P-Stereogenic monophosphines in Pd-catalysed enantioselective hydrovinylation of styrene

Two groups of P-stereogenic monodentate phosphines containing pendant groups bearing functionalities capable of having secondary interactions, PArPhR (Ar = 2-[(2′,6′-dimethoxy)-1,1′-biphenylyl] and R = OMe (6) or Me (8)) and PMeR(CH2R′) (13, R = t-Bu or 14 R = Fc and R′ = SiMe3 (a), SiMe2Ph (b) or CH2(2-naphthyl) (c)) have been prepared in enantiomerically pure form by the Jugé and Evans methodologies and characterised, including the crystal structure for the borane adduct of 12b. Reaction of the phosphines with the Pd dimer [Pd(η3-2-Me-allyl)(μ-Cl)]2 produced neutral allylic complexes C [PdCl(η3-2-Me-C3H4)P∗], which have been characterised in solution and shown to be a mixture of isomers with different absolute configuration at the Pd atom. After abstraction of the chloride ligand by AgBF4, the solutions of cationic complexes have been used as catalyst precursors in the hydrovinylation of styrene under mild conditions. Very good activities (up to 1015 h−1), moderate to good selectivities to 3-phenyl-1-butene and low to moderate enantioselectivities (<45% ee) have been found. Deep differences in the activity and enantioselectivity have been found depending on the structure of the ligand. In spite of that the results did not permit to confirm the improvement of the selectivities of the reaction by stabilization of the catalyst through secondary coordination with the potentially hemilabile ligands.

Novel neutral palladium allylic complexes with P-stereogenic monophosphines bearing groups susceptible of secondary coordination interactions with the metal have been prepared and tested in hydrovinylation. Ligand effects on the activity and stereoselectivity have been observed but do not confirm the potential hemilability of the ligands.Figure optionsDownload as PowerPoint slideHighlights
► Monodentate phosphines with pendant functionalities for secondary interactions.
► P-stereogenic phosphines with different basicity and hindrance.
► Neutral allylic complexes [Pd(allyl)ClP∗].
► Hydrovinylation of styrene.