J. Am. Chem. Soc. 1997, 119, 3087-3092
3087
First X-ray Crystallographic Study of a Benzyl Cation, Cumyl
Hexafluoroantimonate(V), and Structural Implications
Thomas Laube,*,† George A. Olah, and Robert Bau
Contribution from the Loker Hydrocarbon Research Institute and the Department of Chemistry,
UniVersity of Southern California, UniVersity Park, Los Angeles, California 90089
ReceiVed September 9, 1996. ReVised Manuscript ReceiVed February 3, 1997X
Abstract: The crystal structure of the cumyl cation (1; 2-phenyl-2-propyl cation) has been determined at -124 °C
(as hexafluoroantimonate, 1‚SbF6; R1 ) 0.0502, wR2 ) 0.1054). The cation 1 is nearly planar and has a short
C+-Cipso bond (1.41(2) Å) and bond lengths in the phenyl ring which agree with strong benzylic delocalization.
The weak but distinct shortening of the C+-CH3 bonds by 0.025(12) Å indicates weak C-H hyperconjugation.
Nearly all H atoms are involved in H‚‚‚F contacts to SbF6 anions, and one close C+‚‚‚F contact (3.11(2) Å) is
observed. The phenyl rings form infinite stacks and are shifted against each other in the stack.
The cumyl cation (2-phenyl-2-propyl cation, 1; see Chart 1)
is the simplest R,R-dialkylbenzyl cation and has been prepared
in solution and investigated for the first time under superacidic
conditions by Olah in 1964.1
Chart 1
Attempts to prepare methyl-substituted benzyl cations with
aluminum chloride2 were not successful, as has been shown by
comparison with results from superacidic media.3,4 The tertiary
cation 1 is a benzylic cation related to the unsubstituted parent
benzyl cation 2 which was, together with the isomeric tropylium
cation 3, the subject of intensive gas phase and theoretical
studies.5 Originally it was thought that the ion C7H7+ obtained
from toluene and other precursors is 3. It is now known that
both 2 and 3 are formed in the gas phase in a fairly complex
isomerization/dissociation scheme.5 Tropylium cations have so
far not been observed as rearrangement products of benzyl
cations in the condensed phase. Ab initio studies have shown
that 3 lies 29 kJ mol-1 lower in energy than 2.6 The electronic
properties of 1 are expected to be similar to those of 2 or ring-
methylated primary benzyl cations, although C-H hypercon-
jugation should provide additional stabilization of 1. The
unsubstituted benzyl cation 2 is kinetically very unstable in
condensed phases, as it can undergo ready head-to-tail conden-
sation, and only its UV spectrum could be obtained by pulse
radiolysis.7 In contrast para (or other) substituted benzyl cations
were obtained in superacid solutions. The benzylic delocal-
ization in R,R-bis-(1-adamantyl)benzyl cations can be minimized
by sterically forced twisting of the C+-Cipso bond.8,9 Cumyl
cations have been used as polymerization initiators,10,11 and their
solvation12 and reactions12,13 have been studied recently. The
polycyclic R,R-dialkylbenzyl cations 4-6 and the bridged ion
7 have been investigated by X-ray crystallography.14-17 The
indanyl cation 8 has been observed in 78% sulfuric acid, while
1 required 30% oleum;18 i.e., 4 and 5 are probably more stable
-
than 1, and 4 could indeed be crystallized as an AlCl4 salt.
Also the larger π system of 4 and the higher degree of
substitution of 5 should increase the stability of these ions with
respect to 1. According to a recent review article,19 the
Hammett acidity of 1 is even higher than that of the tert-butyl
cation. However, it must be pointed out that simple comparison
of observed ions in different acids does not give real stabilities.
We report here the crystal structure of the tertiary cation 1 which
is of substantial interest in our understanding of the structure
of benzylic carbocations. Up to now, only solution data of 1
† Visiting scientist at USC.
X Abstract published in AdVance ACS Abstracts, March 1, 1997.
(1) Olah, G. A. J. Am. Chem. Soc. 1964, 86, 923-934.
(2) Hanazaki, I; Nagakura, S. Tetrahedron 1965, 21, 2441-2452.
(3) Cupas, C. A.; Cosimarov, M. B.; Olah, G. A. J. Am. Chem. Soc.
1966, 88, 361-362.
(11) Thomas, L.; Tardi, M.; Polton, A.; Sigwalt, P. Macromolecules 1993,
26, 4075-4082.
(12) Richard, J. P.; Jagannadham, V.; Amyes, T. L.; Mishima, M.; Tsuno,
Y. J. Am. Chem. Soc. 1994, 116, 6706-6712.
(13) Lee, I.; Koh, H. J.; Hong, S. N.; Lee, B.-S. Gazz. Chim. Ital. 1995,
125, 347-351.
(4) Bollinger, J. M.; Cosimarov, M. B.; Cupas, C. A.; Olah, G. A. J.
Am. Chem. Soc. 1967, 89, 5687-5691.
(5) Lifshitz, C. Acc. Chem. Res. 1994, 27, 138-144.
(6) Nicolaides, A.; Radom, L. J. Am. Chem. Soc. 1994, 116, 9769-
9770.
(14) Borodkin, G. I.; Nagi, Sh. M.; Gatilov, Yu. V.; Shubin, V. G. Dokl.
Akad. Nauk. SSSR 1985, 280, 881-884.
(15) Solari, E.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. J. Chem. Soc.,
Chem. Commun. 1991, 841-843.
(7) Fujisaki, N.; Comte, P.; Ga¨umann, T. J. Chem. Soc., Chem. Commun.
1993, 848-849.
(16) Laube, T.; Hollenstein, S. HelV. Chim. Acta 1994, 77, 1773-1780.
(17) (a) Laube, T. J. Am. Chem. Soc. 1989, 111, 9224-9232. (b) Laube,
T.; Lohse, C. J. Am. Chem. Soc. 1994, 116, 9001-9008 (improved
refinement).
(8) Olah, G. A.; Heagy, M. D.; Prakash, G. K. S. J. Org. Chem. 1993,
58, 4851-4854.
(9) Heagy, M. D.; Olah, G. A.; Prakash, G. K. S. J. Org. Chem. 1995,
60, 7355-7356.
(18) Olah, G. A.; Pittman, C. U.; Waack, R.; Doran, M. J. Am. Chem.
Soc. 1966, 88, 1488-1495.
(19) Haw, J. F.; Nicholas, J. B.; Xu, T.; Beck, L. W.; Ferguson, D. B.
Acc. Chem. Res. 1996, 29, 259-267.
(10) Kaszas, G.; Puskas, J. E.; Kennedy, J. P. Macromolecules 1992,
25, 1775-1779.
S0002-7863(96)03151-4 CCC: $14.00 © 1997 American Chemical Society