Interference of the Surface of the Solid on the Performance of Tethered Molecular Catalysts

Abstract

The catalytic performance of cinchonidine in the promotion of thiol additions to conjugated ketones was used as a probe to assess the tethering of molecular functionality onto solid surfaces using well-known "click" chemistry involving easy-to-react linkers. It has been assumed in many applications that the tethered molecules retain their chemical properties and dominate the chemistry of the resulting solid systems, but it is shown here that this is not always the case. Indeed, a loss of enantioselectivity was observed upon tethering, which could be accounted for by a combination of at least three effects: (1) the nonselective catalytic activity of the surface of the solid itself; (2) the activity of the OH species generated by hydrolysis of some of the Si-alkoxy groups in the trialkoxy moieties used to bind many linkers to oxide surfaces; and (3) the bonding of the molecule to be tethered directly to the surface. Several ideas were also tested to minimize these problems, including the silylation of the active OH groups within the surface of the oxide support, the selection of solvents to optimize silane polymerization and minimize their breaking up via hydrolysis or alcoholysis reactions, and the linking at defined positions in the molecule to be tethered in order to minimize its ability to interact with the surface.

Recyclable organocatalysts based on hybrid silicas

“…As a result of the pore size decrease and of the higher density, a decrease of the specific surface area and pore volumes is observed. This passivation strategy is generally used to inhibit hydrogen bonding activation of the substrate by the silica surface, 90,91 but it is also believed to facilitate the diffusion of organic species, particularly in very polar environments, through enhanced hydrophobicity of the materials. 92 Amine-grafted silicas are being intensively studied as heterogeneous catalysts for Henry and aldol reactions.…”

Shape‐Controlled Nanostructures in Heterogeneous Catalysis

“…[250] Because of their larger pore sizes, ordered mesoporous materials can be used to overcome the constraints of zeolites and to allow more facile diffusion of bulky substrates, but they also offer less promise as agents to control catalytic selectivity on the basis of pore size and shape. [18,[255][256][257] In most cases, the added selectivity originates from the particular chemical properties of the added molecules, but it is also possible, in principle, to use tethered molecules to partially block the inside volume of the pores in a specific manner to create a sculpted space for the active catalytic site. [24,251] Tethering of molecular functional groups to the surfaces of SiO 2 has been developed extensively in recent years.…”

“…[252][253][254] Enantioselectivity can be added in this way as well by tethering chiral compounds. [18,[255][256][257] In most cases, the added selectivity originates from the particular chemical properties of the added molecules, but it is also possible, in principle, to use tethered molecules to partially block the inside volume of the pores in a specific manner to create a sculpted space for the active catalytic site. To the best of our knowledge, this has not yet been attempted.…”

Click functionalization of magnetite nanoparticles: A new magnetically recoverable catalyst for the selective epoxidation of olefins

“…One way to address this problem is to employ solid supports for the immobilization of homogeneous catalysts . The selection of support is critical, as it affects catalytic efficiency …”

“…[3,4] The selection of support is critical, as it affects catalytic efficiency. [5][6][7][8] Recent studies have indicated that magnetite nanoparticles with superparamagnetic behaviour are appropriate supports for immobilizing catalysts and may facilitate isolation of catalysts from reaction mixtures with an external magnetic field. Functionalization of the surface of magnetite nanoparticles is a well-known method for the preparation of magnetically recyclable heterogeneous catalysts.…”