Axial:Equatorial Rate Ratios. 1. Eliminations Leading to the Exocyclic Methylene Group

Abstract

Phenyl cis-4-t-butylcyclohexanemethyl sulfoxide (4) thermolyzes to 1-t-butyl-4-methylenecyclohexane (5) at 116 and 130° about five to six times faster than its trans (equatorial) epimer 6. cis-4-t-Butylcyclo-hexanemethyl bromide (7) undergoes elimination with potassium t-butoxide in t-butyl alcohol at 100° about nine times faster than its trans (equatorial) epimer 8. The rate difference found for the sulfoxides 4 and 6 is the first clear demonstration of steric acceleration (without any contribution from any reagent repulsion effect) in the formation of an exocyclic carbon–carbon double bond in a simple anchored cyclohexane System. The faster elimination from 7 (vs. 8) is ascribed primarily to steric acceleration; the experiment neither requires nor excludes the influence of non-bonding repulsions between the substrate and the attacking base.