Catalysts systems inorganic-supported

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 results for these three catalyst systems clearly show that the confinement effects exerted by the inorganic support are crucial to the enantiodiscrimination displayed by the catalyst this is represented in Figure 5.10, which shows the interactions between the incoming substrate, the support wall and the chiral catalyst. This graphical representahon of the steric interactions experienced by the substrate gives some indication of the reduction in the degrees of freedom available to the substrate (especially bulky substrates) when interacting with the catalyst. 

The discovery of a zirconium-based catalyst able to promote polyolefin depolymerization encourages the search for more electrophilic catalytic systems that could be obtained either by changing the metal center or the inorganic support. 

In terms of availability, number, and nature of surface groups, surface area, pore size, pore volume, and form and size of the particles, silica has been undoubtedly the most preferred inorganic support. Suitable modification is possible via the surface silanol groups, which can react either directly with an appropriate metal complex or with an intermediate ligand group. Direct surface bonding has often been practiced, e. g., for the anchoring of metal carbonyl complexes [14] (eq. (11)), carbonyl clusters [26], polymerization catalysts [21, 62], or other special systems, e. g., 7r-allyl complexes [63] or metalloporphyrins [64]. 

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. 

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