Enantioselective Syntheses of Sulfoxides in Octahedral Ruthenium(II) Complexes via a Chiral-at-Metal Strategy

The preparation of chiral 2-(alkylsulfinyl)phenol compounds by enantioselective coordination-oxidation of the thioether ruthenium complexes with a chiral-at-metal strategy has been developed. The enantiomerically pure sulfoxide complexes Delta-[Ru(bpy)(2){(R)-LO-R}](PF6) (bpy is 2,2'-bipyridine, HLO-R is 2-(alkylsulfinyl)phenol, R = Me (Delta-1a), Et (Delta-2a), iPr (Delta-3a), Bn (Delta-4a), and Nap (Delta-5a)) and Delta-[Ru(bpy)(2){(S)-LO-R}](PF6) (R = Me (Delta-1a), Et (Delta-2a), iPr (Delta-3a), Bn (Delta-4a), and Nap (Delta-5a)) have been synthesized by the reaction of Lambda-[Ru(bpy)(2)(py)(2)](2+) or Lambda-[Ru(bpy)(2)(py)(2)](2+) with the prochiral thioether ligands 2-(alkylthio)phenol (HL-R), followed by enantioselective oxidation with m-CPBA as oxidant. The X-ray crystallography was used to verify the stereochemistry of ruthenium complexes and sulfur atoms. The configurations of the ruthenium complexes are stable during the coordination and oxidation reactions. Moreover, the chiral sulfoxide ligands are enantioselectively generated by controlling of the configuration of ruthenium centers in the course of oxidation reaction. That is, the Delta configuration at the ruthenium center generates the S sulfoxide ligand; on the contrary, the Delta configuration of the ruthenium complex originates the R sulfoxide ligand. Acidolysis of Lambda-[Ru(bpy)(2){(R)-LO-R}](PF6) and Delta-[Ru(bpy)(2){(S)-LO-R}](PF6) complexes in the presence of TFA-MeCN afforded the chiral ligands (R)-HLO-R and (S)-HLO-R in 96-99% ee values, respectively. Importantly, the chiral ruthenium complexes can be recycled as Delta/Lambda-[Ru(bpy)(2)(MeCN)(2)](PF6)(2) and reused in a next reaction cycle with complete retention of the configurations at ruthenium centers.