Epoxide polymerization selectivity

Polymerization using epoxides as monomers includes the ring opening of epoxides via C-O bond cleavage. Thus, a mode of G-O bond cleavage (Sn2 or SnI) and selectivity, that is, which C-O bond is cleaved, coupled with the symmetry of epoxides (symmetrical or unsymmetrical), cause regio- and stereochemical issues to be controlled in the epoxide polymerization. 

Tsuruta found that the optically pure complex [(/ -salcy)Co] (15) was active for epoxide polymerization (Scheme 24.15) when activated with ALEt3. Although the system exhibited no enantioselectivity for the polymerization of propylene oxide, it was moderately selective (r = 1,5) for the kinetic resolutions of tert-butyl ethylene oxide and epichlorohydrin (Scheme 24.15). 

Although significant advances in stereoselective epoxide polymerization have been achieved over the last half-century, few known catalysts are capable of excellent levels of stereocontrol. Historically, most catalysts for epoxide polymerization have been of the heterogeneous variety and have exhibited poor selectivity. It is our opinion that the most fertile area for future catalyst exploration involves homogeneous, discrete catalysts that are capable of involving multiple metal centers in the polymerization mechanism. If the spatial environment of the active catalyst is precisely controlled, new generations of stereoselective epoxide polymerization catalysts will become available. Our current research focuses on the search for such catalysts. 

The asymmetric-selective polymerization of epoxyethyltrimethylsilane (ETMS) and of (2,3 epoxypropoxy)trimethylsilane (EPOS) (LX) were studied using a ZnEt2/(0-menthol initiator system. OA polymers were obtained in both cases but the asymmetric selectivity of EPOS polymerization was low, considering the weak optical activity of glycidol obtained from the hydrolysate of the unconverted monomer. The presence of a silicon atom, which is considered to have a stronger interaction with initiators than a carbon atom, did not improve the degree of asymmetric selectivity in epoxide polymerization [150 bis]. 

Extensive studies of stereoselective polymerization of epoxides were carried out by Tsuruta et al.21 s. Copolymerization of a racemic mixture of propylene oxide with a diethylzinc-methanol catalyst yielded a crystalline polymer, which was resolved into optically active polymers216 217. Asymmetric selective polymerization of d-propylene oxide from a racemic mixture occurs with asymmetric catalysts such as diethyzinc- (+) bomeol218. This reaction is explained by the asymmetric adsorption of monomers onto the enantiomorphic catalyst site219. Furukawa220 compared the selectivities of asymmetric catalysts composed of diethylzinc amino acid combinations and attributed the selectivity to the bulkiness of the substituents in the amino acid. With propylene sulfide, excellent asymmetric selective polymerization was observed with a catalyst consisting of diethylzinc and a tertiary-butyl substituted a-glycol221,222. ... 

Peroxidases have been used very frequently during the last ten years as biocatalysts in asymmetric synthesis. The transformation of a broad spectrum of substrates by these enzymes leads to valuable compounds for the asymmetric synthesis of natural products and biologically active molecules. Peroxidases catalyze regioselective hydroxylation of phenols and halogenation of olefins. Furthermore, they catalyze the epoxidation of olefins and the sulfoxidation of alkyl aryl sulfides in high enantioselectivities, as well as the asymmetric reduction of racemic hydroperoxides. The less selective oxidative coupHng of various phenols and aromatic amines by peroxidases provides a convenient access to dimeric, oligomeric and polymeric products for industrial applications. 

Other examples involve the immobilization of ruthenium porphyrin catalysts [74]. While Severin et al. generated insoluble polymer-embedded catalysts 16 by co-polymerizing porphyrin derivatives with ethylene glycol dimethacrylate (EGD-MA) [74 a], Che et al. linked the ruthenium-porphyrin unit to soluble polyethylene glycol (PEG) 17 [74b]. Both immobilized catalysts were employed in a variety of olefin epoxidations with 2,6-dichloropyridine N-oxide (Gl2pyNO), providing similar conversions of up to 99% and high selectivities (Scheme 4.9). 

Week et al. [65] further reported the Co salen complex supported on norbomene polymers (23, 24) with stable phenylene-acetylene linker (Figure 8). The polymer-supported salen catalysts were investigated for HKR of the racemic terminal epoxides that showed outstanding catalytic activities and comparable selectivities to the original catalysts reported by Jacobsen. However, the polymeric catalyst was recycled only once after its precipitation with diethylether as the catalyst became less soluble and less reactive in subsequent catalytic... 

Selective polymerization, enantiomers, 185 Semico rrin-copper complexes, 199 Sharpless epoxidation, racemic alcohols, 45 Side-chain units, prostaglandins, 310 Sigmatropic reactions, 222 Silanes, oxidative addition, 126 Silica gel, 285, 352... 

Because the petrochemical industry is based on hydrocarbons, especially alkenes, the selective oxidation of hydrocarbons to produce organic oxygenates occupies about 20% of total sales of current chemical industries. This is the second largest market after polymerization, which occupies about a 45% share. Selectively oxidized products, such as epoxides, ketones, aldehydes, alcohols and acids, are widely used to produce plastics, detergents, paints, cosmetics, and so on. Since it was found that supported Au catalysts can effectively catalyze gas-phase propylene epoxidation [121], the catalytic performance of Au catalysts in various selective oxidation reactions has been investigated extensively. In this section we focus mainly on the gas-phase selective oxidation of organic compounds. 

Aluminum-alkynyl covalent bonds, characteristics, 9, 249-250 Aluminum-aluminum bonds in A1(I) compounds, 9, 261 in Al(II) compounds, 9, 260 Aluminum aryloxides, reactivity, 9, 254—255 Aluminum(III)ates, in organic group-selective transfers, 9, 279 Aluminum(I)-boron bonds, characteristics, 9, 263 Aluminum(III)-boron exchange, process, 9, 266 Aluminum-calix[4]arene catalyst, for alternating epoxide-CC>2 co-polymerization, 11, 617... 

All peptide-catalyzed enone epoxidations described so far were performed using insoluble, statistically polymerized materials (neat or on solid supports). One can, on the other hand, envisage (i) generation of solubilized poly-amino acids by attachment to polyethylene glycols (PEG) and (ii) selective construction of amino acid oligomers by standard peptide synthesis-linked to a solid support, to a soluble PEG, or neat as a well-defined oligopeptide. Both approaches have been used. The former affords synthetically useful and soluble catalysts with the interesting feature that the materials can be kept in membrane reactors for continuously oper-... 

---