Wolff rearrangement regioselectivity

The cycloaddition reactions of the unsymmetrical a-diazo-/3-diketone, 2-diazo-l-phenyl-l,3-butanedione 330, with diaryl imines 331 took place with high regioselectivity, affording exclusively the 6-methyl-5-phenyl-substituted 477-l,3-oxazin-4-ones 332 via the acetylphenylketene, generated by the thermal Wolff rearrangement of 330 (Equation 32) <2002HAC165>. 

Due to its high efficiency and regioselectivity, the Wolff rearrangement was extensively studied and reviewed, with copper or silver being the most efficient metals for this transformation (121). Similar reactivity can be achieved via ultraviolet (UV) light, sonication, or microwave irradiation (122). A free carbene-based pathway was proposed as a general mechanism. Basic additives are usually required when silver is used as the catalyst Silver nanoclusters are known to form under such conditions, which may serve as the real catalyst (Fig. 24) (123,124). 

One further application of the microwave-assisted Wolff rearrangement of 2-diazo-1,3-dicarbonyl conpounds involves the direct Friedel-Crafts a-ketoacylation of heteroaromatic compounds in the absence of any catalyst or additive and under halogen-free conditions tScheme 3.26k In contrast to standard Friedel-Crafts reactions that are often conplicated by regioselectivity and/or efficiency issues, the microwave-assisted Friedel-Crafts a-ketoacylation via a-oxo-ketenes proceeded cleanly and with high regioselectivity. It should be noted at there are only a few known exanples of formation of carbon-carbon bonds by... 

Irradiation of j8-phenylethynyl-a-diazo-jS-ketoester with 300 or 350 nm light causes regioselective Wolff rearrangement resulting in alkynyl migration (Scheme 102). ... 

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