Solvent-controlled g-C3N4 visible-light photoredox benzylic C-H oxidation: selective synthesis of fluorenones and benzo[c]coumarins from 2-methyl-1,1'-biaryls.

Solvent-controlled reactivity, which enables the selective formation of distinct products from the same starting precursors by choosing the appropriate reaction media, is a crucial green chemistry approach. In fact, it reduces the need for additional reagents or precursors, restricts waste formation, and improves reaction efficiency and selectivity. Herein, using heterogeneous graphitic carbon nitride (g-C3N4) visible-light photoredox catalysis, we describe a novel solvent-regulated method to selectively synthesize a series of fluorenones (2) and benzo[c]coumarins (3) from a common precursor, 2-methyl-1,1'-biaryls (1), using dry CH3CN and CH3CN-water (9 : 1, v/v), respectively. Under anhydrous conditions, the reaction preferentially produces fluorenones (2) via intramolecular radical cyclization of in situ generated 2-phenylbenzyl radical intermediates followed by oxidation. In contrast, water and a surplus quantity of oxygen cause the chemical pathway to switch to oxidative lactonization, which results in the production of benzo[c]coumarins (3) with high selectivity. To the best of our knowledge, this is the first photocatalytic system that uses facile solvent control to produce two highly valuable heterocyclic frameworks from the same precursor.