Relative rates of the concurrent reactions in the addition of a substituted benzonitrile oxide to arylacetylenes

The reaction of 3,5-dichloro-2,4,6-trimethylbenzonitrile oxide with some arylacetylenes gives acetylenic oximes, besides 3,5-diarylisoxazoles. Relative rates of concurrent formation of the two kinds of product were measured for some para-substituted phenylacetylenes (substituent, fraction of oxime: NMe2, 0·62; OMe, 0·29; Me, 0·24; H, 0·15; Cl, 0·09).The effect of solvent on the relative rates appears to be small. In the case of phenylacetylene, reaction-temperature variation and deuterium isotope substitution were also employed. There is evidence for a two-step mechanism, with a labile intermediate, for the 1,3-addition. It seems possible that the same intermediate is also on the pathway of the 1,3-cycloaddition, and that the product distribution depends on the conformational distribution of the intermediate.