Intermediates in the decomposition of aliphatic diazo-compounds. Part IX. Kinetics and mechanism of the decomposition of diaryldiazomethanes catalysed by zinc halides

Abstract

The kinetics and products of decomposition of diphenyldiazomethane, 9-diazofluorene, and of some of their substituted analogues induced by zinc chloride, bromide, and iodide in acetonitrile solution have been investigated.

With zinc chloride or bromide, diphenyldiazomethane yields after work-up a mixture of benzophenone and benzophenone azine, the proportions depending on the initial concentration of diazoalkane and the nature (but not the concentration) of the zinc halide. The kinetics and product proportions are consistent with rate-limiting formation of a dipolar diphenylmethylene–zinc halide complex, Ph₂C,ZnX₂. This intermediate reacts with a further diazoalkane molecule yielding the azine and zinc halide or gives rise to ketone, plausibly by way of an α-halogenoalkylzinc halide formed by rearrangement.

The principal products from reaction of diazofluorene are fluorenone, fluorenone azine, and 9,9′-bifluorenylidene. The proportions of products are affected by the presence of oxygen as well as by the factors mentioned for diphenyldiazomethane. The kinetic form of the reaction is similar to that for diphenyldiazomethane but under given conditions, diazofluorene reacts 5–10 times more slowly.

When zinc iodide is used, the products isolated from reactions of both diazo-compounds are qualitatively the same as those found using the other zinc halides. However, kinetic experiments show that the reactions take place in two phases, one fast and one slow. Comparison of the results with those from the reaction of 9-iodofluoren-9-ylzinc iodide (prepared from 9,9-di-iodofluorene and zinc–copper couple) with diazofluorene suggest that the organometallic reagent is one product of the fast step in the diazofluorene–zinc iodide reaction and that its reaction with unchanged diazofluorene is the second, slower phase.

Quantitative aspects of the reactivities of the zinc halides and of the diazoalkanes are discussed.