Intermediates in the decomposition of aliphatic diazo-compounds. Part X. Kinetic and mechanistic studies of the decomposition of diphenyldiazomethane catalysed by copper(II) bromide

Abstract

The kinetics and products of decomposition of diphenyldiazomethane catalysed by copper(II) bromide in acetonitrile have been investigated. It is shown that the catalyst solution reacts rapidly with an equivalent of the diazoalkane in a rapid two-step process, stage (i), yielding a copper complex which may have diphenylmethylene as a ligand. Stopped-flow measurements on the kinetics of this reaction are described. The copper complex formed in stage (i) then consumes the excess of diazoalkane in a much slower stage (ii). The effects of the initial diazoalkane and catalyst concentrations and of added lithium bromide on the rate and kinetic form of this stage lead to the proposal of a reaction scheme involving three main pathways, one leading to benzophenone and two to benzophenone azine. Using this proposed scheme, it is shown that both initial rates of disappearance of diphenyldiazomethane and the final product proportions can be approximately predicted under most reaction conditions. During this stage of the reaction, some 10% of the catalyst is precipitated as an unstable paramagnetic complex. After all the diazoalkane has been consumed, this precipitate slowly redissolves, and the original catalyst solution is regenerated.