C-H Bond Sulfonylation from Thianthrenium Salts and DABCO·(SO 2 ) 2 : Synthesis of 2-Sulfonylindoles.

A three-component reaction of 1-(1 H -indol-1-yl)isoquinolines or 1-(pyridin-2-yl)-1 H -indoles, DABCO·(SO 2 ) 2, and thianthrenium salts under synergistic photoredox and palladium catalysis is accomplished. This direct C-H bond sulfonylation of indoles with the insertion of sulfur dioxide under mild conditions works efficiently, giving rise to a wide range of 2-sulfonated indoles in moderate to good yields under mild conditions. In this protocol, the generality of aryl/alkyl thianthrenium salts is demonstrated as well. A photoredox radical process combined with palladium catalysis is proposed.