The Immobilization of Rhodium-4-(diphenylphosphino)-2- (diphenylphosphinomethyl)-pyrrolidine (Rh-PPM) Complexes: A Systematic Study

A modular toolbox for the immobilization of homogeneous catalysts to various supports is described. It consists of functionalized chiral diphosphines and three different linkers based on isocyanate chemistry and it is used to attach the 4‐(diphenylphosphino)‐2‐(diphenylphosphinomethyl)‐pyrrolidine (PPM) ligand to a large variety of soluble, swellable and non‐swellable solid organic polymers and to silica gels. As model reaction the hydrogenation of acetamidocinnamic acid derivatives, catalyzed with high enantioselectivity was chosen. Besides information on the usefulness of a particular type of support for synthetic applications, the experiments were also designed to address the question how parameters such as solubility, swellability, cage or pore size and solvent affect the rate and enantioselectivity of an immobilized catalyst. Rhodium complexes of ligands attached to soluble polymers and inorganic supports achieved ees up to 95 % and turnover frequencies between 700 and 1400 h−1, very close to the values of the homogeneous Rh catalyst (ee 95 %, TOF 1320 h−1). Insoluble or strongly cross‐linked organic polymers led to catalysts with lower enantioselectivity and activity. PPM ligands attached to water soluble dendrimer fragments allowed hydrogenation in water solution with ees up to 94 %, albeit with much lower activity compared to reactions in methanol with the homogeneous catalyst.