Abstract

Lithium bromide is solubilized in benzene by the addition of an equivalent amount of hexamethylphosphoramide (HMPA) or other phosphine oxide. The resultant complex is an efficient catalyst for the rearrangement of epoxides to aldehydes and/or ketones. The addition of a second equivalent of HMPA quenches this catalytic activity. Evidence is presented which supports a mechanism involving the lithium salt of the bromohydrin as an intermediate in the LiBr-catalyzed rearrangement. Lithium perchlorate is partially solubilized in benzene by epoxides and effects the rapid rearrangement of those systems involving a tertiary center in the oxirane ring. Combined kinetic and product analyses for cyclohexene, 1-methylcyclohexene, and 1,2-dimethylcyclohexene oxides show that the LiClO₄-catalyzed reaction occurs by a different mechanism, presumably involving a carbonium ion. This mechanistic dichotomy allows flexibility in choosing the product of rearrangement and enhances the synthetic utility of the reaction. Kinetic data and product analyses are presented for a number of simple cyclic and acyclic epoxides, illustrating the scope and limitations of the reaction.