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

Abstract 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.