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:fa 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 pi -acceptor ligand 1c, incorporating two p-trifluoromethyl substituents. Substrate electronic variations using 4-methoxystyrene and 4-chlorostyrene showed a pronounced influence on turnover frequencies, branched/linear aldehyde product ratios, and enantiodiscrimination, whereas in the hydroformylation of 1-octene ligand electronic perturbations did affect only the rate, but not the selectivity of the reaction.