Aluminum Oxidation Catalysis under Aqueous Conditions: Highly Enantioselective Sulfur Oxidation Catalyzed by Al(salalen) Complexes

Asymmetric oxidation catalysis with an aluminum-based complex was achieved by a combination of newly synthesized chiral aluminum(salalen) complexes (salalen = half-reduced salen = salan/salen-hybridized [ONNO]-type tetradentate ligand; salan = reduced salen, salen = N,N'-ethylenebis(salicylideneiminato)) derived from binol (1,1'-bi-2,2'-naphthol) with aqueous hydrogen peroxide as the oxidant. The combination was found to be efficient for asymmetric sulfur oxidation. Various sulfides were smoothly converted into the corresponding sulfoxides with high to excellent enantioselectivity. Thioacetals are also good substrates for the oxidation.