Allylpalladium Complexes with P-Stereogenic Monodentate Phosphines. Application in the Asymmetric Hydrovinylation of Styrene

A group of P -stereogenic monodentate phosphines S -PPhRR‘ (R = 1-naphthyl, 9-phenanthryl, or o -biphenylyl and R‘ = CH 3 −, i -C 3 H 8 −, and Ph 3 SiCH 2 −) have been prepared by succesive substitution reactions on the oxazaphospholidineborane obtained from (−)-ephedrine and bis( N, N -diethylamino)phenylphosphine. The reaction with binuclear allyl compounds [Pd(μ-Cl)(allyl)] 2 gives neutral [PdCl(allyl)P*] complexes. When allyl = 2-CH 3 -C 3 H 4 ( 5 ), two isomers appeared in solution due to the R - or S -geometry around the palladium atom. The discrimination effect of the phosphines is small and the maximum isomeric ratio is observed for PPh( o -Ph 2 )(CH 2 SiPh 3 ). The molecular structure determined by X-ray diffraction of two complexes with P* = PPh( o -Ph 2 )( i -Pr) and PPh( o -Ph 2 )(OMe) showed a very similar nonsymmetric coordination of the allyl moiety according to the greater trans influence of the phosphorus atom. When allyl = 1-C 6 H 5 -C 3 H 4 ( 6 ), the NMR spectroscopy showed up to four isomers due to the R - or S- geometry around palladium and the Z - or E -disposition of P* and the phenyl substituent of the allyl moiety. The E -isomers are the major species in solution, unique with PPh( o -Ph 2 )(CH 2 SiPh 3 ). The usual, well-defined dynamic exchanges by π−σ−π and pseudorotation of the allyl moiety have been observed. The codimerization reaction between styrene and ethylene has been tested using filtered CH 2 Cl 2 solutions of [PdCl(2-CH 3 -C 3 H 4 )P*] ( 5 ) complexes and AgBF 4 as catalytic precursors. Moderate activity (TOF < 225 h - 1 at 25 °C) and good selectivities to 3-Ph-1-butene (∼90% at 80% conversion) are obtained. The ee is moderate (<40% ee) and different from the discrimination effects observed in the solutions of neutral complexes [PdCl(ally)P*]. The reaction carried out with deuterated styrene shows the clean C−H addition to the vinyl double bond of stryrene and confirms the irreversible nature of the insertion of styrene in the palladium hydride intermediate. The hydrovinylation reaction using substituted styrene with a potentially secondary coordination atom occurs only when the substitution is in the phenyl ring and without significant improvements of the ee.