Mechanistic insights highlight regioselective Cobalt-catalyzed [3+3] annulation of anilides with benzylallenes

Abstract

C-H annulation of anilide with benzylallene under the catalysis of Co(III)-complex has been studied theoretically by employing density functional theory (DFT) to find out the plausible mechanistic path. Our findings reveal that the C-H activation prefers the formation of a four-membered cobalta benzoazetidine intermediate rather than six-membered 1,2,4-cobalta benzoxazine intermediate to generate the product. This fact reveals a new mechanism in contrast to the mechanism proposed earlier. The metal mediated annulation of various anilides include six steps - coordination of substrate with catalyst, N-H metalation, C-H metalation, insertion of benzylallene, proton abstraction and migration insertion. Moreover, it reveals regioselectivity of cobalt for allene insertion. The effect of substituents on the mechanism has also been studied and the high energetic span obtained for this catalytic cycle implies that very high temperature is required for this reaction. It is consistent with the experimental result.