Polystyrene-based catalysts

These polystyrene-based catalysts are effective for the cyanide displacements of 1-bromooctane and 1-chlorooctane, and also for the generation of dichlorocarbene from chloroform and aqueous sodium hydroxide, giving quantitative yields of (2,2-dichlorocyclopropyl)benzene from styrene. The catalysts may be recovered simply by filtering the reaction mixture. Unfunctionalized polystyrene does not catalyse these reactions. As well as improving product purification and catalyst recovery, this approach also avoids... 

So far, few data are available which allow the comparison of differences in efficacy and selectivity of one catalytic system attached to different supports. As far as the TADDOLate complexes are concerned, no clear rules can be drawn. Polystyrene-based catalysts derived from (8) and (10) show similar enantioselectivities and reaction rates. Differences appear, however, when comparing them with a polystyrene-embedded dendritic ligand system, generated by co-polymerization from TADDOL-derivative (32) (Scheme 4.18) which is described in Section 4.3.2.1. Re-cydabihty seems to be easier for the dendritic catalyst and the enantioselectivity. 

The nature of the support can have a very profound influence on the catalyst activity. Thus, phosphinated polyvinyl chloride supports are fairly inactive (75), and phosphinated polystyrene catalysts are considerably more active (57), but rather less active particularly when cyclic olefins are the substrates than phosphinated silica supports (76). Silica-supported catalysts may be more active because the rhodium(I) complexes are bound to the outside of the silica surface and are, therefore, more readily available to the reactants than in the polystyrene-based catalysts where the rhodium(I) complex may be deep inside the polymer beads. If this is so, the polystyrene-based catalysts should be more valuable when it is desired to hydrogenate selectively one olefin in a mixture of olefins, whereas the silica-based catalysts should be more valuable when a rapid hydrogenation of a pure substrate is required. 

The other main support used for solid base catalysts is polystyrene, which while it does not have a well-defined porous structure, does swell in solvents providing an accessible high surface area on which to carry out reactions. One common method of chemically attaching groups to polystyrene involves incorporation of specific amounts of styrene contain-... 

The use of such an oxazaborolidine system in a continuously operated membrane reactor was demonstrated by Kragl et /. 58] Various oxazaborolidine catalysts were prepared with polystyrene-based soluble supports. The catalysts were tested in a deadend setup (paragraph 4.2.1) for the reduction of ketones. These experiments showed higher ee s than batch experiments in which the ketone was added in one portion. The ee s vary from 84% for the reduction of propiophenone to up to >99% for the reduction of L-tetralone. The catalyst showed only a slight deactivation under the reaction conditions. The TTON could be increased from 10 for the monomeric system to 560 for the polymer-bound catalyst. 

Syndiotactic Polystyrene. Syndiotactic polystyrene is an interesting material because it has a Tg of 95 °C and a Tm of 260 °C [38], Polystyrene made via radical polymerisation may show some syndiotacticity, but its heat distortion temperature is too low to allow its use in important applications requiring temperatures around 120 °C or higher, such as medical equipment which requires sterilization or hot water storage containers. Idemitsu and Dow have reported titanium-based catalysts such as the one shown in Figure 10.23. We presume that the mechanism is a chain-end controlled "2,1" insertion. 

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]. 

C. A. McNamara, M. J. Dixon, M. Bradley, Recoverable Catalysts and Reagents Using Recyclable Polystyrene-based Supports, Chem. Rev. 102 (2002) 3275-3300. 

Solid-phase synthesis is of importance in combinatorial chemistry. As already mentioned RuH2(PPh3)4 catalyst can be used as an alternative to the conventional Lewis acid or base catalyst. When one uses polymer-supported cyanoacetate 37, which can be readily obtained from the commercially available polystyrene Wang resin and cyanoacetic acid, the ruthenium-catalyzed Knoevenagel and Michael reactions can be performed successively [27]. The effectiveness of this reaction is demonstrated by the sequential four-component reaction on solid phase as shown in Scheme 11 [27]. The ruthenium-catalyzed condensation of 37 with propanal and subsequent addition of diethyl malonate and methyl vinyl ketone in TH F at 50 °C gave the adduct 40 diastereoselectively in 40 % yield (de= 90 10). 

Ionic meso-tetraarylporphyrins can be bound to a range of charged supports including cross-linked polystyrene based anions,78 cations,79 exchange resins, cross-linked and quaternized poly(4-vinylpyridine)80 and the oxides of silica, alumina and magnesia.81 Metalloporphyrins may be attached to supports by coordination of a ligand on the support to the metal centre. Such catalysts mimic... 

McNamara CA, Dixon MJ, Bradley M (2002) Recoverable catalysts and reagents using recyclable polystyrene-based supports. Chem Rev 102 3275-3300... 

The polymeric oxazaborolidine prepared from the linear copolymer of 29 and styrene was used in membrane reactor and resulted in high total turnover number with high enantioselectivity [44]. Another polystyrene-based soluble polymeric oxazaborolidine 38 was used in the same system. Polysiloxanes are also useful polymeric supports of catalyst 39 for the same purpose [45]. 

Solid acid catalysts, consisting of polysiloxane bearing alkylsulfonic acid groups (MCM-41), are comparable in their catalytic activity to those of the polystyrene-based cation exchange resins. These catalysts can be used in the preparation of para-Bisphenol-A by the alkylation of phenol with acetone. Other application of these catalysts lie in the alkylation of phenol with isobutene at 90-130 O . 

Also in 2000, attachment of the Jacobsen catalyst to polymeric supports such as poly(ethylene glycol) and different polystyrene-based resins through a glutarate spacer was described [28]. Soluble as well as insoluble polymer-bound complexes were employed as catalysts in the epoxidation of styrene, cfs-2-methylstyrene, and dihydronaphthalene with wx-CPBA/NMO. Results were similar to those achieved with the nonsupported catalyst. Catalyst recycling was shown to be possible either by filtration or by precipitation and one catalyst system could be used for three cycles without significant loss of activity and enantioselectivity. 

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