Isocyanates from acyl nitrenes

Common methods for generating nitrene intermediates are the photolysis and thermolysis of azides. Generation of nitrenes from acyl azides can only be effected photochemically thermolysis of an acyl azide gives the corresponding isocyanate. 

The parallel to the photochemistry of azides found in the arylimines is also observed on irradiation of 1,2,4-triazole 4-acyl-,129 133 4-sulfonyl-,81 and 4-carbamoylimines.133 In methanol the same products are generated as in the photolysis of the corresponding azides. Their formation is consistent with the mechanism given in Scheme 15 the AT-acyl- and sulfonyl-imines afford 1,2,4-triazole and a nitrene which yields the typical nitrene products 107a-d with methanol. Furthermore, carbamic esters (109) are formed, by addition of methanol to an isocyanate (108). These isocyanates can originate directly from the N-imine by a synchronous reaction or from the nitrene by a sextet rearrangement. Although irradiation of s-triazole... 

In Summary Treatment of primary and secondary amides with base leads to deprotonation at nitrogen, giving amidate ions. Bases abstract protons from the a-carbon of tertiary amides. In the Hofmann rearrangement, primary amides react with halogens in base to furnish amines with one fewer carbon. In the conrse of this process an alkyl shift takes place, which converts an acyl nitrene into an isocyanate. 

Like diazoketones, acyl azides lose nitrogen when heated or irradiated. The end result is an isocyanate, formed either directly from the acyl aade, in a reaction called the Curtius rearrangement after Theodore Curtius (1857-1928), or by intramolecular rearrangement of a nitrene, which is the nitrogen analogue of a carbene (Fig. 18.70). For most acyl azides, it is known that the isocyanate is formed by direct rearrangement of the acyl compound and that the nitrene, though also formed from the azide, is not involved in isocyanate production. 

---