Curtius rearrangement, acylnitrenes

In contrast to the somewhat limited synthetic utility of nitrenes, there is an important group of reactions in which migration occurs to electron-deficient nitrogen. One of the most useful of these reactions is the Curtius rearrangement 16 This reaction has the same relationship to acylnitrene intermediates that the Wolff rearrangement does to acylcar-benes. The initial product is an isocyanate, which can be isolated or trapped by a nucleophilic solvent. 

However, Lwowski and co-workers demonstrated that photolysis of 7 in cyclohexane, cyclohexene- or 2-methylbutane does lead to the formation of adducts. Therefore, the acylnitrene 8 is a trappable intermediate. The thermal Curtius rearrangement does not involve free nitrenes and it must be a concerted process. 

The intramolecular reaction of acylnitrenes suffers from the same competing Curtius rearrangement as the intermolecular reaction, and therefore the yields of insertion product are often low. In general, the formation of 8-lactams is preferred to 7-lactams by a factor of about 2 1, where the possibility for competing intramolecular insertion into similar C—H bonds exists. Thus irradiation of the azides (12 R = Me, Pr) gives a mixture of 8- and 7-lactams in a ratio of 2 1 in lowish overall yield of 30-35% (Schente... 

The Curtius degradation of acyl azides (Figure 14.43) expels molecular nitrogen and at the same time leads to the [1,2]-rearrangement of the substituent that is attached to the carboxyl carbon. It is the simultaneous occurrence of these two events that prevents the formation of an energetically unacceptably disfavored acylnitrene intermediate. The rearranged product is an isocyanate. 

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