Rates and products from thermolysis of 5-azidoisoxazoles in solution

Abstract

Thermolysis in solution of 5-azidoisoxazoles with formyl, acetyl or N-phenylimino in the 4-position led to the formation of bicyclic products; in the absence of such 4-substituents, ring-opening resulted and in two instances the expected nitrosoalkene intermediate was successfully trapped with 2,3-dimethylbuta-1,3-diene.

Rate measurements indicate that there are small neighbouring group effects exerted by those 4-substituents which lead to cyclization, and it is argued that these effects are necessarily small in these 5-azidoisoxazoles. Nevertheless, these small effects appear to play a key role in directing the reaction towards cyclization instead of ring-opening.

With ring substituents which cannot exert neighbouring group effects, electron-withdrawing groups at position 4 reduce the reaction rate, while electron-donating groups increase it. Substituents at position 3 have little effect on rate. These results are interpreted in terms of a nitrene-like transition state which is substantially stabilized by electron release from the heteroaromatic ring.