Kinetics and mechanism of 1,3-cycloaddition of a substituted benzonitrile oxide to a series of arylacetylenes

3,5-Dichloro-2,4,6-trimethylbenzonitrile oxide, which is stable and does not dimerise, was made to react with some para-substituted phenylacetylenes, in carbon tetrachloride and in chloroform, in kinetic experiments followed by infrared analysis. Products were shown to be 3,5-diarylisoxazoles by their n.m.r. spectra. Rate constants are correlated by a V-shaped Hammett plot, with ρ-values –0·34 and +0·61. Change of solvent has a small effect.Also ethylacetylene gives 3,5-disubstituted isoxazoles, both with 3,5-dichloro-2,4,6-trimethylbenzonitrile oxide and with benzonitrile oxide. Molecular orbital calculations were done on some arylacetylenes and on benzonitrile oxide, the Huckel approximation and the ω-technique being applied. On the basis of theoretical and experimental results an interpretation of the cycloaddition is advanced.