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

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. The photodimerization of chromone-2-carboxylic esters in the presence of Rose Bengal and triethanolamine (TEOA) gives access to various alkyl substituted 2,2′-bichromanones.