Electrochemical Oxygen Reduction-Enabled Rhodium-Catalyzed [5+1] Annulation Forming 2H-Chromenes

Keywords

[5+1] annulation - rhodium catalysis - electrocatalysis - oxygen reduction - 2H-chromenes

Significance

Rhodium-catalyzed [5 + 1] C–H/O–H annulation of 2-vinylphenols with a wide variety of electron-deficient olefins was achieved under electrochemical conditions. The annulation reactions were carried out in acetonitrile with 2.5 mol% of [Cp*RhCl₂]₂ in the presence of nBu₄OAc (electrolyte) and Cp₂Fe (10 mol%; electrochemical mediator), using electrodes of reticulated vitreous carbon( + )/platinum(–), in an undivided cell under air at 85 °C to give high yields of the corresponding 2-substituted 2H-chromenes.

Comment

The annulation proceeded efficiently under air, where molecular oxygen served as a green oxidant through the electroreduction to regenerate the catalytically active rhodium(III) species in situ. A wide variety of functional groups, including aromatic halides, heteroaromatics, carboxy amides, esters, alkenes, etc., were tolerated under the reaction conditions. Acrylates bearing steroids, carbohydrates and terpenes were also compatible.

Publication History

Article published online:
30 October 2025