BCl3-Mediated Ene Reaction of Sulfur Dioxide and Unfunctionalized Alkenes

The first ene reactions of SO2 and unfunctionalized alkenes are reported. Calculations suggest that the endergonic ene reactions of SO2 with alkenes can be used to generate beta,gamma-unsaturated sulfinyl and sulfonyl compounds. Indeed, in the presence of one equivalent of BCl3, the unstable sulfinic acid form stable sulfinic acid center dot BCl3 complexes that can be reacted in situ with NCS to generate corresponding sulfonyl chlorides, or with a base to generate corresponding sulfinates. The latter can be reacted with electrophiles to generate sulfones, or with silyl chloride to form beta,gamma-unsaturated sill sulfinates. The sulfinic acid center dot BCl3 complexes can be reacted with ethers that act as oxygen nucleophiles to produce corresponding sulfinic esters. Thus one-pot, three-component synthesis of beta,gamma-unsaturated sulfonamides, sulfinyl esters and sulfones have been developed starting from alkenes and sulfur dioxide (reagent and solvent).