Cobalt-Catalyzed Regioselective Synthesis of Indenamine from o-Iodobenzaldimine and Alkyne: Intriguing Difference to the Nickel-Catalyzed Reaction

A new and convenient method for the regioselective synthesis of indenamine from the corresponding o-halobenzyaldehyde, amine, and alkyne in the presence of zinc powder catalyzed by cobalt complexes, was reported. The indenamines and indene-enamine gave the corresponding imine derivatives upon treatment with TBAF in THF at 40°C. The results show that chelation-assisted oxidative addition of 2-iodo-benzaldinine to CobaltI, which is formed from the reduction of CoIII by zinc, gives a five-membered ring cobaltacycle. The nickel-catalyzed reaction and reductive elimination takes place to afford an isoquinolinium salt and then the isoquinoline product. The regiochemistry for the cobalt-catalyzed indenamine and nickel-catalyzed isoquinoline formation is opposite to each other when alkynes with an electron-withdrawing group are used.