Modular Approach to New Chiral Monodentate Diamidophosphite Ligands. Application in Palladium-Catalyzed Asymmetric Hydrovinylation of Styrene

Abstract

A new group of chiral monodentate diamidophosphite ligands (2 P−N/1 P−O bond) based on diazaphospholidine backbones derived from N,N′-dibenzylcyclohexane-1,2-diamine (7) and N,N′-dimethylcyclohexane-1,2-diamine (8), and diazaphosphepine backbones derived from N,N′-dimethyl-[1,1′-binaphthyl]-2,2′-diamine (9) and various chiral alkoxy groups (coming from phenylethanol a, borneol b, methyllactate c, allylic alcohol d, and methanol e) were prepared. The ligands have a highly modular structure, which is well suited to the synthesis of a small library. Preparation was readily accomplished by the successive addition of pure enantiomeric substituted diamine and pure enantiomeric alcohol to phosphorus trichloride. The corresponding diamidophosphite selenides Se(7−9) were prepared and the JPSe was calculated in order to evaluate the σ-donor ability of the new ligands. The reaction of [Pd(μ-Cl)(η3-2-CH3C3H4)]2 with the new diamidophosphite ligands (7−9) led to the monomeric allylic neutral complexes 11−13. Two isomers appeared in solution due to the R- or S-geometry around the palladium atom. The molecular structure determined by X-ray diffraction of the neutral complex [PdCl(η3-2-CH3C3H4)(7a)] (11a-(S,S,Sal)) showed a nonsymmetric coordination of the allyl moiety due to the greater trans influence of the phosphorus atom. The asymmetric hydrovinylation reaction between styrene and ethylene was tested using filtered CH2Cl2 solutions of [PdCl(η3-2-CH3C3H4)L] complexes and AgBF4 as catalytic precursors. The reaction performed at 15 °C and 15 bar of ethylene starting pressure led to good selectivities and moderate to good activities of 3-phenyl-1-butene. The best results were obtained when the cationic catalytic precursor contained the 9b-(R,Sal) diamidophosphite with a binaphthyl backbone and bornyloxy as the third substituent. With this system the TOF reached 595 h−1 of 3-phenyl-1-butene, whereas the ee was 90% toward the R-isomer.