Asymmetric Coordination Chemistry by Chiral‐Auxiliary‐Mediated Dynamic Resolution under Thermodynamic Control

Abstract

A method is presented for the asymmetric synthesis of chiral ruthenium polypyridyl complexes that starts from racemic cis-[Ru(pp)(2)Cl(2)] (pp=2,2'-bipyridine or 1,10-phenanthroline ligands). The chiral bidentate ligands (R)-2-(isopropylsulfinyl)phenol, (R)-SO, and preferably the more electron-rich derivative (R)-2-(isopropylsulfinyl)-4-methoxyphenol, (R)-SO', serve as convenient chiral auxiliaries for the conversion of racemic starting complexes (1a: pp=2,2'-bipyridine; 1b: pp=5,5'-dimethyl-2,2'-bipyridine; c: pp=1,10-phenanthroline) into single diastereomers Λ-[Ru(pp)(2){(R)-SO}]PF(6) (Λ-(S)-2a-c) or Λ-[Ru(pp)(2){(R)-SO'}]PF(6) (Λ-(S)-2a') under a thermodynamically controlled dynamic transformation. The complexes Λ-(S)-2a-c and Λ-(S)-2a' themselves are direct precursors for the generation of optically active ruthenium-polypyridyl complexes by trifluoroacetic-acid-induced replacement of the sulfinylphenolate auxiliaries with bidentate pp ligands under retention of configuration, thereby affording Λ-[Ru(pp)(3)](PF(6))(2) (3a-c) complexes with high enantiomeric ratios of ≥98:2. In particular, by employing the methoxy-modified chiral auxiliary (R)-SO', enantiomeric ratios of >99:1 were reached. In the strategy introduced here, the high steric crowding of an octahedral coordination sphere was exploited by placing a sulfur-based stereocenter in direct proximity to the ruthenium stereocenter, thereby leading to a large difference in the stabilities of the intermediate Λ-S and Δ-S diastereomers and thus providing the opportunity to find suitable reaction conditions for conversion of the destabilized diastereomer into the thermodynamically more-stable one. This method should be of high practical value for the asymmetric synthesis of ruthenium-polypyridyl complexes because it allows one to use readily available racemic ruthenium complexes as starting materials.