Catalysts polymer-bound, reuse

The polymer-bound catalyst was recyclable by filtration and showed just slightly decreased activity when reused. Catalyst (33) also promotes asymmetric Reissert-type reactions [106]. 

The syntheses of most chiral Mn-salen complexes are simple, and thus their recovery and reuse have not been studied extensively. Only a few studies of epoxidation with a polymer-bound Mn-salen complex or a Mn-salen complex embedded in nano-porous materials as catalyst has been performed. It has been disclosed, however, that the microenvironment provided by the macromolecule adversely affects the asymmetric induction by the Mn-salen catalyst to a considerable extent although reuse of the catalysts for several cycles are realized [68]. 

The homogeneous chiral phosphine/DPEN-Ru catalyst can be immobilized by use of polymer-bound phosphines such as polystyrene-anchored BINAP (APB-BINAP) [57, 98], Poly-Nap [99], and poly(BINOL-BINAP) [100], poly(BINAP) [101]. These complexes hydrogenate T-acetonaphthone and acetophenone with S/C of 1000-10 000 under 8 10 atm H2 to give the corresponding secondary alcohols in 84-98% e.e. The recovered complexes are repeatedly used without significant loss of reactivity and enantioselectivity. Immobilization allows the easy separation of catalyst from reaction mixture, recovery, and reuse. These advantages attract much attention in combinatorial synthesis. 

Seebach s group demonstrated the utility of polymer-bound, chiral titanium TADDOLates in preparing chiral secondary alcohols (Figure 3.24).The polymeric catalyst 37 was contained inside a mesh tea bag, and was reused by simply charging fresh reagents and solvent. The ruggedness of the system was shown when the product enantioselectivity dropped from 96% (S) to only 92% (S) over 20 successive runs, and the average yield was 90% [50]. 

Potential recycling of the polymeric catalysts is a very important feature of supported systems. According to this, all polymer-bound catalysts prepared were recovered after the initial reaction, washed, dried and reused for the same reaction, under similar conditions. This procedure was repeated for several cycles. Results obtained showed that all resins partially lose... 

Doyle s chiral rhodium (II) carboxamidates have proved to be exceptionally successful for asymmetric C-H insertion reactions of diazoacetates and some diazoacetamides leading to lactones and lactams, respectively. With 2-alkoxyethyl diazoacetates and the Rh2(5S- and 5R-MEPY)4 catalysts, for example, highly enantioselective intramolecular C-H insertion reactions occur, the 5S-catalyst, Eq. (40), and 5R-catalyst furnishing the S- and R-lactone, respectively [58]. A polymer-bound version of Rh2(5S-MEPY)4 has also been applied to the cycliza-tion in Eq. (40) to yield the lactone with 69% ee (R=Me) the catalyst could be recovered by filtration and reused several times, but with decreasing enantiose-lection [59]. 

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. ... 

Microwave-assisted Suzuki coupling using a reusable polymer-supported palladium complex has been achieved in a more recent study [135]. The reaction mixture was treated with the polystyrene-bound palladium catalyst and irradiated in an open flask for 10 min in a domestic microwave oven (Scheme 16.88). After cooling, the mixture was filtered and the catalyst extracted with toluene and dried. The recycled polymer-bound catalyst can be reused five times without loss of efficiency. 

A second nucleophilic catalyst supported by PtBS is the polymer-bound di-methylaminopyridine analog that was also used in latent biphasic catalysis with the poly(JV-alkylacrylamide) support 129 [131]. This example of a nucleophilic catalyst (133) was used to catalyze formation of a t-Boc derivative of 2,6-di-methylphenol (Eq. 70). In this case, the extent of recovery of the catalyst and the yields of product were directly comparable to those seen with thermomorphic systems. The isolated yield for the first five cycles of this reaction were 34.3, 60.9,82.2,94.6, and 99%. In this case we reused catalyst 133 through 20 cycles. Yields after the first few cycles were essentially quantitative (ca. 93% average for each of 20 cycles). Separation of the polymer from the aqueous ethanol phase was quantitative as judged by either visual observation or UV-visible spectroscopic analysis. 

The idea of using polymers as supports for catalysis chemistry is not new. Prior work with heterogeneous polymer bound transition metal catalysts extends back to the 1960s.(/-i) At that time there was considerable industrial interest in insoluble polymer supports. That interest continued through the 1970s but dwindled because homogeneous and heterogeneized catalysts were not always as comparable to one another as anticipated and because catalysts could not always be recovered and reused with the expected simplicity. Since that time there has been a rejuvenation of... 

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. 

Given the utility of chiral Cu(II)/bisoxazoline complexes in enantioselective Mukaiyama aldol reactions, a number of reports detailing the development of polymer-bound or dendritic bisoxazoline copper (I I) complexes have been developed. Development of such catalyst systems provides the potential for easy recovery and reuse of the relatively expensive catalyst. To this end, Salvadori and CO workers reported Mukaiyama aldol addition of ketene thioacetal (57) to methyl pyruvate catalyzed by a Cu(OTf)2 complex of polystyrene-supported bisoxazoline (89) (Scheme 17.18) [23]. The enantioselectivity of the addition remained high over eight cycles of the catalyst, however, reactivity was gradually reduced over time. 

The system consists of solvent mixture that is biphasic at room temperature and becomes homogeneous when heated (e.g., heptane and 90% DMA/water become miscible in all proportions above 65°C). After completion of the reaction and cooling to room temperature the reaction products are staying in the nonpolar phase and can be isolated by simple phase separation. The polar phase that contains the polymer bound catalyst can be reused in further runs by adding fresh substrate solution in heptane. 

The oxidation of alcohols to aldehydes and ketones is one of the most widely practiced of synthetic transformations. Ge Wang of the University of Science and Technology in Beijing has developed Chem. Lett. 2007, 36, 1236) a Mo catalyst that used aqueous to effect this transformation. Secondary alcohols are oxidized more rapidly than primary alcohols. Vinod K. Singh of the Indian Institute of Technology, Kanpur, has found Synth. Comm. 2007, 37, 4099) that the solid, inexpensive 6 can take the place of oxalyl chloride in the Swem oxidation. Viktor V. Zhdankin of the University of Miimesota, Duluth has devised J. Org. Chem. 2007, 72, 8149) a polymer-bound hypervalent iodine reagent that is easily separated after use, and reoxidized for reuse. 

Polymer-bound Cinchona alkaloids (53) are catalysts for asymmetric Michael additions of carbanions and thiols to a,j -unsaturated ketones, esters and nitro compounds as shown in Scheme 22. Attachment of the alkaloids to the polymer through the remote vinyl group by copolymerization with acrylonitrile or by addition of a polymeric thiol to the vinyl group was required for catalytic activity and stereoselectivity. " " The enantiomeric excesses attained with the polymeric catalysts (28-51 %) are lower than with the soluble alkaloids (36-63%), but the ease of recovery and reuse of the polymers is usually an advantage. 

Second, Sigamide was attached to Merrifield resin A 6. The polymer bound catalyst needs higher catalyst loadings (15mol%) and shows approx. 10% lower yields and selec-tivities. On the other hand, they also showed that the catalyst can be reused five times without any loss in activity. Performing blank experiments with only the resin, it was found that the lower enantioselectivities originate from the... 

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