Intermediates in the decomposition of aliphatic diazo-compounds. Part VII. Mechanisms for formation of benzophenone azine and diphenylmethanol in the thermal decomposition of diphenyldiazomethane

Abstract

An investigation has been carried out of the thermal decomposition of diphenyldiazomethane in solution in acetonitrile containing hydroxylic additives (ROH). The principal products are benzophenone azine and Ph₂CHOR. Experimental determination of the apparent relative reactivity of methyl and t-butyl alcohols by two methods, one direct and one indirect, provides strong evidence that there are two mechanisms for the formation of benzophenone azine, namely, reaction of diphenylmethylene with diphenyldiazomethane, and direct reaction of two diazoalkane molecules without the intermediate formation of a carbene.

The mechanism of the reaction of diphenylmethylene with water has been re-examined in the light of this conclusion. From studies of hydrogen isotope effects on the product-forming steps of the thermal decomposition of diphenyldiazomethane in aqueous acetonitrile, using both isotopically pure water (H₂O or D₂O) and tritiated water, it has been deduced that the intermediate carbene reacts at the oxygen atom of water, giving an ylide, subsequent rearrangement of which yields the observed product, diphenylmethanol. Abstraction of the hydroxylic proton by the carbene to give a carbonium ion, and direct insertion of the carbene into the O–H bond, can be excluded.