Reactions of nucleophilic carbenes with enols

Abstract

The formal insertion of dimethoxycarbene (1a) into the acidic C–H bond of pentane-2,4-dione (9a), methyl acetoacetate (9b), 3-methylpentane-2,4-dione (9c) and 1,3-diphenylpropane-1,3-dione (9d) is reported as well as the insertion of 3-benzoyloxazolidin-2-ylidene (1b) into 9c. The β-dicarbonyl compounds 9 are known to be equilibrated with their corresponding enols in benzene solution and the insertions appear to proceed by proton abstraction from the enol tautomers of 9 to generate enolate anions and either a dimethoxymethyl cation (from 1a) or a 3-benzoyloxazolidin-2-ium cation (from 1b). Collapse of these ion pairs at the carbon atom of the enolate yields the major product. Formal insertion of 1a into the O–H bond of the enol tautomer of anthrone (12) is also reported.