Nitrenes and the Decomposition of Carbonylazides

Abstract The decomposition of organic carbonylazides can lead to the formation of nitrenes. Ethoxycarbonylnitrene is formed in the photolytic and thermal decomposition of ethyl azidoformate and by α‐elimination from N‐(p‐nitrobenzenesulfonyloxy)urethan. Both of the possible electronic states of this nitrene take part in intermolecular reactions. Pure singlet nitrene is formed by α‐elimination from the urethan and on thermal decomposition of ethyl azidoformate, but changes so rapidly into the triplet form that the reactions of both forms are observed. Singlet ethoxycarbonylnitrene undergoes selective and stereospecific insertion into CH bonds and adds stereospecifically to olefins. Triplet ethoxycarbonylnitrene, however, does not undergo insertion into CH bonds, and adds to olefins with complete loss of the geometric configuration. By following quantitatively the stereospecificity of the addition reaction and by selective interception of the triplet and singlet forms of the nitrene, it can be shown that the photolysis of ethyl azidoformate leads directly to nitrene of which one third is in the triplet state. In the decomposition of aryl‐ and alkylcarbonylazides (acid azides), the removal of nitrogen is accompanied by a synchronous rearrangement to isocyanates (Curtius rearrangement). In this system, nitrenes are obtained only by photolysis. They add to double bonds and undergo very selective insertion into CH bonds, but do not rearrange at a measurable rate to isocyanates. The photolytic Curtius rearrangement is also a concerted reaction.