Immobilization systems inorganic support

In the MOF PIZA-3 (PIZA, porphyrinic Illinois zeolite analog), Mn(III) is found both in the porphyrin struts and as a structural metal node. The framework is structurally stable and is used for the oxidation of cycUc alkanes and alkenes with iodosylbenzene or peracetic acid as the oxidant [117]. Reaction is found to take place at the outer surface, which is justified by the authors by the unfavorable hydrophilic properties of the pore interior. Yields were similar to those obtained with homogeneous Mn(III) porphyrin systems or those immobilized inside inorganic supports as heterogeneous catalysts. Less than 0.1 mM of metalloporphyrin or degradation products were observed in the reaction mixtures, with no loss of oxidation activity observed in a second run when peracetic acid was used. 

Asymmetric epoxidation (AE) of unfunctionalized alkenes catalyzed by chiral (salen)Mn(III) complex 38 (Scheme 2.13), developed by Jacobsen et al., is one of the most reliable methods [50]. As shown in Table 2.2, several different strategies have been formulated to immobilize Jacobsen s catalysts on inorganic supports [37-42]. Facilitation of catalyst separation, catalyst reuse, an increase in catalyst stability (e.g. minimization of the possibility of formahon of inachve g-oxo-manganese(lV) species [51a,b]) and sometimes improvement in enanhoselectivity are the main objectives of such research. Heterogenized Mn(salen) systems have recently been reviewed by Salvador et al. [51c] and Garcia et al. [5 Id]. Some selected cases are therefore described herein on the basis of the immobilizahon methods. 

The principles and apphcations of SLM separation processes have been reviewed several times [4-7]. Briefly, in an SLM system an organic solvent is immobihzed in the pores of a porous polymer or inorganic support material by capillary forces, separating two aqueous solutions the feed (donor) and the strip (receiving, acceptor) phase (Fig. 3.1). The compounds are separated from the aqueous sample feed phase into an organic solvent immobilized in a support diffusing through the membrane phase, and then they are continuously back extracted to the other side of the membrane into the... 

V(L)Cl2(TpMs )] (L = N Bu L = O) were in situ supported onto SiC>2 and onto MAO and trimethylaluminum. All catalyst systems were shown to be active in ethylene polymerization. The systems were stable at different [A1]/[V] molar ratios and polymerization temperatures.21 Branched polyethylene/high-density polyethylene blends were prepared using the combined [NiChfa-diimine)] and V(T(Tp-vls )(N Bu) catalysts. The polymerization reactions were performed in hexane or toluene at three different polymerization temperatures (CPC, 30°C and 50°C) and several nickel molar fractions, using MAO as cocatalyst.22 TpMs- and TpMs -imido vanadium (V) were immobilized onto a series of inorganic supports All the systems were shown to be active in ethylene polymerization in the presence of MAO or TiBA/MAO mixture.23... 

Of the inorganic supports, best results were reported for a mesoporous MCM-41 [337]. Support on ionic-liquid phases has been studied by different groups with variable results [338, 339], Of the non-conventional organic polymers, non-covalent immobilization on poly(diallyldimethylammonium) is notable [340], Catalysts 133 (15 mol.%) promoted the aldol reaction of acetone and benzaldehydes to afford the corresponding (i-hydroxyketones in 50-98% yields and 62-72% ee, which are clearly lower than those reported for other polymer-supported systems. Recycling of the catalysts was possible at least six times without loss of efficiency. More recently, proline has been attached to one DNA strand while an aldehyde was tethered to a complementary DNA sequence and made to react with a non-tethered ketone [341], To date, the work has focused more on conceptual development than on the analysis of its practical applications in organic synthesis. 

Numerous attempts have been made to immobilize rhodium or its complexes on polymeric or on inorganic supports. Immobilization without ligands leads to systems that mainly hydrogenate the alkene [13]. Recently van Leeuwen and coworkers described the immobilization ofRh - Xantphos complexes in silicagel [14]. The system can be recycled 8 times without significant loss ofRh or activity, but the reaction rate is very low. 

These silica immobilized systems prepared via the sol-gel method are very promising organic-inorganic hybrid materials. The chemistry takes place at the interface of the materials suppressing the detrimental influence of the support [46c]. A series of network modifiers for the sol-gel process are available, which can be used to optimize the support. This tool to further improve the catalyst performance already proved to be valuable for several reactions [46c], For the hydroformylation reaction this still needs to be explored. 

The nanoparticles prepared in the reverse micellar systems need to be recovered from reverse micelles and immobilized onto stable supports [122, 123]. There have been several reports relating to the immobilization of nanoparticles from reverse micellar systems onto supports made from inorganic and organic materials. For example, dithiol molecules can also be used for the immobilization of metal and semiconductor nanoparticles. Colvin et al. have demonstrated the immobilization of CdS nanoparticles, prepared in the reverse micelles, onto the self-assembled dithiol monolayers on an Au substrate [124]. This approach was used to prepare an Au-CdS nanoparticle multilayer [125] and a CdS nanoparticle multiplayer on Au substrate [126]. Nakanishio et al. demonstrated the layer-by-layer self-assembly of CdS and ZnS nanoparticles on Au substrate, from a reverse micellar solution, using dithiol molecules [127, 128]. In these methods, the preparation of the CdS nanoparticles is completely separated from the immobilization procedure and hence the chemical and physical properties of the nanoparticles can be well controlled when using an appropriate reaction medium such as a reverese micellar system. 

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