A Hydrogen Evolution Catalyst Co2O2 Metallacycle Enables Regioselective Allene C(sp2)‐H Functionalization

Abstract Selective functionalization of allenic C(sp 2 )‐H is an ideal approach to upgrading simple allenes to synthetically useful allenes, albeit suffering from challenges associated with inert reactivity and inferior selectivity. Inspired by energy chemistry, a catalytic hydrogen evolution reaction (HER) strategy was leveraged to selectively activate weakly acidic allene C(sp 2 )‐H bonds in a reductive mode. An array of Co 2 O 2 metallacycle complexes were readily devised starting from amino acids, and they were demonstrated as robust HER catalysts, which would selectively break allenic C(sp 2 )‐H bonds to release hydrogen. With the newly developed HER catalyst, regioselective electrochemical functionalization of allenic C(sp 2 )‐H with alcoholic α C(sp 3 )‐H was unprecedentedly achieved. This strategy features excellent regioselectivity, unconventional chemoselectivity, good functional‐group tolerance (62 examples), and mild conditions. Mechanism experiments revealed a reactive hydroxy‐coordinated cobalt(II) species in the reaction. Density functional theory (DFT) calculations were also conducted to rationalize the regioselectivity observed in the reaction.