The kinetics and mechanisms of additions to olefinic substances. Part 14. Reactions of cholest-5-en-3-one with electrophilic brominating agents

Abstract

Electrophilic brominations of cholest-5-en-3-one with molecular bromine and with bromine chloride have been studied in several solvents. The major products are 5α,6β-adducts and the 6-bromocholest-4-en-3-ones; in some circumstances a small proportion of 5β,6α-adduct was recognised. Results obtained by using bromine chloride establish that the diaxial adducts arise by electrophilic attack on both the α- and the β-face of the steroid. 6α-Bromocholest-4-en-3-one is formed by proton loss from the ionic α-bromonium intermediate; study of 4β-H/D isotope effects on the rates and products of bromination establish that 6β-bromocholest-4-en-3-one is obtained not only by this route, in which the 4α-proton is preferentially released, but also by way of a concerted, stereoselectively syn, S_E2′ mechanism. In the presence of excess of halide ion, substitution is reduced in proportion through catalysis of an Ad_E3 mechanism involving electrophilic attack on the α-face of the molecule to give the diaxial adduct. The pathways available for substitution and addition in cholest-5-en-3-one are compared with those found for the 3β-substituted cholest-5-enes (accompanying paper).