A Hydrogen Evolution Catalyst [Co2 O2 ] 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 [Co2 O2 ] 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.