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478 M. Milun - N. Trinajstiö • Cyclobutadiene, Benzocyclobutadiene, and Biphenylene

Cyclobutadiene, Benzocyclobutadiene, and Biphenylene1

M . M i l u n

The Pharmaceutical and Chemical Works Pliva, Zagreb, Croatia, Yugoslavia

and

N . T r i n a j s t iĆ

Institute Rugjer Boskovic, Zagreb, Croatia, Yugoslavia

(Z. Naturforsch. 28 b, 478^82 [1973]; received March 21, 1973)

Arom atic stabilization, benzocyclobutadiene, biphenylene, cyclobutadiene The difference in the ground state reactivity of cyclobutadiene, benzocyclobutadiene,

and biphenylene is a ttribu ted to the com bination of their arom atic stabilization (ordestabilization), the presence of the strain i appearance of the localized double bonds.

Cyclobutadiene(l) and its benzo (benzocyclobuta­diene, 2) and dibenzo-derivatives (biphenylene, 3) present an interesting set of molecules to challenge both experimental and theoretical researchers. The schematic diagrams of these molecules are given in Fig. 1.

0 1 (q j u \ c o r n o )1 2 3

Fig. 1. Schematic diagram s of cyclobutadiene (1), ben­zocyclobutadiene (2), and biphenylene (3).

For almost a century a number of organic chemists attempted in vain, the synthesis of cyclobutadiene2 until the preparation of the first cyclobutadiene-metal complex (tetra-methylcyclo- butadiene-nickel complex) by C r ie g e e and S c h r ö ­d e r 3 after the suggestion by L o n g u e t -H ig g in s and O r g e l4 that cyclobutadiene should form stable complexes with transition metals, and the prepara­tion of free cyclobutadiene as a short lived species by W a t t s , F it z p a t r ic k , and P e t t i t 6. The interest in cyclobutadiene chemistry is still very much present6.

Similarly, benzocyclobutadiene eluded synthesis for a number of years7 and so far has been detected only as an unstable reaction intermediate8.

Requests for reprin ts should be sent to Dr. N. T R iN A jsT id , Rugjer Bo§kovic Institu te , Y-41001 Z a­greb, Croatia, Postfach 1016, Yugoslavia.

the four-membered ring system, and the

It thus appears that cyclobutadiene and benzo­cyclobutadiene are far too reactive molecules to have any prolonged existence. On the other hand, biphenylene has been known for some time9 and is a reasonably stable molecule10.

In view of current interest6’8>10>11»42 in cyclobuta­diene, benzocyclobutadiene, and biphenylene che­mistry we report our work which was carried out in order to understand the origin of differences in the ground state reactivities of these molecules. These differences may be attributed of the combination o f

several effects. These are the aromatic stabilization (or destabilization), the presence of the strain in the four-membered ring system and the appearance o f

the localized double bonds.

Theoretical Studies and Discussions

a. Aromatic stabilization

In order to obtain the information about the aromatic stabilization of cyclobutadiene, benzo­cyclobutadiene, and biphenylene D e w a r ’s variant12 (variable /^-procedure13) of P o p le SCF MO theory14, has been applied to this problem. This method is known16 to be very reliable for predicting the ground state energy values (heats of atomization, aromatic stabilization, ionization potentials) of conjugated systems. Details of the method can be found elsewhere15 and do not need to be repeated here. Similarly, an extensive discussion about the aromatic stabilization is given, for example, in ref.16. Note, that some recent studies17 indicated that

M. Milun - N. Trinajstid • Cyclobutadiene, Benzocyclobutadiene, and Biphenylene 479

there is a good correlation between the aromatic stabilization of conjugated systems and their ther­modynamic stability.

Calculated heats of atomization, indices of aroma­tic stabilization and ionization potentials of cyclo­butadiene, benzocyclobutadiene, and biphenylene are given in Table I. HMO resonance energies (HMO RE)18, REPE19, and As (HMO)20 values of 1, 2, and 3, are also indluded in Table I.

Results are rather interesting. All indices of aromatic stabilization indicate that cyclobutadiene should be unstable molecule having large conjugati- ve destabilization, while biphenylene should be stable molecule having large conjugative stabiliza­tion. Thus, cyclobutadiene is predicted to be antiaromatic species, while biphenylene aromatic species. This is, of course, in agreement with their chemistry. Benzocyclobutadiene, on the other hand, is predicted by REPE and As (HMO) indices to be non-aromatic, and by As (SCF) index to be weakly aromatic species having a small conjugative stabili­zation. This is also in agreement with its high reactivity.

It is interesting to note that the estimation of the conjugative destabilization of cyclobutadiene was attempted experimentally by B r e s l o w et al.21. Their estimation is > 12-16 kcal/mole. Some other theoretical calculations22 gave this value as high as33 kcal/mole. This may be a question of academic interest, however, it is certain that conjugative destabilization in cyclobutadiene is very large. Perhaps of the same absolute magnitude as the conjugative stabilization in benzene (20.0 kcal/ mole)12. Hence, cyclobutadiene should be thermodi- namically highly unstable species as it is the case.

Experimental ionization potentials of these com­pounds are not known. Some estimates from the photoelectron spectrum of cyclobutadiene-iron tri­

carbonyl (8.5 eV)23 closely agree with the predicted adiabatic ionization potential (8.58 eV) of cyclobu­tadiene.

b. Calculation of bond lengths

The bond lengths have been calculated using the D e w a r - S c h m e is in g linear relation24:

Lu = 1.512-0.176 p i}

where L l} is the calculated bond length in Ä, and p ti the jr-bond order between atoms i and j. The calculated bond lengths of cyclobutadiene and benzocyclobutadiene are given in Fig. 2, while those of biphenylene in Table II.

1.512 X N 1-------1

u n i1.338 X 1.379 X 1.418 X

Fig. 2. Calculated bond lengths of cyclobutadiene and benzocyclobutadiene.

The experimental values25 of biphenylene and some other theoretical results are also given in TableII.

In cyclobutadiene and in the four-membered ring system of benzocyclobutadiene the localized double bonds (~1.34 Ä) are quite distinct. In biphenylene, of course, there is no possibility of forming the ex­ternal localized double bonds in four-membered ring.

The appearance of localized double bonds in cyclobutadiene and benzocyclobutadiene causes that these molecules have ideal geometries for D iels- A lder reactions. Thus, both molecules elude isola­tion as stable substances, because in absence of other reagents with which they can react they un­dergo D iels-A lder dimerization giving unstable adducts which then isomerize to more stable dimers26. Similarly, phenyl-derivatives of cyclobutadiene (o-

Table I. Calculated heats of atom ization (A H a25 °C), indices of arom atic stabilization, and ionization potentials (IP).

Molecule z!Ha250C(eV)12

HMO R EOS)18

As[SCF](kcal/mole)

R E P E (ß )19 As[HMO](ß)

I P (eV)

Cyclobutadiene 36.75 0.00 -18.030 -0.268 -1.04 v:8.6730*a:8.58

Benzo-Cyclobutadiene 70.88 2.38 8.1 -0.027 -0.22 8.43

Biphenylene 104.87 4.50 31.012 0.027 0.33 8.34

♦This is in a good agreement w ith the value (8.50 eV) estim ated from the photoelectron spectrum of cyclobuta­diene iron tricarbonyl23.

480 M. Milun - N. Trinajstiö • Cyclobutadiene, Benzocyclobutadiene, and Biphenylene

diphenylcyclobutadienes) have been trapped with powerful dienofile tetracyanoethylene27 giving two adducts:

1>-CN

\~CN /

,CN

CN HjC, ^CN

Another interesting point is worth mentioning here. Three K ek u l6 structures can be written for benzocyclobutadiene. Similarly, five canonical forms can be written for biphenylene. Two Kekul£ structures of cyclobutadiene are, of course, identi­cal. K ekulä structures of cyclobutadiene (1A, IB), Benzocyclobutadiene (2A, 2B, 2C), and biphenylene (3A, 3B, 3C, 3D, 3E) are given in Fig. 3.

O1A

0.8661B

0.866

Q a C d2A

0.9052B

0.9412C

0.832

0=0 0=03A

0.9183B

0.895

C n O 0 = 03C

0.9063D

0.906

0 = 03E0.865

Fig. 3. K e k u le structures and K e k u le indices.

Calculated bond lengths of benzocyclobutadiene and biphenylene indicated that 2B and 3A are their predominant forms. Recently, we have developed a theoretical index named K ek u l6 index which defines the order of the relative importance of various valence bond structures of conjugated poly cyclic systems28. K ek u l6 indices (K) can be calculated for individual valence structure (L ) from given molecular orbital wavefunction using expres­sion:

K(L) = Z [?i + ?! + P»]112 2N («,j)el

where q\ and q) are ^-electron charge densities on atom i and j, while pi\ is the corresponding n-bond order. Various K eku l6 structures of a particular polycyclic molecule can be than ordered according to the magnitude of this index.

The structures with the largest index corresponds to K eku 16-type structure with the greatest number of formal benzene K ek u l6 formula, i.e. to a structure which the empirical F r ie s rule29 predicts as the most stable. The values of K eku l6 index for valence structures of cyclobutadiene, benzocyclobu­tadiene, and biphenylene are also given in Fig. 3. These results also indicate 2B and 3A as the predominant K eku l6 forms of benzocyclobutadie­ne and biphenylene*.

c. Strain energies The ring strain of the four-membered ring has

been approximatively estimated using the modified D a u b e n ’s formula17:

Strain energy = \ \ 2 A £ (Aon)i

Where Aai is the angle strain at the atom i. The summation goes over all atoms within the particular

Table II. Calculated bond lengths in biphenylene by various authors.

BONDExperim ental25 A li and Coulson43

BOND LEN G TH [A] C alcu lated

Skancke44 Warren and Yandle45

VB46 Our result

1-2 1.423±0.003 1.408 1.421 1.413 1.38 1.409

1-10 1.372±0.002 1.395 1.379 1.385 1.41 1.386

2-3 1.385±0.004 1.394 1.375 1.385 1.41 1.385

9-10 1.514±0.003 1.467 1.509 1.476 1.45 1.485

10-11 1.426±0.003 1.414 1.417 1.422 1.41 1.412

* Note added in proof: Recent maximum overlap calculations (M. R a n d i <5 and Lj. V u jisid , J. org. Chemistry 37, 4302 [1972]) reached the same result, the structures 2B and 3A being energetically the m ost favoured.

M. Milun - N. Trinajstiö • Cyclobutadiene, Benzocyclobutadiene, and Biphenylene 481

ring. A is a constant (0 .024 kcal/deg2) obtained considering experimental energies of monocyclic olefins and paraffins. The strain angle in the poly cyclic conjugated hydrocarbons represents the angle distortion from 120°. Only carbon skeleton of a molecule is taken into account. Calculated angles in the four-membered rings of cyclobutadiene, benzocyclobutadiene, and biphenylene are given in Fig. 4.

Wa6° -^ 1 5 0 ° [K 7 .8 ° ]

Ao°

Fig. 4. Calculated angles in the four-membered ring. Experim ental angles (ref. 25) of biphenylene are given in brackets.

Experimental angles25 of biphenylene agree quite well with the calculated values.

The calculated strain energies are given in TableIII.

Table I I I . S train energies in the four-membered ring systems.

Compound Strain energy [kcal]

Cyclobutadiene 43

Benzocyclobutadiene 57

Biphenylene 86 [79]*

* This value is obtained when the experim ental angles are used.

In literature there are scarce data about the strain in the four-membered ring of cyclobutadiene, benzocyclobutadiene and biphenylene. The earlier estimate of the ring strain of cyclobutadiene is far too low (19 kcal/mole)30. However, this is not suprising because the M IN D O /2 version31 used in the above work80 underestimates the strain energy in strained ring systems for approximatively 20 kcal/mole32. The strain energy in cyclobutadiene with this correction (40 kcal/mole) is then close to our value (43 kcal/mole). The strain energy in the four membered ring of benzocyclobutadiene is not known although it was suggested33 that the strain energy might be higher than 50 kcal/mole. Several researchers tried to estimate the ring strain in the four-membered ring of biphenylene. C o u ls o n 34 estimated this strain to be 100 kcal/mole. Later, C o u lso n and M o f f i t 35 calculated it to be 74 kcal/ mole. D e w a r and G l e ic h e r 38, using the earlier

version of their variant of the SCF MO theory13»37, arrived to the value of 27.5 kcal/mole. C ass, S p r in g - a l l , and Q u in c e y 38 obtained a value of 59+5 kcal/ mole comparing the heats of formation of biphenyle­ne and biphenyl. Some of these results differ considerably from our values (86,79 kcal/mole). It is difficult to say how far our results are from the truth but some experimental results show39 that the four membered ring in biphenylene can be ruptured although under the fairly vigorous conditions.

Conclusions

Apparently there are mainly three effects (aroma­tic destabilization, appearance of localized double bonds, ring strain) responsible foi different ground state stabilities of cyclobutadiene, benzocyclobuta­diene, and biphenylene. The degree of stability depends on how many of these effects are combined in particular molecule. These can be seen in TableIV.

Table IV. Combination of effects responsible for lower stability.

Molecule Effects responsible for lower stability

Aromaticdestabili­zation

Localised double bond

Ringstrain

Cyclobutadiene + + +Benzo­cyclobutadiene

— + +

Biphenylene — — +

The prediction extracted from Table IV nicely parallels the experimental facts. The case of naphto- (a) cyclobutadiene and naphto(b) cyclobutadiene which are a closely related isomers of biphenylene shows that our results may have certain practical use.

OCaThe appearance of localized double bond in

addition to the ring strain make these molecules much less stable then biphenylene. In fact, the chemistry of naphto(a)- and naphto(b)-cyclobuta­diene is closely related to benzocyclobutadiene chemistry and thus, both molecules have only been observed as unstable intermediates10-40.

It is interesting to note that recently developed41 topological expressions for total ^-energy of conju­

482 M. Milun - N. Trinajstid • Cyclobutadiene, Benzocyclobutadiene, and Biphenylene

gated hyd rocarbons show th a t four-m em bered rings should destab ilize fused polycyclic con jugated m ole­cules for ab o u t ten tim es m ore th a n six-m em bered ring w ould stabilize it, b u t, four-m em bered ring w ith four b ranches,

(e,9* f ^ J biphenylene)

w ould less destab ilize m olecule th a n one w ith two branches

Such sim ple g raph -theo re tica l th ink ing is obvi­ously in ag reem ent w ith our results.

1 Presented in p a rt a t the Meeting of Croatian Che­mists, Zagreb, Croatia, Yugoslavia, February 1973.

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(e.g.