Water-soluble polymer-bound catalysts

Recently a rhodium water-soluble polymer-bound catalyst, based on the commercially available copolymer of maleic anhydride and methyl vinyl ether, was shown to be very active in the hydrogenation of various substrates in basic aqueous media [25]. 

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. 

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-... 

Last but not least, the success of aqueous-phase catalysis has drawn the interest of the homogeneous-catalysis community to other biphasic possibilities such as or-ganic/organic separations, fluorous phases, nonaqueous ionic liquids, supercritical solvents, amphiphilic compounds, or water-soluble, polymer-bound catalysts. As in the field of aqueous-phase catalysis, the first textbooks on these developments have been published, not to mention Job s book on Aqueous Organometallic Catalysis which followed three years after our own publication and which put the spotlight on Job s special merits as one of the pioneers in aqueous biphasic catalysis. Up to now, most of the alternatives mentioned are only in a state of intensive development (except for one industrial realization that of Swan/Chematur for hydrogenations in scC02 [2]) but other pilot plant adaptations and even technical operations may be expected in the near future. 

Water-soluble polymer-bound catalysts represent an interesting alternative [86], in particular when they are attached to smart polymers, which can undergo a complete... 

Poly(A alkyl acrylamide)s and poly(7V-isopropylacrylamide) in particular are the other type of LCST polymers our group has studied. Poly(iV-isopropylacrylamide) is soluble below 31 C in water but insoluble above that temperature. Our group has used this temperature induced phase change has been used as a way to isolate, recover and reuse water-soluble polymer-bound catalysts. It is also a way to make a smart catalysts, catalysts that can turn off an exothermic reaction without external temperature control. Such on/off behavior is seen for both catalysts and substrates. 

A first application using ferroceneboronic acid as mediator [45] was described for the transformation of p-hydroxy toluene to p-hydroxy benzaldehyde which is catalyzed by the enzyme p-cresolmethyl hydroxylase (PCMH) from Pseudomonas putida. This enzyme is a flavocytochrome containing two FAD and two cytochrome c prosthetic groups. To develop a continuous process using ultrafiltration membranes to retain the enzyme and the mediator, water soluble polymer-bound ferrocenes [50] such as compounds 3-7 have been applied as redox catalysts for the application in batch electrolyses (Fig. 12) or in combination with an electrochemical enzyme membrane reactor (Fig. 13) [46, 50] with excellent results. 

Bergbreiter, D. E. Liu, Y-S. Water-Soluble Polymer-Bound, Recoverable Palladium(0)-Phosphine Catalysts, Tetrahedron Lett. 1997, 38, 7843. 

A water-soluble polymer-bound Pd(0)-phosphine catalyst was also efficiently used in aqueous or mixed aqueous/organic media, the catalyst being recycled by... 

Water-soluble polymer-bound Pd(0)-phosphine catalyst has also been efficiently used in aqueous or mixed aqueous/organic media, the catalyst being recycled by solvent or thermal preparation methods [17]. Amphiphilic resin-supported palladium-phosphine complexes show high catalytic activity in allylic substitution reactions of various allylic acetates with different nucleophiles in aqueous media [18, 19]. Enantiomeric excess up to 98% is obtained using amphiphilic resin-supported MOP ligand or resin-supported P,N-chelating palladium complexes, the catalyst being recyclable [20,21]. The catalyst could be recovered by simple filtration and re-used without any loss of activity and enantioselectivity. 

Substitution of allylic alcohols with active methylene compounds as a suspension in water was achieved in excellent yields using Pd(PPhj)4 in the presence of a carboxylic add such as l-adamantanecarboxyUc add. Bergbreiter and Liu used a water-soluble, polymer-bound paUadium/phosphine catalyst in water or in mixed aqueous/organic solvents. This polymeric catalyst showed high activity in nucleophilic allylic substitution affording high yields of coupled products, and it could be recycled by solvent or thermal precipitation. 

Bergbreiter, D.E. and Liu, Y.-S., Water soluble polymer-bound, recoverable palladium(0)-phosphine catalysts, Tetrahedron Lett., 1997, 38, 7843-7846. 

Abstract. Water-soluble polymers of acrylamide and acrylic acid with high extent ( 90%) of Ceo consumption are obtained by technique of low-temperature radiation living radical polymerization. In absorption spectra of these copolymers one can see gradually descended unstructured absorption in the range 240-700 mn, characteristic for fullerene covalent-bound or its klasters. The way of radiation initiation is used to obtain the products of high purity, because it is not necessary to embed into the system any initiators or catalyst. Latter is very important in the case of synthesis of polymers for medical purposes. Also at radiation initiation a rate of initiation reaction does not depend on the temperature and the sterilization of products takes place simultaneously. 

The inverse temperature-dependent solubility in aqueous media of polymer-bound palladium(0)-phosphine catalysts, based on the water-soluble polymer poly(Wisopropyl)acrylamide (PNIPAM) 28, was also used to recycle and reuse these catalysts in nucleophilic allylic substitutions (Equation (8)) and cross-coupling reactions between aryl iodides and terminal alkynes (Equation (9)). The catalyst was highly active in both reactions, and it was recycled 10 times with an average yield of 93% in the allylic nucleophilic substitution by precipitation with hexane. ... 

Poly(ethylene oxide)s are the only water-soluble polymers which can be terminally functionalized and from which we can obtain complexes bound to the polymer tail. Thereby, several problems encountered in producing conventional polymer-immobilized catalysts can be obviated. The metal complexes synthesized retain the properties of, on the one hand, homogeneous low molecular weight metal complexes, and on the other, poly(ethylene oxide)s or ethylene oxide-propylene oxide block copol)miers. Among these properties are, first of all, water solubility and also the ability to concentrate nonpolar substances in polymer globules or micelles formed by polymer ligands. 

Several microwave-assisted protocols for soluble polymer-supported syntheses have been described. Among the first examples of so-called liquid-phase synthesis were aqueous Suzuki couplings. Schotten and coworkers presented the use of polyethylene glycol (PEG)-bound aryl halides and sulfonates in these palladium-catalyzed cross-couplings [70]. The authors demonstrated that no additional phase-transfer catalyst (PTC) is needed when the PEG-bound electrophiles are coupled with appropriate aryl boronic acids. The polymer-bound substrates were coupled with 1.2 equivalents of the boronic acids in water under short-term microwave irradiation in sealed vessels in a domestic microwave oven (Scheme 7.62). Work-up involved precipitation of the polymer-bound biaryl from a suitable organic solvent with diethyl ether. Water and insoluble impurities need to be removed prior to precipitation in order to achieve high recoveries of the products. 

At least two groups examined the issue of catalyst recycle by using polymer-bound or water-soluble ligands." " ... 

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