Polymer-supported catalysts, example

Examples of the Michael-type addition of carbanions, derived from activated methylene compounds, with electron-deficient alkenes under phase-transfer catalytic conditions have been reported [e.g. 1-17] (Table 6.16). Although the basic conditions are normally provided by sodium hydroxide or potassium carbonate, fluoride and cyanide salts have also been used [e.g. 1, 12-14]. Soliddiquid two-phase systems, with or without added organic solvent [e.g. 15-18] and polymer-supported catalysts [11] have been employed, as well as normal liquiddiquid conditions. The micellar ammonium catalysts have also been used, e.g. for the condensation of p-dicarbonyl compounds with but-3-en-2-one [19], and they are reported to be superior to tetra-n-butylammonium bromide at low base concentrations. 

Selected examples of the synthesis of aldehydes using polymer-supported catalyst... 

Note 2 Typical examples of polymer-supported reactions are (a) reactions performed by use of polymer-supported catalysts, (b) solid-phase peptide synthesis, in which 240... 

Note 2 Examples of polymer-supported catalysts are (a) a polymer-metal complex that can coordinate reactants, (b) colloidal palladium dispersed in a swollen network polymer that can act as a hydrogenation catalyst. 

Anchoring the catalyst to polymeric materials has some advantages in easy product separation and catalyst recovery for recycling. The first example of a polymer-supported rhodium catalyst for hydroformylation was reported in 1975. Since then, many reports have been published on polymer-supported catalysts here, we focus on examples of normal-sc QcxiY or enantioselective hydroformylation. 

Ruvn—>RuIV) the fact that it selectively oxidizes cyclobutanol to cyclobutanone and ferf-Bu(Ph)CHOH to the corresponding ketone, militates against free-radical intermediates and is consistent with a heterolytic, two-electron oxidation [103, 104]. Presumably, the key step involved /1-hydride elimination from a high-valent, for example, alkoxyruthenium(VII), intermediate followed by reoxidation of the lower-valent ruthenium by dioxygen. However, as shown in Fig. 18, if this involved the Ru(VII)/Ru( V) couple the reoxidation would require the close proximity of two ruthenium centres, which would seem unlikely in a polymer-supported catalyst. A plausible alternative, which can occur at an isolated ruthenium centre, involves the oxidation of a second molecule of alcohol, resulting in the reduction of ruthenium(V) to ruthenium(III), followed by reoxidation of the latter to ruthenium(VII) by dioxygen (Fig. 18). 

The direction of addition (syn or anti) with amine nucleophiles has been tested and fonnd to be dependent on the reaction conditions. The sitnation is clearest with the homochiral cyclohexane derivative (Scheme 15). In this example, a simple Pd catalyst gives a mixture of isomers owing to competitive syn and anti addition, while the polymer-supported catalyst produces only the (usual) anti addition. ... 

Finally, the last few years have seen the first examples of the use of molecular-imprinted, polymer-supported catalysts for achieving product selectivity. The imprinted cavities are tailored in such a way that the course of a chemical reaction is directed towards one of the possible products. In the previous section it has already been shown that molecularly imprinted polymers used as microreactors are able to impart to a given reaction a different regio- and stereo-selectivity with respect to the same reaction in solution. Attempts towards an imprinted enantio-selective catalyst were reported by Gamez and co-workers who employed as template monomer an optically active, polymerisable ruthenium complex bearing in its coordination sphere an enantiomerically pure alkoxide [121]. After polymerisation, the alkoxide was split off and the resulting polymer-supported catalyst was used for enantio-selective hydride transfer reductions. The obtained selectivity was higher than for a polymer prepared without the optically active alkoxide but lower than for the same ruthenium complex in solution. 

An example of a polymer-supported catalyst was produced from a tailor-made resin based on N,N-dimelhylacrylamide with 4 mol% methylene bis(acrylamide) as the cross-linker and 12 mol% methacrylic acid as the functional, metal binding comonomer. Treatment of the resin with a solution of Cu(OAc)2 in methanol resulted in a ligand exchange reaction with partial substitution of the acetates with polymer-bound carboxylate groups (Scheme 11.3) [4], The use of the catalyst is discussed further below. 

The catalyst activity decreased with increasing polymer crystallinity. A high regioselectivity of the catalyst in the hydrosilylation of alkenes towards formation of the linear products was achieved due to the favorable microporous structure of the polyamide supports with pore size of 10-20. The stereoselectivity of the reaction can be reversed by a proper choice of donor functions in a polymer support, for example the traditional cis-selectivity of Rh catalysts in hydrosilylation of phen-ylacetylene was changed to trans-selectivity by use of a 2,5-py instead of a 2,6-py moiety. The polyamide-supported catalysts showed high stability through 6-9 synthesis runs [25]. 

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 vast area of polymer-supported catalysts is too broad to cover systematically in this chapter and readers are referred to Chapter 12.14. In addition, as the loadings of metal ions on the polymers are generally very low, the cited reviews are a more appropriate source of information. We have already discussed supported catalytically active metallocene complexes in Section 2.1. Some illustrative examples of the types of transformations that have been achieved are shown in Table 1. 

Polymer supported catalysts have advantages because of the ease of catalyst recovery and the opportunity for simultaneously using otherwise incompatible catalytic systems. Indeed, the immobilization of several catalysts onto a polymer matrix is a unique way of avoiding antagonistic reactions between them, and of lowing reagents to participate in a cascade of reactive processes. For example, polymer-supported catalysts have been used as the Lewis acid catalysts in the carbocationic polymerization of isobutylene. After the reaction, polyisobutylene is obtained by simply filtering the supported catalyst. The reaction cycle can be repeated many times. 

The products of these reactions form the basis for an entire methodology—polymer-supported chemical reactions—wherein the modified polystyrene serves as a reactant, reagent, or catalyst. The reactions are the usual ones of organic chemistry. In the following equation, for example, the modified polystyrene serves as a phase-transfer catalyst (see Section 21.5). The main advantage of using a polymer-supported reagent, or in this case a polymer-supported catalyst, is that it makes isolation of the reaction product easier. 

The three major catalytic reduction procedures which have emerged are enantioselective hydride reduction, hydrogenation and hydrogenation transfer reduction (HTR) close to the hydrosilylation which represent only few examples in asymmetric polymer-supported catalyst. 

Figure 9.2 Examples of polymer-supported catalysts and ligands 27-29 [107-109].

Dendrimer- and polymer-supported catalysts can be employed in the ATH of sultam precursors [71, 72], including examples where the supporting material has sulfonyl groups and therefore assists reactions in water [73]. The synthesis of an amphiphilic polystyrene-type immobilized TsDPEN ligand and its application in ATH of cyclic sulfonimines has been reported (Fig. 16) [74]. 

---