Preparation of Polymer-supported Catalysts

Preparation of Polymer-supported Catalysts. Three principal methods of attaching metals to complexes exist  

Functionalization through phosphine moieties has been preferred as they form strong bonds with Rh. However other donor ligands have also been used for immobilizing metal complexes. For example, styrene-divinylbenzene 

Scurrell, in Catalysis ed. C. Kemball and D. A. Dowden (Specialist Periodical Reports), The Chemical Society, London, 1978, Vol. 2, p. 215. 

Poly(vinyl pyridine) has been used as a support for the preparation of a variety of Rh and Co immobilized systems and [Rh2Cl2(CO)4] has been attached to poly(benzimidazole).  

This method has been used by Stille and co-workers to incorporate 2(p-styryl)-4,5-bis(tosyloxy)methyl-l, 3-dioxolane (11) into styrene-divinylbenzene polymers. The tosylate groups were then converted to the polymer equivalent of optically active 4,5-bis[diphenylphosphinomethyl]- 

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Chiral aminoalcohols have been widely used for the preparation of polymer-supported catalysts and reagents [8, 9]. Polymer-bound aminoalcohols can be easily obtained from simple... 

The second general method, IMPR, for the preparation of polymer supported metal catalysts is much less popular. In spite of this, microencapsulation of palladium in a polyurea matrix, generated by interfacial polymerization of isocyanate oligomers in the presence of palladium acetate [128], proved to be very effective in the production of the EnCat catalysts (Scheme 3). In this case, the formation of the polymer matrix implies only hydrolysis-condensation processes, and is therefore much more compatible with the presence of a transition metal compound. That is why palladium(II) survives the microencapsulation reaction... 

The preparation of polymer-supported iridium catalysts (61) and (62) for the stereoselective isomerization of double bonds using polystyrene based immobilized triphenyl phosphine were recently reported by Ley and coworkers (Fig. 4.5). The immobilized catalyst is potentially useful for deprotection strategies of aUyl ethers [130]. 

Preparation of polymer-supported ru-tsdpen catalysts and their use... 

PREPARATION OF POLYMER-SUPPORTED RU-TSDPEN CATALYSTS AND THEIR USE FOR ENANTIOSELECTIVE SYNTHESIS OF (5)-FLUOXETlNE... 

M. Moghadam, S. Tangestaninejad, M. H. Habibi, V. Mirkhani, A convenient preparation of polymer-supported manganese porphyrin and its use as hydrocarbon monooxygenation catalyst, /. Mol. Catal. A 217 (2004) 9. 

The final step in the preparation of polymer-supported metal nanoparticles is the generation of the nanoparticles within the polymer, which is usually accomplished by reduction of the polymer-bound metal precursors. Often, techniques similar to the preparation of conventional metal catalysts supported on inorganic solids are employed. 

Abstract. Three types of polymer-supported rare earth catalysts, Nafion-based rare earth catalysts, polyacrylonitrile-based rare earth catalysts, and microencapsulated Lewis acids, are discussed. Use of polymer-supported catalysts offers several advantages in preparative procedures such as simplification of product work-up, separation, and isolation, as well as the reuse of the catalyst including flow reaction systems leading to economical automation processes. Although the use of immobilized homogeneous catalysts is of continuing interest, few successful examples are known for polymer-supported Lewis acids. The unique characteristics of rare earth Lewis acids have been utilized, and efficient polymer-supported Lewis acids, which combine the advantages of immobilized catalysis and Lewis acid-mediated reactions, have been developed. 

The number and diversity of transition metal phosphine complexes is vast and a wide range have been used as catalysts for synthetic organic transformations for many years. It therefore comes as little surprise that the preparation of polymer-supported metal phosphine complexes and assessment of their catalytic activity has attracted much attention. Supported phosphine ligands and their metal complexes prepared from 1981 to 2001 can be found in a review published in 2002. Discussed here are examples in the literature from 1996 to the present together with a selected number of those from 1981 to 1996 where particularly notable synthetic methods have been used or where key points should be raised. 

Scheme 5.6-4 Preparation of polymer-supported imidazolium salt catalysts, where DVB = divinylbenzene and MX = NaBp4 or KOTf [80].

In the same way as many other reactions, there are advantages in using a chiral ligand that is attached to a polymer, as the ligand can normally be recovered by simple filtration. This makes for easier purification of the product and the possibility to recycle the catalyst. On the other hand, this approach typically suffers from the need to prepare the polymer-supported catalyst, the difficulty in its characterization and, crucially, often a reduction in the selectivity of the desired reaction. 

The preparation of a supported catalyst from crystalline MgCl2 was described by J.C.W.Chien, J.C.Wu, C.I.Cuo, J.Polym.Sci.Polym.Chem.Ed., 20,2019(1982) the preparation of a higher activity catalyst from soluble MgCl2 alcoholates is given in ref. 3b. 

Allylic alcohols can be converted to epoxy-alcohols with tert-butylhydroperoxide on molecular sieves, or with peroxy acids. Epoxidation of allylic alcohols can also be done with high enantioselectivity. In the Sharpless asymmetric epoxidation,allylic alcohols are converted to optically active epoxides in better than 90% ee, by treatment with r-BuOOH, titanium tetraisopropoxide and optically active diethyl tartrate. The Ti(OCHMe2)4 and diethyl tartrate can be present in catalytic amounts (15-lOmol %) if molecular sieves are present. Polymer-supported catalysts have also been reported. Since both (-t-) and ( —) diethyl tartrate are readily available, and the reaction is stereospecific, either enantiomer of the product can be prepared. The method has been successful for a wide range of primary allylic alcohols, where the double bond is mono-, di-, tri-, and tetrasubstituted. This procedure, in which an optically active catalyst is used to induce asymmetry, has proved to be one of the most important methods of asymmetric synthesis, and has been used to prepare a large number of optically active natural products and other compounds. The mechanism of the Sharpless epoxidation is believed to involve attack on the substrate by a compound formed from the titanium alkoxide and the diethyl tartrate to produce a complex that also contains the substrate and the r-BuOOH. ... 

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