New Chiral Tetradentate Oxazolinylphosphine Ligands for Nickel and Palladium. Coordination Behavior and Catalytic Activity in Allylic Alkylations (original) (raw)

Efficient stereochemical communication in phosphine-amine palladium-complexes: Exploration of N-substituent effects in coordination chemistry and catalysis

Journal of Organometallic Chemistry, 1997

Six-membered chelate complexes of type [Pd(1a-e)Cl 2 ] (2a-e) and [Pd(1a-e)(h 3-PhCHCHCHPh)]BF 4 (3ae) of a series of systematically varied chiral phosphine-amine ligands (S,S)-Ph 2 PCH(CH 3)CH 2 CH(CH 3)NHR 1a-e (R ¼ benzyl 1a, R ¼ ethyl 1b, R ¼ (S)-a-phenylethyl 1c, R ¼ isopropyl 1d, R ¼ methyl 1e) have been studied. The complexes 2a-e were characterized by X-ray crystallography, DFT analysis and in solution by 1D and 2D NMR spectroscopy. A very good correlation has been observed between the steric demand of the N-substituent and the distortion of the chelate. Furthermore, it has been proved that sterically more demanding N-substituents are capable of distorting the chelate to a larger extent along one single, well defined conformational pathway. The careful variation of the N-substituent thus allows a precise stereochemical fine tuning of the metal's coordination sphere. As a substantiation of this concept, the investigation of complexes 3a-e revealed that the exo/endo ratio as well as the twisting of the allyl moiety around the Pd-allyl axis can easily be modified by the proper choice of the nitrogen substituent. Furthermore, Pd-catalysts with ligands 1a-e provided high enantioselectivities (up to 96%) in asymmetric allylic alkylation reactions.