Immobilization on Silica Supports

One can exclusively obtain species 19 grafted by the two phosphine chains when DIPHOS in very dilute solutions of toluene is added dropwise to a silica suspension at room temperature. Petrucci used the same approach but started from a tri-alkoxyphosphine (Eq. 4) [37], or reacted an amine with a terminal alcohol containing a phosphine [37, 38]. 

An example is provided by complex 20. Placed in an acidic medium, the reaction can easily be carried out in biphasic conditions. 

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Fig. 29 Effect of the linker chain lengths on the intermolecular interaction of catalytic dendrimers immobilized on silica supports [62]...

Natural products such as cellulose and amylose have proved to be of great versatility for enantiomer separations. Though poor chiral selectors in their native state, they become highly effective when their hydroxyl functions are derivatized, particularly with aromatic moieties through ester or carbamate linkages, and they are suitably immobilized on silica supports [11]. 

The above example outlines a general problem in immobilized molecular catalysts - multiple types of sites are often produced. To this end, we are developing techniques to prepare well-defined immobilized organometallic catalysts on silica supports with isolated catalytic sites (7). Our new strategy is demonstrated by creation of isolated titanium complexes on a mesoporous silica support. These new materials are characterized in detail and their catalytic properties in test reactions (polymerization of ethylene) indicate improved catalytic performance over supported catalysts prepared via conventional means (8). The generality of this catalyst design approach is discussed and additional immobilized metal complex catalysts are considered. 

A.K. Singh, A.W. Flounders, J.V. Volponi, C.S. Ashley, K. Wally, and J.S. Schoeniger, Development of sensors for direct detection of organophosphates. Part I immobilization, characterization and stabilization of acetylcholinesterase and organophosphate hydrolase on silica supports. Biosens. Bioelectron. 14, 703-713 (1999). 

The use of heterogeneous catalysts in this reaction has also been achieved palladium-montmorillonite clays [93] or palladium/activated carbon [94] in the presence of dppb transformed 2-allylphenols into lactones, the regiose-lectivity of the reaction being largely dependant on the nature of the support. Very recently, palladium complexes immobilized onto silica-supported (polyaminoamido)dendrimers were used as catalysts in the presence of dppb for the cyclocarbonylation of 2-allylphenols, 2-allylanilines, 2-vinylphenols, and 2-vinylanilines affording five-, six-, or seven-membered lactones and lactams. Good conversions are realized and the catalyst can be recycled 3-5 times [95]. 

The rhodium complex with bis(diphenylphosphino)phenoxazine was immobilized on silica using the sol-gel technique or by a direct grafting to commercially available silica [127]. Under standard hydroformylation conditions (CO/H2 atmosphere), a neutral hydridic complex (57) and cationic species (58) (Scheme 4.35) coexist on the support and act as a hydroformylation/hydrogenation sequence catalyst, giving selectively 1-nonanol from 1-octene 98% of 1-octene were converted to mainly linear nonanal which was subsequently hydrogenated to 1-nonanol. The... 

For this complex, molecular chemistry does not adequately model the surface reactivity and the latter is strongly influenced by the presence of surface hydroxyl groups [22]. The organometallic fragments immobilized on silica have been reacted with trimethylphosphine to afford different silica-supported phosphine complexes of rhodium. The course of the reaction depends strongly on the hydroxyl content of the silica surface [23] (Scheme 7.2). 

The use of supported (i.e., heterogenized) homogeneous catalysts offers another possibility for easy catalyst separation. New examples include polymer-anchored Schiff-base complexes of Pd(TT),446 PdCl2(PhCN)2 supported on heterocyclic polyamides,447 various Pd complexes supported on crosslinked polymers 448 sol-gel-encapsulated Rh-quatemary ammonium ion-pair catalysts,449 and zwitterionic Rh(T) catalysts immobilized on silica with hydrogen bonding.450... 

Further improvement both of the activity of the catalyst and of its ease of handling and recycling was achieved by immobilization on polystyrene supports [78] and, in particular, on silica gel [79, 80]. The latter procedure affords poly-amino acids on silica catalysts , the so-called PaaSiCats . The silica-supported peptide catalysts are sufficiently active to enable use of catalyst loadings as low as 2.5 mol% (one equivalent corresponds to one poly-amino acid chain). 

Researchers turned their attention to applications of silica gel as a new electrode material. Silica gel, which has a three-dimensional structure with high specific surface area and is electroinactive in an aqueous medimn can be used as a support for electroactive species during their formation and/or enzymes by adsorption or entrapment [92,93]. Patel et al. recently reported application of poljwinyl ferrocene immobilized on silica gel particles to construct glucose sensors. Efficiency of carbon paste electrodes prepared with these polymeric electron mediators and GOx was comparable to electrodes constructed with other ferrocene based polymeric electron transfer systems. The fact that 70% of initial anodic current was retained after a month when electrodes were kept in the buffer at room temperature shows that polymerization of monomer vinylferrocene in the pores of silica gel and entrapping GOx in the matrix of poljwinyl ferrocene appears to have added stability to the sensors [94]. 

Analytical chromatographic separation of enantio-merie acids and N-substituted amino acids has been effeeted using immobilized a-chymotrypsin supported on aetivated siliea. Similarly, cellulase immobilized on silica gel ean bring about chromatographic... 

Irrespective of the reaction conditions used (/. e. ultrasound, microwave, changing reaction times, temperature and solvents), the maximum turnover number (TON) that was achieved was 75. In principle, second generation Grubbs-type initiators immobilized on non-porous silica should behave similar to those immobilized on monolithic supports[16]. In fact, catalysts immobilized onto monolithic supports give similar maximum TONs (< 65) in the absence of any chain transfer agent (CTA). Ruthenium measurements by means of ICP-OES revealed quantitative retention of the original amount of ruthenium at the support within experimental error ( 5%), thus otfering access to metal free products. 

The linear/branched ratio was 7.5 1, which was greater than that found with the control catalyst in solution. Rhodium(III) chloride has been immobilized on a support made by polymerization of vinylpyridine and divinylben-zene in the presence of silica. The best activity for the conversion of methanol to acetic acid by carbon monoxide was obtained after 20% of the pyridine groups were quaternized with methyl iodide. This suggests ionic bonding of a tetra-halorhodate ion to the polymer.211... 

The first example of the immobilization of a chiral ketone to promote the enan-tioselective epoxidation of alkenes with Oxone has been reported by Sartori and coworkers [322]. They anchored a-fhiorotropinone on KG-60 silica, MCM-41 and a Merrifield resin. The catalysts were tested for the epoxidation of 1-phenylcyclo-hexene but the polymer-supported fhiorotropinone 121 showed a low activity and selectivity. The catalyst immobilized on inorganic supports promoted the stereoselective epoxidation of alkenes with ee values up to 80% and could be reused with the same performance for three runs. 

Information about the secondary structures (a-helices, /5-sheets, random coil) can be useful for understanding conformation changes of proteins upon the immobilization process. More specifically, circular dichroism (CD) [70] and FT-IR spectroscopy [56, 58, 61, 71-73] have been applied to study the structural characteristics of various proteins adsorbed on mineral surfaces. Kondo and coworkers [70] have studied the modification in a-helix content of proteins adsorbed on ultrafine silica particles with CD and found a decrease upon immobilization. Circular dichroism is not usually used because this technique is applicable only for the study of enzymes immobilized on nano-sized mineral particles due to problems arising from light scattering effects. On the other hand, infrared spectroscopy does not suffer from light scattering perturbations and has thus been used for the study of the conformation of proteins when they are immobilized on solid supports [57, 58]. 

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