Photo-Curtius rearrangement

Acyl azides can undergo photolytic cleavage and rearrangement upon irradiation at room temperature or below. In that case acyl nitrenes 8 have been identified by trapping reactions and might be reactive intermediates in the photo Curtius rearrangement. However there is also evidence that the formation of isocyanates upon irradiation proceeds by a concerted reaction as in the case of the thermal procedure, and that the acyl nitrenes are formed by an alternative and competing pathway " ... 

The outstanding thermal reaction of the acyl azides is their rearrangement to isocyanates. This Curtius rearrangement occurs also in the photodecomposition of the acyl azides, but the yield of isocyanate is lower than in the corresponding thermal process, and other products appear which can be accounted for by a nitrene precursor. A considerable amount of work has been directed towards an elucidation of the mechanism of the photo-Curtius rearrangement and the bulk of the evidence seems to favour the view that it ° is a concerted process not involving a discrete nitrene intermediate. Nitrenes are nevertheless formed in the photolysis of acyl azides and the products of this alternative route will now be discussed. 

Lwowski s work on pivaloyl azide has some interesting implications for the mechanism of the photo-Curtius rearrangement photolysis of pivaloyl azide in a variety of solvents results invariably in about 40% isocyanate, irrespective of the medium in which the reaction is conducted. Even in mixed solvents the isocyanate yield is hardly affected... 

Denmark, S. E., Dorow, R. L. The stereochemical course of migration from phosphorus to nitrogen in the photo-Curtius rearrangement of phosphinic azides (Harger reaction). J. Org. Chern. 1989, 54, 5-6. 

Photolysis of carbonyl azides gives rise to two types of reactions. The photo-Curtius rearrangement proceeds to form isocyanate. In addition, bimolecidar trapping products, typical of the reactions of singlet carbonylnitrenes, are also observed. 

As in the case of azides 31 and 32, irradiation of nitro-substituted aroyl azides 33 and 34 leads both to photo-Curtius rearrangement (32 and 38% yields) and to the formation of products derived from the capture of singlet aroylnitrenes (37 and 38). The chemical and spectroscopic properties of nitrenes 37 and 38 indicate the singlet nature of their ground states. 

In the above discussion, it was presumed that one of the intermediates involved in the photo-Curtius rearrangement is indeed a phosphinoyl nitrene. Although the formation of the alkyl phosphonamidic esters constitutes one line of evidence for the intermediacy of... 

The mechanisms of photo-Curtius rearrangements have long been debated. 

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