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 Δ-[Ru­(bpy)2­{(R)-LO-R}]­(PF6) (bpy
is 2,2′-bipyridine, HLO-R is 2-(alkylsulfinyl)­phenol,
R = Me (Δ-1a), Et (Δ-2a), iPr (Δ-3a), Bn (Δ-4a), and Nap (Δ-5a)) and Λ-[Ru­(bpy)2­{(S)-LO-R}]­(PF6) (R = Me
(Λ-1a), Et (Λ-2a), iPr (Λ-3a), Bn (Λ-4a), and Nap
(Λ-5a)) have been synthesized by the reaction of
Δ-[Ru­(bpy)2­(py)2]2+ or
Λ-[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 Λ configuration at the ruthenium
center generates the S sulfoxide ligand; on the contrary,
the Δ configuration of the ruthenium complex originates the R sulfoxide ligand. Acidolysis of Λ-[Ru­(bpy)2­{(R)-LO-R}]­(PF6) and Δ-[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 Δ/Λ-[Ru­(bpy)2­(MeCN)2]­(PF6)2 and reused in a next reaction
cycle with complete retention of the configurations at ruthenium centers.