Acyl azides and azido formates

Azide formates and acyl azides decompose at substantially lower temperature than do alkyl and aryl azides, owing to a decrease in the double bond character of the N-N2 bond as a result of conjugation between the carbonyl and triazo groups. Acyl azides, in particular decompose at or slightly above room temperature and in some cases a concerted mechanism has been proposed , viz. 

The Curtius rearrangement also occurs in the direct photolysis of acylazides, but additional products have been isolated and have been shown to arise from typical nitrene reactions . For example, Horner et have trapped benzoyl nitrene with a variety of scavengers. Similarly to the Wolff rearrangement (vide infra) it has been shown ° that the Curtius rearrangement occius via a singlet intermediate this intermediate can be a nitrene or an excited azide molecule in which a concerted decomposition leads to isocyanate derivatives. This reaction could occur in competition with nitrene formation or, alternatively, originate from a vibrationally-excited ground state azide molecule. 

From the above general discussion, it is obvious that the proposed mechanisms of the Curtius rearrangement must be regarded as tentative. In particular, measurements of lifetimes and rates of intersystem crossing of intermediate nitrenes, coupled with spectroscopic data on the available excited states of acyl azides, should resolve the problem. 

The thermal decomposition of sulfonyl azides, like azido formates, appears to involve nitrene intermediates . The thermolysis of benzenesulfonyl azide is first order and the rate coefficient is relatively insensitive to solvent changes or substituent effects . o-Nitrobenzenesulfonyl azide decomposes with evolution of nitric oxide and nitrogen the rate of azide disappearance however, is about the same as that for other benzenesulfonyl azides and it appears that the reaction leading to NO can be explained by an intramolecular rearrangement in the nitrene intermediate. This is in contrast to the decomposition of o-nitrophenyl azide (vide infra) where rearrangement occurs in the transition state of the rate-determining step. 

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