Photochemistry of organic nitrogen compounds. Part III. The formation of allenes and 1,3-dienes from pyrazolenines: a photochemical reaction involving ion-pairs

Abstract

The photolysis of pyrazolenines normally gives cyclopropenes via a vinyldiazoalkane; but, in the present work 3-(1′-acyloxyalkyl)-5,5-dimethylpyrazolenines have been found to give allene and 1,3-diene esters in good yield, together with small quantities of a vinylallene. Attempts to trap an intermediate diazoalkene by photolysis in the presence of acid were not successful, leading merely to partial randomisation of the acyloxy-group in the products. The extent of randomisation has been studied quantitatively, and the results are consistent with an ionic mechanism involving both inter- and intra-molecular processes, and similar in some respects to the mechanism which has been suggested for the thermal decomposition of N-nitroso-amides. A possibly general route to vinylcarbonium ions is suggested.