Radical sulfonylation/cyclization of 1-acryloyl-2-cyanoindoles/2-cyano-N-arylacrylamides with aryldiazonium tetrafluoroborates and Na2S2O5 has been developed for the synthesis of pyrrolo1,2-aindoles and quinoline-2,4-diones. Na2S2O5 serves as a dual-function reagent, acting as both a reducing agent and a sulfur dioxide surrogate for the generation of sulfonyl radicals, which underwent a cascade sequence, including radical addition to the C═C bonds, intramolecular cyclization, and hydrolysis to deliver the final products.
Luo et al. (Tue,) studied this question.