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

Selective functionalization of allenic C(sp2)‐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(sp2)‐H bonds in a reductive mode. An array of [Co2O2] metallacycle complexes were readily devised starting from amino acids, and they were demonstrated as robust HER catalysts, which would selectively break allenic C(sp2)‐H bonds to release hydrogen. With the newly developed HER catalyst, regioselective electrochemical functionalization of allenic C(sp2)‐H with alcoholic α C(sp3)‐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. [ABSTRACT FROM AUTHOR]