Catalysts polymer-tethered

To enable the attachment to a polymer, the original Gao-Noyori catalyst was tethered as shown in Eq. (2) and described in the literature [4]. As polymer backbone a polysiloxane polymer was chosen simply because the attachment of the monomer through its vinylic functionality by means of a hydrosilylation reaction could easily be achieved [Eq. (2)]. 

Supported catalysts involving palladium on carbon and dendrimer-encapsulated palladium and a polymer-supported phosphine palladium catalyst have facilitated C-C coupling reactions in SCCO2. Polymer-tethered substrates or amine bases have also been successfully used for the Mizoroki-Heck and Suzuki-Miyaura reactions in SCCO2. For example, REM resin underwent a Mizoroki-Heck reaction with iodobenzene to yield, after cleavage, ( )-methyl cinnamate 48 (74%) (Scheme 88). It is assumed that SCCO2 acts as a good solvent that swells the polymers and exposes reactive sites. 

Equation (8.1) is the continuity equation for the polymer Eq. (8.2) describes the evolution of v due to the BZ reaction and the transport of the catalyst with the movement of the polymer network Recall that the catalyst is tethered to the chains and, thus, does not diffuse through the solution. Equation (8.3) characterizes the changes in u due to the BZ reaction, the transport of this activator with the solvent, and the diffusion of the activator within the solvent We assumed for simplicity that it is solely the polymer-solvent interdiffusion that contributes to the gel dynamics hence, in Eq. (8.3), we took into account that < vfPi + (1 — )vf i = 0, where vf i is the solvent velocity [2]. 

REMP, the acronym for ring expansion metathesis polymerisation is a special case of ROMP, where the growing polymer chain stays attached to the catalyst at both ends nntil a macrocycle is released. This requires that the active carbene be tethered... 

Biological catalysts in the form of enzymes, cells, organelles, or synzymes that are tethered to a fixed bed, polymer, or other insoluble carrier or entrapped by a semi-impermeable membrane . Immobilization often confers added stability, permits reuse of the biocatalyst, and allows the development of flow reactors. The mode of immobilization may produce distinct populations of biocatalyst, each exhibiting different activities within the same sample. The study of immobilized enzymes can also provide insights into the chemical basis of enzyme latency, a well-known phenomenon characterized by the limited availability of active enzyme as a consequence of immobilization and/or encapsulization. 

In addition, Buchwald has reported polymer-bound versions of the biphenylyl dialkyl-phosphines. As shown in Chart 4, the ligand is tethered to Merrifield resin through a 2 -OH on the biphenylyl group. The scope for reactions was similar to that of homogeneous catalysts bearing these types of ligands. Catalysts were recycled four times starting with 2 mol %... 

Different conditions (including additives and solvent) for the reaction have been reported,often focusing on the palladium catalyst itself," or the ligand." Catalysts have been developed for deactivated aryl chlorides," and nickel catalysts have been used." Modifications to the basic procedure include tethering the aryl triflate or the boronic acid to a polymer, allowing a polymer-supported Suzuki reaction. Polymer-bound palladium complexes have also been used." " The reaction has been done neat on alumina," and on alumina with microwave irradiation." Suzuki coupling has also been done in ionic liquids," in supercritical... 

This section has focused on liquid—liquid biphasic catalysis in which catalysts are supported in different solvents to the substrates and products. Considerable efforts have also been directed to supporting homogeneous catalysts on solid supports including silica, alumina, and zeolites as well as functionalized dendrimers and polymers.33 It has also been found that synergic effects sometimes prevail between particles embedded in the support and the tethered molecular catalyst, increasing the activity of the catalyst.34... 

Zhao and Brittain also used the grafting from technique to grow polymers off silicon surfaces [329]. In their example, however, they first prepared tethered pSt cationically, which was terminated with a chlorine chain end, then used it as the initiator for the ATRP of MMA using the CuBr/PMDETA catalyst system. Use of a chlorine-terminated pSt chain in conjunction with a CuBr catalyst, however, produces a mismatch between the rates of cross-propagation and the rate of po-... 

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. 

The best results were obtained using 10 mole % of the catalyst in toluene. The results of work with N-methylephederine are summarized in Table 1. As the table shows, the reaction with unattached N-methylephedrine gave up to 82 % ee of the product, while lower ee were obtained when it was tethered to polymer and used for the same reactions. Clearly, this indicates that it does not provide the necessary cavity for the selective addition of diethylzinc to aldehyde. 

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