Phosphorus-Chiral Diphosphines as Ligands in Hydroformylation. An Investigation on the Influence of Electronic Effects in Catalysis.

The phosphorus-chiral diphosphine 1,1‘-bis(1-naphthylphenylphosphino)ferrocene (1a) and its new electronically modified derivatives 1b−d bearing methoxy and/or trifluoromethyl groups in para positions of the phenyl rings were investigated as ligands in rhodium-catalyzed (asymmetric) hydroformylation. Depending on ligand basicity, high-pressure NMR and IR characterization of the respective (diphosphine) rhodium dicarbonyl hydride precursor complexes revealed subtle differences in the occupation of bis-equatorial (ee) and equatorial−apical (ea) coordination geometries. The high ee:ea ratio of the four complexes contrasted with the clear ea preference observed for the related achiral compound dppf (1,1‘-bis(diphenylphosphino)ferrocene). In the hydroformylation of styrene the best result (50% ee) was obtained by employing the best π-acceptor ligand 1c, incorporating two p-trifluoromethyl substituents. Substrate electronic variations using 4-methoxystyrene and 4-chlorostyrene showed a pronounced influence on t...