Pyrolysis of aryl azides. IV. Neighbouring group effects by ortho carbonyl groups

Abstract

The question of the mechanism by which ortho carbonyl groups enhance the rate of pyrolysis of aryl azides is examined. Rate measurements on sterically hindered 1-(2-azidophenyl)ethan-1-ones are consistent with the pericyclic mechanism proposed by Dyall and Kemp1 but not with the intramolecular 1,3-dipolar addition route put forward by Hall, Behr and Reed.2 A definitive test of mechanism is also provided by 1- azidoanthraquinone and 1-(2-azidophenyl)-2,2-dimethylpropan-1-for both of which the formation of a 1,3-dipolar adduct is precluded on steric grounds but the pericyclic process is not. At 393.2 K, these two azides pyrolyse 7385 and 61.7 times faster than phenyl azide, respectively.