Applications of polymer-bound catalysts

Reagents and conditions i) L-(+)-diethyl tartrate. Ti(0-iPr)4, tBuOOH, molecular sieves 4A, CHjCt -20X 

Hydroxylation of benzene using a polymer-supported catalyst 

The direct formation of phenols from benzene has been studied with several transition metal complexes. However, the complexes are usually used in stoichiometric amounts and cannot be recycled. 

Although the reaction proceeded with only 30% conversion it offers some potential for further investigations. The catalyst could be recovered by simple filtration and re-used another ten times before deterioration was observed. 

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Besides these technologies, the application of polymer-bound or immobilized palladium catalysts has found widespread interest (see Sect. X.2). 

A similar strategy was developed by Kureshy et al. [45]. In 1998, Sherrington et al. [46] reported the application of polymer-bound Mn(lll)-salen catalyst in the epoxidation of 1-phenylcyclohex-l-ene, resulting in 49% yield and 91% ee, which are comparable to results obtained with the nonimmobilized Jacobsen catalyst. The synthesis of these catalysts started from polymer-bound salicy-laldehyde derivatives, which were first treated with 1,2-diamino cyclohexane and then with a second salicylaldehyde derivative. In the last step, the manganese complex was formed. Polymethacrylate was used as polymeric backbone. A similar approach was employed by Peukert and Jacobsen [47] to immobilize this catalyst to polystyrene. 

Support-bound transition metal complexes have mainly been prepared as insoluble catalysts. Table 4.1 lists representative examples of such polymer-bound complexes. Polystyrene-bound molybdenum carbonyl complexes have been prepared for the study of ligand substitution reactions and oxidative eliminations [51], Moreover, well-defined molybdenum, rhodium, and iridium phosphine complexes have been prepared on copolymers of PEG and silica [52]. Several reviews have covered the preparation and application of support-bound reagents, including transition metal complexes [53-59]. Examples of the preparation and uses of organomercury and organo-zinc compounds are discussed in Section 4.1. 

C. MacMillan, /. Am. Chem. Soc. 2000, 122, 4243-4244. For the synthesis of polymer-bound derivatives of the MacMillan catalyst and applications in Diels-Alder reactions, see M. Benaglia, G. Celentano,... 

In reactions with polymer-bound catalysts, a mass-transfer limitation often results in slowing down the rate of the reaction. To avoid this disadvantage, homogenous organic-soluble polymers have been utilized as catalyst supports. Oxazaborolidine 5, supported on linear polystyrene, was used as a soluble immobilized catalyst for the hydroboration of aromatic ketones in THF to afford chiral alcohols with an ee of up to 99% [40]. The catalyst was separated from the products with a nanofiltration membrane and then was used repeatedly. The total turnover number of the catalyst reached as high as 560. An intramolecularly cross-linked polymer molecule (microgel) was also applicable as a soluble support [41]. 

BQC is derived from quinine, which is a member of the cinchona family of alkaloids. Ammonium salts derived from quinidine, a diastereomer of (1) at the hydroxyl substituent, have been used less frequently in catalysis than BQC. Quini-dinium salts often give rise to products with enantioselectivity opposite to that from (1). Other related compounds, such as those derived from cinchonine and cinchonidine (which lack the methoxy substituent on the quinoline nucleus), have found application in organic synthesis. The cinchona alkaloids, as well as salt derivatives in which the benzyl group bears various substituents, have also been studied. Results from polymer-bound catalysts have not been promising. ... 

In 2002, Ley reported the application of resin-bound reagents and polymers towards the synthesis of carpanone [57]. In the final steps towards carpanone, a resin-bound Co(salen) catalyst was used to give the desired intermediate along with the formation of a small amount of aldehyde by-product. To remove this byproduct, a resin-bound tris-amine scavenger was used, yielding the desired product in high purity (Scheme 8.42). 

Catalysis with water-soluble polymer-bound catalysts in a single homogeneous aqueous phase, the subject to this section, can be of interest for the conversion of water-soluble organic substrates. With a view to applications, the use of water as a nonhazardous, environmentally benign solvent can be advantageous. 

Soluble polymer-bound catalysts can be expected to receive continued attention as they offer specific advantages. By comparison to aqueous two-phase catalysis, a range of substrates much broader with respect to their solubility can be employed. By comparison to heterogenization on solid supports, the selectivity and activity of homogeneous complexes can be retained better. However, it must also be noted that to date no system has been unambiguously proven to meet the stability and recovery efficiency required for industrial applications. 

The various properties of water in different aspects (being important for the reactivity, reaction kinetics or mechanisms, reaction engineering, or other concerns) are discussed elsewhere. The procedures for tailoring the water-solubility of the catalysts are many-sided and may be generalized much more easily than the corresponding methods for SHOP (cf. Section 7.1), fluorous phase (Section 7.2), supercritical solvents (Section 7.4), water-soluble polymer-bound catalysts (Section 7.6), or NAIL utilization (Section 7.3) no wonder that all other biphasic applications remain singular or are still just proposals. Both the scientific and industrial com-... 

Polymer-bound Catalysts Various quaternary phosphonium salts are known to be excellent catalysts for trans-esterification and condensation polymerizations, e.g. melt polymerization of polycarbonates and polyesters. The ionomers from Exxpro elastomer can be used in those applications with added advantages such as solubility in the melt, ease of recovery and thermal stability. 

Using soluble polymers under conditions where the products and polymer supports both remain in solution during the separation stage is a scheme that is uniquely applicable to soluble polymers. It is, for the most part, the scheme used in nature where enzyme catalysts are used and separation is based on size. To date, hquid/hquid separations remain a less common way of recovering soluble polymer-bound species. However, the earhest examples where soluble polymers were used in catalysis separated the solutions of soluble polymer-bound catalyst from the products with membranes [ 106,107]. While solid/Uq-uid separations (vide infra) stiU predominate, that situation may change as new separation strategies are invented and perfected or as new more durable and improved membranes are developed. Indeed, as can be seen from the discussion below, a variety of new and improved approaches have recently been developed where soluble polymer-bound catalysts are isolated as solutions. 

The applications reported for polymer-supported, soluble oxidation catalysts are the use of poly(vinylbenzyl)trimethylammonium chloride for the autooxidation of 2,6-di-tert-butylphenol [8], of copper polyaniline nanocomposites for the Wacker oxidation reaction [9], of cationic polymers containing cobalt(II) phthalocyanate for the autooxidation of 2-mercaptoethanol [10] and oxidation of olefins [11], of polymer-bound phthalocyanines for oxidative decomposition of polychlorophenols [12], and of a norbornene-based polymer with polymer-fixed manganese(IV) complexes for the catalytic oxidation of alkanes [13], Noncatalytic processes can also be found, such as the use of soluble polystyrene-based sulfoxide reagents for Swern oxidation [14], The reactions listed above will be described in more detail in the following paragraphs. 

In these cases, the polymer was used as an asymmetric support to induce the formation of optically pure product (cf. Worster et al., 1979). Few reports of the use of polymer-bound asymmetric reagents seem to exist in the literature. In this application, the reagent is used either to promote the asymmetric coupling of two groups or to add a group to a compound in an asymmetric manner. By far the largest number of applications have been those in which the polymer-bound asymmetric centers act as catalysts. Asymmetric catalysts, based on either amino acids or cinchona alkaloids, have been used to catalyze the Michael reaction in an... 

Although an enantioselective reaction carried out on solid phase, in principle, also includes the reactions wherein a chiral catalyst or a chiral auxiliary is polymer bound, in this chapter we have mainly focused on reactions wherein a substrate is bound to a solid support and the chemical transformation is carried out by chiral reagents. However, we have included few examples of employing solid-supported catalyst and auxiliaries to give readers an idea of these alternative synthesis strategies. Readers are, however, advised to refer to Refs 6 and 7 for a review on the applications of immobilized chiral catalysts and solid-phase-bound chiral auxiliaries, respectively, and Refs 1,5, and 8 for diverse solid-phase synthesis approaches directed toward small molecules and natural products. ... 

The highly important hydroformylation of olefins to aldehydes is a fruitful area for the development and application of new techniques. Photochemically initiated high-pressure formylation and the use of polymer-bound ruthenium catalysts are but two examples. The complex role of HCo(CO)4 in hydroformylation has been reviewed. Nickel(O) complexes catalyse an acetylation of aryl... 

Stable indefinitely in polymer-bound catalysts under phase transfer conditions that require the presence of hydroxide ion at the ion exchange site. Benzyltrialkylammonium and phosphonium ions are much less stable in base than non-benzylic tetraalkylammonium and phosphonium ions. Industrial applications of polystyrene-supported onium ion catalysts under strongly basic conditions will require catalysts such as 7, 1 rather than the usual commercially available... 

New developments in the Suzuki cross-coupling reaction include the application of microwave, polymer-bound catalysts, nanoparticles, and ionic liquids as reaction medium. A discussion of these methods exceeds the scope of this chapter. 

This chapter is divided into six subsections which cover the most prominent applications of the metathesis reaction in solid-phase organic chemistry RCM for cleavages from the resin synthesis of small rings to macrocycles on the resin via RCM the use of the metathesis reaction for dimerization of polymer-bound small molecules RCM to constrict peptide conformations (while this is formally an RCM, it is treated separately because of the unique considerations of peptide chemistry) CM between a polymer-bound alkene and an alkene in solution and ene-yne metathesis. As the field evolved relatively rapidly with major advances in the catalysts, each subsection is organized in chronological order for the sake of clarity. 

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