Radical Addition Reaction between Chromenols and Toluene Derivatives Initiated by Brønsted Acid Catalyst under Light Irradiation

A Bronsted acid-catalyzed benzylic C(sp3)–H functionalization of toluene and its derivatives was accomplished through the photo-excitation of benzopyrylium cation intermediates. Light irradiation promoted the reaction of chromenols with toluene and its derivatives in the presence of a Bronsted acid catalyst, affording the corresponding benzylated chromene derivatives having tetrasubstituted carbon centers. Experimental and theoretical studies elucidated that the reaction proceeds through a radical addition pathway in which the benzopyrylium cation intermediate acts as both an electrophile and a photo-redox catalyst. The use of a chiral phosphoric acid catalyst enables the enantioselective radical addition reaction through a chiral anion-controlled asymmetric induction via an ion-pairing interaction to provide an addition product in an enantioenriched form.