Bromination of ketones in the presence of epoxides as a method for regiospecific synthesis of bromo-ketones

Abstract

Regioselective free-radical introduction of bromine at the α-position of an unsymmetrical ketone has been achieved simply by photocatalytic bromination in the presence of 1,2-epoxycyclohexane. Under these conditions function-alization of steroid ketones is also observed. The selectivity of these reactions is due to the epoxide which, by scavenging the halogen acid produced during the reaction, inhibits any ionic acid-catalysed bromination of the ketone. Whereas the more highly substituted ketones are brominated easily, the less substituted ones are unchanged because of competing free-radical ring opening of the epoxide.