Chiral Bis(oxazoline) Ligands as C₂-Symmetric Chiral Auxiliaries for the Synthesis of Enantiomerically Pure Bis-Cyclometalated Rhodium(III) Complexes

Abstract

The synthesis of enantiomerically pure bis-cyclometalated rhodium(III) complexes using chiral bis(oxazoline) ligands as C 2 -symmetric chiral auxiliaries is described. Bis(oxazolines) are versatile chiral ligands for asymmetric catalysis but have not been applied to the resolution of racemic mixtures of transition-metal complexes. Due to their C 2 symmetry, chiral bis(oxazolines) are particularly useful for the synthesis of nonracemic transitionmetal complexes with lower symmetry, and this is demonstrated with the synthesis of an enantiomerically pure rhodium(III) complex containing two different cyclometalated ligands.

Enantioselective α-Fluorination and α-Chlorination of N-Acyl Pyrazoles Catalyzed by a Non-C2-Symmetric Chiral-at-Rhodium Catalyst

“…With the synthesis of the racemic complex rac - RhNP in hand, we next engaged in the synthesis of single enantiomers using a recently established chiral auxiliary-mediated approach employing a simple C 2 -symmetric bis­(oxazoline) (BOX) ligand as the chiral auxiliary . Due to the non- C 2 -symmetry of the racemic catalyst, the particular advantage of the C 2 -symmetric nature of the BOX ligand becomes evident.…”

“…With the synthesis of the racemic complex rac-RhNP in hand, we next engaged in the synthesis of single enantiomers using a recently established chiral auxiliary-mediated approach employing a simple C 2symmetric bis(oxazoline) (BOX) ligand as the chiral auxiliary. 18 Due to the non-C 2 -symmetry of the racemic catalyst, the particular advantage of the C 2 -symmetric nature of the BOX ligand becomes evident. The amount of possible diastereomers resulting from coordination of the chiral bidentate ligand is reduced to only two, thereby significantly facilitating the resolution of the stereoisomers formed.…”

Planar Chiral Rhodium Complexes of 1,4‐Benzoquinones

“…Recently, Meggers et al. have successfully applied C 2 ‐symmetric bis(oxazoline) ligands for the separation of chiral‐at‐metal Rh(III) complexes [32] . The use of C 2 ‐symmetric moieties potentially doubles the steric interactions and therefore may indeed increase discriminating ability of the auxiliary ligands.…”

“…[31] Recently, Meggers et al have successfully applied C 2 -symmetric bis(oxazoline) ligands for the separation of chiral-at-metal Rh(III) complexes. [32] The use of C 2symmetric moieties potentially doubles the steric interactions and therefore may indeed increase discriminating ability of the auxiliary ligands. For our study we selected bis(oxazoline) ligand S,S-BOX (Scheme 2), which bears two methyl substituents in the bridge that protect it from unwanted deprotonation.…”

Chiral-at-Ruthenium Catalysts with Mixed Normal and Abnormal N-Heterocyclic Carbene Ligands

“…In an analogous fashion, we also synthesized a ruthenium complex 3′ in an isolated yield of 61% in which the CF 3 substituent of the pyridyl moiety was replaced with a bromide. According to our established chiral-auxiliary-mediated strategy for the synthesis of nonracemic chiral metal complexes, we first tried to react racemic 3 or 3′ with chiral salicyloxazoline ligands but did not obtain stable complexes, while the reaction with chiral bis­(oxazoline) ligands afforded mixtures of diastereomers which in our hands could not be resolved by chromatography. When we reacted nonracemic N -benzoyl- tert -butansulfinamide , with racemic 3 in MeCN at 40 °C for 16 h in the presence of K 2 CO 3 , we were able to resolve the resulting diastereomers to some extent.…”