Sequential Transformation of Terminal Alkynes to 1,3-Dienes by a Cooperative Cobalt Pyridonate Catalyst

We describe the cobalt(II) catalyst 1 bearing a phosphinopyridonate ligand for sequential transformation of aryl terminal alkynes to (E,Z)-1,3-dienes with excellent stereoselectivity. By cooperative metal–ligand reactivity, 1 reacts readily with the terminal alkynes to afford the alkynyl cobalt intermediate for dimerization, producing (E)-1,3-enynes, and then to smoothly catalyze reduction of the acetylene unit to the olefin by H3N·BH3. A plausible catalytic cycle for both dimerization and transfer hydrogenation was proposed on the basis of stoichiometric reactions and DFT calculations, which suggest the ability of the cobalt catalyst to activate the amine–borane as well as the terminal alkyne through cobalt pyridonate cooperation.