Wolff rearrangement synthetic utility

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. 

In the case of 2-diazo-1,3-diketone and 2-diazo-l,3-ketoester substrates I Scheme 3.51. the Wolff rearrangement produces a-oxo-ketenes, a reaction that Ludwig Wolff himself studied in his early work. The control of the relative migratory aptitude of the R and R groups is a crucial parameter for the synthetic utility of this approach and essentially depends on the nature of the R groups, the geometry of the diazo substrate, and the activation method employed (see Section 3.3). Similarly to other ketenes, a-oxo-ketenes readily react with a large array of... 

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