Methylcyclohexyl cation

This transition-state, 22, has a calculated energy only slightly higher (0.48 kcal/mol at B3LYP/6-31G ) than the transition-state for isomer 18, and the geometry is also slightly different, for example, a somewhat shorter H—H bond. Loss of dihydrogen from this transition-state will lead to a secondary cation intermediate, and after a 1,2-hydride shift, to the same 1-methylcyclohexyl cation product that is directly produced from the other route. 

Formic acid as reagent for carboxylation of 1-methylcyclohexyl cation to form 1-methylcyclohexanecar-boxylic acid, 46, 72... 

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.

The two chair conformations of tertiary methylcyclohexyl cation 43 and 44 are in rapid equilibrium. 

For the cyclopentyl cation (20) formation, the reaction is slightly endothermic, while for methylcyclohexyl cation (21), the reaction profile itself is slightly exothermic. However, relative to the most stable 2-heptyl cation, (the 1,5-bridged structure 16) this reaction is also slightly endothermic. Based on calculated AG values at 298K, both reaction profiles are exergonic due to a larger entropy contribution for the product side in each case. 

This was shown for fcrf-alkyl, norbomyl, and methylcyclohexyl cations. 

Figure 9. B3LYP/6-3IG Energy profile (LE) for 6 ring (- - ) vs. 5 ring (—) dehydrocyclization in the 2-heptyl cation system. The arbitrary zero reference point is the cis 1-5-p-H-bridged cation 16 (see footnote (a) in Table II). The transition-state structures have been shown in Figure 6. The product ions are the I-2-dimethylcyclopentyl cation 20 and the I-methylcyclohexyl cation 21, respectively, and these have almost the same energy at B3LYP/6-3IG level.

Hewett (1975) has reported a lower rearrangement rate for the 1,3-di-methylcyclohexyl cation [74] than for [11], which demonstrates the sensitivity of direct 1,3-hydride transfer to geometrical constraints. 

A particularly interesting example of the effect of hyperconjugation is found in the 1 -methylcyclohexyl carbocation. The NMR spectmm of this cation reveals the presence... 

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