Elimination reactions. Part I. Decomposition of menthyltrimethylammonium ion in strongly basic media

Abstract

Hydroxide-promoted elimination from menthyltrimethylammonium ion is more favoured in the absence of solvent or in dipolar aprotic solvents than in protic media or under the classical conditions of Hofmann degradation, but the proportions of isomeric menthenes in products are little changed. Menth-3-ene, the isomer forbidden on the usual requirement of an anti-periplanar conformation for reaction, is formed in 0–27% of the olefin yield under a variety of conditions.

Tracer studies of Hofmann degradation of the 'onium hydroxide exclude α-elimination or E1cB routes involving a relatively long-lived carbanion, but indicate about 15% of the total elimination to be of the α′–β type. This last mechanism also probably accounts for essentially all the elimination effected by phenyl-lithium in ether. A concerted cis-E2 mechanism, permitted by the carbanionic transition state, is probable both in the Hofmann degradation and in the other strongly basic media, and leads to menth-3-ene and menth-2-ene from the ground-state conformation of the substrate. Concerted trans-E2 appears unimportant under these conditions, but occurs in less-basic media, where a less carbanionic transition state is utilised to give uniquely menth-2-ene from a skew-boat conformation.

Studies of decomposition of n-butyltrimethylammonium ion in similar basic media allow optimum conditions for elimination from primary 'onium compounds to be delineated.