Harnessing photo-excited anions in BHAS reactions and mechanistic insights into borrowing hydrogen catalysis

Abstract

Introduction to C-C cross-coupling reactions via base promoted homolytic aromatic substitution (BHAS). The synthesis of biaryls bears great importance in organic chemistry, as the unit is seen in a large number of core structural motifs that are found in molecules having tremendous medicinal, agrochemical, and pharmaceutical importance. Henceforth, there is a strong interest in furnishing C−C bonds in biaryls by easily accessible routes. Thus, direct C−H arylation of arenes has gained tremendous attention as it can bypass double preactivation for both participating substrates. In the domain of single electron transfer catalysis, simple organic additives in conjunction with KO t Bu are very effective for facile electron transfer. SET from potential electron donor species generates an aryl radical from Ar-X substrate with the liberation of leaving group (X). The aryl radical then attacks a coupling partner benzene ring to form a cyclohexadienyl radical. Cyclohexadienyl radical can be further deprotonated in the presence of a base, and the radical anion will be sufficiently reducing to promote SET to another substrate. In this way, the catalytic cycle becomes sustainable. There are several literature reports with tailor-made, well-designed electron donor species being super electron donors. Also there are several methods involving electrochemical reduction, photoexcitation.