperature changes from 104 to 100°C. 178'23g At T > T, the value of ~ is similar to and sometimes higher than the yield in solution, x~7 suggesting that under these conditions, the formation of the six-membered abstraction ring is highly efficient in spite of the short triplet lifetime; 1~7 furthermore, the results show that the biradieal must have enough time to change its geometrical conformation prior to spin-flip.

When T < Tg the rotations of the main chain are restricted. The sharp decrease in ~x may be due to the following causes: (i) the biradical generation is inefficient because the internal conversion processes become competitive with hydrogen abstraction, now limited by the rate of bond rotation; (ii) the biradicals formed are considerably less mobile and decay mainly by reabstraction; in the case of the triplet state reaction this means that spin-flip occurs in its initial conformation; and (iii) fragmentation of the biradical becomes considerably slower; this could be related to the activation volumes for reaction 5 (Schemes 1 and 2), which should be positive, 24° leading to an unfavorable process in the solid state.

An interesting effect is the enhancement of the polymer luminescence caused by the restriction of molecular motions. 16°'241 For example, Guillet et aL 2~ have observed that the phosphorescence of poly(ethylene--CO) increases dramat- ically when the temperature is reduced. This suggests that the effect originates from the abstraction process, i.e., the change in phosphorescence which has an activation energy of ca. - 7.8 kcal mol- x would largely reflect the activation energy of the hydrogen abstraction by the triplet state. 55

D. Assisted intramolecular processes

The behavior of Type II biradicals is strongly dependent on interactions with the medium. For example, the Type II reaction of simple phenyl alkyl ketones (see Table 9) usually leads to ca. 60% reabstraction in nonpolar solvents. The fraction of biradicals leading to photoproducts, in particular to fragmentation products, can be increased considerably in the presence of alcohols, 8 pyridine, s dioxane, 242 organophosphorus (V) compounds, 2~8 thiols, 2s and oxygen/a'2~4 This marked sensitivity to the medium is largely characteristic of the triplet state reaction and in general can be regarded as being the result of "protection" of one of the reaction sites involved in the reabstraction reaction, i.e., the hydroxylic hydrogen and/or the y-radical site.

As we have already pointed out, the triplet and biradical lifetimes are not


very dependent on the nature of the solvent (see Table 6). In general, the changes in quantum yields reflect changes in aPu, aPcy and apt due to variations in the rates of the different reactions of 1B (see Scheme 1). Hornback 44 (see reaction (3)) has shown that a similar increase in the yields is also observed when the biradical is derived from the corresponding olefin.

We will discuss in this section only those reagents which change the product ratios in the Norrish Type II reaction, but which do not form any new products. The interaction of biradicals with thiols, 7s which also involves hydrogen abstraction, and the interaction with oxygen, 24+ which is accompanied by the formation of a hydroperoxide, are discussed in Section VI.

Some examples have been reported where the introduction of a new solvent substantially changes the mode of decay of the triplet state. For example, when butyrophenone is irradiated in benzene the quantum yields for acetophenone formation and butyrophenone consumption are 0.34 and 0.40 respectively, xx7 In methanol the values are 0.35 and 0.80. The large difference between the two values reflects the photoreduction of triplet butyrophenone by methanol, i.e.,

OH I

PhCO*CaH7 + CHaOH "-+ Pht~---CaH7 + (~H20H (21)

This type of effect is expected to be observable in systems involving long lived triplets and good hydrogen donors. Methanol increases the probability of fragmentation of the biradical but at the same time it decreases the quantum yield of biradical formation.

In the case of aromatic compounds, ~mc is usually unity; if the limiting quantum yields of fragmentation plus cyclization at high concentrations of polar solvents do not approach unity, it can frequently be inferred that intramolecular abstraction is not the only process in which the triplet state partici- pates. 2~5 Wagner 7 has indicated that acetonitrile containing 2% water is a convenient solvent in which limiting quantum yields can be measured.

Rauh and Leermakers 13a have found that while the quantum yield of fragmentation of butyrophenone increases in moderately polar solvents (e.g., t-butyl alcohol and pyridine), highly polar solvents (e.g., 85°70 phosphoric acid) cause a decrease in ~=. The effect results from a change in the nature of the excited triplet, i.e., in highly polar solvents the 3zr, ~r* state lies at lower energies than the 3n, 7r* state. 2+6 A similar effect is observed when t-butyl alcohol is added to p- methoxyva le rophenone . 247 W a g n e r a3,24a has suggested that the enhancement of the quantum yields by polar solvents is the result of hydrogen bonding of the hydroxyl proton in the biradicai to the solvent, which prevents the otherwise efficient return of the H-atom to the 7-carbon. Scheme 3 shows the mechanism of the triplet state reaction assuming that the overall process is kinetically controlled; in other words, k~6 has been designated as an irreversible process. In fact, there is no way to establish, from quantum yield studies, whether one should include such a reaction in the mechanism. 24~

176 J. C. Scaiano, E. A. Lissi, and M. F. Eneina

kls aK + X ~ quenching

a B + X k~,> BX

BX k . K + X

BX k~,> fragmentation + X

BX k : . eydization + X k~ BX > new products (Y)

Scheme 3. T h e r e a c t i o n s in th i s S c h e m e occu r in a d d i t i o n to t h o s e s h o w n in S c h e m e I.

From Scheme 3, eq. (22) can be derived,

~o 8p0 n 1 ~ ~o~ = 3 . * ~ - 8.o (1 +

- -

or, alternatively, a

(22)

~ _ ~o = 1 + kxe~'e[X] (23)

where the superscript 0 refers to the results in the absence of X and oo to those at "infinite" X concentration, i.e., they represent the probability (e.g. 3P~z) of reaction from BX.

Table 16 shows the data for valerophenone interacting with t-butyl alcohol, pyridine and several organophosphorus compounds. Limiting quantum yields of fragmentation for aromatic ketones have already been included in Table 9. The value of ~oy/~,, is usually smaller in polar solvents than in hexane or benzene. The effect is mostly the result of the increase in ~:x. Table 17 shows some representative values selected from the literature)

In the case of organophosphorus compounds there is evidence suggesting

Table 16. Interaction of several polar substrates with the biradical from valerophenone in benzene

Substrate klera ~ kle a ~n/~% ~ ~cul~u ~ Reference

t -C,HgOH 0.63 1.8 x 107 1.03 - - 8 Pyddine 4.4 1.3 x 10 a 1.15 - - 8 PhaPO 18 5.3 x 10 a 1.48 0.16 243 (CHaO)aPO 12 3.5 x I0 a 1.35 0.19 243 (CaHTO)aPO 16 4.7 x 10 a 1.44 0.18 243 (PhO)aPO 1.7 5.0 x 107 1.06 ~0 .22 243

Units of I r ee l - 1, based on the assumption of kinetic control. l r ee l - 1 s - x; see reference 243 for the estimation of kle. for IX] = 0.02 M.

Chemistry o f the biradicals produced in the Norrish Type H reaction

Table 17. Fraction o f cyclobutanols from aromatic ketones in benzene and alcohols s

~7o cyclobutanols 4 Ketone Benzene Alcohol

Butyrophenone 12 -- Valerophenone 18 12 ?-Methylvalerophenone 11 6 Hexanophenone 20 -- ~-Methylhexanophenone 23 17 a-Methylvalerophenone 68 64 p-Methoxyvalerophenone 20 15 m-(CFo)-valerophenone 24 23

"Cyclobutanol/(acetophenone + cyclobutanol).

177

that not only the hydroxyl proton but also the 7-radical site is involved in the interactions with the substrate. 24a For example the values ofkl6~'B are 16, 12 and 6 1 tool -1 for butyrophenone, valerophenone and 7-methylvalerophenone respectively for their interaction with trimethylphosphate; the order of the interaction is the reverse of that observed in the cleavage of phosphoranyl radicals. 25°'251 The enhancement of the yields is only observed with substrates bearing Pm-O bonds. P(III) compounds are excellent quenchers of singlets and triplets.252, 253

In the case of aliphatic compounds where the Type II reaction occurs from the singlet as well as the triplet state, the polarity of the solvent affects only the triplet state reaction. The conclusion that the singlet state process is largely independent of the solvent is based on the observation that the yield of the reaction which cannot be quenched by dienes is not affected by the polarity of the solvent. 2.5~.15~.165.2~8.25~.255

The viscosity of the solvent has little or no effect on the behavior of the biradicals, as suggested by Guillet's study of the photochemistry of 2-undecanone. 181

A number of studies have been concerned with the addition of polar solvents, dienes, olefins, tributylstannane, etc. to 2-pentanone, 2°~'258'257 2-hexanone, ~65 2-octanone, 2°~ 4-octanone, 16s 5-methyl-2-hexanone 2°7 and S(+)-5-methyl-2- heptanone; ~s~ the reader is referred to the original publications for the details of these results. The ketones having longer lived excited states might engage in direct interactions between the excited state (in particular the triplet) and the solvent. Such is probably the case for 2-pentanone in cyclohexane and cyclo- hexene)S4.256

The easiest way to differentiate between interactions of the solvent with the biradical and with the triplet state is to study ketones having different triplet lifetimes. While an interaction with the biradical will not be affected by the change in triplet lifetime, the probability of an interaction with the triplet is

178 .l. C. Scaiano, E. A. Lissi, and M. V. Encina

inversely proportional to the triplet lifetime (see also Section VI D). An analysis of this type has been carried out by Encina and Lissi 2s4 for 2-pentanone, 4-methyl-2-pentanone, 2-heptanone, 2-octanone, 5-methyl-3-heptanone and 5- methyl-2-hexanone.

Table 18 shows a summary of Np values in different solvents. In order to be able to estimate the effect of the solvent it is necessary to know the value of ,~mc and the fraction of excited states quenched by the solvent. In solvents of

Table 18. Reaction probabilities in different solvents

Refer- Ketone Solvent xPx ~Px 1Pu 3Pn 2Pc, ~Po, ence

2-Hexanone Hexane 0.13 0.36 165 t-Butanol 0.13 0.74 165

2-Hexanone Hexane 0.13 0.30 60 Acetonitrile 0.13 0.39 60

4-Methyl-2-pentanone Hexane 0.14 0.23 60 Acetonitrile 0.18 0.35 60

2-Octanone Benzene 0.004 0.13 207 Methanol 0.003 0.19 207 t-Butanol 0.006 0.26 207

4-Octanone Hexane 0.93 0.43 0.062 0.45 0.007 0.115 165 t-Butanol 0.93 0.12 0.056 0.73 0.007 0.146 165

5-Methyl-2-hexanone Benzene 0.0035 0.29 207 Cyclohexane 0.0023 0.40 207 Methanol 0.007 0.57 207 t-Butanol 0.009 0.70 207

5-Methyl-2-heptanone Hexane 0.89 0.47 0.083 0.394 0.007 0.131 159 t-Butanol 0.92 0.145 0.068 0.68 0.004 0.172 159

high dielectric constant 3Pra increases and ap,~ decreases. Alcohols also lead to an increase in 3poy. The enhancement of 3Pxi is larger in good hydrogen bonding solvents, in agreement with the results for the case of aromatic ketones, 93.x38 again because of hydrogen bonding.

The magnitude of solvent effects is larger for the case of aromatic biradicals than in that of aliphatic compounds. This difference has been attributed to a longer lifetime of the biradicals derived from aromatic ketones. 2.258

Eqs. (22) and (23) also hold in the case of aliphatic ketones, with the only difference that the quantum yields refer to those based only on the triplet s tate contribution to the reaction. This has been confirmed for 4-methyl-2-pentanone and 2-heptanone in n-hexane, interacting with t-butyl alcohol, ethanol and amyl alcohol. 26° As in the case of the aromatic biradicals, 8'2~3 the linearity of these plots implies a 1 : 1 interaction between the biradicals and the solvent molecules.

The values of 3p~, for 2-heptanone are 0.5 in t-butyl alcohol and amyl alcohol and 0.54 in ethanol (based on 3P a = 0.36), while the values of kx6~'8 (or the corresponding equilibrium parameter) 2~ are 0.5, 0.5 and 0.41tool - t respectively.


VL Intermoleculax reactions

179

The biradicals involved in the Type II reaction can undergo a number of intermolecular reactions characteristic of both radical sites. These reactions can produce a new biradical (e.g., addition to double bonds) or two radicals (e.g., atom or electron transfer). This section is subdivided according to the type of reaction involved. All the examples considered refer to trapping of triplet derived biradicals. No examples of trapping of singlet derived biradicals have been reported.

A. Hydrogen abstraction

The first study of biradical trapping involving a kinetic analysis of the process was reported by Zepp and Wagner in 1972. aa The authors showed that thiols are rather inefficient quenchers of carbonyl triplets. 281 However, thiols do quench the formation of fragmentation products from ~,-methoxybutyrophenone and valerophenone in benzene containing t-butyl alcohol or pyrid- inc. 7s.~8 The information obtained in kinetic and deuterium labelling experiments provides clear evidence that the species being trapped by the thioI is the biradieal. The experiments led to the evaluation of kTrB, the product of the rate of trapping and the biradical lifetime as 1.8 1 mol-x for dodecyl mercaptan reacting with the biradical from valerophenone, and butanethiol reacting with the biradical from ~,-methoxybutyrophenone. Butanethiol-5-d~ is only one third as efficient. Reaction (24) illustrates the process in the case of ~,-methoxybutyrophenone

Ph Ph

OMe OMe

+ RS- (24)

One of the possible reaction paths of the radical pair produced in reaction (24) is disproportionation to RSH and the parent ketone; if RSD is used this leads to ~,-deuterium labelling of the ketone. 98

Wagner 75,98 has pointed out that the values reported for the rates of reaction of alkyl radicals with alkyl mercaptans range from l0 s to 108 1 mo1-1 s- l , 262 making the estimation of ~'B from krrB rather uncertain. Wagner T M estimated rB to be 1/~s in polar solvents, based on kT ~ 106 1 mo1-1 s -1. A comparison with values of rB obtained in time resolved experiments (Table 6) suggests that the correct value ofkT should be ~ 1.8 x 107 1 mol -~ s-L

The trapping experiments with thiols have been limited to polar solvents because in non-polar media the addition of small concentrations of thiols results in an enhancement of the yields 2s similar to that caused by alcohols.


Wagner et aL TM have tried to trap the biradical from valerophenone with tri-n-butylstannane/5 No trapping could be observed. From this Wagner con- eluded 2 that the biradical lifetime is shorter than 100 ns. Studies of this type are usually faced with the problem that the products of the triplet state reaction and those of the biradical reaction are the same. This is common to most hydrogen abstraction reactions, i.e.,

+ XH

(2s)

Therefore, if the production of fragmentation products is used as a monitor, a decrease in quantum yields can result from either biradical trapping or triplet quenching. This type of mechanism leads to curvature of the Stern-Volmer plots, but this is difficult to detect. We have shown that even in eases where optimum conditions are met, it is still difficult to establish the presence of significant curvature. 2e3 In general, the yields of photofragmentation are unsuitable monitors of biradical reactions whenever triplet quenching is important.

O'Neal et al. 28 have studied the trapping of the biradical from triplet 2- pentanone by hydrogen bromide in the gas phase and obtained kTr~ = 8 x 103 I tool-1 at 115°C. The value was essentially temperature independent from 35 to 194°C. If kr is assumed to be identical to the rate of reaction of ethyl radicals with hydrogen bromide, 29~ then the biradical lifetime can be estimated to be 10 -4.9 s, a value considerably longer than the lifetimes in solution (see Table 6).

Table 19 gives a summary of kr~'B values obtained from hydrogen abstraction experiments, as well as from other reactions which are discussed below.

We have also obtained some evidence indicating that Type II biradicals are also capable of abstracting hydrogen atoms from phenols. 2e5

B. Addition to double bonds

A number of experiments describing the addition of biradicals to double bonds have been reported.

Torselrs trapping experiment with 2-methyl-2-nitrosopropane constitutes the first report on the subject. 2s6 Although the experiments did not conclusively establish the type of species trapped (and have been questioned for this reason), 9s we can now, with hindsight, assume that they were almost certainly 1,4-biradicals.


N

i

r4

0 0 0 0 ~ ~ ..... " ' " 0 0 0 0 0 ~ ..........

.... oooooooo~ "~'~'-'~'~

Z<<<<<<<<666

~ 0 0 0 0

0 0 0

~ o ~ - - ~ ~ ~.~.~.~ ~ = 2 o o o o 2 o o ~

m ~ z ~ ~ ~ o o o o o

O o

8 8

~ o o o o o o o o ~

~ ~ ~ o ~ ~ ~ ~ ~

g ?

o 0 o 0 ( 0

O Q O ~ O

m

o

o

o

..=

0

~.~ °

a~

u~

182 J. C. Scaiano, E. A. Lissi, and M. II. Encina

The biradicals from butyrophenone, valerophenone and 7-methylvalerophenone initiate the polymerization of methylmethacrylate. 267 It is not yet clear which is the mechanism of propagation, but it is certain that the initiation involves the reaction of the biradical with the monomer. The rates of propagation, under constant experimental conditions, follow the following order:

acetophenone < monomer alone < butyrophenone < valerophenone < ~,-methylvalerophenone

while the triplet lifetimes follow the order: s0

acetophenone > butyrophenone > valerophenone > 7-methylvalerophenone.

The low yield of polymerization in the presence of acetophenone, as compared with monomer alone, was attributed to uv screening by the ketone. For the three ketones producing biradicals, it is clear that the efficiency of the process increases as the triplet lifetime decreases, and therefore the possibility of quenching also decreases. The initiation of polymerization cannot be attributed to the triplet otherwise acetophenone, having a long lived triplet and no possibility of giving biradicals, would be expected to be the most efficient ini- tiator in the group.

Osborne and Sandner 26a have reported that 2,2-diethoxyacetophenone (DEAP) initiates the photopolymerization of 2-ethylhexyl acrylate. The effect was accompanied by an increase in the rate of consumption of DEAP which the authors suggest results from biradical trapping thus preventing the reabstraction reaction. The interpretation is probably correct, although a similar effect on the rate of DEAP consumption could result if the monomer is involved in solvent effects of the type discussed in Section V D.

The kinetics of the reaction of Type II biradicals with di-t-butylselenoketone have been examined in detail. 26a The selenoketone is known to be a very efficient spin trap. 269,27° Because of its low triplet energy (probably ET < 40 kcal mo1-1) it is also a diffusion controlled quencher for carbonyl triplets. Reaction (26) illustrates the mechanism of trapping in the case of 7-methylvalerophenone.

Ph Ph Ph

+ But2C = Se kr , ..,A" , Se - CL,A Se - CHButz

64 65 (26) The structure of the intermediate 64 was inferred on the basis of that of 65

and the reactivity of radical centers towards di-t-butylselenoketone. The values of kr~'s in different solvents have been included in Table 19.

The plots of OT (the quantum yield of trapping) vs [Bu~CSe] show a maximum; 26a this is a consequence of the two opposing modes of action of the selenoketone, i.e., quenching of the carbonyl triplet and biradical trapping. The location and quantum yield at the maxima can be used to gather information on the behavior of the biradical.


The values of kr~'s (see Table 19) tend to increase with increasing solvent viscosity, an effect which is also observed in polar solvents (see Table 6 and reference 76).

The quantum yields of trapping are larger for ketones having shorter lived triplets, as is usually the case in biradical processes. 26a'267

Preliminary results using di-t-butylthioketone 265 as a biradical trap (also known to be an efficient spin trap) 271 indicate that the mechanism is similar to the one observed in the selenoketone case.

The reactions of the biradicals with oxygen and SO2 are discussed in Sections VI D and VI E.

C. Electron transfer reactions

Ketyl radicals are good electron donors. For example diphenylhydroxy- methyl2~Z and hydroxymethyl radicals 27a will transfer an electron to paraquat dication 66. The rate constant for reaction (27) is 3 x 10 a I mol -x s-X: 27s

H2t~OH + M e + N / ~ @ / / N + M e

66, PQ + + (27)

M e + N ~ N ' M e + H + + H2CO

67, PQ+"

The ketyl radical sites in biradicals derived from phenyl alkyl ketones have been shown to react in the same way, 76'~°'274 i.e.,

Ph Ph

1R 2 R 2 R 1R

+ PQ+" + H + (28)

Electron transfer reactions of this type are in fact the only examples where the reactions of the biradicals reflect the chemistry of the ketyl radical site. The quantum yields of radical ion formation (monitored at h=a x = 603 nm, emax = 1.2 x 104 1 mo1-1 cm-1) 272"2~s show an inverse dependence with the triplet lifetime 274 which, as pointed out before, 283"2e7 is indicative of a biradical process.

If the formation of the radical ion PQ +" is monitored in laser flash photolysis experiments the corresponding experimental rate constants, derived from the oscilloscope traces, incorporate information on both the lifetime of the biradical and the rate of trapping.

ke~pt = ~'~1 + kT[PQ + +] (29)


From a series of studies of this type we have been able to determine absolute values for ra and kT. 7a,76,9° The values of ra have already been included in Table 6, while those of kT are given in Table 20. The values of kT for other monoketyl biradicals, e.g., from valerophenone, hexanophenone and butyrophenone can be assumed to be identical to the value for y-methylvalerophenone.

A rather interesting example is the case of the biradicals derived from 1,4- diphenyl-4-hydroxybutan-l-one. These are diketyl biradicals 68 and are trapped at both ends, reaction (30): 7e

Ph Ph

~ . O H + pQ++ kT ~ O H OH

I Ph Ph

68 69

+ PQ+" + H + (30a)

Ph /

kRT, C O + pQ+. + H + (30b) 69 + pQ++

] Ph

The experimental values ofkz and ksT are 9 x 10 a and 4.8 x 10 a 1 mol - t s - t 7s suggesting that path degeneracy is important in determining the rate constants.

The rate of reaction (28) is temperature dependent, and in the case of the biradical from y-methylvalerophenonc in methanol kz can be expressed as:

kw = I0 I~-4 exp(-2,500/RT) 1 tool -1 s -I

that is, the activation energy largely reflects the temperature dependence of the viscosity of the solvent.

Other suitable electron acceptors include benzyl viologen 70 and 2,5-dinitro- benzoic acid 71.

PhCH2 + N ~ N + CH2Ph

70

CO2H

71

In a related system, paraquat has been used to trap the biradicals generated in the photoenolization of o-methylacetophenoneY 7


b

X x x x X oo'~ll-~p,,P'.

0

. . . , - - . . ~ •

0 0

"~ ~ ~ .~ ~

0

i

>.~ ¢.,, e.~

r~ O~

¢J

.o

<

0

- i

0 t ~

t~

0

t~

185


D. Interaction with oxygen

Oxygen interacts with Type II biradicals in a rather unusual way. For example, it causes an increase in the yields of photofragmentation of y-methylvalerophenone in benzene. 7a'a44 That the effect should be attributed to a reaction of the biradical and not, for example, of the triplet state, has been con- eluded from kinetic studies (using for example eq. 22). In addition to this, laser photolysis experiments have confirmed the rapid interaction of oxygen with the biradicals and provided direct measurements of the rate of interaction/TM Perhaps the most convincing argument is that in the case of y-methylvalerophenone one atmosphere of oxygen more than doubles the yield of fragmentation (~/~o "" 2.3), while a simple calculation shows that this concentration of oxygen can only interact at most with 77o of the triplets.

In the case of y-methylvalerophenone in methanol the biradical interacts with oxygen with k = 6.5 x 109 1 tool - t s - t determined using the paraquat technique 7a and 7.3 x 1091 tool- t s- ~ by direct detection; 9t laser flash photolysis techniques were used in both eases.

The interaction can be interpreted according to Scheme 3, where the new product which is produced in reaction 20 is a hydroperoxide which, in the case of ~,-phenylbutyrophenone, corresponds to 72Y a

Ph

OOH

Ph 72

Kinetic studies have shown that of the BX complexes formed (Scheme 3), and referred here as BO2, 25yo yield peroxide 72 or a similar one according to the structure of the parent ketone, while the remaining 7570 yield Type II products. The photofragmentation is usually responsible for most of the increases in Type II yields observed in non-polar solvents, while the yield of cyclobutanols is usually increased only slightly.

The interaction of oxygen with biradicals derived from long lived earbonyl triplets is not readily observable. For example, oxygen causes a decrease of the yield of acetophenone produced by photolysis of butyrophenone. The decrease is due to the fact that triplet quenching is the predominant (but not the only) effect of oxygen on the reaction.

We have observed 278 that the modes of interaction of oxygen with the biradical and the ketone triplet can be differentiated using what we have called the "triplet-lifetime-tuning" technique. In this technique the lifetime of the triplet state is " tuned" to a conveniently short value by addition of a chemically inert triplet quencher. The addition of different concentrations of oxygen to


samples containing a constant concentration of quencher results in changes of quantum yields which reflect the interaction of oxygen with the biradical. Quantum yield and kinetic studies lead to the kT~'B values listed in Table 19 and to the following observations.

(a) In the case of non-polar solvents, where reabstraction is usually a major mode of biradical decay, oxygen enhances the yields of fragmentation of the biradicals from all the ketones examined (Table 19).

(b) The effect of oxygen on the yields tends to be masked by triplet quenching as the triplet lifetime increases. Significant compensation of the enhancement as a result of triplet quenching is observed even with relatively short lived triplets, as is the case for valerophenone (rT ~ 7 ns).

(c) In the case of polar solvents, e.g., wet acetonitrile, the interaction results in a moderate decrease of the yields.

(d) The interaction of oxygen with the biradicals results in the production of Type II products with 75% probability and a hydroperoxide (see structure 72) with 25% probability.

We have suggested that the constancy of the product ratio (Type II/hydroperoxide = 3/1) regardless of the nature of the ketone and solvent is due to the fact that the ratio is controlled by spin-statistical factors, as shown in Scheme 4. The mechanism resembles the one usually accepted for oxygen quenching of triplet states. 279'2s° The quintet states are dissociative, while the singlet and triplet states yield products in spin allowed reactions, and therefore lead to the 3:1 ratio observed. In fact the enhancement of the yields can be quantitatively predicted if 3Pn is knownY s

3B + aO 2 k~l> 5BO 2 > Dissociative (3B + aO2) k~

> aBO2 ~ Type II products + aO2 kaa> 1BO2 > hydroperoxide

Scheme 4.

The enhancement of the quantum yields by oxygen has also been observed in the photochemistry of 2-pentanone adsorbed on Vycor glass. 2al

We note that assisted intersystem crossing in itself would not be a sufficient explanation for the effect of oxygen on the behavior of the biradicals, because all the modes of unimolecular decay require spin inversion. The formation of BO2 probably results in the protection of one of the biradical ends, therefore preventing reabstraction.

Finally, a number of reports have indicated that oxygen has no effect on the Norrish Type II reaction; quite frequently these reports have referred to valerophenone and poly(phenyl vinyl ketone). 92.94'147"14a'~Sa'za2 These are incorrect or at least misleading. We have suggested that in many cases the authors have assumed that a non-degassed solution could be considered to be air saturated and this is not true if oxygen is being consumed, as is the case here. 27a Apart


from this, the effects of triplet quenching and interaction with the biradical tend to be compensatory, leading to smaller changes in quantum yields than could be expected from the large values of the rate constants. The rate constant for the reaction of the biradical from PPVK with oxygen is smaller than that of mono- ketones, but still in the neighborhood of 109 1 tool -1 s-X. 26s

E. Trapping by S02

Farid 2sa has shown that the biradicals produced in the photolysis of 73 at A _> 436 nm can be trapped in liquid SO2 at - 5 0 °.

O

O

7 3 ; R = H , t - C 4 H 9

Reaction(31) shows the mechanism proposed.

HO CH2 OH 0 SO2 SO2H

SO2 ~,

R R R ~[ ~ O. O. 0 (31)

A related example involving the trapping by SO2 of the biradicals produced in the addition ofp-benzoquinone to olefins has also been reported. 284

VII. Concluding remarks

Biradicals are proposed as reaction intermediates in thermal and photochemical reactions with increasing frequency. This probably reflects the fact that they are nowadays more "acceptable" intermediates than they were some ten years ago. For example, in the Type 11 reaction from the triplet manifold, the presence of the biradicals has been inferred from studies of photoracemization; they have been trapped in reactions reflecting the chemistry of both radical ends and have been observed directly in laser flash photolysis experiments; it is hard to think that anyone could question the intermediacy of biradicals in the reaction.

Not all the reactions where biradicals have been proposed as intermediates are as well documented as the Norrish Type II process from the triplet state. We do not need to look very far for a less conclusive example: the Type II


reaction from the singlet manifold yields the same products as the triplet state process, but there are obvious differences in the mechanism. While the evidence discussed in Section V supports the involvement of biradicals, these have never been trapped nor observed; furthermore, the evidence only requires the "involvement" of biradicals, but these could represent only a few percent of the total reaction from the singlet state.

There are no reports on the esr spectra of Type II biradicals. This is rather unfortunate because esr has proved a useful tool in the study of other types of biradicals. 28~-288 The explanation for this is very simple: in order to examine Type II biradicals by esr one has to do low temperature experiments which, as a result of the activation energy requirement for the generation of the biradicals, simply prevents their formation.

While the involvement of biradicals in the Type II process is widely accepted, one can find many examples where their possible role is totally ignored. Such is the case in polymer chemistry where in most cases the intermediacy of biradicals is first acknowledged and then ignored. Considering that the use of molecules of high reactivity towards free radical centers is quite common in polymer chemistry, one wonders whether the implicit assumption that biradicals do not undergo intermolecular reactions is a safe one. Most probably it is not. This is probably one of the areas of biradical chemistry which can be expected to undergo substantial progress during the next few years. Another area where more research is warranted is the singlet state process. So far these biradicals have escaped trapping and detection. Quite likely the lifetime might be so short as to make any attempts to trap or detect them fruitless.

We have tried to present an objective view of the information available on the behavior of Type II biradicals. A substantial fraction of the data has been obtained from reports where the primary interest was the behavior of the biradical precursors; in this case it is sometimes difficult to evaluate biradical parameters. Some authors will not totally agree with some of our interpretations; when we were aware that this is the case, we have tried to point out other possible points of view. We realize that we might have failed to present the opposing points of view in a manner reflecting the full merit that others might find in them; for this we apologize.

Finally, our search of the literature is reasonably complete until the end of 1976. We have included a number of 1977 references, but the literature has not been thoroughly checked during this period and some important references published from January to October of 1977 might have been involuntarily omitted.

Acknowledgments

The authors are grateful to Dr. R. D. Small, Jr., for reading the preliminary manuscript and for many valuable discussions during its preparation. Thanks are also due to Professor P. J. Wagner for a copy of the material presented at the Second International Symposium on Organic Free Radicals.


References and Notes

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Press, N.Y., 1974, p. 157.


(96) (97) (98) (99)

(IO0) (101) (I02)

(121) (122) (123) (124) 025) (126) (127) (128) (129) (130) (131) (132)

H. Lutz and L. Lindqvist, Chem. Commun., 493 (1971). H. Lutz, E. Breheret, and L. Lindqvist, J. Phys. Chem., 77, 1758 (1973). P. J. Wagner and R. G. Zepp, 3'. Am. Chem. Soc., 94, 287 (1972). L. M. Stephenson and J. I. Brauman, J. Am. Chem. Soc., 93, 1988 (1971). P. D. Bartlett and N. A. Porter, J. Am. Chem. Soc., 90, 5317 (1968). C. P. Casey and R. A. Boggs, J. Am. Chem. Soe., 94, 6457 (1972). Photoracemization studies involving the intermediacy of Type II biradicals have been included in Section V C.

(103) N.C. Yang, A. Morduchowitz, and D.-D. H. Yang, J. Am. Chem. Soc., 85, 1017 (1963). (104) N. C. Yang and D.-D. H. Yang, J. Am. Chem. Soc., 80, 2913 (1958). (105) P. J. Wagner and K.-C. Liu, J. Am. Chem. Soe., 96, 5952 (i974). (106) H. Morrison, V. Tisdale, P. J. Wagner, and K.-C. Liu, J. Am. Chem. Soc., 97, 7189

(1975). (107) This rate is based on a rate of cyclization of 26 of I x l0 s s -x at room temperature,

based on scavenging studies with BuaSnH ~°8 and esr experiments at low tempera- tures x°9 (see also ref. I10). As noted by Wagner l°s the conclusions reached depend "On the critical assumption that each site in 1,4-biradicals react independently of and unaffected by the other . . . . . " Recent studies of free radical isomerizations indicate that the rate of cyclization of 21 could be faster than initially assumed: 11

(108) D.J . Carlsson and K. U. Ingold, J. Am. Chem. Soe., 90, 7047 (1968). (109) D. Lal, D. Griller, S. Husband, and K. U. Ingnld, J. Am. Chem. Soc., 96, 6355 (1974). (110) C. Walling and A. Coiffari, J. Am. Chem. Soe., 94, 6059 (1972). (111) P. J. Wagner, presented at the Second International Symposium on Organic Free

Radicals, Aix-en-Provence, France, July 1977. (112) M.J . Perkins, N. B. Peynircioglu, and B. V. Smith, J. Chem. Soc. Chem. Commun.,

222 (1976). (113) A. Padwa, D. Cromrine, R. Hartman, and R. Layton, J. Am. Chem. Soc., 89, 4435

(1967). (114) R .A. Cormier and W. C. Agnsta, J. Am. Chem. Soc., 96, 1867 (1974). (115) W. C. Agosta and S. Wolff, J. Am. Chem. Soc., 98, 4182 (1976). (116) The term "iner t" is of course a relative one. We refer mainly to solvents which will

not strongly solvate the biradicals. This excludes alcohols, amines, pyridine and aqueous solvents.

(117) P. J. Wagner and A. E. Kemppainen, J. Am. Chem. Soc., 90, 5896 (1968). (118) J. N. Pitts, Jr., D. R. Burley, J. C. Mani, and A.D. Broadbent, J. Am. Chem. Soc., 90,

5900 (1968). (119) W. A. Noyes, Jr., and P. A. Leighton, "The Photochemistry of Gases," Reinhold,

New York, 1941, p. 365. (120) Rice and Teller T M in a theoretical study of free radical reactions already suggested

the initial formation of the enol, as early as 1938. F. O. Rice and E. Teller, J. Chem. Phys., 6, 489 (1938). P. Ausloos and R. E. Rebbert, J. Am. Chem. Soc., 83, 4897 (1961). E. B. Abuin, M. J. Encina, and E. A. Lissi, J. Photochem., 3, 143 (1974). F. S. Wettack J. Phys. Chem., 73, 1167 (1969). J. E. Wilson and W. A. Noyes, Jr., J. Am. Chem. Soc., 65, 1547 (1943). W. Davis, Jr., and W. A. Noyes, Jr., J. Am. Chem. Soc., 64, 2676 (1942). F. S. Wettack and W. A. Noyes, Jr., J. Am. Chem. Soc., 90, 3901 (1968). J. L. Michael and W. A. Noyes, Jr., J. Am. Chem. Soc., 85, 1027 (1963). J. G. Calvert and C. H. Nicol, J. Am. Chem. Soc., 89, 1970 (1967). R. P. Borkowski and P. Ausloos, J. Am. Chem. Soc., 65, 2257 (1961). P. Ausloos, J. Phys. Chem., 65, 1616 (1965). Rauh and Leermakers xaa reported a study of solvent effects on the phosphorescence and photoreactivity of butyrophenone. Phosphorescence data on ring substituted derivatives were also reported.

(133) R. D. Rauh and P. A. Leermakers, J. Am. Chem. Soc., 90, 2246 (1968). (134) P. J. Wagner and A. E. Kemppainen, J. Am. Chem. Soc., 94, 7495 (1972). (135) P.J. Wagner, R. G. Zepp, K.-C. Liu, M. Thomas, T.-J. Lee, and N. J. Turro, J. Am.

Chem. Soc., 98, 8125 (1976). (136) F. D. Lewis, J. Am. Chem. Sot., 92, 5602 (1970). (137) P. J. Wagner and M. J. Thomas, J. Am. Chem. Soe., 98, 241 (1976).


(138) J. A. Barltrop and J. D. Coyle, J. Am. Chem. Soc., 90, 6584 (1968). (139) E. J. Baum, J. K. S. Wan, and J. N. Pitts, Jr., J. Am. Chem. Soc., 88, 2652 (1966). (140) P. J. Wagner and G. Capen, Mol. Photochem., 1, 173 (1969). (141) R. B. Gagosian, J. C. Dalton, and N. J. Turro, J. Am. Chem. Soc., 92, 4752 (1970). (142) D. S. Weiss, N. J. Turro, and J. C. Dalton, Mol. Photochem., 2, 91 (1970). (143) N. Sugiyama, K. Yamada, and H. Aoyama, J. Chem. Soe., C., 830 (1971). (144) E. C. Alexander and J. Uliana, Tetrahedron Letters, 1551 (1977). (145) E. C. Alexander and J. Uliana, J. Am. Chem. Soc., 98, 4324 (1976). (146) E. C. Alexander and J. Uliana, J. Am. Chem. Soc., 96, 5644 (1974). (147) F. J. Golemba and J. E. Guillet, Macromolecules, 5, 212 (1972). (148) G. Beck, J. Kiwi, D. Lindenau, and W. Schnabel, Europ. Polym. J., 10, 1069 (1975). (149) G. Drobrowolski, J. Kiwi, and W. Schnabel, Europ. Polym. J., 12, 657 (1976). (150) J. Kiwi and W. Schnabel, Maeromolecules, 9, 468 (1976). (151) C. David, W. Demarteau, and G. Geuskeus, Europ. Polym. J., 6, 1405 (1970). (152) L Luke,, P. Hrdlovi~, Z. Mafiitsele, and D. Bellu~, J. Polym. Sci., A1, 9, 69 (1971). (153) R. Salvin, J. Meybeck, and J. Faure, J. Photochem., 6, 16 (1976/77). (154) P.-F. Casals, J. Ferard, R. Ropert, and M. Keravec, Tetrahedron Letters, 3909 (1975). (155) R. Salvin, H. Balard, and J. Meybeck, L'actualit~ chimique, 6, 38 (1975). (156) The synthesis of the dimers has been discussed in L. Merle-Aubry, Y. Merle, and E.

Selegny, Comptes Rendus, C, 276, 249 (1973). (157) For 1,4-diphenyl-4-hydroxy-l-butatone, Lewis lae reports a y-isotope effect on the rate

of triplet decay of 1.7 while Wagner 75 reports 4.8 in the case of y-deuterated phenyl-n- octyl ketone. The difference probably reflects the higher exothermicity of the reaction studied by Lewis. 15e

058) See, for example, H. S. Johnston, "'Gas Phase Reaction Rate Theory," Ronald Press, New York, 1966, p. 242.

(159) N. C. Yang and S. P. Elliott, J. Am. Chem. Soc., 91, 7550 (1969). (160) J. E. Guillet, Polymer Eng. Sci., 14, 482 (1974). (161) F. J. Golemba and J. E. Guillet, Maeromolecules, 5, 63 (1972). (162) P. J. Wagner, Mol. Photochem., 3, 169 (1971). (163) M. Heskins and J. E. Guillet, Macromolecules, 3, 224 (1970). (164) M. Heskins and J. E. Guillet, Macromolecules, 1, 97 (1968). (165) P.J. Wagner, Tetrahedron Letters, 5385 (1968). (166) S. R. Kurowsky and H. Morrison, J. Am. Chem. Soc., 94, 507 (1972). (167) G. H. Hartley and J. E. Guillet, Macromolecules, 1, 413 (1968). (168) A large number of studies have been concerned with the photochemistry of aliphatic

polymers containing carbonyl groups; only a few representative references have been included~eT, le9-~7

(169) A. M. Trozzolo and F. H. Winslow, Macromolecules, 1, 98 (1968). (170) G. H. Hartley and J. E. Guillet, Macromolecules, 2, 165 (1968). (171) D. J. Carlsson and D. M. Wiles, Macromolecules, 2, 587 (1969). (172) D.J. Carlsson and D. M. Wiles, Macromolecules, 7, 259 (1974). (173) M. U. Amin, G. Scott, and L. M. K. Tillekeratne, Europ. Polym. J., 11, 85 (1975). (174) P. I. Plooard and J. E. Guillet, Macromolecules, 5, 405 (1972). (175) J. E. Guillet, B. Houvenagbel-Defoort, T. Kilp, N. J. Turro, H.-C. Steinmetzer, and

G. Schuster, Macromolecules, 7, 942 (1974). (176) K. F. Wissbrun, J. Am. Chem. Soc., 81, 58 (1959). (177) J. E. Guillet and R. G. W. Norrish, Proc. Roy. Soc. (London), A233, 153 (1955). (178) Y. Amerik and J. E. Guillet, Macromolecules, 4, 375 (1971). (I 79) J.E. Guillet, J. Dhanraj, F. G. Golemba, and G. H. Hartley, ,4dvan. Chem. Ser., No. 85,

272 (1968). (180) J. E. Guillet, "Proceedings of the International Symposium on Macromolecules,"

E. Mano, Ed., Elsevier Scientific Co., Amsterdam, 1975, p. 183. (181) In the case of 2-pentanone the value of ~sc is high enough that most of the reaction

proceeds from the triplet manifold. For this ketone, the results of Ausloos and Rehbert ~22 show that between - 6 5 ° and 76 ° 3Px~/3Pc, is almost independent of temperature suggesting that E5 ,,, Ee (Scheme 1).

(182) Barltrop and Coyle ~3e measured the following values of e(3Px:) in benzene: Butyro- phenone, 5.8 + 1.2 kcai tool-l; valerophenone, 0.0 + 0.5 kcal mol -~ and ~,-methylvaleropbenone, -2.1 + 0.6 kcal tool -~.

194

(183)

(184) (185)

(186) (187)

(188) (189) (190) (191) (192) (193) (194) (195) (196) (197) (198) (199) (200)

(2Ol)

(202)

(203) (2o4) (2o5)

(206) (207) (208) (209) (210) (211) (212) (213) (214) (215)

(216) (217) (218) (219) (220)

(221) (222) (223) (224) (225) (226) (227) (228) (229) (230) (231) (232)

J. C. Scaiano, E. A. Lissi, and M. V. Encina

For a related calculation of entropic contributions in the hydrogen abstraction process see reference 55. S. W. Benson and H. E. O'Neal, J. Phys. Chem., 71, 2903 (1967). Estimating the rate constants for intramolecular hydrogen abstraction and intersystem crossing, aT" xee

J. C. Dalton and R. J. Sternfels, Mol. Photochem., 6, 307 (1974). See for example reports 118.x:3"288 concerned with the photolysis of ketones having epoxy and cyclopropyl rings. Some of these have already been mentioned (see structures 27 and 28). D. I. Schuster and T. M. Well, Mol. Photochem., 6, 69 (1974). L. M. Stepbenson, P. R. Cavigli, and J. L. Parlett, J. Am. Chem. Soc., 93, 1984 (1971). R. B. Gagosian, J. C. Dalton, and N. J. Turro, J. Am. Chem. Soc., 97, 5189 (1975). J. D. Coyle, J. Chem. $oe. B, 2254 (1971). C. H. Nichols and P. A. Leermakers, J. Org. Chem., 35, 2754 (1970). A. A. Scala and J. P. Colangelo, Chem. Commun., 1425 (1971). A. A. Scala and G. E. Hussey, J. Org. Chem., 36, 598 (1971). T. Mori, K. Matsui, and H. Nozaki, Tetrahedron Letters, 1175 (1970). K. Matsui, T. Mori, and H. Nozaki, Bull. Chem. Soc. Japan, 44, 3440 (1971). M. Barnard and N.-C. Yang, Proe. Chem. Soc., 302 (1958). B. Camerino and B. Patelli, Experientia, 20, 260 (1964). D. S. Weiss and P. M. Kochanek, Tetrahedron Letters, 763 (1977). K. H. Schulte-Elte, B. Willhalm, A. F. Thomas, M. Stoll, and G. Ohloff, Heir. Chim. Acta, 54, 1759 (1971). N. J. Turro and D. S. Weiss, presented at the 156th meeting of the ACS, Atlantic City, N.J., September 1968, abstract ORGN, 11. J. C. Dalton, K. Dawes, N. J. Turro, D. S. Weiss, J. A. Barltrop, and J. D. Coyle, J. Am. Chem. Soc., 93, 7213 (1971). P. J. Wagner and P,. W. Spoerke, J. Am. Chem. Soe., 91, 4437 (1969). I. Fleming, A. V. Kemp-Jones, and E. J. Thomas, Chem. Commun., 1158 (1971). I. Fleming, A.V. Kemp-Jones, W. E. Long, and E.J. Thomas, J. Chem. Soc.Perkin. Trans. II, 7 (1976). I. Fleming and W. E. Long, J. Chem. Soe. Perkin. Trans. II, 14 (1976). J. A. Barltrop and J. D. Coyle, Tetrahedron Letters, 3235 (1968). G. A. Segal, J. Am. Chem. Soc., 96, 7892 (1974). J. A. Barltrop and J. D. Coyle, Chem. Commun., 390 (1970). R. Srinivasan and S. E. Cremer, J. Phys. Chem., 69, 3145 (1965). C. H. Bamford and R. G. W. Norrish, J. Chem. Soe., 1521 (1938). N. J. Turro and D. S. Weiss, J. Am. Chem. Soc., 90, 2185 (1968). P. M. Collins, P. Gupta, and R. Iyer, J. Chem. Soc. Perkin Trans. I, 1670 (1972). P. M. Collins and P. Gupta, Chem. Commun., 90 (1969). S. Wolff, W. L. Schreiber, A. B. Smith, III, and W. C. Agosta, J. Am. Chem. Soc., 94, 7797 (1972). A. B. Smith, III, and W. C. Agosta, J. Am. Chem. Soc., 96, 1961 (1973). A. B. Smith, III, and W. C. Agosta, J. Am. Chem. Soc., 96, 3289 (1974). W. C. Agosta and A. B. Smith, III, J. Am. Chem. Soc., 93, 5513 (1971). A. B. Smith, III, and W. C. Agosta, J. Org. Chem., 37, 1259 (1972). P. S. Engel, M. E. Schroeder, and M. A. Schexnayder, J. Am. Chem. Soc., 98, 2683 (1976). K. N. Houk, Chem. Revs., 76, 1 (1976). P. S. Engel and M. A. Schexnayder, J. Am. Chem. Soc., 94, 9252 (1972). N. C. Yang and D.-M. Thap, Tetrahedron Letters, 3671 (1966). E. F. Kiefer and D. A. Carlson, Tetrahedron Letters, 1617 (1967). T. Matsui, A. Komatsu, and T. Moroe, Bull. Chem. Soe. Japan, 40, 2204 (1967). R. C. Cookson, J. Hudec, A. Szabo, and G. E. Usher, Tetrahedron, 24, 4353 (1968). R. C. Cookson and N. R. Rogers, J. Chem. Soc. Chem. Comm., 809 (1972). J. C. Dalton and H.-F. Chan, J. Am. Chem. Soe., 95, 4085 (1973). R. R. Sauers, A. D. Rousseau, and B. Byrne, J. Am. Chem. Soc., 97, 4947 (1975). A. W. Jackson and A. 3. Yarwood, Can. J. Chem., 49, 987 (1971). N. J. Turro and T. L Lee, J. Am. Chem. Soc., 91, 5651 (1965). W. H. Urry and D. J. Trccker, J. Am. Chem. Soc., 84, 118 (1962).


(233) W. H. Urry, D. J. Trecker, and D. A. Winey, Tetrahedron Letters, 145 (1970). (234) A possible reaction path might involve ~-hydrogen abstraction by the 1-keto group. (235) Padwa et al. have used similar arguments to explain the photochemical behavior of

arylaroylazetidines: A. Padwa and R. Gruber, J. Am. Chem. Soc., 92, 107 (1970). (236) R. R. Sauers, M. Gorodetsky, J. A. Whittle, and C. K. Hu, J. Am. Chem. Soc., 93,

5520 (1971). (237) The report ~e also includes a study of the behavior of several ketones under electron

impact in a mass spectrometer. (238) J. Slivinskas and J. E. Guillet, J. Polym. Sci. AI, 11, 3043, 3057 (1973). (239) E. Dan and J. E. Guillet, Macromoleeules, 6, 230 (1973). (240) For studies of the pressure dependence of the Type II reaction see: R. C. Neuman, Jr.,

and C. Berge, presented at the Second International Symposium on Organic Free Radicals, Aix-en-Provence, France, July 1977. For a related study of the thermolysis of 3,6-diphenyl-3,4,5,6-tetrahydropyridazine see: R. C. Neuman, Jr., and E. W. Ertley, J. Am. Chem. Soe., 97, 3130 (1975).

(241) A. C. Somersall, E. Dan, and J. E. Guillet, Macromolecules, 7, 233 (1974). (242) P. J. Wagner, Pure AppL Chem., 49, 259 (1977). (243) J. C. Seaiano, J. Org. Chem., 43, 568 (1978). (244) J. Grotewold, C. M. Previtali, D. Soria, and J. C. Scaiano, J. Chem. Soe. Chem.

Commun., 207 (1973). (245) An example of an exception is 1,4-diphenyl-4-hydroxybutan-l-one 13e where the

resulting 1,4-diketyl biradical is probably strongly hydrogen bonded intramolecularly. (246) Usually the ~r, ¢* triplet is rather unreactive toward hydrogen abstraction because it

has less radical-like character than the n, ~r* triplet. 33 (247) P.J. Wagner and H. N. Schott, J. Am. Chem. Soc., 91, 5383 (1969). (248) P. J. Wagner, Tetrahedron Letters, 1753 (1967). (249) For the equilibrium controlled process, klerB should be replaced by kxern(kxe +kx, +

k~o)/kxe. (250) G. B. Watts, D. Griiler, and K. U. Ingold, J. Am. Chem. Soe., 94, 8784 (1972). (251) A.G. Davies, D. Griller, and B. P. Roberts, J. Chem. Soe. Prelim. Trans. II, 2224 (1972). (252) R. H. Lema and J. C. Seaiano, Tetrahedron Letters, 4361 (1975). (253) J. C. Seaiano, J. Photochem., 2, 471 (1973/74). (254) M. V. Encina and E.A. Lissi, J. Photoehem., 5, 287 (1976). (255) P.J. Wagner, presented at the 154th ACS meeting, Chicago, Illinois, September 1967,

Abs. # $21. (256) P. Borrell and J. D. Holmes, J. Photochem., 2, 315 (1973/74). (257) N. C. Yang and E. D. Feit, J. Am. Chem. Soc., 90, 505 (1968). (258) An alternative explanation could be based on the higher acidity of the hydroxylic

proton in aromatic compounds. The importance of the acidity of this proton in determining the behaviour of the biradical has been recognized. 259

(259) For example, P. J. Wagner and H. N. Sehott, presented at the 158th ACS meeting, New York, N.Y., September 1969, Abstract ORGN, 43.

(260) M.V. Encina and E. A. Lissi, unpublished results. (261) P. J. Wagner and R. G. Zepp, Chem. Commun., 167 (1972). (262) R.D. Burkhart, J. Phys. Chem., 73, 2703 (1969); C. Sivertz, J. Phys. Chem., 63, 34

(1959); B. Smaller, J. R. Remko, and E. C. Avery, J. Chem. Phys., 48, 5174(1968); R. D. Burkhart, J. Am. Chem. Soe., 90, 273 (1968).

(263) J. C. Seaiano, J. Am. Chem. Soc., 99, 1494 (1977). (264) A.F. Trotman-DickensonandG. S. Milne, Nat.Stand. Ref Data Ser., Nat. Bur. Stand.,

No. 9 (1968). (265) J. C. Scaiano, unpublished work. (266) K. Torsell, Tetrahedron, 2759 (1970). (267) M. Hamity and J. C. Scaiano, J. Photochem., 4, 229 (1975). (268) C.L. Osborn and M. R. Sandner, presented at the 167th ACS meeting, Los Angeles,

April 1974, abstract ORPL, 85 (see also CoatingsandPlastiesPreprints, 34, 660(1974)). (269) J. C. Seaiano and K. U. Ingold, J. Chem. Soc. Chem. Commun., 205 (1976). (270) J. C. Scaiano and K. U. Ingold, J. Phys. Chem., 80, 1901 (1976). (271) J. C. Scaiano and K. U. Ingold, J. Am. Chem. Soe., 98, 4727 (1976). (272) P. Hyde and A. Ledwith, J. Chem. Soc. Perkin. Trans. 11, 1768 (1974). (273) L. K. Patterson, R. D. Small, Jr., and J. C. Scaiano, Rad. Res., 72, 218 (1977).

196 J. C. Scaiano, E. A. Lissi, and M. 1I. Encina

(274) R. D. Small and J. C. Scaiano, 1. Photochem., 6, 453 (1976177). (275) J.A. Farrington, M. Ebert, E.J. Land, and K. Fletcher, Biochim. Biophys. Aeta, 314,

372 (1973). (276) R. D. Small, Jr., and J. C. Scaiano, unpublished work. (277) R. D. Small, Jr., and J. C. Seaiano, 1. Am. Chem. Soc., 99, 7713 (1977). (278) R. D. Small, Jr., and J. C. Scaiano, 1. Am. Chem. Sac., submitted for publication. (279) J.B. Birks, "Photophysics of Aromatic Molecules," Wiley-Interscienee, London, 1970,

p. 500. (280) B. Stevens and B. E. Algor, Annals of the New YorkAeademyofSeienees, 171, 50(1970). (281) Y. Kubokawa and M. Anpo, I. Phys. Chem., 79, 2225 (1975). (282) I. Luk~ic, J. Pilka, M. Kulickovi, and P. Hrdlovic, J. Poly. Sei., Polym. Chem. Ed.,

15, 1645 (1977). (283) S. Farid, Chem. Commun., 73 (1971). (284) R. M. Wilson and S. W. Wundedy, 1. Am. Chem. Soc., 96, 7350 (1974). (285) M.S. Platz and J. A. Berson, J. Am. Chem. Sac., 95, 6743 (1974). (286) S. L. Budswalter and G. L. Closs, J. Am. Chem. Soe., 97, 3858 (1975). (287) R. K. Waring and G. J. Sloan, 1. Chem. Phys., 40, 772 (1964). (288) E.F. Ullman, J. H. Osiecki, D. G. B. Boocock, and R. Daray, J. Am. Chem. Sac., 94,

7049 (1972).

chemistry of the biradicals produced in the norrish type...

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