Thermodynamic Resolution and Enantioselective Synthesis of C2‑Symmetric Bis-sulfoxides Based on Chiral Iridium(III) Complexes
journal contribution
posted on 2019-09-04, 17:37 authored by Li-Ping Li, He-Long Peng, Bao-Hui YeEnantiopure
Λ-[Ir(dfppy)2(MeCN)2](PF6)
and Δ-[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 Λ-[Ir(dfppy)2(MeCN)2](PF6) and Δ-[Ir(dfppy)2(MeCN)2](PF6) with C2-symmetric bis-sulfoxides led to diastereoselective formation of
the corresponding diastereomers Λ-[Ir(dfppy)2(R,R)-bis-sulfoxide)](PF6) in
90–92% and Δ-[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 Λ-[Ir(dfppy)2(MeCN)2](PF6) and Δ-[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 C2 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.
