Chiral Recognition and Dynamic Thermodynamic Resolution of Sulfoxides by Chiral Iridium(III) Complexes

The optically active Ir(III) complex Lambda-[Ir(ppy)(2)(MeCN)(2)](PF6) (ppy is 2-phenylpyridine) with a chiral-at-metal was first demonstrated to preferentially react with (R)-configuration sulfoxides 2-(alkylsulfinyl)phenol (HLO-R, R= Me, Et, Pr-i, and Bn) rather than (S)-configuration sulfoxides under thermodynamic equilibrium due to the hydrogen-bonding interaction and the differences in the steric interference, and thus act as a highly efficient enantioreceptor for resolution of sulfoxide enatiomers. Treatment of Lambda[Ir(ppy)(2)(MeCN)(2)](PF6) with 2 equiv of rac-HLO-R offered (S)-HLO-R in yields of 46-47% with 97-99% enantiomeric excess (ee) values and Lambda[Ir(ppy)(2){(S)-LO-R}] complex in yields of 89-93% with 98% diastereomeric excess (de). The (R)-HLO-R chiral sulfoxides were obtained by the acidolysis of Lambda-[Ir(ppy)(2){(S)-LO-R}] complexes with trifluoroacetic acid (TFA) in the presence of coordinated solvent MeCN in yields of 45-47% with 98-99% ee values. Moreover, the enantioreceptor Lambda-[Ir(ppy)(2)(MeCN)(2)](PF6) can be recycled in a yield of 86-91% with complete retention of the configuration at metal center and can be reused cycle without loss of reaction activity and enantioselectivity. The absolute configurations at the metal centers were determined by X-ray crystallography.