Harnessing the Energy of Visible Light to Promote LMCT of Cu(II) Species for Intramolecular Dehydrogenative Lactonization of Biaryl-2-carboxylic Acids

Earth-abundant copper salts, in recent years, have attracted the attention of the scientific community with their ability to perform ligand-to-metal charge transfer (LMCT) upon light excitation, thereby eliminating the necessity of a high activation energy barrier required in thermal approaches. Herein, we have reported an application of photoinduced LMCT of Cu(II) species toward the synthesis of benzocoumarin derivatives from 2-aryl benzoic acids via intramolecular dehydrogenative coupling. Existence of light-induced LMCT is confirmed by using detailed mechanistic studies, which include lowering of the d–d transition intensity upon photoexcitation (in UV–visible spectroscopic analysis) and drop-down of the EPR signal intensity on sequential irradiation. Moreover, X-ray photoelectron spectroscopy (XPS) studies reveal the photochemical conversion of Cu(II) to Cu(I) through LMCT. A large number of diverse functional groups could easily be tolerated under this newly developed protocol affording very good yield of the desired benzocoumarins. Additionally, the bulk-scale synthesis of a metabolite compound urolithin A further extends the applicability of this light-induced methodology.