Copolymerization asymmetric alternating

It is also possible to desymmetiize a meso epoxide in the alternating copolymerization. Thus, asymmetric alternating copolymerization of cyclohexene oxide with CO2 catalyzed by a dimeric zinc complex provides a polycarbonate in which the diol unit is optically active with 80% ee. (See Scheme 4.24.)... 

However, there are numerous reported instances of stereocontrol by a site-control mechanism involving chiral metal catalysts. That is, Nozaki and coworkers first illustrated the asymmetric alternating copolymerization of cyclohexene oxide and CO2 employing a chiral zinc catalyst derived from an amino alcohol (Fig. 2a) [13-16]. This was soon followed by studies of Coates and coworkers utilizing an imine-oxazoline zinc catalyst (Fig. 2b) [17]. Both investigations provided isotactic poly(cyclohexene carbonate) (Fig. 3) with enantiomeric excess of approximately 70%. 

Shi L, Lu X-B, Zhang R, Peng X-J, Zhang C-Q, Li J-E, Peng X-M (2006) Asymmetric alternating copolymerization and terpolymerization of epoxides with carbon dioxide at mild conditions. Macromolecules 39 5679-5685... 

In a recent report, Nozaki and coworkers isolated presumed intermediates in the asymmetric alternating copolymerization of cyclohexene oxide with CO2P Reaction of a 1 1 mixture of ZnEt2 and (5)-a, a-diphenylpyrrolidine-2-yl-methanol (11, Scheme 24.13) yielded dimeric 12, which was structurally characterized by X-ray diffraction studies. At 40 °C and 30 atm CO2, 12 catalyzed... 

SCHEME 24.13 Chiral zinc catalysts for the asymmetric, alternating copolymerization of cyclohexene oxide and CO2. 

Nozaki, K. Nakano, K. Hiyama, T. Optically active polycarbonates Asymmetric alternating copolymerization of cyclohexene oxide and carbon dioxide. J. Am. Chem. Soc. 1999, 121,... 

Brookhart first reported the asymmetric alternating copolymerization of 4-tert-butylstyrene with carbon monoxide using [Pd(Me)(MeCN)(biox)][B(3,5-(CF3)2C6H3)4] (Figure 9). In this copolymerization, the enantioface of the olefin was selected by the chiral ligand instead of the chain end as a result, the polymer was completely isotactic. Since one enantioface was... 

The asymmetric alternating copolymerization of olefin and CO has been achieved by using chiral ligands Me-DUPHOS and BINAPHOS for the functionalized olefins possessing hydroxyl, carboxy, carbamate, amide, fluoroaryl, epoxyalkyl,perfluoroalkyl, and para-chlorophenyl groups (Figure 10). 

An alternative method for generating enriched 1,2-diols from meso-epoxides consists of asymmetric copolymerization with carbon dioxide. Nozaki demonstrated that a zinc complex formed in situ from diethylzinc and diphenylprolinol catalyzed the copolymerization with cyclohexene oxide in high yield. Alkaline hydrolysis of the isotactic polymer then liberated the trans diol in 94% yield and 70% ee (Scheme 7.20) [40]. Coates later found that other zinc complexes such as 12 are also effective in forming isotactic polymers [41-42]. 

The catalyst [Pd(Me-DUPHOS)(MeCN)2](BF4)2 was also effective in the alternating asymmetric copolymerization of aliphatic a-olefins with carbon monoxide [27,28]. The polymer synthesized in a CH3N02-CH30H mixture has both 1,4-ketone and spiroketal (10) units in the main chain. The propylene-CO copolymer consisting only of a 1,4-ketone structure shows [ ]D +22° (in (CF3)2CHOH), and the optical purity of the main chain chiral centers is over 90% as estimated by NMR analysis using a chiral Eu shift reagent. 

In addition to hydrogenation reactions, modular phospholane ligands are being applied in a growing rank of other useful asymmetric catalytic transformations. For instance, Jiang and Sen reported the discovery of a dicationic Me-DuPhos-Pd catalyst for the alternating copolymerization of aliphatic a-olefins and carbon monoxide (Scheme 13.21).67... 

An alternative approach is the use of a PS support bearing sulfonate pendant groups. For this, a quaternary ammonium salt of styrenesulfonic acid was copolymerized with a N-(p-styrenesulfonyl)-l,2-diphenylethylenediamine monomer. The polymeric chiral Ru complex was prepared from 177 and [RuCl2(p-cymene)]2 and applied to the asymmetric transfer hydrogenation of aromatic ketones in water (Scheme 3.55) [114]. The polymeric chiral complex was evenly suspended in water and the reaction proceeded smoothly to produce the alcohol in quantitative yield and with high enantioselectivity. For several of the aromatic ketones tested, higher... 

Crowther has reported the copolymerization of ethylene and norbornene using asymmetric cyclo-pentadienyl—amido complexes (132, 133) to yield alternated polymers that melt at 250 °C (Scheme... 

Supercritical carbon dioxide represents an inexpensive, environmentally benign alternative to conventional solvents for chemical synthesis. In this chapter, we delineate the range of reactions for which supercritical CO2 represents a potentially viable replacement solvent based on solubility considerations and describe the reactors and associated equipment used to explore catalytic and other synthetic reactions in this medium. Three examples of homogeneous catalytic reactions in supercritical CC are presented the copolymerization of CO2 with epoxides, ruthenium>mediated phase transfer oxidation of olefins in a supercritical COa/aqueous system, and the catalyic asymmetric hydrogenation of enamides. The first two classes of reactions proceed in supercritical CO2, but no improvement in reactivity over conventional solvents was observed. Hythogenation reactions, however, exhibit enantioselectivities superior to conventional solvents for several substrates. 

Using a mixture of two kinds of olefins, a terpolymer can be generated by alternating olefin/CO copolymerization (Scheme 22.7). An olefin and carbon monoxide are incorporated in a completely alternating manner, and the order of the two olefins is mostly random. Asymmetric... 

The combination of silanes and silanols is not limited to simple methylsilicones or arylsilicones but can be extended to much more complicated systems. Kawakami and coworkers described the synthesis of complex double decker siloxanes and their alternating copolymerization with a dimethylsilicone chain (Fig. 15a) [65]. They have also shown that asymmetrical copolymers can be prepared enantiose-lectively using this strategy (Fig. 15b) [66]. 

Let us also mention some asymmetric-selective copolymerizations with propylene oxide (PO) it could be expected that another chiral co-monomer would favor the asymmetric-selection of one of both co-monomers. Tsuruta etal.[ S 2i] reported the asymmetric-selection copolymerization of 3-phenyl-4-tetrahydrophthalic acid anhydride with PO (LXI). Minoura et al [152] described the alternating copolymerization of PO and d-camphoric acid anhydride with diethylzinc triethylamine as optically inactive initiator the... 

A different method for obtaining macromolecules having, in their main chains asymmetric centers which can give optical activity to the polymer, is based on the alternate copolymerization of monomers having a suitable structure. 

Arcus [8] discussed the conditions of the optical activity of polymers in alternate copolymerizations and underlined that an asymmetry was brought into main chains in the polymerization between vinyl monomers and a,j3-substituted monomers. Accordingly, the main chains of the polymers obtained by vinyl homopolymerizations do not have optical activities since they are pseudo-asymmetric. 

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