Cobalt/Lewis Acid Catalysis for Hydrocarbo­functionalization of Alkynes via Cooperative C–H Activation

A catalytic system comprising a cobalt–diphosphine complex and a Lewis acid (LA) such as AlMe3 has been found to promote hydrocarbo­functionalization reactions of alkynes with Lewis basic and electron-deficient substrates such as formamides, pyridones, pyridines and related azines, imidazo­[1,2-a]­pyridines, and azole derivatives through site-selective C–H activation. Compared with known Ni/LA catalytic systems for analogous transformations, the present catalytic systems not only feature convenient setup using inexpensive and bench-stable precatalyst and ligand such as Co­(acac)3 and 1,3-bis­(diphenylphosphino)­propane (dppp) but also display distinct site-selectivity toward C–H activation of pyridone and pyridine derivatives. In particular, a completely C4-selective alkenylation of pyridine has been achieved for the first time. Meanwhile, the present catalytic system proved to promote exclusively C5-selective alkenylation of imidazo­[1,2-a]­pyridine derivatives. Mechanistic studies including DFT calculations on the Co/Al-catalyzed addition of formamide to alkyne have suggested that the reaction involves cleavage of the carbamoyl C–H bond as the rate-limiting step, which proceeds through a ligand-to-ligand hydrogen transfer (LLHT) mechanism leading to an alkenyl­(carbamoyl)­cobalt intermediate.