N-Radical-Initiated Cyclization through Insertion of Sulfur Dioxide under Photoinduced Catalyst-Free Conditions

A N-radical-initiated cyclization involving the sulfonylation of unactivated alkenes through insertion of sulfur dioxide in the presence of visible light under catalyst-free conditions is accomplished. A range of sulfonated 3,4-dihydro-2H-pyrroles can be generated in good yields under photoinduced sulfonylative conditions. Additionally, the corresponding 2-(3,4-dihydro-2H-pyrrol-2-yl)methylsulfonyl-1-arylethanones can be easily converted to 3,4-dihydro-2H-1,4-thiazine 1,1-dioxides. This photoinduced transformation occurs efficiently at room temperature under catalyst-free conditions, and various functional groups can be tolerated. A tandem radical process is involved through the iminyl radical-mediated cyclization with the insertion of sulfur dioxide; this process shows high efficiency and good selectivity.