Asymmetric hydroformylation using Taddol-based chiral phosphine-phosphite ligands

A small library of 17 modular and easily accessible phenol-derived chiral phosphine−phosphite ligands was evaluated in the asymmetric Rh-catalyzed hydroformylation of styrene. It was found that the stereochemical outcome of the reaction is highly dependent on the chiral phosphite moiety and the substituents on the phenolic backbone. Among the ligands studied, Taddol-based ligands of type 10 bearing bulky substituents in ortho-position to the phosphite performed best, with enantioselectivities of up to 85% ee and regioselectivities of ≥98:2. High-pressure NMR of the active catalyst [HRh(P−P)(CO)2] (P−P = 10h) revealed an equatorial-apical coordination of the ligand at rhodium. Temperature dependency of the coupling constants observed during the experiment indicates equilibrium between the two equatorial-apical isomers, with the isomer in which the phosphite occupies the equatorial position being the dominant species.