1-Methyl-1-cyclohexyl Cations. Equilibria in Search of Structures

Article abstract of DOI:10.1021/ja00259a033

Solutions of the 1-methyl-1-cyclohexyl cation 1, by four independent criteria, show unequivocal evidence for a populated equilibrium involving two different "structures" of this cation.One of these criteria makes use of a remarkably large new equilibrium isotope effect, a method that should have general structural use in other carbocation systems.Unfortunately, our previous assignment of a chair and twist-boat conformation for the structures dose not accord with the new data.From an analysis of the 13C and 1H NMR shifts and K_H/K_D isotope effects for 1 and a series of ring-methylated analogues, it can be deduced that both structures are chair conformers and that one structure must involve extensive α-C-H hyperconjugation, while the other structure involves predominantly C-C hyperconjugation.The very existence of this type of isomerism in observable carbocations is unprecedented and has important implications for the field in general.