Reactions of cyclohexane derivatives in superacids

Abstract

The reactions of menthol with FSO₃H–SO₂ and neomenthyl chloride with SbF₅–SO₂ClF follow different routes. The first yields a substituted cyclopentyl cation and the second a substituted cyclohexyl cation. Experiments on substituted cyclohexyl chlorides show that replacement of all the hydrogen atoms on the next but one carbon atom to the reaction centre blocks formation of a carbocation. It is suggested that ionisation of an equatorial chlorine atom takes place with assistance from an intramolecular electronic interaction, forming the methyl cyclopentyl carbocation in a synchronous process, rather than stepwise via the cyclohexyl carbocation. Reasons for contrasting behaviour in solvolytic reactions are discussed.