Immobilization, polymer-supported

Polymer supported reagents, catalysts, protecting groups, and mediators can be used in place of the corresponding small molecule materials (Sherrington, 1991 Sundell and Nasman, 1993). The reactive species is tightly bound to a macromolecular support which immobilizes it. This generally makes toxic, noxious, or corrosive materials much safer. The use of polystyrene sulfonic acid catalyst for the manufacture of methyl r-butyl... 

Polymer-supported catalysts often have lower activities than the soluble catalysts because of the intraparticle diffusion resistance. In this case the immobilization of the complexes on colloidal polymers can increase the catalytic activity. Catalysts bound to polymer latexes were used in oxidation reactions, such as the Cu-catalyzed oxidation of ascorbic acid,12 the Co-catalyzed oxidation of tetralin,13 and the CoPc-catalyzed oxidation of butylphenol14 and thiols.1516 Mn(III)-porphyrin bound to colloidal anion exchange resin was... 

The insoluble polymer-supported Rh complexes were the first immobilized chiral catalysts.174,175 In most cases, however, the immobilization of chiral complexes caused severe reduction of the catalytic activity. Only a few investigations of possible causes have been made. The pore size of the insoluble support and the solvent may play important roles. Polymer-bound chiral Mn(III)Salen complexes were also used for asymmetric epoxidation of unfunctionalized olefins.176,177... 

The most well-developed recent examples of catalysis concern catalysts for oxidation reactions these are essentially achiral or chiral metal-salen complexes. Taking into account a number of results suggesting the importance of a degree of mobility of the bound complex, Sherrington et al. synthesized a series of polymer-supported complexes in which [Mn(salen)Cl] units are immobilized in a pendant fashion by only one of the aromatic rings, to polystyrene or poly(methacrylate) resin beads of various morphology (Figure 6).78,79... 

Figure 6 Immobilization of [Mn(salen)Cl] derivatives in a pendant fashion (P — polymer support).

A variation of this method led to the generation of bis-benzimidazoles [81, 82], The versatile immobilized ortho-phenylenediamine template was prepared as described above in several microwave-mediated steps. Additional N-acylation exclusively at the primary aromatic amine moiety was achieved utilizing the initially used 4-fluoro-3-nitrobenzoic acid at room temperature (Scheme 7.72). Various amines were used to introduce diversity through nucleophilic aromatic substitution. Cyclization to the polymer-bound benzimidazole was achieved by refluxing for several hours in a mixture of trifluoroacetic acid and chloroform. Individual steps at ambient temperature for selective reduction, cyclization with several aldehydes, and final detachment from the polymer support were necessary in order to obtain the desired bis-benzimidazoles. A set of 13 examples was prepared in high yields and good purities [81]. 

Lithium ester enolate addition to imines has been used for the construction of optically active p-lactams, e.g. 64 and the lithium enolates have been found to be superior to other metal derivatives for both yields and diastereoselectivity in some cases <00H(53)1479>. Immobilized lithium ester enolates have been utilized for the first time <00OL907> and soluble polymer supported imines were used to obtain N-unsubstituted azetidin-2-ones under mild conditions <00CEJ193>. Both lithium and titanium enolates have been employed to obtain cholesterol absorption inhibitors <99TA4841>. Lithium ynolates 65 add to imines to provide P-lactams in good to excellent yield <00TL5943>. 

The cyclopentadienyl group is another interesting ligand for immobilization. Its titanium complexes can be transformed by reduction with butyl lithium into highly active alkene hydrogenation catalysts having a TOF of about 7000 h 1 at 60 °C [85]. Similar metallocene catalysts have also been extensively studied on polymer supports, as shown in the following section. 

Fig. 42.13 Immobilization of cyclopentadiene for the preparation of polymer-supported titanocene complex. 

The polymer-supported chiral phosphine obtained (Fig. 42.15) was treated with an Rh precursor and used for the enantioselective hydrogenation of dehydroamino acid derivatives. The obtained catalyst gave up to 82% ee, albeit with still low activity. Stille has developed this immobilization technique further by even more careful tuning of the polarity of the support with that of the reaction medium. For example, he introduced DIOP to a monomer vinylbenzalde-hyde in reactions analogous to those shown for the polymer in Figure 42.11. 

S. Kobayashi and R. Akiyama, Renaissance of immobilized catalysts. New types of polymer-supported catalysts, micro-encapsulated catalysts, which enable environmentally benign and powerful high-throughput organic synthesis, Chem. Commun., 2003, 449. 

Only recently a selective crossed metathesis between terminal alkenes and terminal alkynes has been described using the same catalyst.6 Allyltrimethylsilane proved to be a suitable alkene component for this reaction. Therefore, the concept of immobilizing terminal olefins onto polymer-supported allylsilane was extended to the binding of terminal alkynes. A series of structurally diverse terminal alkynes was reacted with 1 in the presence of catalytic amounts of Ru.7 The resulting polymer-bound dienes 3 are subject to protodesilylation (1.5% TFA) via a conjugate mechanism resulting in the formation of products of type 6 (Table 13.3). Mixtures of E- and Z-isomers (E/Z = 8 1 -1 1) are formed. The identity of the dominating E-isomer was established by NOE analysis. 

Ohtaki, M., Komiyama, M., Hirai, H., and Toshima, N., Effects of polymer support on the substrate selectivity of covalently immobilized ultrafme rhodium particles as a catalyst for olefrn hydrogenation, Macromolecules, 24, 5567, 1991. 

With a view to producing catalysts that can easily be removed from reaction products, typical phase-transfer catalysts such as onium salts, crown ethers, and cryptands have been immobilized on polymer supports. The use of such catalysts in liquid-liquid and liquid-solid two-phase systems has been described as triphase catalysis (Regen, 1975, 1977). Cinquini et al. (1976) have compared the activities of catalysts consisting of ligands bound to chloromethylated polystyrene cross-linked with 2 or 4% divinylbenzene and having different densities of catalytic sites ([126], [127], [ 132]—[ 135]) in the... 

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