Asymmetric Sulfoxidation of Thioanisole by Helical Ti(IV) Salan Catalysts

A new helix-directing salan ligand (R,R)-2 with the (1R,2R)-diaminocyclohexyl backbone and benz[a]anthryl sidearms was synthesized by borohydride reduction of the corresponding salen ligand. Formylation and reduction of (R,R)-2 yielded the N-Me counterpart, (R,R)-3. Complexation of these flexible ligands to TiCl4 produced the [TiCl2(salan)] complexes (R,R)-4 and (R,R)-5, which were characterized by NMR, IR, and HRMS techniques. The complexes were tested as catalysts for the asymmetric sulfoxidation of thioanisole with cumene hydroperoxide and hydrogen peroxide as the oxidants. Modest selectivity was observed, with the N-H salan complex showing somewhat better chiral induction.