nmr spectroscopy of carbanions and carbocations

Pro<jress ,,I NMR &U?CtrOSCOpJ~, Vol. 12, pp 261-286 C Pergamon Press Ltd., 1979. Printed m Great Britain

NMR SPECTROSCOPY OF CARBANIONS AND CARBOCATIONS

R. N. YOUNG Chemistry Department, University of Sheffield, U.K.

(Received 20 December 1978)

CONTENTS

I. Introduction 2. Chemical Shift-Charge Density Correlations 3. Alkyl Cations 4. Allylic Ions 5. Polyenylic Ions 6. Aromatic Ions 7. Arylalkyl Carbanions and Ion Pairing Phenomena 8. ‘Li NMR and Ion Pairing

8.1 Exchange Phenomena in Ion Pairs 9. Homoaromatic Ions

10. 1,2-Hydrogen and 1,2-Alkyl Shifts 11. Norbornyl and Related Cations 12. The Cyclopropylcarbinyl Cation References

1. INTRODUCTION

The chemistry of carbocations is so extensive that it constitutes a major subject of study in its own right. To only a slightly lesser degree, the same may be said of carbanions and in consequence, the two subjects are usually treated quite separately. Whilst the organic chemistry of these two classes of ions is indeed often quite dissimilar, several features of their physical chemistry are closely allied. The present review is an attempt to explore some of the similarities and differences in their NMR spectroscopy, and to illustrate the power of the NMR technique to probe bonding and other structural details.

Because of the sheer volume of data which is appearing at an ever-increasing rate, it is necessary to be selective rather than exhaustive. To this end, I have concentrated largely on ‘H and 13C spectra and have omitted reference to other nuclei, with the exception of ‘Li which has proved to be particularly informative about the structures of a number of organolithium compounds. The carbanionic species surveyed include only those which are substantially ionic; in effect, this limits the held to the alkali-metal salts. Finally, cations such as acyl in which a hetero atom plays an essential role have also been excluded.

The great reactivity of carbanions towards oxygen, carbon dioxide and water necessitates the use of high vacuum techniques. Fortunately, such technology has been brought to an advanced state of development and the preparation of carbanion salts presents few major problems. In most cases tetramethylsilane (TMS) is unreactive towards carbanions and can be used as an internal standard.

At the beginning of the present century, sulphuric

261 262 263 267 270 272 274 277 279 279 280 281 283 284

acid was the most frequently used solvent for the preparation of carbocations. The conveniently high melting point (10.4”C) facilitated the cryoscopic deter- mination of the number of ions produced per solute molecule introduced. The strong hydrogen-bonding supports Grotthuss chain-conduction by the HSO; ion; because of the high viscosity of the solvent, this is the only ion of significant mobility and its concentration can thus be determined directly by con- ductimetry. The development of visible and ultraviolet spectrophotometers provided an additional powerful tool for the investigation of the solute species. How- ever, the interpretation of such spectra is fraught with difficulties and the literature abounds with fallacious conclusions. It is regrettable that electronic spectroscopy has fallen so far from grace in this field that few authors now bother to record the absorption spectrum. In the light of experience, it is clear that suiphuric acid possesses several features which render it far from being an ideal choice of solvent. Apart from its ability to act as an oxidizing agent, it is not sufficiently acidic to convert adequate fractions of the weaker bases to their conjugate acids, i.e. the equilibrium

H+ + CH2=CHR=CH3CHR+

does not lie far to the right. When ion and olefin are simultaneously present in comparable concentrations, rapid reaction almost invariably occurs between them to yield polymeric products. Still further complications arise from the ability of sulphuric acid to sulphonate a wide range of organic compounds.

This scene of mixed success and failure has been completely transformed by two factors: (a) the

261

262 R. N. YOUNG

development of “super-acids” which permit the clean preparation of a host of carbocations in high concentrations and(b) the application of NMR techniques which permit the unambiguous identification of the structure of the solute species. Many of the super- acids are based on antimony pentafluoride. Antimony pentafluoride has a low dielectric constant (cu. 3) and the cation is present as the ion pair :

BuF + 2SbFS + Bu+Sb2F,,.

The association with the counterion does not seem to significantly affect the ‘H NMR spectrum, e.g. there is no evidence of spin coupling with i9F. The high melting point of SbF5 (2°C) prevents direct low temperature studies, but this problem is readily

circumvented by dissolution in solvents such as SO* (liquid to -75°C) or S02ClF (liquid to - 125°C). These solutions have the important property of low viscosity; the resulting rapid tumbling of the solute molecules gives rise to the observation of sharp NMR spectra. Usually, the reference employed is external (capillary) TMS. Suitable techniques have been described.“) When HzS04 is employed as medium it is convenient to introduce the solute from the vapour phase.(3)

2. CHEMICAL SHIFT-CHARGE DENSITY CORRELATIONS

It was noticed early in the development of NMR spectroscopy that the chemical shifts of the protons in aromatic molecules reflect the electron density at the carbon atoms to which they are attached. Sub- stitution of an aromatic ring by an electron- withdrawing group decreases the shielding of the ring protons whereas substitution by an electron-releasing group has the converse effect. In discussing the spectra of aromatic ions, it is convenient to adopt benzene as a reference for the definition of the “excess charge” Ap on the carbon atoms, Thus for benzene Ap is zero, for the cyclopentadienyl anion Ap = - l/5 and for the cycloheptatrienyl cation Ap = + l/7. Several groups of workers recognized’4-7’ more or less simultaneously that there appears to be a linear relationship between chemical shift 6 and the charge :

6 = 7.27 + kAp.

The experimental evidence available at the time was very meagre, but a considerable effort has since been directed to extend the range.@’ Figure 1 shows a plot of proton chemical shift vs Ap for a series of sym- metrical aromatic ions. The gradient corresponds to a value for k of about 1Oppm per unit of charge. Figure 2 shows the analogous plot for i3C data for the same series of ions and is in accord with a value for k of about 160ppm(4*7~10) per unit charge. An interesting extension of this work has been made.“” Whereas all the previous studies had been of planar and cyclic ions of Dnh symmetry, a linear i3C shift vs charge density plot was obtained also for a range of acylic ions when an average chemical shift was

I

6-

I I I

a2 0 -0.2

Excess charge relative to benzene

FIG. 1. Dependence of proton chemical shifts on excess charge density.

computed. By way of illustration, the benzyl anion

(KC salt in THF at 22°C) was found to have the following shifts: Cl 6 = 152.7, C2 and C6 = 110.7, C3 and C5 = 130.6, C4 = 95.7 and Ccc = 52.7. The average shift is thus 6 = 112.0; the average n-electron density is the excess of charge relative to benzene, divided by the number of carbon atoms over which the charge is delocalized, i.e. Ap is -l/7. The potassium salts of a number of hydrocarbons, only some of which were aromatic, were studied, and the results are shown in Fig. 3. The linearity is excellent and the shifts conform to the equation

6 = 133.2 + 156.3Ap.

The authors noted, however, that alkyl substituted anions did not fit this equation unless an appropriate modification was made to the averaging.

The shielding constant G of an atom A is determined by the sum of several terms :

Excess charge density relative to benzene

FIG. 2. Dependence of carbon chemical shifts on excess charge density.

NMR Spectroscopy of carbanions and carbocations 263

Excess charge relative to benzene

FIG. 3. Dependence of averaged carbon chemical shifts on excess charge density.

where 0:: is the contribution arising from the diamagnetic shielding due to electrons localized on A, erg allows for any departure from spherical symmetry of these localized electrons, and c? represents the effect on atom A of the circulation of electrons either localized on other atoms, or in delocalized n-systems (i.e. ring currents). The terms aAB are independent of the nature of A and of any charge it may bear and their magnitudes are the same for ‘H and for r3C. In the case of “C, o is generally more than an order of magnitude greater than crAB and, in consequence, the latter is often ignored. In contrast, the screening of protons is often substantially determined by the term oAB. In practice, it is difficult to make reliable theoretical predictions of aAB.

The term ai is similar in magnitude to uAB for both ‘H and ’ 3C and its calculation involves allowing for the effect of the electric field due to dipoles (and charges) upon the circulation of electrons on A. In the case of protons, the calculation is indirect, being based on the charge located on the carbon atom to which the proton is attached.

Finally, the term 0; is computed by mixing the ground state with excited states. This term is accordingly insignificant for ‘H because of the large difference in energy between the 1s and 2s and 2p orbitals. In

the case of carbon, 0; is a dominant term, but its charge dependence is not easily predicted!@

In summary, therefore, the apparent linear dependence of iH and 13C shifts has no theoretical foundation. Nonetheless, within series of closely related compounds, it undoubtedly provides a useful basis for the prediction of data for unknown members.@‘12i

Valuable structural information can often be obtained from an examination of the coupling constants. The 13C-‘H coupling constants, relating to these atoms separated by a single bond length, are generally large and are very sensitive to the s- character of the bonding of the carbon atom. For hydrocarbons, a useful guide is that JCH is approximately 125Hz for sp3 carbon, 160Hz for sp2 and 250Hz for sp. Indeed, for hydrocarbons the coupling constant JCH is linearly related to the s-character. In

neutral molecules, substitution by electronegative atoms causes a significant increase in JCH (e.g. the

values for CH3CH3, CH30CH3 and CH3F are,

respectively, 125, 140 and 149 Hz). In the case of the carbocations, JCH is generally

increased by the presence of the charge. For methyl groups attached to a positive carbon atom, the increase is usually in the range 7-lOHz, e.g. for the methyl

groups in (CH3)*CH and (CH3)& JCH is 132Hz. A similar increase is found for JCH where the ‘H is directly bonded to the positively charged carbon atom, for

+ + example in (CH3)*C-H and (CsHs)&-H the values are, respectively, 169 and 164 Hz. Conversely, for ‘H atoms attached to the charged atom in a carbanion, the JCH values are decreased, e.g. for C6Hsg2Li, CH2 = CHCHLi and (C6H5)2CHLi the values are respectively 134, 146 and 141 Hz(in THF solution). Even smaller values of JCH (cu. 90Hz) are found for alkyl and vinyl lithium compounds which must in part be a reflection of the high degree of self- association of these species. A similar effect has been noted’13) for C6H5g2Li ; in benzene solution this species is dimeric and Jcu is only 116 Hz.

One final factor which influences JCH is ring strain. For large rings the departure from the values found for acyclic alkanes is quite small, but is very marked for the three and four membered rings: cyclopropane (160.5 Hz), cyclobutane(l36 Hz), cyclopentane (128Hz), cyclohexane (125Hz). A similar effect is noted in simple cyclic ions

Ions CaH: C,H; C,H, &Hi- CsH; J&Hz 265 157 171 145 137 Charge density 0.66 1.20 1.14 1.25 1.11

There is clearly no regular dependence of JCH upon charge density.

Of the remaining coupling constants, Jcc is only observable in samples enriched in 13C whilst JHH values are most conveniently dealt with in the following text.

3. ALKYL CATIONS

The alkyl cations have long been postulated as intermediates in a wide variety of reactions. However, because of their extremely high reactivity towards olefins, the direct observation of these species had to await the development of super-acid solvents, notably by Olah and co-workers. (14) Their preparation has

been reviewed.“” The t-butyl cation can be prepared by dissolving butyl fluoride in SbFs or via pivalyl hexafluoroantimonate which undergoes facile loss of carbon monoxide :

(CH&CCO+SbF; -+ (CH3)$+SbF; + CO.

The ‘H NMR spectrum of the t-butyl cation consists of a singlet at 4.15 6 (Table l), the downfield location reflecting the positive change. Even more striking evidence of cationic character is provided by the 13C NMR shift of the central carbon atom of the

264 R. N. YOUNG

TABLE 1. Chemical shifts” and coupling constantsh for alkyl cations in S02CIF-SbFS at -70’

Ion CH+ KHZ KH, jCH3 J+CH J+CCH C’ YCH.3

(CH&C + 4.15 3.6 330.0 48.3 (CH&C+H 13 4.5 169 3.3 319.6 61.8 (CH&C+C2H5 4.5 4.1 1.94 333.8 44.5 CHJ+(C&L 4.44 4.16 1.87 334.0

‘Chemical shifts in ppm. h Coupling constants in Hz.

ion viz. 3306, the corresponding carbon atom of the neutral parent being observed at 25 6. Under identical conditions, the chemical shift of the central carbon of the isopropyl cation is 320 d ; the ’ 3C-H coupling constant, 169Hz, is in accord with sp’ hybridization. Comparison of the 13C NMR shifts of the central carbon atoms of these two cations would seem to suggest that the t-butyl ion bears the greater positive charge. Extended Hiickel calculations also show this relative charge density. (I ‘) Since these calculations refer to the gas phase, and in the light of the certainty that chemical shifts are determined by factors additional to charge density, too much significance ought not to be placed on this apparent concordance. It is perhaps worth noting that replacement of hydrogen by methyl in alkanes in general results in a downfield shift of about 7 ppm.“‘)

The isopropyl cation is quite stable in super-acids up to about 40°C. However, 13C scrambling does occur quite quickly (half-life one hour at -60°C) as was discovered when the ion was generated from 2- ‘3C-2-chloropropane. ‘I+-) A protonated cyclopropane was proposed as the reaction intermediate.“‘) The methyl groups in the diethylmethyl carbenium ion undergo a similar interchange process, which was revealed”b’ by irradiating the two equivalent methyl ‘H NMR signals and noting the consequent rapid fall in the intensity of the signal due to the methyl group attached to the charged carbon atom.

The n-butyl cation has not been observed to date. When rl-butanol is dissolved in SbFs--S02-HS03F the initial product is the ion CH3CH2CH2CH20’H2. By monitoring the -OH: triplet it was possible to

follow the conversion to the t-butyl cation. The rate- determining step was the cleavage of the C-O bond, the re-arrangement of the n-butyl cation to the t-structure being too rapid to observe in the temperature range studied(5-25”). Saunders and co-workers(21) found that by the use of low temperatures (- 110°C) and high vacuum techniques it was possible to prepare a mixture of set- and t-butyl cations.

Attempts to prepare the primary ions CH: and C2Hl have not so far been successful. Methyl fluoride does interact with SbFS, but only to form a complex in which the carbon atom is slightly deshielded. Ethyl fluoride seems to form a slightly more highly ionized complex which, however, readily decomposes to form a mixture of t-butyl and t-hexyl cations.(15)

The INDOR spectrum of the cyclopentyl cation”‘)

in SbF5---S02ClF consists of a multiplet of ten lines corresponding to the coupling of nine protons with (five) equivalent carbon atoms. The shift, 99ppm, and the coupling constant, 28SHz, are unusually small. The ‘H NMR spectrum consists of a singlet at 6 4.68 which remains unchanged down to - 130”, showing the equivalence on the NMR time scale of all nine protons.

Cl-l,-CH< + ,C-CH,

CH,-CH<+

/C-CH,

CH,-CH, CH+H,

I

2a 2b 2c 2d 2e

Arising from the coupling of five equivalent carbons with the set of equivalent protons, the 13C satellites are five times as intense as would otherwise be expected. These features indicate that the nine protons are equivalent because of degenerate 1,2-hydride shifts (2a* 2be 2c etc.) which are fast in comparison with

the NMR time scale. In general, degenerate ions, may be defined as those which can rearrange .through a finite energy barrier to produce an identical ion. This process of rendering hydrogen or carbon atoms equivalent is thus quite distinct from resonance in which no interchange of atoms is involved.

The coupling constant of the cyclopentyl cation can be predicted by comparison with model compounds, i.e. as the average of one sp2 and four sp3 carbon-hydrogen coupling constants. Using the isopropyl cation (JCH 169Hz) as a model for the former, and cyclopentane (JCH 131 Hz) for the latter, the JCH for the cyclopentyl cation is expected to be (169 + 131 x Q/(9 x 5), i.e. 27 Hz. Similarly, the chemical shift should be one-fifth of the sum of the isopropyl cationic carbon shift (6 319.6) plus twice the methylene shift (cyclopentane 6 27.6) plus twice the shift for methylene alpha to a positive carbon (6 52). The predicted value is S 96 in good agreement with that observed viz. 6 99.

The benzenium cation 3, formed by the protonation of benzene, is an analogous degenerate ion.‘22) At -80°C and above, the 13C NMR spectrum consists of a single line at 146ppm. The ‘H NMR spectrum is a single sharp line at 8.096 with unusually intense

13C-H satellites (3.3%); JcH is 26Hz. These data accord well with a degenerate time-averaged species


scrambling of 13C in the ion prepared from methylcyclopentyl chloride enriched with 13C in the methyl group demonstrates unambiguously(24) the involvement of the equilibration 4a+ 4b.

Cl +

CsH: as can be shown by simple calculation. For. example, considering the coupling constant, the ion may be regarded as composed of four olefinic carbons C2,3,5 and 6 having JcH 167Hz (a typical value), one secondary carbon C4 with JcH 169Hz (the value for the isopropyl cation) and two sp3 hybridized bonds on Cl with J 123 Hz. Averaging

JcH _ (4 x 167) + (2 x 123) + (1 x 169) = 26Hz - 6x7

in agreement with observation.

+

a I I

H3H

When the temperature is lowered to - 134°C the degeneracy is removed: the ‘H NMR spectrum then has resonances at 5.69 (CH& 9.58 (H2,6), 8.22 (H3,5) and 9.42 (H4).

The ‘H NMR spectra(23-25’ of the ions derived from cyclohexyl chloride and from I-methylcyclopentyl chloride following reaction with excess SbF5-SO#F are identical at + 110°C viz. a singlet at 4.68 6. The

4 4a 4b

The hexyl cation 5 is interesting in that all four methyl groups are equivalent,“@ the protons appear-

ing as a doublet at 3.326. This observation could be accounted for by (a) rapid hydride shifts 5a + 5b, (b) a hydrogen-bonded structure 5c or (c) n complexation

3

50 5b 5c 5d

5d. That the equivalence is due to (a) is demon- strated by analysis of the 13C NMR spectrum. The two centrai carbon atoms appear at 1986 and the INDOR spectrum is a doublet having JcH 65Hz. The methyl and central carbon atoms of the t-butyl cation provide models for 5a: these have respective shifts of 48.3 and 330ppm, the average of which

TABLE 2. 13C NMR data for central carbon atoms of degenerate ions

Ion

+

CH3CHCH2CH3

CH CH3 3\+ / c-c-

CH’ \ciH3 3 3

CH3 H

‘&- ’ CH/ “q

3 3

C H,- CH, / \

%_\CH-- CH+ P

H H

C+/6 JcdHz Solvent Temp. Ref.

173.4 70 SO#ZIF--SbFS -120 16

206.1

198.0 65 SbFS-SO2

99.2 28.5 SbFs-SOCIF

146 26 SbFs-HF-SO$IF

Bridgehead cations

202.2 50.8 FS03H-SbFS-SO#ZIF

205.3 53.3 FS03H-SbFS-S02CIF

218.7 51.3 FS03H-SbFs-SO#ZIF

16

-20 16

-70

-78 22

-70

- 70

-70

266 R. N. YOUNG

(189 ppm) is in satisfactory agreement with observation. The heptyl cationo6’ 6 is analogous to the hexyl

ion in that the ‘H NMR spectrum corresponds to complete equivalence of all five methyl groups, viz a single resonance at 2.906. The r3C NMR spectrum shows a single signal at 206ppm which is in accord with rapid 1,2-methide shifts. The shifts remain

sufficiently fast at - 160°C to preserve the appear- ance of the ‘H NMR spectrum, implying a barrier of less than 20 kJ mol- r,

6

Equilibration has also been observed”‘) for some

bicyclic systems bearing the positive charge on a bridgehead carbon atom. The 13C NMR spectrum

of the bicycle (3.3.0) octyl cation is particularly simple, consisting of only three signals: the bridgehead carbons at 218.7ppm (Jcn 51.3Hz), the c( carbons at

41.9 ppm (Jcn 135.7 Hz) and the fl carbons at 32.9 ppm (Jcn 137.0 Hz).

The adamantyl cation 7 is another species of interest which has the positive charge on a bridgehead carbon atom. The resonances of the /I, y and 6 protons are located at 4.50, 5.42 and 2.67ppm, respectively. “” It has been proposed that the unexpected greater deshielding of the y protons than the /II may be due to a “cage” effect whereby the lobe of the vacant p-orbital overlaps the blacklobes of the three bridgehead C-H bonds causing their deshielding. The 13C NMR spectrum also shows the y carbon to be more deshielded than the /I, the resonances being at 300.6, 66.6, 87.6 and 34.6ppm for the LY, b, y and 6 carbons, respectively.i30’

+

22 B Y

7 B A cation involving a non-classical bridged structure

is the ethylene bromonium ion 8. The ‘H NMR spectrum consists of a single peak at 5.53 6 (solution in

Br + /\

CH,-CH,

8

SbFS-SO2 at -60”C).‘31) The 13C NMR signal is

at 73.8 6 with JcH = 185Hz. The 13C shift of such an ion undergoing a degenerate 1,2-bromine shift can be estimated by taking the CH*Br moiety to be repre- sented by BrCH2CH2Br (S 39). The choice of a model for -CH: is more difficult since no primary ion is known. However, using the generalization that substitution by a methyl group causes shielding of some 10 ppm, extrapolating from the t-butyl and iso-propyl cations provides the value 3106. Accordingly, the degenerate ion would be expected to have a shift of

5 4 3

w-n

FIG. 4. The proton magnetic resonance spectrum of tertiary hexyl cation (dimethylisopropylcarbenium ion).

ca. 1746. The observed value is so grossly different that degeneracy must be excluded, leaving the bridged structure as the only plausible alternative. The Jcn value lends support to this conclusion, the values for sp3 analogues being similar, e.g. cyclopropane 162 Hz

and ethylene oxide 176 Hz. Cyclopropyl groups attached to a positively charged

carbon atom have a strong stabilizing effect much

greater than that of an alkyl group. This is clearly illustrated by the ‘H NMR data(32’33’ for the ions 9- 14 ; compared with cyclopropane (6 0.22) the x and fi protons of the ions lie at markedly lower fields. The shifts of the p protons are much too large to be simply ascribed to the inductive effect but constitute evidence for the ability of the cyclopropyl residue to

4 3 2

wm

FIG. 5. The proton magnetic resonance spectrum of tertiary heptyl cation (dimethyl-tert-butylcarbenium ion).


TABLE 3. 13C NMR shiftS (ppm) for cyclopropylmethyl cations

Ion CH3 CH2 CH

9 56.6 109.6 56.6 10 34.6 58.6 68.6 253.7 11 (40.6 31.6) 54.6 60.6 281,4 12 38.7 45.7 253.7 13 38.4 38.4 45.2 275.4 14 30.9 32.6 271.6

Ions 9, 10 and 11 at -70 ° in SbFs--SO2CIF. Ions 12, 13 and 14 at -60 ° in SbFs--SO2--

FSO3H.

actually delocalize charge. In the tricyclopropyl methyl cation 14 all fifteen protons are equivalent and are upfield of those in the dicyclopropyl methyl cation 12 as is to be expected from the relative numbers of rings. Extrapolation of this trend to the cyclopropyl methyl cation 9 is not, however, per- missible since this ion, unlike 10 through 14, has been shown not to have a classical structure (see below). The ~3C NMR spectra (shifts in Table 3) support the conclusions drawn from the ~H work, the ~ and fl resonances being well downfield of the cyclopropane carbons.

6.50 4.58 3.83 338 H 9.6 [ , ~ F4_ ..H ...o"3 L'~ \ci.4 I"I ~CH3

4.2 -4.6 H 432-4.45 ~"3 3.57-3.68 2.70 4.2 -4.8 3.34

9 I0 II

3.00 8.14 2.92-3.45 H

2.35-2,74 2.28 't"~-~ 2.14

12 13 14

Olah et al. (32"34) have used 13C NMR studies to probe the relative abilities of methyl, phenyl and cyclopropyl substituents to delocalize positive charge. Two series give particularly clear evidence that the order of effectiveness is C6Hs > c-C3Hs > CH3

(CH3)2CH + (c-C3Hs)2CH + (C6Hs)2CH + 319.6 253.7 200.2

(CH3)3C + (c-C3H5)3C + (C6H5)3 C+ 330.0 271.6 212.7

The chemical shifts quoted are those for the carbon formally marked as positive. The data refer to solutions in SO2CIF--SbF5 at - 6 0 to -90°C.

The dimethylcyclopropyl methyl cation 11 has decidedly inequivalent methyl groups as shown by both 1H and 13C data. The structure is believed ~33) to be the one in which the CH3--C--CH3 plane is orthogonal to the cyclopropane plane. The vacant p orbital thus lies parallel to the cyclopropane ring. The methyl group cis to the cyclopropyl ring appears at higher field due to the diamagnetic anisotropy of

the ring, and the rotational barrier to the interchange of the methyl groups has been estimated (37) as 57 kJ mol - 1.

The cyclopropyl residue seems also to have some ability to delocalize negative charge, although this is less marked. An example is provided by the diphenyl- cyclopropyl methyl anion ;(38) the fl protons are about 0.27 6 upfield of those of the parent hydrocarbon. An unusual feature of this ion is that it can be converted reversibly to the 1,1-diphenylbutenyl anion by a change of solvent provided that the counterion is lithium:

L i+ Et20 L i P C6H5-C--C6H5 ~' THF (C6 H5 )2C=CH - - CH-CH3

4. ALLYLIC IONS

The allylic cations constitute a large and important class of ions of diverse types and stabilities. The parent ion 15, stable only in super-acid media, was first prepared (39) by treating allyl fluoride with SO2--SbF5 at - 60 °. Theoretical calculation (4°) antici- pates that there should be a substantial amount of positive charge on C2 and this is confirmed by the IH NMR spectrum. Indeed, the central proton is actually shifted rather more than the terminal protons in going

H 8"97 H H H TM

H ""~H ...J~-. +//'L.3.45 CH;~ "7" "c.~ H H 9.64 8.30

15 16

from allyl fluoride (CH~C 5.14, ~---CH-- 5.52 and --CH2F 4.56 in SO2 solution) to the allyl cation. When methyl substituents are progressively introduced into the terminal positions, the resonance of the central proton moves upfield from 6 9.6 to 7.6 a similar effect is noted when the 2-methylallyl cation is terminally methylated, the 2-methyl 1H NMR resonance moving from 3.85 to 2.126. Some represen- tative (41) data are shown in Table 4. The stabilities

TABLE 4.

cH3 cH3 H u % ?

c ~ C H 3 H ~ H C ~ C H 3 12361 11.3

H 3 H 8.65 (147) 7.83 3.85

~7 18 19

3 ~ / ( 2 4 9 ) /12621 / 2.87 10.6 3.61 CH CH 3 12191 CH3 49

6.36 CH 3 ( 148]

21 22

/ / 2 2 9

(206) 8,08 CH3143) (139l

24 25

2O

~2191 10.2

8.32 (138)

23

268 R. N. YOUNG

TABLE 5. 13C NMR data for ally1 cations R,CH’cCA’zCH Rz at -70” in FS03H--S02C1F

ally1 cyclopropyl phenyl

RI RZ CI CZ C3 C, cp ciplu C0 Clfl CP

CH, CH3 231.3 147.0 231.3 CHx GHs 194.7 139.4 227.0 45.8 42.2 CHx CsHs 205.1 136.5 196.9 135.1 148.0

{ i

132.2 149.9 137.0 132.0

GH, &Hz 211.3 135.6 211.3 32.9 23.4 GH5 CsHs 186.8 129.3 186.8 135.3 143.3

i i

131.6 144.7 133.5 131.3

of the ions are increased by methylation and the A detailed study ‘47,48r has been made of 3-neo- more highly alkylated members can readily be pre- pentylallyl lithium in ds-toluene. The cis and trum

pared in sulphuric acid. isomers do not interconvert on the NMR time scale, The influence of methylation on the spectra of and the ‘H NMR parameters are summarized in the

cyclic allylic cations is illustrated by the cyclopentenyl table below. As in the case of ally1 and crotyl lithium,

(CD,),CCH,CH=CHCH,Li

Isomer 6% 6,

TWl.5 0.775 6.059 Cis 0.801 6.112

6;

4.637 4.495

‘H NMR data in toluene at 30”

6, J $ Jg; J,, ~__

1.935 9.4 14.5 7.0 1.886 10.0 10.0 6.9

ions 20, 21 and 22 and the cyclohexenyl ions 23, 24 and 25 for which data have been recorded’41P43’ for both ‘H NMR and (in parentheses) r3C NMR. The shifts of the allylic 2-position carbon and the proton attached to it are relatively insensitive to substitution. Methylation of Cl causes a downfield shift of some 20ppm and increases the shielding of C3 and its attached proton.

The relative abilities of methyl, phenyl and cyclopropyl substituents to disperse charge can be seen from Table 5 to decrease in the order phenyl N cyclopropyl > methyl. (43) All of the ions appear to adopt exclusively the tram, tram conformation. A number of allylic cations having other, less stable, conformations have been obtained and in certain cases the isomerization to the tram, truns form has been studied by ‘H NMR. Cis, cis 1,3-dimethylallyl ion converted to the cis, trLIn.s isomer’25’ with an E,

of 73 kJ molt ’ and log A = 11.8 ; the cis, trclns converted to tram, rruns with E, = 100 kJmol_’ and log A = 14. Similar results were obtained(44’ for the corresponding processes undergone by the 1,2,3- trimethylallyl cation.

The ‘H NMR spectrum’45’ of ally1 lithium in THF or diethyl ether is a simple AB4 spin system at ambient temperatures but at low temperatures (- 87”) it changes to AA’BB’C. The shifts dA = 64, = 2.24, 6~ = 6~. = 1.78 and & = 6.38 correspond to an essentially delocalized ionic species. Somewhat similar behaviour is shown (46) by crotyl lithium in tetramethyl- ethylene diamine where the x protons become inequivalent at -90”. Two conformers, cis and tram,

are present in the ratio 85: 15 at 35”; these are distinguishable by the J,,, coupling constants which are 10.2 Hz for the ~.i.s and 13.4 Hz for the trans.

the large upfield shift of the 71 proton of both conformers indicates some degree of delocalization of charge from the a position. This effect is very much greater if diethyl ether or THF is used as solvent, when the y resonances are at 4.1 and 3.6 6, respectively. Computer simulation of the spectrum suggests that the cis: tram ratio is 3 : 1 in toluene. The preference of terminally alkyl-substituted ally1 anions for the cis conformation contrasts with the behaviour of the carbocations.

The study of allylic lithium compounds has a

special relevance for the very considerable industry based on the polymerization of dienes by organo- lithiums. This synthetic route has the virtue of afford- ing flexible control over the stereochemistry of the propagation reaction, hydrocarbon solvents leading to high degrees of 1,4 enchainment whilst solvating media such as the amines and ethers give high pro- portions of 1,2 or 3,4 concatenation. It is clear that detailed knowledge of the geometry and bonding of the propagating centre is required to understand the factors governing the stereochemistry of the polymer.

In order to study the anionic propagating centre polybutadiene, it is necessary to employ polymers of a reasonably high molecular weight as it has been shown’49’ that the initiation reaction and the first few propagating steps proceed predominantly in a 1,2 fashion which is unrepresentative of the subsequent propagation steps. However, with higher molecular weight polymers the intensity of the signals due to the chain-end protons is so outweighed by the in-chain protons that observation is difficult. This problem was overcome by preparing polybutadiene with a per- deutero backbone. This was achieved”‘) in benzene solution using d,-ethyl lithium and de-butadiene to


5 3 I

B/w-n

FIG. 6. The proton magnetic resonance spectrum of polybutadienyl lithium in d6-benzene.

generate “transparent” polybutadienyl lithium which

was finally capped by the addition of a small quantity of protic butadiene. The resulting product had the average composition

The ‘H NMR spectrum is shown in Fig. 6 and is in accord with virtually 100’4 1,4 chain-end structure, the lithium being o-bonded to the a-carbon. Changing the solvent to THF dramatically changes the spectrum ; they proton resonance shifts from 4.7 to 3.3 6 indicating the conversion to the delocalized rr-ally1 structure. At lower temperatures, in THF, two distinguishable r-protons (two overlapping doublets) were observed- in accord with restricted rotation about the x-/i

partial double bond (Fig. 7). Similar studies of polyisoprenyl lithium’50) showed

that the active chain-end is present exclusively in benzene as the 4,l adduct with the lithium a-bonded to the terminal carbon atom. The cis and trans methyl

groups and the cis and tram y protons were distinguishable’52’ and corresponded to a cis: tram ratio of about 2: 1. Studies of the ageing of polyisoprenyl lithium in benzene showed that the chain-ends do not undergo cis-trans interconversion although this does occur in THF solutions. The latter have NMR spectra indicative of x-ally1 structures.

A satisfactory explanation of the stereochemistry of the propagation reactions cannot, however, be founded on a simple classification of hydrocarbon solutions as containing only o-bonded organo-lithium compounds and the etheral solutions as n-allylic. Despite being apparently exclusively endowed with 1,4 (or 4,l) active centres in hydrocarbon solvents, the overall structure of the resulting polybutadiene has 9% of 1,2 concatenation; polyisoprene has lo”/, of 3,4 and poly-2,3-

dimethylbutadiene about 20% of 1,2. It has been suggested’51’ that these unexpected structures arise from highly reactive rc-allylic chain ends, 26 and 27, in

cis 26

2

trans 27

d&THF SOhJtiOn

(*impurity) 23”

6 4 2

S/ppm

FIG. 7. The proton magnetic resonance spectrum ofpolybutadienyl lithium in ds - THF at 23 and - 70°C.

270 R. N. YOUNG

Y /‘\ CH,

C,H,[CH,CHCHCKH3,], CH, WC< (1 0

k Li t-l

L L

7.0 5.0 3.0

Chemical shift/ppm

FIG. 8. The proton NMR spectrum of 1,3_pentadienyl lithium in de-benzene.

amounts too small to be detected by NMR, in equilibrium with the ~7 bonded form.

The ability of the cis and trans isomers to interconvert in ethers is believed to be due to the presence of a minute equilibrium concentration of o-bonded isomer which is expected to have a low barrier to

rotation. Support for these proposals comes from the ‘H NMR spectrum of the anionic oligomer of penta- 1,3-diene (Fig. 8). The simple triplet signal of the /I proton shows that the chain-end has 1,4 rather than a 4,l structure. However, a resonance at 4.6 6 could be discerned, particularly when the chain length was

very small; this was ascribed to the y protons of a small amount of 4,l chain-end. The similarity of the location of the y chain-end protons of the 4,l adduct to those of butadienyl-lithium and polyisoprenyl lithium in benzene suggests that all these species have a covalent structure. The corresponding re-

0.9 0.5 0.1

l/(n+2)

FIG. 9. Dependence of proton chemical shift for the methyl

protons in the cations (CH&6-CHz(CxC)rz-C(CH3)z upon l/(,1 + 2). For n = 0, 1, 2, 3, 4 and cc. The shift for the

t-butyl cation is also shown.

semblance of the y protons of butadienyl lithium and polyisoprenyl lithium in THF to those of the 1,4 chain-end of pentadiene in benzene suggests these species are all predominantly rr-allylic. The unusual

bonding of the 1,4 chain-end was attributed to the high free energy of secondary carbon-lithium bonds. The 1,4 chain-ends were found to be replaced completely by the 4,l at higher chain lengths. No such conversion of secondary to primary carbanion is possible with the ion derived from the precursor 2,6- hexadiene which has its y resonance at 3.1 6, in accord with a delocalized n-ally1 structure, in benzene solution.‘* 3,

5. POLYENYLK IONS

The aliphatic polyenylic cations are formed readily by the protonation of the appropriate conjugated polyene by sulphuric acid!3,54,55’ The spectra are relatively simple and the p and y protons are easily identifiable. It was presumed that the cations predominantly, if not exclusively, adopted the all-trans conformation.

’ H NMR data for polyenylic cations”

(CH~)Z~_CH~C~~C~)_C(CH~)~

n &,,, 6, 6, -

0 2.95 ~ 7.60 1 2.67 9.02 7.19 2 2.42 8.58 7.05 3 2.21 8.20 6.97 4 2.14 7.87 6.60

‘In HZS04 solution.

The charge is delocalized over (n + 3) carbon atoms and it was found that a plot of the chemical shift of the methyl protons against l/(n + 2) was linear (Fig. 9). The chemical shift for the terminal group =C(CH3)2 in the neutral polyenes was found to be close to 1.8 (in CC14) irrespective of the length of


L I * I

Io 5 o

ppm

FIG. 10. The ~H NMR spectra of the 1,1,5,5-tetramethyl-pentaenyl cation and its cyclization product.

the conjugated sequence. This gives an estimate for the 6 value for the methyl protons on an infinitely long cation. This, together with the value for the t-butyl cation provides two further points on the plot which are gratifyingly close to the same straight line. The analogous plot for the y protons was also linear, but that for the fl protons exhibited curvature.

The polyenylic cations cyclize (54'56) with varying degrees of rapidity to form cyclopentenyl ions, these processes being clearly evident on monitoring the NMR spectrum. Figure 10 illustrates a typical example.

The acyclic pentadienyl carbanions can, in principle, exist in three planar conformations: the all-cis U, the

all-trans W and the cis, trans sickle. By studying a series of ions of fixed and known geometry, Bates and his co-workers t57) were able to formulate some general rules for the interpretation of the 1H NMR spectra. In accordance with the predictions of MO theory, t58~ the protons on C 1,3 and 5 are more highly shielded than those on C 2 and 4. The inner two carbon- carbon bonds are of 1.33 order and the coupling constants for their protons are 6.4 to 6.5 Hz for cis

and ca. 12Hz for trans. The outer carbon-carbon bonds are 1.67 order and the corresponding JHH are 7.5 to 9Hz for cis and 16Hz for trans protons (Table 6). Compound 29 is of interest t57) in that the ct protons

TABLE 6. 'H NMR data (ppm) for some pentadienyl carbanions

3.9 2.3 3.7 2.5 4.1

28 29 30

CH 3

3.0 4.1 3 - 0 1 ~ 1 ~ 7 4 . 0 7 2.7[~ ~4"09 ~3"39

• CH 6,2 6.09 5.84 " 6.02 5.75 3

31 32 33

3.56 4.59

6.14 6.01

34

~ 3.05

5,72

3.50

37

C H 3,45 4.6{ 4.46 13 3.32 4.43 4,47 4.09

6.14 6,00 5.94 6.11 5.93 5.80 "~H3

35 36

3.51

5.67 5.65

3-14 2,33

38 39

272 R. N. YOUNG

TABLE 7. ‘%Z NMR chemical shift data (ppm) for pentadienyl carbanions at - 37°C in THF

Cl c2 c3 c4 c5

66.2 143.8 86.9 143.8 66.2

85.7 137.3 81.4 143.6 56.7

CH3 w 89.2 139.6

(CH,),C~ 94.7 133.1

101.2 126.4 77.2 138.2 49.2

83.2

81.9

142.9

142.7

51.5

50.8

become inequivalent at 0°C. The unsubstituted pentadienyl carbanion 31 has coupling constants in accordance with a W conformation. As with 29, lowering the temperature (to 15”) causes the protons on the terminal carbon atoms to become inequivalent; the coupling constant of the b proton with the c( proton which is trans to it is 16Hz whereas the coupling constant with the cis c( proton is 9 Hz. Substitution by a methyl group in the /? position reduces the energy difference between the W and the sickle conformation and resonances due to both forms were evident in the spectrum. If a methyl substituent is placed on Cl of the pentadienyl carbanion it is found’5vs60) that structure 32 with the methyl in the cis position is of lower energy than when tram as in 33. A similar situation prevails when the heptatrienyl ion 34 is methylated on Cl : the isomers 35 and 36 are present in a 4: 1 ratio at O”.‘61) A further common observation is that a cis methyl group is less able to repel electron density to the terminal pair of hydrogens in the pentadienyl and heptatrienyl carbanions than is a trans methyl substituent.

The carbanions cyclohexadienyl 37, cyclohepta- dienyl 38 and cyclooctadienyl 39 have been studied’62) and the resonances are markedly sensitive to ring size. A linear relationship was found between bond order and chemical shift.

The barriers to rotation have been measured by NMR for a number of cross-conjugated polyenyl carbanions.‘63)

Studies of the r3C NMR spectra of the polyenylic anions are somewhat less extensive than those of ‘H NMR spectra, whose principal features they mirror. In accordance with MO theory, the odd- numbered carbon atoms absorb at higher field than the even, which bear very little charge.‘64) Some illustrative data’65) are given in Table 7.

6. AROMATIC IONS

Simple Hiickel theory predicts that only those cyclic conjugated polyenes having trigonally-hybridized

carbon atoms associated with (4n + 2)rc-electrons should be aromatic. Different views prevail as to precisely what the term “aromatic” should be taken to imply. For the present purpose, an ion will be regarded as aromatic if it is capable of sustaining an induced diamagnetic ring current. Considerable efforts have been directed towards testing the Huckel theory and a number of reviews are available. Particular impetus has been provided by the elegant syntheses of many annulenes by Sondheimer and his co- workers.@6)

The simplest ionic example, where n = 0, is the cyclopropenyl cation. (66) This is readily formed by the reaction of 3-chlorocyclopropene with SbCIs in di- chloromethane. The ‘H NMR spectrum consists of a singlet at 11.1 6, which is consistent with a delocalized aromatic species.

40 41

The remarkable four-membered ring di-cation 41 has been prepared, (67) although the uncomplexed neutral parent has not. The ‘H NMR spectrum is a singlet, S 3.68 at -65” in SbFs-S02. Comparison with the 1,2,3,4,4,5-hexamethylcyclopentenyl cation@) in which the l- and 3-methyl groups (2.816) are similarly attached to sp’ carbon atoms with a charge density of approximately 0.5 suggests that the deshielding of 41 is due to the ring current.

There are two very familiar ions belonging to the case n = 1, viz. the cyclopentadienyl anion 42 and the cycloheptatrienyl (tropylium) cation 43. The ‘H NMR spectra” r) are respectively singlets at 5.4 and 9.18 6. Allowing for an approximate dependence of the shift of 1Oppm per unit charge then correcting these shifts for charge yields the values 6 7.4 for CsH; and 7.7 for C,H: in apparent concordance with the prediction that larger rings should exhibit larger ring current effects.


42 43 44

The bis tropylium species 44 has also been prepared : I”) the ‘H NMR spectrum is a singlet at 6 9.95 and the two rings are twisted out of mutual coplanarity. More recently, the 1,3,5,7_tetramethyl derivative of the elusive cycle-octatetraene di-cation

CH3 ,/-- ‘\ Cl-l, 0 1 +2 ’ Cl-t\ ? 3 \__J

C% 45

45 has been prepared. (“i In SO#ZIF--SbFS solution the ‘H NMR spectrum was independent of temperature in the range -30 to - 100°C and comprised two singlets at 6 4.27 and 10.80 in the intensity ratio 3 : 1. On the basis that a change in the n-electron charge of one unit on a carbon atom gives a shift of 10 ppm to the attached proton, the difference between the neutral parent and the di-cation should be 2.5 ppm; that the observed difference is 4.6ppm is a reflection of the diamagnetic ring current. The first 10 rr-electron species prepared was the di-anion of cyclo-

octatetraene’74’46. When this tub-shaped hydrocarbon was progressively reduced by potassium in THF, the singlet comprising its ‘H NMR spectrum diminished in intensity. When only 10% of the neutral hydrocarbon remained, the spectrum consisted of a very broad signal due to the di-negative ion, superimposed on which was a shurp signal due to the parent hydrocarbon Although the concentration of the mono- negative ion remained low throughout reduction because the disproportionation equilibrium

2COT-‘+ COT + COT- 2

lies far to the right, it is striking that electron exchange broadened the resonance due to the di- negative ion but was without significant effect on that due to the neutral molecule. It was estimated that electron exchange between COT and COT-’ is 10,000 times slower than that between COT-’ and COTm2. This difference in rates is readily accounted for if it is assumed that COT-’ and COT-’ are planar species. When COT is completely reduced to COT 2, the ‘H NMR spectrum of the resulting solution is scarcely different from that of the parent hydrocarbon. The failure to observe increased shielding is in accordance with flattening of the carbon skeleton on reduction and the induced diamagnetic ring current thereby made possible.

The great stability of the 10 rc-electron con-

figuration is illustrated by the rearrangement of the skeleton of the Spiro j2.7) decatrienyl anion 47 on deprotonation resulting”” in the formation of 48.

The nonatrienyl anion 49, readily prepared from 50 by reduction in THF by potassium,‘76’ is stable at -40” for several days. The ‘H NMR signal of the

outer protons is a multiplet at 6 6.4-7.27 whilst that of the solitary intra-annular is at 6 - 3.52. Calcu- lations suggest this species is non-planar. Warming the solution to 30” causes the conformation to change to 51 whose ‘H NMR spectrum consists of a singlet at 6 7.0. If it is assumed that the ring currents are the same in &Hi- and CgHg, the latter would be predicted to be some 1.4 ppm downfield of the former, in good agreement with observation.

The bicyclo(4.3.1 ldecatetraenyl anion”‘) 52 and bi- cyclododecapentaenylium cation”‘) 53 are hypothetic- ally related to 1,6-methano { 10) annulene in the same way as are the cyclopentadienyl anion and the tropy- hum cation to benzene. In all three species the very large shielding of the bridge protons is in accord with a diamagnetic ring current as are their large geminal coupling constants-respectively 7.5, 10 and 6.9 Hz.

52 53

54 55

Treatment(79’ of bicycle [ 5.4.1) dodecapentaene 54 with KNH2 in liquid ammonia results in the removal of a proton to form the 12 n-electron anion 55. The anti-aromatic nature of 55 is revealed by the far-downfield location of the signal due to the bridge protons at 6 14.2-almost 16 ppm downfield of the corresponding proton in the aromatic cation 53. Similarly, the observation of the “olefinic” protons substantially upfield from the position expected on the basis of charge alone is due to the paramagnetic ring current. The geminal coupling constant for the bridge protons is large (9.6 Hz) but the vicinal coupling between those on C8 and C9 is unusually low (4.2 Hz) in comparison with the corresponding coupling for cation 53 (9.46Hz). It seems that the anion is twisted out of planarity, the H8-CGC9-H9 dihedral angle being greater than 40”, such a deformation reducing the degree of anti-aromaticity,

J P N.M.R.S. 12/4--o

274 R. N. YOUNG

TABLE 8. 13C NMR data for aromatic ions

Ion Solvent Counterion G(ppm) JcH(W Ref.

40 176.5 262 10 41 SOL SbF, 210.3 67 42 THF Li+ 102.8 157 4 43 CH&N Br- 156.1 171 4 46 THF K+ 85.5 145 4 51 THF Li’ 109.5 137 69.70 45 SOfJF SbF, 182.7” 73

1 70.0h

d (Cl, 3, 5, 7). ’ (C2, 4, 6, 8).

The theory of paramagnetic ring currents has been discussed.@O’ In general, HMO theory predicts that planar cyclic systems of 4n rr-electrons have their highest-occupied MO doubly degenerate. As a result,

these systems have triplet ground states unless Jahn- Teller distortions remove the degeneracy by bond length alteration or by the development of non- planarity. An example of a triplet species is provided by the cyclopentadienyl cation 56 which exhibits an ESR spectrum.(“)

The 14 n-electron { 12) annulene dianion (57) is not a truly aromatic species because the three intra- annular hydrogen atoms prevent planarity.‘8z’ None- theless, this anion is very much more stable than the parent hydrocarbon ; the former is quite stable at +30” whereas the latter decomposes above -5O”, and has to be prepared by a low temperature photo- lytic reaction. The reduction is accompanied by a striking change in the ‘H NMR spectrum, the intra- annular protons moving from 6 7.86 to -4.60 due to the replacement of a paramagnetic ring current by a diamagnetic one. The extra-annular protons move in the opposite direction, but to a smaller extent, from 6 5.91 to 6.23 and 6.98.

Treatment of { 16) annulenecg3) with S02-FS03H at - 80” generates the violet di-cation 58 whose intra- annular protons resonate at S - 4.1 (2H), -4.48 (1H) and -2.58 (2H) and whose extra-annular protons are at 6 10.7, demonstrating the existence of a diamagnetic ring current. Reduction of (16) annulene by potassium in THF yields the 18 n-electron aromatic di-anion (59) having ‘H NMR resonances(84) at 6 8.83 (8H) and 7.45 (4H) for the outer protons and at

-8.17 for the four intra-annular protons again pro- viding clear evidence for the diamagnetic ring current. Unlike the parent 116) annulene and the aromatic { 18) annulene, the spectrum of the di-anion is indep- dent of temperature, suggesting that it has a greater degree of resonance stabilization. It is also interesting to note that the di-anion and di-cation of (16} annulene have different conformations. A final 18 rr-electron system is the { 17) annulene anion 60; this ion is stable at 100°C and the diamagnetic ring current is indicated by the resonance of the intra-annular protons at 6 -7.97.(85’ The di-anion of { 181 annulene 61 (potassium salt in THF) exists as an equilibrium mixture of two conformers. The intra-annular protons are located at 6 29.5 and 28.1, the extra at 6 - 1.13. The ion is non-planar and exhibits n-bond localization.

62

A particularly informative study’87’ has been made

of the di-anions derived from the aromatic and rigid truns-15,16-dialkyldihydropyrenes 62. The results obtained for the methyl, ethyl and n-propyl compounds are summarized in Table 9.

The enormous downfield shift of the alkyl CI protons

of 25 ppm provides unequivocal evidence of the change in the character of the ring current from diamagnetic to paramagnetic on reduction.

Studies of i3C NMR spectra have been generally

less useful than ‘H spectra in testing for aromaticity. Although i3C resonances are sensitive to stereo-

chemical factors such as conformation, ring size and ring strain, they are less sensitive to ring current effects.‘88’

7. ARYLALKYL CARBANIONS AND ION PAIRING PHENOMENA

The influence of ion pairing upon the NMR spectra of carbanions and carbocations has been largely ignored in the preceding sections of this review, on the grounds that such effects as have been observed in ions of the foregoing classes have been relatively trivial. However, in the cases of arylalkyl carbanions with their extensive x-electron systems, ion pairing with the alkali-metal and alkaline-earth cations

usually employed as counterions, may cause signi-

ficant perturbation of their NMR spectra. Such cations tend to form relatively well-defined solvates,

NMR Spectroscopy of carbanions and carbocations

TABLE 9. ‘H NMR chemical shift data* (ppm) l&16-dimethyl- dihydropyrenes and their dianions

Alkyl substituent hydrocarbons a B Y Ring protons

_ CH3 -4.25 7.95-8.67

C&-&I* - 3.96 - 1.86 7.95-8.67

C&-C&H, - 3.95 - 1.87 -0.65 7.95-8.69

Di-anions CH3 21.00 3.19-3.96 CzHs 21.15 10.70 2.50-3.14 C3H, 21.24 12.59 5.51 2.56-3.14

* Determined in ds - THF at - 65°C with potassium counter ion.

275

and in consequence, more than one kind of thermo- dynamically-distinct ion pair can generally be identified. In contrast, the carbocations almost invariably have polyatomic counterions, SbF;, BF, or the like, which have low polarizing power and very limited tendencies to form specific solvates. Accordingly, carbocations seem to form only one kind of distinguishable ion pair.

The more highly-conjugated and polarizable carbanions generally exist in solution as equilibrium mixtures of two types of ion pairs: the tight (contact) and the loose (solvent-separated). The formation of the latter category is favoured by lowering the temperature, by the use of strongly-solvating solvents and by employing as counter ions those derived from the lighter metals.

At the most fundamental level, there is clearly much interest attached to probing such structural details as the location of the cation with respect to the anion, or how many molecules of solvent constitute an essential part of an ion pair. On a more pragmatic level, the frequent observation that tight and loose ion pairs differ in reactivity by as much as four or five orders of magnitude makes it important to quantify their relative concentrations. Since the tight and loose ion pairs interconvert rapidly on the

NMR time scale, an averaged spectrum is observed. Quantitative results can be obtained by application of the formula

{loose ion pair)

Keq = (tight ion pair}

6, - 6 =p

i? - 6,

where S,, 6, are respectively the shifts for the tight and loose ion pairs and 6 is that observed for an

equilibrium mixture. Some illustrative results are shown in Table 10 for ion pairs of the triphenylmethyl anion in 1,2_dimethoxyethane (DME).‘89’ A similar study of ion pairs of the 1,3_diphenylpropenyl anion yielded values for AH and AS for the interconversion of the tight and loose ion pairs:‘90’ these values were in satisfactory agreement with those obtained by analysis of the temperature dependence of the electronic absorption spectra which exhibit distinguishable bands for the two ion pairs.“‘)

The ‘H NMR spectrum of the fluorenyl anion is also sensitive to the nature of the ion pairing (Table 11). The spectra of the lithium salt in THF and in DMF are very similar,(92) in accord with the knowledge that the predominant ion pair, in both solvents, is the loose one.(93) The spectra of the sodium and potassium salts are downfield of that of the lithium salt in THF.‘95) The salts of the heavier alkali metals are predominantly tight ion pairs. In general, where only tight ion pairs are present, the ‘H NMR spectrum shifts systematically downfield in the

sequence Cs > Rb P K > Na > Li reflecting the progressively greater attraction of electron density by the cation. When the lithium salt happens to be mostly

present as loose ion pairs and the sodium and potassium salts as tight ion pairs, the ‘H NMR spectra may move downfield in the order Li > K > Na ; such a situation is found with THF solutions of the 43 diphenylallyl’90’ and indoyl’“‘l anions.

When THF or DME solutions were evaporated under high vacuum, the residue tenaciously retained some of the solvent: when taken up in CsD6, the NMR spectrum showed’92) that the ratio (THF): { RLi} was about 3.2 : 1 and { DME} : (RLi} was 1.2 : 1.

TABLE 10. ‘H NMR chemical shifts (ppm) for triphenylmethyl anion in DMEts9’

Cation Temperature (“C) 6, %I 60 9/, loose pair

Li 56 7.277 6.507 5.942 100 Li 27.3 7.268 6.512 5.945 100 K 68.5 7.235 6.601 6.034 41 I( 27.3 7.262 6.546 5.981 68 K - 27.5 1.274 6.495 5.928 94

276 R. N. YOUNG

TABLE 11. ‘H NMR chemical shifts (ppm) of fluorenyl anion

Cation Base Solvent 61 62 63 64 69 6a isp

Li THF THF 7.20 6.72 6.30 7.78 5.80 Li THF CsDs 7.65 1.25 6.91 8.24 6.00 2.81 1.23 Li DME DME 7.21 6.12 6.32 7.78 5.87 Li DME CsDs 7.80 1.36 7.04 8.35 6.21 2.16 2.42 Li Et20 Et,0 7.40 6.98 6.66 7.98 5.76 Na THF THF 7.38 6.90 6.55 8.00 6.04 K THF THF 7.27 6.81 6.44 7.87 5.89

The 2 and fi protons of the ethers were shifted appreciably upfield (Table 11) relative to uncomplexed ether in C6D6 (3.61 and 1.51 respectively for THF and 3.38 and 3.18 for DME). In addition to the upfield shift, it is noteworthy that the relative positions of the CI and fi protons have been reversed. These observations are readily accounted for if it is assumed that the cation, together with its solvation shell of ether, is not associated with any individual carbon

atom of the carbanion, but is located symmetrically above the anion in the rr-electron cloud. The ether protons are thus subject to the influence of the ring current. A similar effect was observed with the diphenylallyl carbanion ion pairs.‘gO’ Strong upfield shifts have also been noted when cyclic polyethers complex the cation in fluorenyl alkali salts.‘96)

Variability in the location of the cation has been proposed to explain the relative ‘H chemical shifts of the indenyl carbanion. The 4,5,6 and 7 protons move to higher field in the sequence Li, Na, K whereas for the 1,2 and 3 protons the corresponding sequence is Na, K, Li. These results can be accounted for if it is accepted that the lithium cation is located above the 6-membered ring and the sodium and potassium ions are above the 5-membered ring.(97’

Comparatively few systematic studies of the temperature and cation dependence of i3C NMR spectra have been made. An interesting exception is that of the diphenylmethyl carbanion.‘98)

The polarization arising from the proximity of the cation to the anion in an ion pair may result in an effective reduction in the rt-electron density which can significantly reduce the barriers to conformational change. Clear examples are provided by THF solutions of the 1-phenylallyl carbanion.‘p9) The Li+, Na+ and K’ salts all exists as tight ion pairs and AG* for methylene rotation is respectively 49.7, 53.9 and 53.9 kJmol_‘, these values reflecting the relative polarizing power of the three cations. The 1,3- diphenyl-2-methylallyl carbanion in THF forms predominantly tight ion pairs with K+ and loose pairs with Li+. As a consequence of the absence of significant polarization effects in the loose ion pair, the AG+ for topomerization is greater for the Li+ salt than for the K+ salt.(“‘) In contrast, the Li+ and K’ salts of the 1,3-diphenylallyl ion which both exist as loose ion pairs in liquid NH3 have been found to have identical barriers to phenyl rotation.““) The AG* for methylene group rotation in the Li+, Na+ and K’ salts of the 1-naphthylmethyl anion in THF, all of which are tight ion pairs, take the values 53.9, 61.9 and 74.4 kJ mol- ’ respectively.(102)

The structures of the benzylic carbanions have particular significance for the polymer chemist since they represent the propagating centres in the anionic polymerization of styrene and its derivatives. The results of studies of several such ions are summarized in Table 12. The ‘H NMR spectrum of benzyl

TABLE 12. Proton chemical shifts (ppm) for benzylic anionstn’-“‘6’

Compound Solvent ortho meta para a P CH3

Benzyl lithium THF 6.09 6.30 5.50 1.62 Styryl lithium” CsH6 5.98 6.53 5.52 Styryl lithium 920’C6H6 /D + 8”/ ’ THF 6.00 6.62 5.32 3.02 2.06 Styryl lithium THF 5.87 6.32 5.12 2.36 1.85 Benzyl potassium THF 5.59 6.12 4.19 2.24 Cumyl potassium THF 5.15 6.08 4.41 1.48 x-Methylstyryl lithium” CsH6 5.61 6.23 4.66 1.60 1.20

5.29 r-Methylstyryl lithium THF 1 5.57 6.10 4.46 1.88 1.51

5.19 Dimer cc-methylstyryl’ THF (;:;; (I;; 4.20 1.32

potassium Dimer l,l-diphenyl-d THF 7.01 6.55 5.67 2.48

ethylene potassium

* Mean composition C4H9(CHzCHPh)iLi+. hMajor component tC4Hg(CH2CMePh)-Li+.

’ KtPhCMeCH,CH2CMePhK’. d K+Ph&H2CH2CPh2K+.


1 1 1 .

6.0 5.0 4.0

8 &pm

FIG. 11. The ‘H NMR spectrum of r-methylstyryl lithium in THF

potassium (lo3) in THF is upfield of that of benzyl lithium(‘06) as expected on the basis of the relative polarizing powers of the two cations. The spectrum of oligomeric polystyryl lithium which is qualitatively similar, is shifted upfield on changing the solvent from benzene to THF. Unfortunately, it is not possible to provide a simple interpretation of this change purely on the basis of ionic character since the active chain-ends aggregate to form dimers in benzene, but not in THF.(lo4) The cc-methylstyryl lithium oligomer

is of special interest since the ortho protons (and to a smaller extent the meta-) are inequivalent, showing at once the sp’ character of the benzylic carbon atom and the existence of a significant barrier to rotation of the phenyl group. Raising the temperature removes this inequivalence (Fig. 11). It is surprising that no such inequivalence is shown by either cumyl potassium”04) nor by the dimeric di-anion of l,l-

diphenylethylene. (lo’) The latter species is also unusual in the marked low-field position of its spectrum, which in part is due to non-coplanarity.

13C NMR spectroscopy of the dimeric di-anions of cc-methyl styrene and diphenylethylene (Table 13) parallels the ‘H NMR spectra in that the ortho carbon atoms of the former ion are inequivalent, in contradistinction to those of the latter.(107) Com- parison of the a-methylstyrene di-anion spectrum with that of the related neutral hydrocarbon 2,5-diphenyl-

hexane provides particularly clear evidence of the deshielding of the a carbon atom arising from the change from sp3 to sp* hybridization.

It is instructive at this point to compare the spectra of arylmethyl carbanions and carbocations. Although the shifts of the benzylic carbanion ring protons appear in the sequence meta, ortho, para in moving from low to high field, the corresponding sequence for the cations is ortho, para, meta.(‘08) Likewise, although the ring protons of the triphenylmethyl anion move to higher field in the order ortho, meta, para, the analogous order is para, meta ortho for the cation(106.10v) (Fig. 12). As Farnumc8) has emphasized, a mirror image relationship between cation and anion is only to be expected if the charge density at a given position is linearly related to the proton shift and is the only factor which determined the shift. Ring current effects will certainly modify the chemical shifts, and in the triphenylmethyl ions the angle of twist may be different in the two ions.

8. ‘Li NMR AND ION PAIRING

Brown’“‘) was the pioneer to introduce ‘Li NMR to the study of organo-lithium compounds, although primarily to the alkyls which, as substantially covalent substances, fall outside the scope of the present review. A useful summary of such data has been given.“’ ‘)

R R TABLE 13. 13C NMR chemical shifts (ppm) for PhCCHZCH2CPh in THF

K K

ipso ortho meta para c( B CH3

R = CsHs 145.8 117.5 129.2 108.0 86.9 30.4 R=CH3 137.5 103.3 129.6 88 78.4 33.6 19.1

107.9 131.5 2,Sdiphenylhexane 148.4 127.5 128.9 126.5 41.4 37.4 22.9

278 R. N. YOUNG

Ph3C+

m 0

Ph,C-

m

0

n /A

I I I

6.0 75 70 6.0

8h-m

FIG. 12. The ‘H NMR spectra of the triphenylmethyl cation and anion.

An early study of fluorenyl lithium showed that the ‘Li resonance moved from 6.24 to 3ppm upfield of 20”/, aqueous lithium chloride when the solvent was changed from diethyl ether to benzene.(g2) This difference was attributed to the formation of tighter ion pairs in benzene than in diethyl ether. The discovery that LiBr and LiC104 have shifts ranging over some 6ppm in different organic solvents necessi- tated there-examination of this interpretation. A series of organo-lithium species was studied in a range of solvents of differing solvating power ; the results”’ 3,114) are summarized in Table 14.

Cyclopentadienyl lithium forms tight ion pairs in all of the solvents listed, except for HMPA (hexamethyl- phosphoramide) in which it is present as the loose pair. The near-constancy of the ‘Li shift in the tight pairs is noteworthy. The greatest range of shift is shown by fluorenyl lithium: visible absorption spectro-

scopy has shown that tight ion pairs are formed in Et20 and loose pairs in the other solvents. The location of the cation over the carbanion, in the a-electron cloud, subjects it to the shielding influence of the induced ring current, the effect of this being very sensitive to the interionic separation, and diminishing

TABLE 14. ‘Li shifts” of organo-lithium compounds’1’4’

Anion Et,0 THF DME HMPA

Cyclopentadienyl 8.68 8.37 8.67 0.88 Indenyl 8.99 6.12 6.65 0.95 1 -Phenylallyl 0.66 0.71 0.96 0.66 Fluorenyl 6.95 2.08 3.06 0.73 1,3-Diphenylallyl 1.44 1.24 2.42 0.54 Triphenylmethyl 1.11 2.41 0.66

‘In ppm upfield from external aqueous 1.0~ LiCI.

in going from a tight to a loose ion pair. The same mechanism is responsible for the upfield ‘Li resonance of the cyclopentadienyl and indenyl lithium ion pairs. The ion pairs of phenylallyl lithium and diphenylallyl- lithium are at much lower field since the cation,

although in the n-electron cloud, is located over the ally1 structure and not over the rings. The lithium salts of fluorenyl, 1,3-diphenylallyl and triphenylmethyl are all loose pairs in DME and in THF ; the systematic upfield location in DME as compared to THF has been ascribed to a solvent effect. In the case of the fluorenyl ion pairs, a linear relationship between the Li shift and the interionic separation was deduced.“’ 3, However, the assumption that this distance can be adequately measured using Dreiding models makes the validity of this conclusion open to some doubt.

The use of ‘Li NMR to study ring currents has been extended’“” to a large number of hydrocarbon di-anions, and the results are in general accord with those obtained from similar studies using ‘H NMR. The large upfield shift of the ion pair of cyclo- octetraene di-anion is a clear reflection of the existence

TABLE 15. ‘Li chemical shifts” for di- anions in THF

Di-anion Shift

Cycle-octatetraene +8.55 Acenaphthylene +4.13 Anthracene +1.15 Azulene - 2.05 15,16-Dimethyldihydropyrene -3.15

* Measured in ppm from the reference aqueous 1.0~ LiCI, positive values indicating upfieid shift.


of the induced diamagnetic ring current in an aromatic orbitals on carbon atoms 1 and 3 in a manner 10 n-electron species. Conversely, the downfield shift intermediate between that corresponding to cr- and n- of 15, 16-dimethyldihydropyrene must arise from the bonding, substantial stabilization can result from the paramagnetic ring current in this anti-aromatic ion. ensuing delocalization. Earlier work in this field has The azulene di-anion is similar. been reviewed.” 1g.120)

The 7Li chemical shift for the lithium salt of the carbazole nitranion is 0.13 ppm downfield from LiCl; it was concluded that the lithium ion is associated with the nitrogen atom.(116)

8.1. Exchunge Phenomena in Ion Pairs

The rate of the exchange reaction

Fl-Li+ + FlHeFlH + Fl-Li+

The simplest two x-electron homoaromatic system is

the homocyclopropenyl cation 64, formed on treating 3- acetoxycyclobutene at - 78” with SbFS--FS03H- S02ClF. The ‘H NMR spectrum is independent of temperature over the range 20” to - 60”, and consists of signals at 4.53, 7.95 and 9.726 with intensities 2: 2: 1, respectively. At lower temperatures, the methylene signal at 4.53 6 broadened, the other signals remaining sharp. This change was attributed to slowing of the ring

where FlH is fluorene and FI-Li+ fluorenyl lithium, has been determined in dimethylsulphoxide solution by the application of double resonance: a value of 0.5mol-‘dm3s-’ was found”’ 7, at 38°C. The analogous exchange between indene and indenyl sodium in HMPA solution was also studied by ‘H

NMR.” I’) The rate constant obeyed the equation k=7 x 107exp(-4800/T); the Ais* is thus -965 molt ’ K _ ‘, representing an enormous loss of entropy in reaching the transition state. When diglyme or DME were employed as solvents, ‘H NMR could detect no exchange below 150°C. The exchange reaction between free dimethyldibenzo { 18}-crown-6 and that complexed to sodium fluorenyl has been studied in THF solution: (g6) it was concluded that k = 1013 exp (-6250/T)mol-’ dm’s-I. When potassium fluorenyl was employed, the exchange rate was much greater, as expected since K’ is less strongly complexed.

c=c E ‘c’

63 OAc

inversion 64ae64b. Comparison of the ‘H NMR shifts with those of other analogous allylic cations is very informative. Because of the involvement of homo- aromaticity in 64, substantial positive charge is located at the 2-position which results in the reversal of the order of proton shifts from that observed in other simple cyclic allylic cations where the greater ring size prevents significant 1,3 interactions. A reversal of the ordering of chemical shift is also found in the ’ 'C NMR spectra (Table 16).

9. HOMOAROMATIC IONS

In contrast to the simple parent ion, the 1,3- diphenyl-2,4-dimethylcyclobutenyl cation 65 has ‘H

Homoaromatic ions are stabilized by the delocalization of charge by homoconjugation. The arche- type is the homoallyl structure 63 in which there is a charged atom located /j to an olefinic residue. If the skeleton can be rotated to allow overlap of the 65 66 67 68

TABLE 16. Chemical shifts (ppm) for cyclic allylic cations’41’

Hl,H3 H2 H4 Cl, c3 c2 c4

A :: 11.1 115.9

1.95 9.12 4.53 130.0 187.6 54.0

11.26 8.65 4.23 236 147 49

10.25 8.32 3.81 219 138 33

280 R. N. YOUNG

and r3C NMR spectra characteristic of allylic ions with no perceptible 1,3-interaction.“22’ Oxidative ring opening of the tetramethylcyclobutadiene dimer by SbFs-S02ClF at - 78” yielded the bis-(tetramethyl- homocyclopropenyl) di-cation 66 ; the NMR spectra were in accordance with substantial 1,3-interaction.“23’

The ion 67 formed by protonating cyclo- octatetraene displays a 5: 2 : 1 : 1 proton pattern in- consistent with a planar structure.“24,‘25) The structure 68 containing a cyclopropyl ring is eliminated on the grounds that the signals for HI and H7 are sub-

stantially deshielded from those of a typical cyclopropane (cu. 6 3). The very large difference between the chemical shifts of Ha and Hb, and their large geminal coupling constant (10 Hz) are in accord with the monohomotropylium structure possessing a diamagnetic ring current. The large difference in the chemical shifts of Ha and Hb, together with the stereospecificity of the protonation in D2S04 (80% of the deuterium being located in the above-ring position) allowed the ring inversion rate to be measured by NMR.‘r2”‘The AG* thus determined was 93 kJ mol- ’ : if the inversion proceeds via the planar cyclo- octatrienyl cation, this value represents the free energy difference between the homoaromatic and planar ions.

The benzo-(127) and dibenzo-homotropylium”28’ ions 69 and 70 also exhibit large differences in Ha and Hb.

69

O.$O

anion relative to the hydrocarbon parent 73 shows that the ion should be regarded as bis homocyclo- pentadienyl with delocalization of negative charge to positions 6 and 7. Similarly, the increased shielding of the S-protons in the anion represents the involvement of a diamagnetic ring current. It has been pointed out that if allowance is made for the negative charge “on the naive assumption that the density is the same at all five carbon atoms” then the “corrected” values are in the range 4.8-7.4 6 which is rather far upfield for a delocalized 6 n-electron system having a diamagnetic ring current. However, comparison with the bicycle ( 3.2.2) nonatrienyl 74 and nonadienyl 75 anions”31) tends to confirm the existence of the diamagnetic ring current. The bridge protons of the former anion are insensitive to the negative charge (1.71 6 as compared with 1.83 in the parent hydrocarbon). This is the contrast to be expected for H8 and H9 of anion 74 which are located over the periphery of the ring and H8 of anion located over the ring centre.‘r3 ‘)

A remarkable bridged di-anion, the bicycle (3.3.2) decatrienyl di-anion 76 has been prepared from bull- valene by reduction with potassium in DME.“32’ The following NMR data were reported:

‘-‘C NMR shifts (ppm) 131.5 106.3 75.3 36.3 JctdW 135 151 159 122 ‘H NMR shifts (ppm) 6.14 4.63 3.06 2.31

71

The ‘H NMR spectrum of the monohomocyclo- octatetraene di-anion 71 (K+ salt in THF at -60”) shows also differential shielding of the protons on C9, the proton above the ring being much the more shielded!rz9’ Further evidence for the diamagnetic ring current is provided by the peripheral protons which are only some 0.5 ppm upfield from the parent hydrocarbon whereas the shift predicted from charge density alone would be 2.5 ppm upfield.

The bicycle j3.2.1\ octadienyl anion 72 is readily

72 73 74

prepared as the potassium salt in THF.‘r3” The upfield shift of CU. 2.3 ppm of protons 6 and 7 in the

75 76 77

Comparison of the 13C NMR spectrum of the

bicyclo[5.4.l]dodecapentaenylium cation 77 with that of the parent hydrocarbon bicyclo[5.4.l]dodecapenta- 2 579 11-ene shows’r3” that the formation of the , 3 1 1 former from the latter causes carbons 8,9,10 and I1 to suffer a downfield shift in the range 9915 ppm. Carbons 1,2,3,4,5,6, and 7 simultaneously suffer much larger downfield shifts, in the range 27-33 ppm, which are remarkably similar to the downfield shift of 29.6 ppm observed when cycloheptatriene is converted to tropylium cation, The narrow range of shifts for carbons 1 through 7 of 77 (140- 158 ppm) suggests” 33) that this ion has a homotropylium structure rather than the 10 n-electron aromatic form indicated by the ‘H NMR spectrum.‘78’

10. 1,2-HYDROGEN AND 1,2-ALKYL SHIFTS

The facile 1,2 hydrogen shifts occurring in alkyl cations such as set-butyl and cyclopentyl have been described earlier. Rather similar shifts are observed for the hexamethylbenzenonium cation 78. At temperatures below - 20”, these are in slow exchange relative to the NMR time scale and the spectrum


I I I 4 3 2

pm

I I I 4 3 2

rwm

FIG. 13. The proton NMR spectrum of the hexamethylbenzenonium cation.

exhibits(134,‘35) separate methyl resonances for the 4-, the 3-, and 5-, the 2- and 6- and the l- groups, (Fig. 13). Above -2o”, the 1,2 hydrogen shift is very rapid ; the four methyl signals coalesce into a doublet, the splitting arising from coupling with the wandering proton. Lineshape analysis gave the rate constant for the shift as k = 10’3.6 exp { -5650/T } s-t. Examina- tion of a series of methylated benzenes revealed no correlation of rate with basicity and it was concluded that a three-centre bonded transition state was involved and not any kind of n-complex.

79 80

The corresponding 1,2-methyl shifts in 79 also occur, but much more ~lowly:(‘~~) k = 10’3.5expf-9,100/ T) s- l. In both 78 and 79 the AS’ was small and the smaller rate of the latter shift is almost solely due to a larger AH+.

In contrast, the 9,lO methyl shift in the 9,10,10 trimethylphenanthrenium cation 80(i3” is almost as fast as the 1,2 hydrogen shift in benzenonium ions. By way of comparison, the AH+ is only of the order of 20 kJ mall 1 for the alkyl cations.

The 1-acenaphthenyl cation 81 in SOzCIF at - 78” has a ‘H NMR spectrum comprising(138) a multiplet 8.4-9.0 6 (protons 3,4,5 and 7), doublets at 9.5 6 (H6) and 9.85 6 (HS) and singlets at 10.65 and 5.2 6, respectively, due to Hl and H2. The spectrum was found to be independent of temperature in the range -90”

to +4W (above which decomposition occurred), demonstrating the absence of a 1,2-hydrogen shift. The observed absence of coupling between the protons on Cl and C2 is explicable in terms of the rigid geometry of the five membered ring which results in a dihedral angle of about 55” for which little vicinal coupling would be predicted by the Karplus ex- pression.“39) This rigidity, in conjunction with the rather large Cl-C2 bond length, presumably raises the energy of the transition state for l-2 hydrogen shift sufficiently to make this process very slow.

82 83 84

The 2-methyl-1-acenaphthenyl cation 82 rapidly iso- merizes to 83 even at low temperatures,‘L38) the ion 83 does not undergo 1,2-proton shift at a rate detectable by ‘H NMR in the range -90” to +80”. In contrast, the 1,2_dimethylacenaphthenyl cation 84 suffers 1,2-

hydrogen shift above -60”; the 1,1,2-trimethyl- acenaphthenyl cation was, however, found to be non- equilibrating.

11. NORBORNYL AND RELATED CATIONS

The ‘H NMR spectrum of the norbornyl cation is remarkable for its simplicity in the range 37 to -5”C-a single band at S 3.75 (SbFS-S02).(140)

282 R. N. YOUNG

Similarly, the i3C NMR spectrum’i4i) consists of a single band at d 59.8. When the temperature was lowered to - 70”, the ‘H NMR spectrum was resolved into three bands, at 6 1.86, 2.82 and 5.01 with relative intensities 6: 1 : 4. On a classical basis, this temperature dependence can be rationalized in terms of a 1,2 hydride shift 85a*85b, a Wagner-Meerwein shift 85b * 85~ and a 3,2 hydride shift 85a+ 85d. At room temperature, these degenerate processes are fast, rendering all the carbon atoms, and all the protons, equivalent. Lowering the temperature to -70°C effectively removes the 3,2 hydride shift and the signals due to the protons on C7, C5 and C3 are equivalent to each other (6 1.86) as are those on Cl, C2 and C6 (6 5.01). An alternative formu1ation(142’ of a non-classical nature is in terms of a 6,1,2 hydride shift 85eg85fti85g together with a 3,2 shift

85d 85a 85b

85h 85e 05f ,. 859

85cti85h. As in the classical scheme, the 3,2 shift is effectively frozen at - 70”.

On further lowering the temperature to - 150”, employing SbF~--S02ClF-S02F2 as a super-acid solvent, the low-field four-proton resonance splits into two peaks of equal area at 6 3.05 and 6.59. The high-field signal also broadens and develops a shoulder at S 1.70 but that at 6 2.82 due to the bridgehead proton is unchanged (Fig. 14). These observations can be interpreted classically, that is, all hydride shifts are frozen, leaving only the Wagner-

Meerwein process. The i3C NMR spectra also exhibit temperature

dependence (Table 17). (142,‘43) At -70” there are

-113” _i_l!lt 6 4 2

FIG. 14. The proton NMR spectrum of the norbornyl cation.

signals at 6 92, 37.7 and 31.3 corresponding to equilibrating Cl, C2, C6; bridgehead C4; and equivalent methylenes C3, C5 and C7. The most deshielded resonance is split into a quintet, in the off-resonance spectrum, because of coupling of each of the cyclopropane carbons with four equivalent protons. The methylene carbons, each attached to two protons, appear as a triplet. At - 150”, the signal at lowest field is separated into two components at 6 125.3 (Cl and C2) and 22.4 (C6). The C4 bridgehead resonance also shifts to slightly higher field.

Olah et a1.(‘43’ have attempted to test the suitability of a classical model for the ‘H NMR data obtained at - 150°C. If a Wagner-Meerwein process is occurring, then the signal at 6 6.59 in the ‘H NMR spectrum must be the average of that of the secondary

TABLE 17. t3C NMR chemical shift (ppm) and coupling constant (Hz) data”43,144’ for the norbornyl cation and derivatives

Ion Temperature Cl c2 c3 c4

Norbornyl - 70” 31.3 31.1

(85)

(q953) (qT9:3)

(t, 140) (d, 153) Norbornyl - 150” 125 125 33.4

(85) (d, 184) (d, 184) (1,41853) (d, 159) Norbornenyl - 70’ 58 126 126

(94) (d, 173) (d, 194) (d, 194) (d5;73) 7-Norbornadienyl - 70” 63 115 115

(87) (d, 181) (d, 192) (d, 192) (&I) 2_Methylnorbornyl(92) -70 80.8 271 55.6 42.8 2,3-Dimethylnorbornyl -70 169 169 48.6 42.1 2_Phenylnorbornyl(93) - 70” 59.8 257 51.0 40.1

Coupling constants (Hz) in parentheses. d = doublet, t = triplet, q = quintet.

C5

31.3

(t, 140)

($0) 26.7

(t, 140) 122

(d, 177) 23.6 25.2 34.6

C6

(q>9:3) 22.4

(t, 146) 26.7

(t, 140) 122

(d, 177) 35.8 40.6 41.8

c7

31.3

(L 140)

(t41853, 34.0

(d, 219) 36.2

(d, 216) 40.2 48.6 25.8


CH and the attached bridgehead proton as in 86. a rate constant of 3 x 10m4s- ‘. Because of the slow- They used as models, the isopropyl and cyclopentyl ness of the scrambling, it was possible to observe cations for the former (S 12) and for the latter the that the label appeared first at the bridgehead positions 2 methylnorbornyl ion (6 4.5), which yielded a pre- 1 and 4 and only thereafter at position 7. Such diction of 6 8.25, in poor agreement with observation. scrambling can be rationalized by the occurrence of They concluded that the structure of the norbornyl ring contraction to the bicyclo(3.2.0) heptadienyl cation cation is the non-classical bridged structure 87. The 90. Warming to 0°C extended the deuterium scrambling penta-coordinated carbon atom should carry little to positions 2 and 3, probably by way of “bridge- charge, accounting for its high field location (6 3.05) flipping” 91ae91b. At equilibrium, the ‘H intensities whereas the nortricyclane-like CH groups which de- approached the ratios 2: 1.6: 1.6:O.S for the bound localize charge have their resonance at 6 6.59. vinyl : unbound vinyl : bridgehead: bridge, respectively.

Kinetic analysis of the 3,2 and the 1,2,6 hydride shifts yielded the Arrhenius A factors 10”.’ and 10’2,7s-1 respectively. Kramer has suggested”45) that these values, being normal, tend to constitute evidence that the norbornyl ion is in fact classical. In particular, the 3,2 hydride shift requires cleavage of the cyclopropyl ring, which might be expected to lead to a larger than normal A factor.

Analysis of the r3C NMR data of the norbornyl cation obtained at -70” and - 150” has also been attempted by Olah and co-workers.(142,144) Using a similar modelling procedure to that described above for the ‘H NMR spectra, they concluded that a classical structure for carbons 1 and 6 undergoing only the Wagner-Meerwein equilibrium 85b + 85~ would be expected to correspond to a chemical shift of about S 185-so far removed from that observed (6 125) as to exclude the classical structure. The non- classical, penta-coordinated bridging methylene carbon, bearing little charge, corresponds to the signal at 6 22 whereas the more highly charged, bridging, tetra-coordinated carbons appear at b 125.

The failure of the classical models to predict the observed 13C NMR shifts should be recognized as evidence against a classical structure rather than evidence for a non-classical structure, until it is possible to predict shifts for the latter by calculation. Kramer’r4*’ and Brown (146) have stressed the un- certainties attending the assumptions of a linear correlation between charge density and chemical shifts and have questioned the suitability of the isopropyl cation as a model for predictions of the chemical shifts of the norbornyl ion when it is a poor model for ions such as the set-butyl cation. These authors present a strong case that, when the results of other techniques such as ESCA and Raman spectroscopy are also considered, the evidence in favour of a non- classical structure must be regarded as, at least, somewhat weak.

The 7-norbornadienyl cation 88 is another particularly interesting system which has been studied exten- sively. (12’) The ester 89 gave rise to this ion on dissolution in FS03H at -73°C. When the ester was deuterated in the 5-position, the ‘H NMR spectrum of the ion formed in FS03H was similar to that of 88, but the intensity of the signal arising from the protons on C5 and C6 was halved. Warming the solution of the labelled ion to -47°C caused deuterium scrambling over positions 1,4,5,6 and 7 with

88 89 90

Comparison of the r3C NMR for the norbornyl, norbornenyl and norbornadienyl cations reveals their basic similarity (Table 17). Olah and co-workers”42’ have concluded that all three cations must be regarded as non-classical. In contrast, the 2-methyl 92

910 91 b

and 2-phenylnorbornyl93 cations seem to have structures which are essentially classical. The r3C NMR signals from the C2 positions of these latter ions are significantly downfield of that of the norbornyl C2 signal. The proton spectra of 92 and 93 show these ions to be static at -6O”C, with, for example, separate exo and endo H6 signals.(r4’) The use of models suggests that there is some degree of G- delocalization in the 2-methyl-2-norbornyl ion(r4*) and the onset of Cl-C6 o-delocalization has been observed when sufficiently strong electron-withdrawing substituents are introduced into the 2-phenylnorbornyl cation.’ ’ 47)

12. THE CYCLOPROPYLCARBINYL CATION

The ‘H NMR spectrum of the cyclopropylcarbinyl cation consists of two three-proton doublets at 6 4.21 (.I 6.5 Hz) and S 4.64 (J 8.0 Hz) together with a one- proton multiplet comprising overlapping quartets (J 6.5 and 8.0Hz) centred at 6 6.50 (Fig. 15). When LY, c( dideuteriocyclopropylcarbinol was used as ion precursor, the intensity of the methylene doublets was reduced by one-third.‘33) The r3C NMR spectrum shows equivalent methylene absorptions at 6 57 (Jcn

284 R. N. YOUNG

7 6 5 4

pm

FIG. 15. The proton NMR spectrum of the cyclopropylcarbinyl cation.

180Hz) and the methine carbon at 6 120. Evidently, the ion contains two sets of equivalent protons coupled to equivalent carbons. As has been remarked earlier, the structure of this ion must be quite unlike that of the related methyl 10 and dimethyl 11 cyclopropyl cations. A consideration of several possibilities led Olah and co-workers(142) to conclude that the ion is non-classical, involving two-electron three-centre

94 95

bonding 95. Brown (14~) has expressed surprise that the well-established bisected geometry adopted by other cyclopropylcarbinyl cations should not also be adopted by the parent ion. He has also drawn attention to the unexpectedly low value of JcH observed for the cyclopropylcarbinyl cation. Extrapolation from the values 187 and 190Hz for ions 11 and 10, and making allowance for the increased strain involved in forming a a-bridge, means the predicted value would be ca. 200 Hz, whereas the observed value is only 180 Hz.(‘~‘) It would seem that the decrease in JcH occasioned by the rotation from the bisected geometry more than offsets the increase due to the inward shift of Cl on forming the cr-bridge.

The 1-methylcyclopropylcarbinyl cation 96(148,‘49’

can be generated from 1-chloro-1-methylcyclobutane or 1-chloromethyl-1-methylcyclopropane by SbFS in SO&lF at -78°C. The ‘H NMR spectrum consists

of two signals only with a quartet at 6 3.89 (6H, J 0.9 Hz). If I-chloro-1-trideuterio-cyclobutane is used as precursor, the spectrum consists solely of a singlet at 6 3.93, and no scrambling of protons to the -CDs group occurs.(148) Saunders and Rosenfeld”48’ interpreted their observations as evidence for a non-static cation rapidly equilibrating between the methylcyclo- butyl structure, 96d and the three degenerate methylcyclopropylcarbinyl cations 96% b and c. The ‘jC NMR spectrum consists of signals at 6 25.3 (CH,), 48.6 (C2, C3, Ca) and 163 (Cl) is also in accord with this conclusion. That the 1 -methylcyclopropylcarbinyl cation has a fundamental difference from the parent cyclopropylmethyl ion is particularly clear from the lack of separate signals for C2, C3 and Ca in the former and by the separate endo and exo proton resonances of the latter. Kelly and co-workers(lSo’ have developed an empirical dependence of JCH upon dihedral angle for groups adjacent to cationic carbon atoms and have concluded that whereas the cyclobutyl cation structure is not a resonance form of the cyclopropylcarbinyl cation, the l-methylcyclobutane cation is a resonance form of the l-methylcyclopropylcarbinyl cation which has a significant weighting.

The 1-phenylcyclobutyl cation is the only static cyclobutyl ion so far reported:‘33’ the Cl resonance is at 273 6.

1.

2.

3. 4.

5.

6.

I. 8.

9. 10.

11.

12.

13.

14.

15.

16.

17. 18.

19.

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nmr spectroscopy of carbanions and carbocations

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