title: Optimizing dirhodium(II) tetrakiscarboxylates as chiral NMR auxiliaries.
abstract: Thirteen enantiopure paddlewheel-shaped dirhodium(II) tetrakiscarboxylate complexes have been checked for their efficiency in the dirhodium method (differentiation of enantiomers by NMR spectroscopy); six of them are new. Their diastereomeric dispersion effects were studied and compared via so-called key numbers KN. Adducts of each complex were tested with five different test ligands representing all relevant donor properties from strong (phosphane) to very weak (ether). Only one of them, the dirhodium complex with four axial (S)-N-2,3-naphthalenedicarboxyl-tert-leucinate groups (N23tL), showed results significantly better for all ligands than the conventional complex Rh* [Rh(II)(2)[(R)-(+)-MTPA](4); MTPA = methoxytrifluoromethylphenylacetate]. On the basis of (1)H{(1)H} NOE spectroscopy and X-ray diffraction, a combination of favourable anisotropic group orientation and conformational flexibility is held responsible for the high efficiency of N23tL in enantiodifferentiation. Both complexes, Rh* and N23tL, are recommended as chiral auxiliaries for the dirhodium experiment.
authors.affiliation: Leibniz University Hannover, Institute of Organic Chemistry, Schneiderberg 1B, D-30167, Hannover, Germany.
authors.forename: Jens T
authors.initials: JT
authors.lastname: Mattiza
authors.forename: Joerg G G
authors.initials: JG
authors.lastname: Fohrer
authors.forename: Helmut
authors.initials: H
authors.lastname: Duddeck
authors.forename: Michael G
authors.initials: MG
authors.lastname: Gardiner
authors.forename: Ashraf
authors.initials: A
authors.lastname: Ghanem
mesh_terms: D002264:Carboxylic Acids / Q000737:chemistry*; D018360:Crystallography, X-Ray; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D012238:Rhodium / Q000737:chemistry*; D013237:Stereoisomerism
chemical_list: D002264:Carboxylic Acids; D009942:Organometallic Compounds; D012238:Rhodium