Palladium-Catalyzed Direct C-H Functionalization of Indoles with the Insertion of Sulfur Dioxide: Synthesis of 2-Sulfonated Indoles.

A palladium-catalyzed direct C-H bond sulfonylation of indoles with the insertion of sulfur dioxide is achieved through a three-component reaction of 1-(pyridin-2-yl)indoles, DABCO·(SO2)2, and aryldiazonium tetrafluoroborates under mild conditions. Diverse 2-sulfonated indoles are generated by using 10 mol % of palladium(II) bromide as the catalyst at room temperature. This synthetic approach is efficient by merging palladium catalysis and insertion of sulfur dioxide via a radical process. 2-Pyrimidinyl can be used as the directing group well as for the C-H bond sulfonylation. Additionally, the directing group can be easily removed.