Photo- and Copper Dual-Catalyzed <i>Z</i> -Selective Chlorosulfonylation of Allenes for Tetrasubstituted Alkenes

The selectivity control of C═C double bonds, especially in tetrasubstituted olefins, is a fundamental issue due to their importance in organic chemistry, medicinal chemistry, and material chemistry. Herein, we observed that the stereoselectivity of chlorosulfonylation of allenoic acids or allenoates was reversed by applying an organic photocatalyst in combination with a copper catalyst Cu(acac)₂, affording <i>Z</i>-selective tetrasubstituted olefins substituted with a heteroatom of sulfur. This reaction tolerates a broad range of functional groups. In addition, the intermediate trapping experiment and the reaction of optically active allenoate confirmed the presence of sulfonyl and allylic radicals. Cyclic voltammetry studies and Stern-Volmer quenching experiments demonstrated that the reaction proceeds via an oxidative quenching mechanism. Cu(acac)₂ was found to play a crucial role in the formation of stereodefined π-allyl Cu species in stereoselective chlorosulfonylation.