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.