Sulfonium Cation in the Service of π-Acid Catalysis

Modern organic synthesis has been strongly affected by recent advances in π-acid catalysis. To increase the π-acidity of catalytically active metal centers tunable electron-withdrawing ligands have become intensively sought-after. In this context cationic ligands, while still rare, offer quite an intriguing research ground. Recently, we introduced sulfonium cations into the list of available strongly π-acidic ancillary ligands, by embedding them in a robust pincer-type framework. However, the M-S bond in the so-formed sulfonium complexes was still easy to dissociate, precluding their application as π-acid catalysts. Herein we demonstrate that this obstacle can be overcome by increasing the rigidity of the sulfonium pincer scaffold. This allows the formation of a unique tris-cationic Pt(II) complex which, despite the strong electrostatic repulsion between its two constituents, features a remarkably short S-Pt bond. With its highly electrophilic Pt(II) center coordinated by a labile ligand this complex is ideally suited for π-acid catalysis. Indeed, in representative cycloisomerization reactions, it exhibits an enhanced catalytic activity compared to an analogous complex of a neutral thioether-based pincer ligand. This very first example of employing sulfonium-based ligands in homogeneous catalysis places sulfonium on par with the state-of-the-art ancillary ligands for such reactions.