Page 450 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

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SECTION 4.4
Structure and Reactions
of Carbocation
Intermediates

C D
Fig. 4.9. Distribution of LUMO orbitals of isomeric 1-methylcyclohexyl cations
showing dominant C−C(C) and C−H(D) hyperconjugation. Reproduced from
J. Am. Chem. Soc., 118, 3761 (1996), by permission of the American Chemical
Society.

of two isomeric cations that are separated by a small barrier. Investigation of the NMR
chemical shifts using the MP2-GIAO method points to an axial and equatorial isomer
∗
of nearly equal energy. B3LYP/6-31G calculations indicate an energy difference of
about 0.6 kcal and suggest that the (nearly planar) TS is about 0.9 kcal above the
79
minimum energy structure. The cationic carbon is slightly pyramidalized toward the
C−CorC−H bonds involved in hyperconjugation. The reason for the pyramidalization
is better alignment with the C−C and C−H bonds that provide hyperconjugative
stabilization. The hyperconjugation is also indicated by the differing shapes of the
LUMO orbitals for the isomeric ions shown in Figure 4.9.

CH 3
CH 3

C H D H
θ = 7.6° θ = 1.4°

θ = Deviation of – CH from the plane of
3
C(1) –C(2) –C(6) of the cyclohexane ring
The 1-adamantyl carbocation provides another example of C−C hyperconju-
gation. The C
-C bond is shortened by 0.06 Å in the crystal structure. 80 The NMR
spectrum also shows characteristics of delocalization of the positive charge. A compu-
tational study also indicates delocalization of the positive charge. 81

+

It is important to note the relationship of C−H and C−C hyperconjugation
to the reactivity as well as the structure of carbocations. Hyperconjugation repre-
sents electron sharing with an empty orbital and can lead to structural changes or

79
A. Rauk, T. S. Sorensen, C. Maerker, J. W. d. M. Carneiro, S. Sieber, and P. v. R. Schleyer, J. Am.
Chem. Soc., 118, 3761 (1996); A. Rauk, T. S. Sorenson, and P. v. R. Schleyer, J. Chem. Soc., Perkin
Trans. 2, 869 (2001).
80 T. Laube, Angew. Chem. Intl. Ed. Engl., 25, 349 (1986); T. Laube and E. Schaller, Acta Crystallog. B,
B51, 177 (1995).
81
G. A. Olah, G. K. S. Prakash, J. G. Shih, V. V. Krishnamurthy, G. D. Mateescu, G. Liang, G. Sipos,
V. Buss, T. M. Gund, and P. v. R. Schleyer, J. Am. Chem. Soc., 107, 2764 (1985).