Kinetics of addition and cycloaddition of phenylacetylene to benzonitrile N-oxides. Competitive concerted and stepwise mechanisms

Abstract

The kinetics of the concurrent processes leading to isoxazoles (1,3-cycloaddition) and acetylenic oximes (1,3-addition) in the reactions of substituted benzonitrile N-oxides with phenylacetylene have been examined under various conditions. The results supplement previous findings and are in accord with the occurrence of two independent pathways, a concerted 1,3-cycloaddition and a two-step 1,3-addition. For the latter reaction the rate-determining step is formation of an intermediate derived from the electrophilic addition of the nitrile oxide to the p electrons of the triple bond of phenylacetylene.