Visible light mediated synthesis of 6-benzo[]chromenes: transition-metal-free intramolecular direct C-H arylation

A synthetic approach towards the 6 H -benzo[ c ]chromene ring under visible light and transition-metal-free conditions has been developed. Benzochromenes are synthesized from the corresponding (2-halobenzyl) phenyl ethers or (2-halophenyl) benzyl ethers using KO t Bu in dimethyl sulfoxide (DMSO) at room temperature (rt) and blue light-emitting diodes (LEDs) as the light source. This methodology replaces the use of ligands or additives, high temperatures and toxic solvents. The photostimulated reaction exhibits very good tolerance to different functional groups and 5 H -dibenzo[ c , f ]chromenes are also effectively obtained. An electron donor-acceptor complex formed by the dimsyl anion and (2-halobenzyl) phenyl ethers was found and it induces the ET as the initial step in the photocyclization reaction. Furthermore, in order to explain the regiochemical outcome of this reaction, a theoretical analysis was performed using DFT methods. A synthetic approach towards the 6 H -benzo[ c ]chromene ring under visible light and transition-metal-free conditions has been developed. A theoretical analysis was performed using DFT methods in order to study the ET mechanism.