Enantioselectivity copolymerization

The enantioselective copolymerization of styrenes and CO has also been achieved (Scheme 12). Using bidentate pyridine-imine ligands (26), Sen synthesized optically active styrene and 4-methylstyrene copolymers [80]. Based on a microstructural analysis, a 36% ee for olefin insertion was reported. Brookhart employed a C2-symmetrical bisoxazoline complex (27) to produce styrene-based... 

Polyesters with high optical purity were synthesized by the lipase CA-catalyzed copolymerization of racemic /3-BL with e-CL or DDL (175). (S)- -BL was preferentially reacted with DDL to give the (S)-enriched optically active copolymer with ee of /3-BL unit = 69%. 5-CL was also enantioselectively copolymerized by the lipase catalyst to give the (R)-enriched optically active polyester with ee up to 76%. 

SCHEME 24.14 Enantioselective copolymerization of cyclohexene oxide and CO2 with 11,13, or 14. 

Cheng, M. Darling, N. A. Lobkovsky, E. B. Coates, G W. Enantiomerically-enriched organic reagents via polymer synthesis Enantioselective copolymerization of cycloalkene oxides and CO2 using homogeneous, zinc-based catalysts. Chem. Commun. 2000, 2007-2008. 

Copolymerization of racemic lactones with an achiral lactone is an effective way to prepare a chiral polyester. The enantioselective copolymerization of racemic lactones with achiral lactones, such as s-CL and DDL, has been established by lipase. As a typical example, fi-BL and 6 -CL were copolymerized by lipase in isooctane to produce the S-enriched optically active copolyester with 69% e.e. of the -BL unit, and R - P-BL with 100% e.e. remained unreacted. This indicated that the S-isomer of -BL preferentially reacted during the copolymerization (Scheme 16a) [114]. Also, during the copolymerization of the racemic y0-BL with DDL, the (S)-isomer of -BL preferentially reacted to give the (S)-enriched optically active copolymer with an enantiomeric excess of the y0-BL unit of 69%, which is much higher than that for the homopolymerization of yS-BL. (5-CL was also enantioselectively copolymerized... 

Kikuchi, H., Uyama, H. and Kobayashi, S. (2000) lipase-catalyzed enantioselective copolymerization of substituted lactones to optically active polyesters. Macromolecules, 33, 8971-5. 

The enantioselectivity was greatly improved by the copolymerization with 7- or 13-membered non-substituted lactone using lipase CA catalyst (Scheme 8) the ee value reached ca. 70% in the copolymerization of (3-BL with DDL. ft is to be noted that in the case of lipase CA catalyst, the (5 )-isomer was preferentially reacted to give the (5 )-enriched optically active copolymer. The lipase CA-catalyzed copolymerization of 8-caprolactone (6-membered) with DDL enan-tioselectively proceeded, yielding the (/ )-enriched optically active polyester with ee of 76%. 

Enzymatic enantioselective oligomerization of a symmetrical hydroxy diester, dimethyl /Lhydroxyglutarate, produced a chiral oligomer (dimer or trimer) with 30-37% ee [24]. PPL catalyzed the enantioselective polymerization of e-substituted-e-hydroxy esters to produce optically active oligomers (DP < 6) [25]. The enantioselectivity increased with increasing bulkiness of the monomer substituent. Optically active polyesters with molecular weight of more than 1000 were obtained by the copolymerization of the racemic oxyacid esters with methyl 6-hydroxyhexanoate. 

Seebach and co-workers copolymerized a dendritically modified TADDOL ligand with styrene (Figure 9). When associated with Ti(OiPr)4, the immobilized catalyst gave a very high ee (98%) for more than 20 runs in the enantioselective addition of diethylzinc to benzaldehyde95 96... 

The synthesis of the first polymer-supported chiral Mn-salen derivatives was reported independently by Sivaram171 and Minutolo.171-173 Different monomeric Jacobsen-type units, containing two polymerizable vinyl groups, were copolymerized with styrene and divinylbenzene to yield the corresponding cross-linked polymers as a monolithic compact block.174-176 The less mobile system (Figure 19) with no spacer between the aromatic ring and the polymer backbone is less enantioselective. 

Kureshy developed a polymer-based chiral Mn-salen complex (Figure 21). Copolymerization of styrene, divinylbenzene, and 4-vinylpyridine generated highly cross-linked (50%) porous beads loaded with pyridine ligands at 3.8 mmol g-1. Once the polymer was charged with the metal complex catalyst, enantioselective epoxidation of styrene derivatives was achieved with ee values in the range 16 46%. 79... 

Dependence on Metallocene Symmetry of E-Z Selectivity for 2-Butene Copolymerizations. We have seen in the Section 3.1.3 that opposite enantiofaces are favored for primary and secondary propene insertion on C2-symmetric metallocenes, whereas the same enantioface is favored for primary and secondary insertion on Cv-symmetric metallocenes. In this framework, if the same steric interactions which rule the enantioselectivity of primary and secondary propene insertions hold for 2-butene, the insertion of... 

For example, a proline-based chiral ligand was attached to a vinyl-substituted monomer (Fig. 42.15) by reacting vinylbenzoyl chloride with the amine functionality of the ligand [106]. As mentioned previously, the apolar Merrifield resin as a support is not swollen in polar solvents. Hence, in order to match the polarity of the resin with that of the typically used substrates in enantioselective hydrogenation, the functionalized monomer was copolymerized with polar units of methacrylic acid 2-hydroxyethyl ester. 

This technology was extended to the preparation of chiral capillary columns [ 138 -141 ]. For example, enantioselective columns were prepared using a simple copolymerization of mixtures of O-[2-(methacryloyloxy)ethylcarbamoyl]-10,11-dihydro quinidine, ethylene dimethacrylate, and 2-hydroxyethyl methacrylate in the presence of mixture of cyclohexanol and 1-dodecanol as porogenic solvents. The porous properties of the monolithic columns can easily be controlled through changes in the composition of this binary solvent. Very high column efficiencies of 250,000 plates/m and good selectivities were achieved for the separations of numerous enantiomers [140]. 

Moreover, in-situ copolymerization approaches of polymerizable chiral cin-chonan carbamate selectors have also been shown to be viable straightforward routes to enantioselective separation media. In one approach, polymethacrylate-type monoliths have been fabricated by copolymerization of functional monomers and crosslinker in presence of porogenic solvents [80-85]. They have been utilized mainly for CEC (and will be described in detail later) while they turned out to be less suitable for HPLC application because of a low crosslinking degree. 

Monolithic columns with the chiral anion exchange-type selectors incorporated into the polymer matrix obtained through in situ copolymerization process of a chiral monomer (in situ approach) [80-83,85] or attached to the surface of a reactive monolith in a subsequent derivatization step (postmodification strategy) [84], both turned out to be viable routes to enantioselective macroporous monolithic columns devoid of the limitations of packed columns mentioned earlier. 

Borovik et al. [70] prepared a highly crosslinked polymeric porous material containing Co-salen units 38 (Figme 13) by template copolymerization method. The authors reported that as the cross-linking degree increases from 5 % to 50 %, the catalyst become more efficient in terms of reactivity, possibly due to the improved proximity of metal centers that work in cooperation. Unfortunately low enantioselectivity for the product epoxide was observed (<42 % ee) while the ee for concomitantly produced diol did not go above 86%. Reusability of the catalyst containing 50 mol% template showed consistent activity and enantioselectivity for three consecutive recycle experiments. 

Reactions where NLE have been discovered include Sharpless asymmetric epoxi-dation of allylic alcohols, enantioselective oxidation of sulfides to sulfoxides, Diels-Alder and hetero-Diels-Alder reactions, carbonyl-ene reactions, addition of MesSiCN or organometallics on aldehydes, conjugated additions of organometal-lics on enones, enantioselective hydrogenations, copolymerization, and the Henry reaction. Because of the diversity of the reactions, it is more convenient to classify the examples according to the types of catalyst involved. 

One of the earliest enantioselective carbon-carbon bond-forming processes catalyzed by chiral transition-metal complexes is asymmetric cyclopropanation discussed in Chapter 5, which can proceed via face-selective carbometallation of carbene-metal complexes. Some other more recently developed enantioselective carbon-carbon bond forming reactions, such as Pd-catalyzed enantioselective alkene-CO copolymerization (Chapter 7) and Pd-catalyzed enantioselective alkene cyclization (Chapter 8.7), are thought to involve face-selective carbometallation of acy 1-Pd and carbon-Pd bonds, respectively (Scheme 4.4). Similarly, the asymmetric Pauson-Khand reaction catalyzed by chiral Co complexes most likely involves face-selective cyclic carbometallation of chiral alkyne-Co complexes (Chapter 8,7). 

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