Synthesis of Tetrahydropyridine Derivatives through a Reaction of 1,6-Enynes, Sulfur Dioxide, and Aryldiazonium Tetrafluoroborates

Sulfonated tetrahydropyridine derivatives are accessed through a radical reaction of 1,6-enynes, sulfur dioxide, and aryldiazonium tetrafluoroborates under mild conditions. Sulfonated pyrrolidine can be generated when terminal alkyne is used as the substrate. This reaction proceeds efficiently in dichloroethane without the addition of any catalysts or additives, providing sulfonated tetrahydropyridine derivatives in moderate to good yields. During this transformation, aryldiazonium tetrafluoroborates cooperate with DABCO·(SO2)2 leading to sulfonyl radicals, which initiate the radical cyclization process. Two molecules of aryldiazonium tetrafluoroborates are involved in the reaction.