Organophotoredox-catalyzed stereoselective reductive dimerization of chromone-2-carboxylic esters

Abstract

The irradiation of chromone-2-carboxylic esters with a blue LED in the presence of Rose Bengal and triethanolamine (TEOA) is used to obtain 2,2′-bichromanone of methyl, ethyl, n-propyl, iso-propyl, n-butyl, n-pentyl, and n-hexyl chromone-2-carboxylic esters in 52–93% yield and dr_{trans : cis} up to 83 : 17. The key role played by Rose Bengal, TEOA, and blue light was established through numerous control experiments and a plausible mechanism is discussed.