Tertiary cyclohexylcations. Definitive evidence for the existence of isomeric structures (hyperconjomers)

Abstract

The 1-methyl-1-cyclohexyl cation 1 has previously been proposed to exist in superacid solution as a rapidly equilibrating pair of structures, one isomer involving C–C hyperconjugation, and a counterpart with axial C–H hyperconjugation (hyperconjomers). Using a combination of three techniques, which successfully compare theoretical results with experimental data, we have now obtained virtual proof for this concept. These three calculational procedures involve matching the experimental energy difference for the C–C and C–H hyperconjomers of 1, together with the cis-3,5-dimethyl 2 and 4,4-dimethyl 3 analogs of 1, to an accuracy of ±2 kJ mol⁻¹ (solvation-simulation studies), a successful simulation of the α-d₄ equilibrium isotope effects for the 1, 2 and 3 cation systems, and, finally, a close simulation of the average ¹³C NMR spectra for mixtures of these C–C and C–H hyperconjomers in the 1, 2 and 3 systems. These results have interesting implications for solvolysis studies of tertiary cyclohexyl systems, a full discussion of which is presented.