Chemodivergent Chalcogenation of Aryl Alkynoates or N-Arylpropynamides Using 9-Mesityl-10-Methylacridinium Perchlorate Photocatalyst

Herein we report a cascaded chalcogenation of aryl alkynoatesor N-arylpropynamides using 9-mesityl-10-methylacridiniumperchlorateas a visible light photocatalyst to obtain selectively either 3-sulfenylated/selenylatedcoumarins or spiro[4,5]trienones. In a radical initiated process,the spiro-cyclization reaction was favored due to the presence ofa -OMe or -F substituent at the para position of the aryl group, whichhelped to stabilize the allylic radical intermediate formed duringthe reaction. Otherwise, 6-endo-trig cyclization led to 3-sulfenylated/selenylatedcoumarins. Overall, the new C-S/C-Se, C-C, andC O bonds were formed in a single step. The Stern-Volmerquenching study, EPR experiments, light ON-OFF experiments, radicaltrapping experiments, etc., helped to understand the radical-basedmechanism.

Automated summary

Here we describe a cascaded chalcogenation reaction for the synthesis of selectively either 3-sulfenylated/selenylated coumarins or spiro[4,5]trienones using a visible light photocatalyst. The reaction is initiated by radicals and the presence of -OMe or -F substituent at the para position stabilizes the allylic radical intermediate, promoting spiro-cyclization. Otherwise, 6-endo-trig cyclization leads to the formation of 3-sulfenylated/selenylated coumarins. The formation of new C-S/C-Se, C-C, and C=O bonds occurs in a single step. Quenching studies, EPR experiments, light ON-OFF experiments, and radical trapping experiments provide insights into the radical-based mechanism.