Gel-immobilized catalytic systems

These requirements are partially met by catalytic systems consisting of a polymer (gel) swelled in the reaction medium and permeable to reagents and reaction products, and in which transition metal complexes are immobilized. In this case, the reaction proceeds not only on the surface but within the whole bulk of catalyst particles. Therefore the effectivness of such active sites can be as high as during homogeneous catalysis. At the same time, the catalyst is readily separated from reaction products. Furthermore, under conditions which regulate polymer gel swelling, it is possible to control the catalytic process [139). 

Gel-immobilized catalytic systems (GCS) represent swelled polymer composites in which active sites of the particular metal complex are inunobilized. Graft copolymers of ethylene-propylene rubber (EPRu) and ligands of 4-vinylpyridine, acrylic acid, vinylpyrrolidone, organophosphorus compounds etc. act as a polymeric supports (polymeric phases) [140]. The structure of metal complex sites immobilized in a polymer gel is presented by the following scheme  

A hierarchical structure of GCS consists of three phases [143]. The first phase represents an elastomer base in which inclusions of phase 2 of the graft copolymer are dispersed. Isolated inclusions of phase 2 contain separate domains of the polymer 

CjHjAlClj (CjHjlzAlCl (C2Hj)3A1, C4H9AICI2 (C4H9)2A1C1 (C4H9)3A1 

The formation of active sites in GCS is probably associated with the necessity for vacating the initial ligand environment of a central ion. One likely mechanism of GCS activation is shown as follows  

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The results obtained show that immobilization of metal complexes in polymer gels allows to prepare physically heterogeneous and chemically homogeneous catalysts and leads to an important increase in their activity, selectivity and stability in the reactions of dimerization of lower olefins. The immobilization of the complexes opens new possibilities of macromolecular design of the catalysts with desired structural organization and will contribute to the development of general principles of synthesis of highly efficient and environmentally friendly catalytic systems for liquid phase processes. 

The Tamao-Kumada-Corriu reaction has also been carried out with supported catalytic systems. One interesting recent example has been the use of a multichannel microreactor to perform this reaction [169], The glass micro reactor was designed so as to increase the catalytic surface area and ensure a uniform distribution of the velocity/temperature field. The sol-gel procedure was used to immobilize the nickel catalyst to the channel walls. The Tamao-Kumada-Corriu reaction was conducted using bromobenzene and phenylmagnesium bromide, and was also carried out in the batch configuration for comparison, and it was observed that the reaction in-flow was four orders of magnitude more rapid than that performed under batch conditions and there was a threefold increase in the yield of the biaryl compound. 

The Heck reaction is another Pdcarbon-carbon bond forming process that is widely employed in organic synthesis and can occur in water. A recent example reported by Cacchi and coworkers was applied to the chemoenzymatic synthesis of (f ) -Rhododendrol (34) and other chiral alcohols (Scheme 4.17) (67]. To aid this work, perfluoro-tagged palladium nanoparticles (Pd p) immobilized on fluorous silica gel or through covalent bonding to silica were used as the catalytic systems. The Heck coupled product could be further treated with (R)-selective LbADH and 2-propanol to address the synthesis of (R)-Rhododendrol in 90% conversion and with 99% ee. 

MIP beads or microspheres are also widely used for sensing purposes [166]. They are prepared by precipitation polymerization and then they are embedded in a dedicated matrix, which is immobilized on the transducer surface. Moreover, the MIP beads are used to serve as stationary phases in HPLC [167] and for catalytic purposes. Other systems, such as self-assembled monolayers, SAMs [168], sol-gel matrices [169] and preformed polymers [170] have also been utilized for fabrication of MIP constructs. 

This cubic silicon-titanium g-oxo complex was also shown to be a model system for insoluble titanosilicates and related catalysts which have been used for epoxidation reactions of olefins. The cubic silicon-titanium g-oxo complex, when immobilized on a silica matrix by dissolving it in tetraethoxysilane and further treatment with acetic anhydride followed by heating up to 60 °C for 20 h, resulted in the formation of a Si02—Ti02 mixed oxide (63). The resulting solid was separated by filtration, and the filtrate formed a gel in about 1 week. This gel showed an enhanced catalytic activity (epoxidation yield of cyclohexene was 72%) as a solid catalysf for epoxidation of cyclohexene in the presence of TBHP in the liquid phase. 

A number of methods are being tested for enzyme immobilization. The method selected depends on the operating details of the enzyme system employed and the nature of the solvent to be used, which is usually water. Enzyme, or inactivated cells, may be encapsulated in a film, or encased in a gel, which is permeable to both the substrate and product, but not to enzyme [77]. Porous glasses or insoluble polymers such as a derivatized cellulose may be used as a support onto which enzyme is adsorbed. Pendant functional groups of a polymer, such as those of the ion-exchange resins, can be used either to ionically bind the enzyme to the resin active sites or to covalently bond the enzyme to the resin [79]. The enzyme may be bonded to a polymer backbone chain using a bifunctional monomer such as glutaraldehyde to react with enzyme sites that do not affect its catalytic activity [80]. 

Inspired by the seminal report of Nagel [34], which described a very active and selective Rh-pyrphos catalyst attached covalently to sihca gel, Pugin and colleagues have developed the modular toolbox which his depicted schematically in Figure 12.4. The main elements of their system are functionalized chiral diphosphines, where three different hnkers are based on isocyanate chemistry and various carriers [37, 45, 58]. This approach allows for a systematic and rapid access to a variety of immobilized chiral catalysts, with the possibility of adapting their catalytic and technical properties to specific needs. 

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