Cathodic oxygen reduction-enabled rhodium-catalyzed (5 + 1) C-H/O-H annulation inspired by fuel cells.

Transition metal-catalyzed electrochemical C-H annulation with alkynes has emerged as a promising method for constructing heterocycles via formal cycloadditions. However, catalytic electrochemical C-H annulation with alkenes has been less explored. In this study, we report a cathodic oxygen reduction-enabled rhodium catalyzed (5 + 1) annulation reaction between readily available alkenylphenols and alkenes, yielding valuable 2-substituted 2H-chromenes. Unlike existing methods that involve direct oxidation of catalysts at the anode, our protocol uses a sacrificial anode to protect the substrate from overoxidation, while the cathode reduces oxygen, coupling with the Rh. to regenerate the rhodium catalyst. This efficient, atom-economical heterocyclization reaction demonstrates a broad scope and functional-group tolerance for diverse biologically relevant molecules, with a Faradaic efficiency greater than 100%.