Poly -bound catalysts

Starting from the commercially available aldehyde (12), styrene (13) was prepared by a straightforward synthetic sequence (Scheme 11.3). Subsequent esterification of the phenol with succinate-derivatized poly(ethylene glycol) monomethyl ether (MeO-PEG) appended the styrene unit to approximately 50% of the free acid groups in (14). The loading in (15) was estimated by 500 MHz NMR spectroscopy to be about 0.1 mmol g . In a final step, the polymer-bound catalyst was ob-... 

Continuous homogeneous catalysis is achieved by membrane filtration, which separates the polymeric catalyst from low molecular weight solvent and products. Hydrogenation of 1-pentene with the soluble pofymer-attached Wilkinson catalyst affords n-pentane in quantitative yield A variety of other catalysts have been attached to functionalized polystyrenes Besides linear polystyrenes, poly(ethylene glycol)s, polyvinylpyrrolidinones and poly(vinyl chloride)s have been used for the liquid-phase catalysis. Instead of membrane filtration for separating the polymer-bound catalyst, selective precipitation has been found to be very effective. In all... 

In contrast to the PAMAM-bound catalysts, chiral dendrimers based on rigid poly(phenylethyne) and flexible carbosilane backbones without heteroatoms proved to be highly efficient catalysts in such alkylation reactions. Excellent enantioselectivities in the asymmetric addition of dialkylzincs to aldehydes (up to 93% ee) and N-diphenylphosphinyli mines (up to 94% ee) were achieved. 

Polyethylene-supported catalysts that are initially insoluble but that become soluble on heating and are separated as insoluble materials on cooling are also used as catalysts in polymerization reactions. Infact, this was the first way a polyethylene-bound catalyst was used (Eq. 11) [34]. However, soluble polymers used in this manner appear to have several deficiencies. First, separation of the products from the catalyst may notalways be as simple as was the case with catalysts like 11 or 12 and low molecular weight products. For example, while a hot solution of a polyethylene-bound neodymium salt was successfully used in the stereoselective polymerization of butadiene to form high molecular weight (Z)-poly( 1,4-butadiene), the product mixture after cooling was a thick, viscous sus-... 

Karakhanov s group has also been exploring poly(ethylene oxide)- and poly(alkene oxide)-copolymer-bound catalysts [99-102]. A notable aspect of this work is the design of polyethers like 39 that contain jS-cyclodextrins and calyx[4]- and calyx[6]arenes. Such polyethers couple the molecular recognition associated with these macrocycles with the catalytic activity of acac, phosphine, dipyridyl, and catechol ligands. Metals complexed to such ligands have been used in reactions like hydroformylation, Wacker oxidations, and arene hy-droxylation. 

With supported Mn(III)-salen complexes [27], the use of polymer-bound catalysts for the asymmetric epoxidation of olefins is possible, allowing once more the easy recovery of the catalyst by precipitation with a suitable solvent [28]. Poly(ethylene... 

Poly(A alkyl acrylamide)s and poly(7V-isopropylacrylamide) in particular are the other type of LCST polymers our group has studied. Poly(iV-isopropylacrylamide) is soluble below 31 C in water but insoluble above that temperature. Our group has used this temperature induced phase change has been used as a way to isolate, recover and reuse water-soluble polymer-bound catalysts. It is also a way to make a smart catalysts, catalysts that can turn off an exothermic reaction without external temperature control. Such on/off behavior is seen for both catalysts and substrates. 

The use of soluble polymers as catalysts was also explored by Bayer. His group showed that both diphenylphosphinated polystyrene and diphenylphosphinated poly(ethylene glycol) could be used as recoverable, reusable hydroformylation catalysts. Separation of the catalyst and the reaction products in these cases was achieved by taking advantage of the properties of the polymer chain. Solvent precipitation or membrane filtration both proved to be acceptable techniques to isolate products free from the polymer-bound catalyst. 

The isomerization of quadricyclene is catalysed by poly(styryl)bipyridine-palladium(o) and is ca. 30-fold as active as 10% Pd/C. Some other work in the development of polymer-bound catalysts of the photochemical reactions of quadricyclene has been reported. This is of interest as a model system for solar energy storage. 

Atactic poly(methallyl)alcohol reacts with PClPha to give a polyphosphite Pn, and on treatment with [Rh2(Cl)2(CO)4] or [RhaCCOaCnorbornadienela] polymer-bound catalysts are obtained which are active in the hydrogenation of a variety of olefins. The species derived from the norbornadiene complex catalyses oct-l-ene hydrogenation at a rate dependent on catalyst concentration with solutions 0.125 mol 1 in olefin and at a rate oc [olefin] / up to a concentration of 0.3 mol 1. No attempt is made to rationalize these results in mechanistic terms. 

Water (10% by volume) has to be added to the DMA phase in order to suppress the miscibility-increasing property of the poly(ethylene glycol) based catalyst 45. In the case of the PNIPAM-bound catalyst 45 the water content in the DMA phase prevented the precipitation of the catalyst while the reaction mixture is hot [increase of the lower critical solution temperature (LCST) above the reaction temperature of 95 °C]. 

In practice, 1—10 mol % of catalyst are used most of the time. Regeneration of the catalyst is often possible if deemed necessary. Some authors have advocated systems in which the catalyst is bound to a polymer matrix (triphase-catalysis). Here separation and generation of the catalyst is easy, but swelling, mixing, and diffusion problems are not always easy to solve. Furthermore, triphase-catalyst decomposition is a serious problem unless the active groups are crowns or poly(ethylene glycol)s. Commercial anion exchange resins are not useful as PT catalysts in many cases. 

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