Ketocarbene complexes

Scheme 69. (Amino)ketocarbene complexes and acid amides.

The reaction of a-diazocarbonyl compounds with nitriles produces 1,3-oxazoles under thermal (362,363) and photochemical (363) conditions. Catalysis by Lewis acids (364,365), or copper salts (366), and rhodium complexes (367) is usually much more effective. This latter transformation can be regarded as a formal [3 + 2] cycloaddition of the ketocarbene dipole across the C=N bond. More than likely, the reaction occurs in a stepwise manner. A nitrilium ylide (319) (Scheme 8.79) that undergoes 1,5-cyclization to form the 1,3-oxazole ring has been proposed as the key intermediate. 

Either a palladadioxetane species (67) formed upon addition of 02 to the ketocarbene metal complex (66), or a dioxirane form (68), has been suggested to be the active intermediate in this reaction (equation 48). 

These dihydrofuran syntheses may be considered as 1,3-dipolar cycloadditions because the ketocarbene (8.117 a) formed in the reaction of diazocarbonyl compounds (8.116), like 8.110, with metal complexes has dipolar character (8.117 b). 

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