Constraining Asymmetric Organometallic Catalysts within Mesoporous Supports Boosts Their Enantioselectivity

Abstract

By constraining tethered asymmetric organometallic catalysts within the nanopores of silica supports so as to increase the interaction between the pore wall and the active center (and hence to restrict access of the reactant to the catalyst), a significant improvement in enantioselectivity is achieved. A schematic illustration of a cationic chiral, organometallic catalyst, [Rh(I)(COD)PMP] {(S)-(+)-1-(2-pyrrolidinylmethyl)-pyrrolidine and cyclooctadiene}, which is noncovalently anchored via a N-H...F hydrogen bond with the triflate ion, CF3SO3-, to the curved inner surface of a 38 A diameter pore of a silica support for the asymmetric hydrogenation of the C=O bond in methyl benzoylformate to the corresponding methyl mandelate is shown. Rh (purple); N (blue); H (white); F (green); C (gray); S (yellow); O (red).

Chiral Metal‐Organic Frameworks and Their Application in Asymmetric Catalysis and Stereoselective Separation

“…99 % conversion with an enantiomeric excess of 93 % were observed for the reaction of diethyl zinc and 1-naphthaldehyde (1-NaphCHO) in the presence of (15) and an excess of Ti(Oi-Pr) 4 (Tab. 5).…”

“…For basic principles of asymmetric catalysis in confined space see [14]. [15] features an impressive study of the dependence of enantioselectivity on the pore size of the mesoporous support used. Thus, in an ideal set-up chiral MOFs may connect the advantages of homogeneous stereoselective catalysts with the advantages of heterogeneous catalysis [16].…”

An Overview of Zeolite, Zeotype and Mesoporous Solids Chemistry: Design, Synthesis and Catalytic Properties

“…Using exactly the same approach and anion, Raja et al reported the immobilization of a range of chiral Rh phosphine complexes onto a set of inexpensive, commercially available silicas. [125] The ee values of the asymmetric hydrogenation of methyl benzoylformate were found to increase upon decreasing pore size of the inorganic host, which reflects the beneficial effect of a constrained environment on the enantioselective performance of a chiral catalyst. This method is, in fact, a combination of two immobilization techniques: the triflate ion is hydrogen bonded to the surface hydroxyl groups, and the cationic metal complex is anchored onto this modified surface via an ionexchange step.…”

Self-assembled materials for catalysis

“…For example, cationic Rh(I) complexes containing a chiral diamino ligand and cyclooctadiene have been tethered on mesoporous silica. When used to catalyze the hydrogenation of methyl benzoylformate, enantiomeric excess (EE) values are actually boosted as compared to the homogeneous systems [114,115].…”