Copper-Catalyzed Formal Carbene Migratory Insertion into Internal Olefinic C═C Bonds with N-Tosylhydrazones To Access Iminofuran and 2(3H)-Furanone Derivatives.

Efficient copper-catalyzed formal carbene migratory insertion into the olefinic C═C bonds of internal olefins, that is, α-oxo ketene N,S-acetals, has been achieved by means of N-tosylhydrazones of ketones as the carbene precursors. Iminofuran derivatives were obtained and further transformed to the corresponding 2(3H)-furanones and 4-oxobutanoates (γ-ketoesters), respectively, under mild conditions. In a similar fashion, α-thioxo ketene N,S-acetals reacted with N-tosylhydrazones of ketones to afford iminothiophenes. It is suggested that formal carbene migratory insertion into the olefinic C═C bond is involved in the overall catalytic cycle, demonstrating a new type of carbene insertion reaction for five-membered heterocycle construction.