Enantioselective Oxidation of Thioethers to Sulfoxides by Means of a Structural Template with Chiral-at-Metal Ruthenium Complexes

Treatment of cis-[Ru(bpy)(2)Cl-2]center dot 2H(2)O or Delta/Lambda-[Ru(bpy)(2)(py)(2)](2+) (bpy = 2,2'-bipyridine, py=pyridine) with the prochiral thioether ligands 2-alkylthiobenzoic acid (HOS-R) produces the corresponding thioether complexes rac-[Ru(bpy)(2)(OS-R)](PF6) (R=Me (rac-1), iPr (rac-2), 2-benzylthiobenzonate (Bn) (rac-3)) and Delta/Lambda-[Ru(bpy)(2)(OS-R)](PF6) (R=Me (Delta-1/Lambda-1), iPr (Delta-2/Lambda-2), Bn (Delta-3/Lambda-3)) with retention of the configurations at chiral metal centers. In situ oxidation of the thioether complexes by meta-chloroperoxybenzoic acid provides the corresponding sulfoxide complexes rac-[Ru(bpy)(2)(OSO-R)](PF6) (OSO-R is 2-alkylsulfinylbenzonate, R=Me (rac-1a), iPr (rac-2a), Bn (rac-3a)), Delta-[Ru(bpy)(2){(R)-OSO-R}](PF6) (R=Me (Delta-1a), iPr (Delta-2a), Bn (Delta-3a)), and Lambda-[Ru(bpy)(2){(S)-OSO-R}](PF6) (R=Me (Lambda-1a), iPr (Lambda-2a), Bn (Lambda-3a)) in yields of 95% with 98% ee values. The absolute configurations at the metal centers and sulfur atoms were determined by means of X-ray crystallography. The results indicate that the configurations of the metal centers are retained and have the function of controlling sulfoxide chirality during the oxidation process. The Delta metal-centered configuration enantioselectively generates an R-configuration sulfoxide, and the L configuration enantioselectively forms an S-configuration sulfoxide in the course of the in situ oxidation reaction, thereby resulting in a predetermined chirality of the sulfoxide ligands. The predetermined chirality of sulfoxides (S)-HOSO-R and (R)-HOSO-R were obtained by the treatments of the corresponding sulfoxide complexes Delta-[Ru(bpy)(2){(R)-OSO-R}](PF6) and Lambda-[Ru(bpy)(2){(S)-OSO-R}](PF6) with trifluoroacetic acid in yields of 90% with 83.5-92.9% ee values.