Photolysis of 5,5-dibenzyl-Δ3-1,3,4-oxadiazolines

Abstract

Photolysis of dibenzyl-Δ3-1,3,4-oxadiazolines (3) in the presence of dimethyl acetylenedicarboxylate (DMAD) gives only modest yields of the expected symmetrical 3,3-dibenzylcyclopropenes (4), but these are accompanied by more than six by-products, including unsymmetrical cyclopropenes, methylenecyclopropanes, and various pyrazoles. The origin of this array of products can be explained by a series of steps starting with photolysis of 3 to form a diazoalkane that undergoes 1,3-dipolar cycloaddition to DMAD, generating a 3H-pyrazole as initial product. The latter is further photolyzed to a symmetrical cyclopropene in competition with benzyl group migration by thermal 1,5-sigmatropic or ion-pair rearrangement to afford a 4H-pyrazole. The 4H-pyrazole in turn undergoes photolysis to an unsymmetrical cyclopropene, which rearranges to a methylenecyclopropane. The 4H-pyrazole also undergoes autoxidation, in the presence of air, to afford a benzoyl-4H-pyrazole. Additionally, in competition with rearrangement, the various pyrazoles lose a benzyl group or a methoxycarbonyl group to afford pyrazoles with one less substituent. Keywords: 5,5-dibenzyl-Δ3-1,3,4-oxadiazolines, photolysis of; 3,3-dibenzyl-3H-pyrazoles, rearrangement of; 3,4-dibenzyl-4H-pyrazoles, autoxidation of; 3,4-dibenzyl-4H-pyrazoles, photolysis of; cyclopropenes, rearrangement to methylenecyclopropanes.