Alkenylthianthrenium Salts as Synthetic Equivalents of (Cyclo)alkynes in Azide Cycloaddition Reactions

Cycloalkynes are highly reactive coupling partners in a variety of cycloaddition manifolds, but the synthetic utility of cycloalkynes with fewer than 8 atoms is limited by their fleeting stability. To expand access to heterocycles derived from formal cycloadditions to cycloalkynes, we have developed a methodology in which alkenylthianthrenium salts are used as synthetic equivalents of cycloalkynes in reactions with azides. This methodology enables expedient access to a variety of 1,2,3-triazoles from alkene building blocks in two steps, including triazoles fused with 5-, 6-, 7-, and 8-membered carbocycles. Rather than serving as precursors to in situ-generated alkynes, we discovered that many alkenylsulfonium salts react with azides as dipolarophiles, generating 1,2,3-triazoles via a distinct cycloaddition-elimination mechanism. We show that this methodology is viable for both acyclic and cyclic alkenylsulfonium salts and demonstrate its applications as a tool for the synthesis of new small molecule heterocycles and functional polymeric materials.