New reaction of enamines with aryldiazoacetates catalyzed by transition metal complexes

The reaction of aryldiazoacetates with enamines catalyzed by copper and rhodium complexes provided γ-keto esters in good yields. A full investigation of the effects of solvents, catalysts, enamines and aryldiazoacetates on the reaction was carried out. Careful analysis of the crude reaction mixture revealed a substituted enamine as the primary product, which was hydrolyzed over silica gel to give a γ-keto ester as the final product. A reaction mechanism involving nucleophilic addition of an enamine to a metal carbene and subsequent hydrogen transfer was proposed. Chiral dirhodium and copper catalysts were examined and found to provide γ-keto esters with no enantioselectivity. The result could be rationalized based on the proposed reaction mechanism. Attempts to trap the enamine intermediate with several electrophilic reagents were not successful.