Thermodynamic Resolution and Enantioselective Synthesis of C2-Symmetric Bis-sulfoxides Based on Chiral Iridium(III) Complexes

Enantiopure Lambda-[Ir(dfppy)(2)(MeCN)(2)](PF6) and Delta-[Ir(dfppY)(2)(MeCN)(2)](PF6) (where dfppy is (4,6-difluoropheny)pyridine) were demonstrated to preferentially react with (S,S)-1,2-bis(arylsulfinyl)ethane and (R,R)-1,2-bis(arylsulfinyl)ethane, respectively, under thermodynamic equilibrium. Sequential treatment of Lambda-Dr(dfppy)(2)(MeCN)(2)](PF6) and Delta-[Ir(dfppY)(2)(MeCN)(2)](PF6) with C-2-symmetric bis-sulfoxides led to diastereoselective formation of the corresponding diastereomers Lambda-[Ir(dfppy)(2)(R,R)-bis-sulfoxide)](PF6) in 90-92% and Delta-[Ir(dfppy)(2)(S,S)-bis-sulfoxide)](PF6) in 88-90%, respectively. The uncoordinated (R,S)-bis-sulfoxides were afforded in 45% with >97% de values. Enantiopure (S,S)-bis-sulfoxides and (R,R)-bis-sulfoxides were respectively obtained by the release of sulfoxide ligands from the corresponding complexes in the presence of glycine in yields of 20-21% with 97-99% ee values. The enantioreceptors Lambda-[Ir(dfppy)(2)(MeCN)(2)](PF6) and Delta-[Ir(dfppy)(2)(MeCN)(2)](PF6) can be recycled and reused in the next reaction cycle. Moreover, a protocol for asymmetric oxidation of prochiral bis-sulfide into enantiopure C-2 symmetric bis-sulfoxide was also developed in a high enantioselectivity. The absolute configurations at the metal centers and sulfur atoms were determined by X-ray crystallography.