RATES AND PRODUCTS FROM THERMOLYSIS OF 5-AZIDOISOXAZOLES IN SOLUTION

Thermolysis in solution of 5-azidoisoxazoles with formyl, acetyl or N- phenylimino in the 4-position led to the formation of bicyclic product s; in the absence of such 4-substituents, ring-opening resulted and in two instances the expected nitrosoalkene intermediate was successfull y trapped with 2,3-dimethylbuta-1,3-diene. Rate measurements indicate that there are small neighbouring group effects exerted by those 4-sub stituents which lead to cyclization, and it is argued that these effec ts are necessarily small in these 5-azidoisoxazoles. Nevertheless, the se small effects appear to play a key role in directing the reaction t owards cyclization instead of ring-opening. With ring substituents whi ch cannot exert neighbouring group effects, electron-withdrawing group s at position 4 reduce the reaction rate, while electron-donating grou ps increase it. Substituents at position 3 have Little effect on rate. These results are interpreted in terms of a nitrene-like transition s tate which is substantially stabilized by electron release from the he teroaromatic ring.