Electroremovable Traceless Hydrazides for Cobalt-Catalyzed Electro-Oxidative C–H/N–H Activation with Internal Alkynes

Abstract

Electrochemical oxidative C–H/N–H activations have been accomplished with a versatile cobalt catalyst in terms of [4 + 2] annulations of internal alkynes. The electro-oxidative C–H activation manifold proved viable with an undivided cell setup under exceedingly mild reaction conditions at room temperature using earth-abundant cobalt catalysts. The electrochemical cobalt catalysis prevents the use of transition metal oxidants in C–H activation catalysis, generating H2 as the sole byproduct. Detailed mechanistic studies provided strong support for a facile C–H cobaltation by an initially formed cobalt(III) catalyst. The subsequent alkyne migratory insertion was interrogated by mass spectrometry and DFT calculations, providing strong support for a facile C–H activation and the formation of a key seven-membered cobalta(III) cycle in a regioselective fashion. Key to success for the unprecedented use of internal alkynes in electrochemical C–H/N–H activations was represented by the use of N-2-pyridylhydrazides, for which we developed a traceless electrocleavage strategy by electroreductive samarium catalysis at room temperature.