Polystyrene-bound catalysts

The use of chiral esters as 2tx-substrates permit the production of diastereomer-ically enriched cycloadducts. For example, the menthyl ester 48 was obtained as a 4 ldiastereomeric mixture. The most intriguing feature of polystyrene-bound catalyst 46 is the fact that it can be removed from the reaction mixture by simple filtration and repeatedly reused. This means that the two basic problems of homogeneous catalysis, separation and recycling of the catalyst, can be solved by using the solid-supported complex 46. Additionally, the environmental problems associated with chromium can also be effectively eliminated. 

A method for the polymerization of polysulfones in nondipolar aprotic solvents has been developed and reported (9,10). The method reUes on phase-transfer catalysis. Polysulfone is made in chlorobenzene as solvent with (2.2.2)cryptand as catalyst (9). Less reactive crown ethers require dichlorobenzene as solvent (10). High molecular weight polyphenylsulfone can also be made by this route in dichlorobenzene however, only low molecular weight PES is achievable by this method. Cross-linked polystyrene-bound (2.2.2)cryptand is found to be effective in these polymerizations which allow simple recovery and reuse of the catalyst. 

The polymer-bound catalysts A-C. (Table 31) are prepared by reaction of the corresponding amino alcohols with partially chloromethylated 1 -2% cross-linked polystyrene. In the case of A, the enantioselectivity of the addition of dialkylzincs to aldehydes is higher than with the corresponding monomeric ephedrine derivatives (vide supra). Interesting insights into the mechanism of the alkylation of aldehydes by dialkylzinc reagents can be obtained from the experi-... 

Thus, by using a mixture of the Grubbs catalyst 6/3-13 and Pd(OAc)2/PPh3, 6/3-84a was transformed into 6/3-85a in 65% yield. With the polystyrene-bound palladium catalyst 71 % yield was obtained in contrast, the use of a biphasic system... 

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. 

Since cobalt on kieselguhr in one of the original Fischer-Tropsch catalysts (1-9), it appeared attractive to investigate the catalytic activity of cobalt complexes immobilized on polystyrene. Although there are many supported cobalt-based Fischer-Tropsch catalysts known (see, for example, references 18-21), no polystyrene-bound systems had been reported. During the course of our work 18% (22,60,61) and 20% (23) crosslinked analogs of CpCo(C0)2 were shown to exhibit limited catalytic activity but no CO reduction. A preliminary disclosure of our work has appeared (2)4). 

MP borohydride catches one equivalent of the titanium catalyst, while the polystyrene-bound diethanolamine resin (PS-DEAM) can scavenge the remaining titanium catalyst. The borohydride reagent also assists in the reductive animation reaction. Final purification of the crude amine product is achieved with a polystyrene-bound toluene sulfonic acid resin scavenger that holds the amine through an ion exchange reaction, while impurities are washed off. The pure amine can be recovered with methanol containing 2M ammonium hydroxide. 

Immobilized catalysts on solid supports inherently have benefits because of their easy separation from the products and the possibility of recycling. They are also expected to be useful for combinatorial chemistry and high-throughput experimentation. The polystyrene-bound BINAP/DPEN-Ru complex (beads) in the presence of (CH3)3COK catalyzes the hydrogenation of l -acetonaphthone with an SCR of 12 300 in a 2-propanol-DMF mixture (1 1, v/v) to afford the chiral alcohol in 97% ee (Fig. 32.35) [113]. This supported complex is separable... 

Copolymeriziation of polystyrene-bound dicyanoketene acetal (DCKA) and ethylene glycol dimethacrylate (EGDMA) yielded a polymer (41) with high n -acidity. It was found to be an effective and completely recyclable catalyst in the high yielding carbon-carbon bond-forming reaction of dimethylacetals with silylated nucleophiles (Scheme 4.26) [118]. 

Various polymer-bound (polystyrene-bound) oxazaboroHdine catalysts for the reduction of secondary alcohols were reported [128]. These can simply be prepared by condensation of the resin-bound boronic acid with chiral 1,2-amino alcohols. The best results as far as enatioselectivity is concerned were obtained with oxaza-borohdine (59) (Scheme 4.36). 

The Pauson-Khand reaction involves the aimulation of an alkene, an alkyne and carhon monoxide to yield cyclopentenones. Recently, it was shown that in this respect polymer-hound species (60) is an effective catalyst which may be generated by heating Co2(CO)g with polystyrene-bound phosphine (Scheme 4.37) [129]. 

Figure 6.12 Polystyrene-bound Schiff base (thio)urea catalysts HTS-optimized in the asymmetric Strecker reaction between N-allyl-protected benzaldimine and TBSCN key results obtained from the different libraries.

Taylor and Flood could show that polystyrene-bound phenylselenic acid in the presence of TBHP can catalyze the oxidation of benzylic alcohols to ketones or aldehydes in a biphasic system (polymer-TBHP/alcohol in CCI4) in good yields (69-100%) (Scheme 117) °. No overoxidation of aldehydes to carboxylic acids was observed and unactivated allylic alcohols or aliphatic alcohols were unreactive under these conditions. In 1999, Berkessel and Sklorz presented a manganese-catalyzed method for the oxidation of primary and secondary alcohols to the corresponding carboxylic acids and ketones (Scheme 118). The authors employed the Mn-tmtacn complex (Mn/168a) in the presence of sodium ascorbate as very efficient cocatalyst and 30% H2O2 as oxidant to oxidize 1-butanol to butyric acid and 2-pentanol to 2-pentanone in yields of 90% and 97%, respectively. This catalytic system shows very good catalytic activity, as can be seen from the fact that for the oxidation of 2-pentanol as little as 0.03% of the catalyst is necessary to obtain the ketone in excellent yield. 

The affinity of the polymer-bound catalyst for water and for organic solvent also depends upon the structure of the polymer backbone. Polystyrene is nonpolar and attracts good organic solvents, but without ionic, polyether, or other polar sites, it is completely inactive for catalysis of nucleophilic reactions. The polar sites are necessary to attract reactive anions. If the polymer is hydrophilic, as a dextran, its surface must be made less polar by functionalization with lipophilic groups to permit catalytic activity for most nucleophilic displacement reactions. The % RS and the chemical nature of the polymer backbone affect the hydrophilic/lipophilic balance. The polymer must be able to attract both the reactive anion and the organic substrate into its matrix to catalyze reactions between the two mutually insoluble species. Most polymer-supported phase transfer catalysts are used under conditions where both intrinsic reactivity and intraparticle diffusion affect the observed rates of reaction. The structural variables in the catalyst which control the hydrophilic/lipophilic balance affect both activity and diffusion, and it is often not possible to distinguish clearly between these rate limiting phenomena by variation of active site structure, polymer backbone structure, or % RS. 

With the less active polystyrene-bound benzyltrimethylammonium ion catalysts 2 (17% RS),... 

Substantial variations of the organic solvent used in triphase catalysis with polystyrene-bound onium ions have been reported only for the reactions of 1-bromo-octane with iodide ion (Eq. (4))74) and with cyanide ion (Eq. (3)) 73). In both cases observed rate constants increased with increasing solvent polarity from decane to toluene to o-dichlorobenzene or chlorobenzene. Since the swelling of the catalysts increased in the same order, and the experiments were performed under conditions of partial intraparticle diffusional control, it is not possible to determine how the solvents affected intrinsic reactivity. 

Spacer chain catalysts 3, 4, and 19 have been investigated under carefully controlled conditions in which mass transfer is unimportant (Table 5)80). Activity increased as chain length increased. Fig. 7 shows that catalysts 3 and 4 were more active with 17-19% RS than with 7-9% RS for cyanide reaction with 1-bromooctane (Eq. (3)) but not for the slower cyanide reaction with 1-chlorooctane (Eq. (1)). The unusual behavior in the 1-bromooctane reactions must have been due to intraparticle diffusional effects, not to intrinsic reactivity effects. The aliphatic spacer chains made the catalyst more lipophilic, and caused ion transport to become a limiting factor in the case of the 7-9 % RS catalysts. At > 30 % RS organic reactant transport was a rate limiting factor in the 1-bromooctane reations80), In contrast, the rate constants for the 1 -chlorooctane reactions were so small that they were likely limited only by intrinsic reactivity. (The rate constants were even smaller than those for the analogous reactions of 1-bromooctane and of benzyl chloride catalyzed by polystyrene-bound benzyl-... 

The data in Table 7 obtained with equimolar amounts of the polymeric catalysts and the 2-naphthoxide ion should be more reliable because all of the reactive anion is contained within the polymer. These conditions (expts 7-9) gave 100 % O-alkylation, indicating that the active site environment of the polystyrene-bound tri-n-butylphos-phonium ion/naphthoxide ion pair or aggregate is aprotic even with the 60%RS polymer. However, the common benzyltrimethylammonium ion found in commercial ion exchange resins is more hydrophilic, giving both C- and O-alkylation (expts 10 and 11 of Table 7). 

Both alumina and silica gel are more stable physically than the common polystyrene supports. The alumina-bound catalysts are particularly promising because of their higher activity and higher selectivity compared with the silica gel-bound catalysts. Alumina also is stable in alkali. The alumina-bound catalysts 32 and 33 worked well for reaction of 1-bromooctane with concentrated aqueous sodium cyanide 118). 

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