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Asymmetric Coordination Chemistry by Chiral‐Auxiliary‐Mediated Dynamic Resolution under Thermodynamic Control
Author(s) -
Lin Zhijie,
Gong Lei,
Celik Mehmet Ali,
Harms Klaus,
Frenking Gernot,
Meggers Eric
Publication year - 2011
Publication title -
chemistry – an asian journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.18
H-Index - 106
eISSN - 1861-471X
pISSN - 1861-4728
DOI - 10.1002/asia.201000555
Subject(s) - coordination complex , chiral auxiliary , chemistry , computational chemistry , enantioselective synthesis , organic chemistry , metal , catalysis
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 )‐ A , 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; 1c : pp=1,10‐phenanthroline) into single diastereomers Λ‐[Ru(pp) 2 {( R )‐ SO }]PF 6 (Λ‐( S )‐ 2a , 2b , 2c ) or Λ‐[Ru(pp) 2 {( R )‐ A }]PF 6 (Λ‐( S )‐ 2a′ ) under a thermodynamically controlled dynamic transformation. The complexes Λ‐( S )‐ 2a , 2b , 2c 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 , 3b , 3c ) complexes with high enantiomeric ratios of ≥98:2. In particular, by employing the methoxy‐modified chiral auxiliary ( R )‐ A , 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.