Palladium(II) Complexes of New Bulky Bidentate Phosphanes: Active and Highly Regioselective Catalysts for the Hydroxycarbonylation of Styrene

The synthesis of four new bulky bidentate phosphines that possess both tert‐butyl and trifluoromethylphenyl substituents is described. Symmetric ligands were readily obtained by alkylation of phosphidoboranes of the type Li[P(BH3)(tBu)(Ar)] with dihaloalkanes. Non‐symmetric ligands were prepared from a new stable precursor, tBu2P(BH3)(CH2)3Br, that should prove useful for other ligand syntheses. Palladium(II) complexes of the four new ligands were prepared and were characterised by spectroscopic methods, microanalysis and X‐ray crystallography. The new [PdCl2(L)] complexes were evaluated as catalysts for the hydroxycarbonylation of styrene and found to give unprecedented regioselectivity and yields for a diphosphine‐based catalyst. A study on promoter effects reveals that the presence of acid and chloride is necessary to achieve such selectivities. It has been proposed in the literature that such conditions result in a new pathway in which styrene is converted into 2‐phenethyl chloride, with the latter being the real substrate in the reaction. However, a deuterium labelling study seems to rule out this mechanism, at least under the conditions used herein. Branching complexity: Palladium complexes of several new bulky bidentate diphosphanes can give extremely high regioselectivity (up to 76:1) towards the branched acid in the hydroxycarbonylation of styrene (see scheme). Key features are the correct degree of steric bulk in the ligand and the use of H+/Cl− promoters. Labelling experiments were used to explore the mechanism of the reaction.