Visible light-induced selective sulfonylation-cyclization versus methylation-cyclization of aryl alkynoate esters to corresponding 3-substituted coumarins under metal-, additive- and catalyst-free conditions

Abstract

With a rise in the pollution and waste produced by chemical processes in industrial and chemical laboratories, chemists are concentrating their efforts toward the use of green catalysts, green solvents, and solvent-free or catalyst-free conditions. Herein, we present a metal-, catalyst-, additive-, and toxic organic solvent-free methodology for the tandem cyclization of aryl alkynoate esters (1) with diaryl disulfides (2), or DMSO (4) under violet LED (16 W, 420 nm) to produce 3-sulfenylcoumarins (3), or 3-methylcoumarins (5) in a selective manner. In addition, based on the control experiments, a plausible reaction mechanism is described. While compared to literature reports, this approach is eco-friendly, cost effective, adaptable to a variety of substrates, affords good product yields, effective with sunlight radiation and applicable for synthesis of two different compounds. Moreover, under visible-light irradiation, this is the first method for the synthesis of 3-methylcoumarins (5) as well as a second method for the synthesis of 3-sulfenylcoumarins (3).