REACTION OF ARYLBROMODIAZIRINES WITH AZIDE ION - EVIDENCE FOR N-AZIDODIAZIRINE INTERMEDIATES

Azide ion reacts with15N-labeled phenylbromodiazirine ([15N]-1) to give benzonitrile that has substantial15N incorporation as determined by15N and13C NMR as well as by mass spectrometry. When unlabeled phenylbromodiazirine (1) is reacted with terminally15N-labeled sodium azide, the benzonitrile product contains no15N label. These results support the intermediacy of an N-azidodiazirine 8 formed by SN2’ attack of azide upon the nitrogen atom in the bromodiazirine. The N-azidodiazirine is suggested to rapidly lose nitrogen to give an analogue of a 1,1-diazene, which also loses nitrogen to give a benzonitrile product, which is consistent with the labeling results. Similar results were obtained when m-CF3, p-CH3, and p-OCH3 analogues of 1 were reacted with azide ion. This argues against the previously proposed mechanism involving C-azidodiazirines having the azide moiety attached to carbon. These experiments present evidence that phenylbromodiazirine can indeed react with nucleophiles via an SN2’ pathway and bypass the previously proposed diaziriniumcation. A rationale for the observed substituent effect on rate of reaction of arylbromodiazirines with azide ion is presented in terms of stabilization of the developing carbon-nitrogen double bond in the transition state leading to 8 by the aryl group. It is concluded that a mechanism involving consecutive SN2’ reactions is preferable to one involving diazirinium cations in substitution reactions of arylbromodiazirines. © 1990, American Chemical Society. All rights reserved.