Enantioselective Oxidation of Thioethers to Sulfoxides by Means of a Structural Template with Chiral‐at‐Metal Ruthenium Complexes

Abstract

Treatment of cis‐[Ru(bpy)2Cl2]⋅2 H2O or Δ/Λ‐[Ru(bpy)2(py)2]2+ (bpy=2,2′‐bipyridine, py=pyridine) with the prochiral thioether ligands 2‐alkylthiobenzoic acid (HOSR) produces the corresponding thioether complexes rac‐[Ru(bpy)2(OSR)](PF6) (R=Me (rac‐1), iPr (rac‐2), 2‐benzylthiobenzonate (Bn) (rac‐3)) and Δ/Λ‐[Ru(bpy)2(OSR)](PF6) (R=Me (Δ‐1/Λ‐1), iPr (Δ‐2/Λ‐2), Bn (Δ‐3/Λ‐3)) with retention of the configurations at chiral metal centers. In situ oxidation of the thioether complexes by meta‐chloroperoxybenzoic acid provides the corresponding sulfoxide complexes rac‐[Ru(bpy)2(OSOR)](PF6) (OSOR is 2‐alkylsulfinylbenzonate, R=Me (rac‐1 a), iPr (rac‐2 a), Bn (rac‐3 a)), Δ‐[Ru(bpy)2{(R)‐OSOR}](PF6) (R=Me (Δ‐1 a), iPr (Δ‐2 a), Bn (Δ‐3 a)), and Λ‐[Ru(bpy)2{(S)‐OSOR}](PF6) (R=Me (Λ‐1 a), iPr (Λ‐2 a), Bn (Λ‐3 a)) in yields of 95 % with 98 % ee values. The absolute configurations at the metal centers and sulfur atoms were determined by means of X‐ray crystallography. The results indicate that the configurations of the metal centers are retained and have the function of controlling sulfoxide chirality during the oxidation process. The Δ metal‐centered configuration enantioselectively generates an R‐configuration sulfoxide, and the Λ configuration enantioselectively forms an S‐configuration sulfoxide in the course of the in situ oxidation reaction, thereby resulting in a predetermined chirality of the sulfoxide ligands. The predetermined chirality of sulfoxides (S)‐HOSOR and (R)‐HOSOR were obtained by the treatments of the corresponding sulfoxide complexes Δ‐[Ru(bpy)2{(R)‐OSOR}](PF6) and Λ‐[Ru(bpy)2{(S)‐OSOR}](PF6) with trifluoroacetic acid in yields of 90 % with 83.5–92.9 % ee values.

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

“…The one-pot synthesis of sulfoxide complexes was carried out with the similar procedure of ours . The yield of Λ- 3a is 81%.…”

“…The data collection and the structure resolution for rac- 2a ·CH 2 Cl 2 , Δ- 2a ·CH 2 Cl 2 , Λ- 2a ·CH 2 Cl 2 , and ( R )- HLO-Bn were carried out as the previous report . The crystal data and parameters for the complexes are presented in Table .…”

“…However, the enantiomeric excess (ee) values are low in 7–18%. In the previous work, we have developed an approach for the preparation of chiral 2-alkylsulfinylbenzonate compounds up to 93% ee value by means of oxidation of the thioether complexes to sulfoxide complexes in a chiral-at-metal asymmetric envionment, followed by release of the oxidated ligand . The configurations of the ruthenium centers are stable during the coordinated reaction of the thioether ligand to the ruthenium center, followed by oxidation of the thioether complexes to the corresponding sulfoxide complexes.…”

“…Moreover, the chirality of the sulfoxide ligand is controlled by the chirality of the ruthenium complexes in the oxidation reaction; therefore, the predetermined chirality of sulfoxide is originated. To the best of our knowledge, though the asymmetric catalysis based on chiral ligands have been well documented, the report for the asymmetric synthesis using a chiral-at-metal complex as the unique source is still rare. , On the other hand, the oxidation of thiolato and thioether complexes has been observed; , however, the enantioselective oxidation in situ of the thioether ligands generation of the predetermined chirallty of the sulfoxide ligands is unexplored . As part of an ongoing study, we extend the approach of “ coordination oxidation in situ ” to synthesize the chiral ortho hydroxyl phenyl sulfoxides which have been used as chiral auxiliaries for asymmetric synthesis .…”

Optically active multi-helical erythrocyte-like Ln(OH)CO3 (Ln = La, Ce, Pr and Sm)

“…7,8 Concomitantly, chiral inorganic materials with new types of absorption-based optical activity, especially in the visible absorption region, 8,9 have become an increasingly vital concern because of their potential applications in asymmetric catalysis, chemical sensing, optical devices and other technology. [10][11][12][13][14] Erythrocyte-red blood cells (RBCs), with characteristic asymmetric biconcave discoid morphology 15 ultimately give rise to their biological functionalities, including the long circulation times, the ability to squeeze through capillary blood vessels and delivery oxygen. [16][17][18] The sophisticated morphology and biofunctions provide a specific challenge for materials design.…”