Ureas from Hofmann rearrangement

Two new syntheses of hydrazines from secondary amines have been reported. In one 1,1-diarylhydrazines are the products of the Hofmann rearrangement of the corresponding ureas (Scheme 33), and in the other ... 

Preparation from Nitrene Intermediates. A convenient, small-scale method for the conversion of carboxyhc acid derivatives into isocyanates involves electron sextet rearrangements, such as the ones described by Hofmann and Curtius (12). For example, treatment of ben2amide [55-21-0] with halogens leads to an A/-haloamide (2) which, in the presence of base, forms a nitrene intermediate (3). The nitrene intermediate undergoes rapid rearrangement to yield an isocyanate. Ureas can also be formed in the process if water is present (18,19). 

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