Chiral catalysts, polymeric

Transition metal coupling polymerization has also been used to synthesize optically active polymers with stable main-chain chirality such as polymers 33, 34, 35, and 36 by using optically active monomers.29-31 These polymers are useful for chiral separation and asymmetric catalysis. For example, polymers 33 and 34 have been used as polymeric chiral catalysts for asymmetric catalysis. Due... 

Polylactides, 18 Poly lactones, 18, 43 Poly(L-lactic acid) (PLLA), 22, 41, 42 preparation of, 99-100 Polymer age, 1 Polymer architecture, 6-9 Polymer chains, nonmesogenic units in, 52 Polymer Chemistry (Stevens), 5 Polymeric chiral catalysts, 473-474 Polymeric materials, history of, 1-2 Polymeric MDI (PMDI), 201, 210, 238 Polymerizations. See also Copolymerization Depolymerization Polyesterification Polymers Prepolymerization Repolymerization Ring-opening polymerization Solid-state polymerization Solution polymerization Solvent-free polymerization Step-grown polymerization processes Vapor-phase deposition polymerization acid chloride, 155-157 ADMET, 4, 10, 431-461 anionic, 149, 174, 177-178 batch, 167 bulk, 166, 331 chain-growth, 4 continuous, 167, 548 coupling, 467 Friedel-Crafts, 332-334 Hoechst, 548 hydrolytic, 150-153 influence of water content on, 151-152, 154... 

As discussed in Section 3.1.6.1., natural biopolymers are useful chiral selectors, some of which are readily available they are constructed from chiral subunits (monomers), for instance, from L-amino acids or D-glucose. If synthetic chiral polymers of similar type are to be synthesized, appropriate chiral starting materials and subunits, respectively, must be found. Chiral polymers with, for example, a helical structure as the chiral element, are built using a chiral catalyst as chirality inducing agent in the polymerization step. If the chirality is based on a chiral subunit, the chirality of the polymer is inherent, whereas if the polymer is constructed from chiral starting materials, chiral subunits are formed which lead to chirally substituted synthetic polymers that in addition may order or fold themselves to a supramolecular structure (cf. polysaccharides). 

Asymmetric oxidative coupling polymerization of hydroxynaphthalene derivatives was investigated by Habaue and Okamoto et al. First, they studied the oxidative coupling polymerization of optically active 3,3/-hydroxy-2,2/-dimethoxy-1,1 -binaphthalene with copper catalysts bearing chiral ligands under an oxygen atmosphere (Scheme 41) [166]. The obtained polymers had molecular weights of 3100-5200. When the polymerization of (J )-monomer... 

Two communications on propene polymerization by non-metallocene catalysts that include DFT/MM calculations have been recently published [60, 61]. They deal with group 4 bidentate non-cyclopentadienyl complexes. In the first communication [60], the topic addressed is the fact that a C2-symmetric precatalyst of titanium leads to a syndiotactic polymer, contrary to observations of metallocene catalysts. The chirality at the metal center is found to play a key role in the stereocontrol of the process. The second communication [61] addresses the fact that a C2-symmetric precatalyst of zirconium very similar to the previous one produces an isotactic polymer, finds out that it is due to a complicated concourse of synergic steric and electronic effects, and emphasizes the key role that serendipity still plays in the design of new catalysts. 

In research with Ziegler catalysts, Cossee (11) and Arlmann and Cossee (12) hypothesized that the insertion of propylene monomer takes place in a cis conformation into a titanium-carbon bond. Natta et al. (8) postulated that in the stereospecific polymerization, chiral centers on the surface are needed to produce isotactic polymers. These and other issues regarding the nature of the active sites have helped to increase the interest in investigations of homogeneous metallocene catalysis. 

However, there is still a lot to do. The chemistry of lanthanide carbonyl and olefin complexes, and the complexes containing a lanthanide to transition metal bond and/or a lanthanide to lanthanide bond is still underdeveloped. To fully utilize these new aspects of reductive chemistry clever approaches will be needed. The development of highly active activatorless olefin polymerization catalysts and chiral versions of these families of complexes, and the catalysts for Cl chemistry are still the challenges. So, organolanthanide chemistry will continue to be an attractive field for organometallic chemists and there are many opportunities for the future. 

One of the most powerful catalysts of the Mukaiyama aldol reaction is a chiral Ti(IV)-Schiff base complex 91 prepared from Ti(0 Pr)4 and enantiomerically pure salicylaldimine reported by Carreira [103-105]. This catalyst furnished aldol adducts in good yields and with excellent enantioselectivity. The Ti(IV)-Schiff base catalyst system is unique among the aldol catalysts yet reported in terms of operational simplicity, catalyst efficiency, chirality transfer, and substrate generality. Because the Ti(IV)-Schiff base complexes are remarkably efficient catalysts for the addition of ketene acetals to a wide variety of aldehydes, the polymeric version of catalyst 92 was prepared [106]. The activity and enantioselectivity of the polymer-supported chiral Ti(IV)-Schiff base complex were, however, much lower than were obtained from the low-molecular-weight catalyst (Eq. 28). 

The soluble polymer-supported catalysts have also been used for asymmetrically catalyzed reactions Following a procedure for the preparation of insoluble polymeric chiral catalysts a soluble linear polystyrene-supported chiral rhodium catalyst has been prepared. This catalyst displays high enantiomeric selectivity compared to the low molecular weight catalyst. Thus, hydroformylation of styrene using this catalyst produces aldehydes in high yields. The branched chiral hy drotropaldehy de is formed in 95% selectivity. 

The enantioselective alkynylation of ketones catalyzed by Zn(salen) complexes has been reported [24]. Polymeric salen ligand 30 was prepared with a polycondensation reaction and subsequently used as a polymeric chiral ligand of Zn. The polymeric Zn(salen) complex (prepared by 30) was then used as a catalyst of asymmetric addihon of phenylacetylene to aldehyde in the presence of 2 equivalents of Et/Zri. Subsequent asymmetric alkynylahon of 31 gave 33 in 96% yield and 72% ee (Scheme 3.9) [25]. 

In recent years, catalytic asymmetric Mukaiyama aldol reactions have emerged as one of the most important C—C bond-forming reactions [35]. Among the various types of chiral Lewis acid catalysts used for the Mukaiyama aldol reactions, chirally modified boron derived from N-sulfonyl-fS)-tryptophan was effective for the reaction between aldehyde and silyl enol ether [36, 37]. By using polymer-supported N-sulfonyl-fS)-tryptophan synthesized by polymerization of the chiral monomer, the polymeric version of Yamamoto s oxazaborohdinone catalyst was prepared by treatment with 3,5-bis(trifluoromethyl)phenyl boron dichloride ]38]. The polymeric chiral Lewis acid catalyst 55 worked well in the asymmetric aldol reaction of benzaldehyde with silyl enol ether derived from acetophenone to give [i-hydroxyketone with up to 95% ee, as shown in Scheme 3.16. In addition to the Mukaiyama aldol reaction, a Mannich-type reaction and an allylation reaction of imine 58 were also asymmetrically catalyzed by the same polymeric catalyst ]38]. 

A similar approach has been examined by using polymer-supported ALB 91 (Scheme 3.25). When this polymeric chiral ALB catalyst was used for the asymmetric Michael reaction, the corresponding chiral adduct was obtained in 91% yield with 96% ee [48]. 

A poly(bmaphthyl metallosalen complex) 128 (Scheme 3.36) was prepared and used as a catalyst for the asymmetric epoxidation of alkene [72]. Although enantioselectivities obtained by using the polymeric catalyst were low, this represented a new type of polymeric chiral complex based on the main-chain hehcity. 

Sharpless asymmetric epoxidation was also conducted by using polymer-supported catalysts. Some very interesting phenomena were observed when methoxy PEG (MeO-PEG) -supported tartrate 147 was used as the polymeric chiral ligand (Scheme 3.43). In the epoxidation of 148 under Sharpless epoxidation conditions, 2S,3S -trans 149 with 93% ee was obtained using 147 (MW = 750), while (2R,3R)-trans 149 with 93% ee was obtained using 147 (MW = 2000) [80]. More recently, Janda studied the precise effects of the molecular weight of the PEG chain on the... 

Among the many examples of asymmetric hydrogenation catalysts that have been developed, chiral complexes prepared from 1,2-dianiines and RuCb/diphosphines provide one example of the most powerful catalysts for this reaction. Polymer-supported fR -BINAP was treated with RuCh and fR,R -l,2-diphenylethylenedi-amrne to give the polymeric chiral complex 180 (Scheme 3.56) this serves as an excellent precatalyst for the asymmetric hydrogenation of aromatic ketones to give the chiral secondary alcohols in quantitative yields with 84—97% ee-values [115]. For example, the asymmetric hydrogenation of I -acetonaphthone with (R,RR)-180 occurred in quantitative conversion within 26 h with 98% ee. The enantioselectivity, turnover number (TON) and turnover frequency (TOF) in this reaction... 

The recycling experiments were examined for the amination of the cycloheptenyl ester 29 with 1 equivalent of dibenzylamine. After the first use of the polymeric chiral catalyst to give 98% ee of 41a, the recovered resin catalyst was taken on to second and third uses, without any additional charge of palladium, and exliibited no loss of its catalyhc activity or stereoselectivity. 

A multi-step asymmetric synthesis of a hydrindane framework was achieved in water via asymmetric allylic alkylation, propargylation and aquacatalytic cycloisomerization of a 1,6-enyne, where all three steps were performed in water with the recyclable polymeric catalysts. The racemic cyclohexenyl ester 27 reacted with diethyl malonate under the conditions mentioned in Table 6.1 to give 90-92% ee of 34b. The polymeric chiral palladium complex 23-Pd was reused four times... 

The tetrahydropyridyl ester rac-30 underwent nitromethylahon in water with the polymeric chiral catalyst 18 under similar conditions to give an 81% isolated yield of (R)-nitromethyl(tetrahydro)pyridine 50 with 97% ee. The resultant nihometliyl (tetrahydro)pyridine 50 was readily converted to the tehahydropyridyl carboxylic acid 51 also in water by the modified Carreira condihons [18], which is a promising synthetic intermediate for Isofagomine and Siastahn B [19]. 

Inoue et al. ( ) found that a porphyrin-Zn alkyl catalyst polymerized methyloxirane to form a polymer having syndio-rich tacticity. The relative population of the triad tacticities suggests that the stereochemistry of the placement of incoming monomer is controlled by the chirality of the terminal and penultimate units in the growing chain. There is no chirality around the Zn-porphyrin complex. Achiral zinc complex forms syndio-rich poly(methyloxirane), while chiral zinc complex, as stated above, forms isotactic-rich poly(methyloxirane). The situation is just the same as that for propylene polymerizations. Achiral vanadium catalyst produces syndiotactic polypropylene, while chiral titanium catalyst produces isotactic polypropylene. 

Ziegler-Natta Various types of Ziegler-Natta catalysts Polymerization mechanism and applications of molecular mechanics to the study of enantioselectivity of some stereospecific catalytic systems having chiral site stereocontrol Mechanism of enantioselectivity and its relevance to catalytic systems 56... 

Linear polymers carrying chiral oxazaborolidine as a pendant group were prepared from a methylhydrosiloxane-dimethylsiloxane copolymer [72]. Borane reduction using the polymeric oxazaborolidine 25 gave (i )-phenylethylalcohol of 97% ee which is as high as in analogous reaction with non-polymeric catalyst. This chiral polymer can be retained by a nanofiltration membrane thus will be suitable for use in a continuously operated membrane reactor. 

The product is obtained in 95% yield and 94% ee. In the counterpart in solution, the ee was only 21-50%. Polymeric chiral catalysts have also been used in the addition of zinc alkyls to aldehydes. Use of a proline-based copolymer in a continuous asymmetrical synthesis with an ultra filtration membrane gave 80% ee (10.61).138 There was no deactivation in 7 days. A boron-containing polymer (10.62) gave only 28-51% ee compared with the 65-75% ee found with model compounds in solution.139... 

A noted earlier, coordination of transition-metal ions to water-soluble polymers can allow for facile catalyst recovery, by ultrafiltration, from water-soluble substrates and/or products. For example, Han and Janda [22] used an osmium complex of the water-soluble polymeric chiral ligand 8 as a catalyst for the asymmetric dihydroxylation of alkenes in aqueous acetone (Eq. 5). However, they suggested that the catalyst should be recovered by precipitation with methylene chloride. Obviously the use of an ultrafiltration membrane for catalyst separation would be far more attractive. nu... 

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