A poly(ethylene glycol)-supported quaternary ammonium salt: An efficient, recoverable, and recyclable phase-transfer catalyst

A quaternary ammonium salt readily immobilized on a soluble poly(ethylene glycol) polymer support efficiently catalyzes different reactions carried out under phase-transfer catalysis conditions; the catalyst, easily recovered by precipitation and filtration, shows no appreciable loss of activity when recycled three times.     

A poly(ethylene glycol)-supported quaternary ammonium salt for highly efficient and environmentally friendly chemical fixation of CO2 with epoxides under supercritical conditions

A PEG-supported quaternary ammonium salt is proved to be an efficient and recyclable homogeneous catalyst for solvent-free synthesis of cyclic carbonates from carbon dioxide and epoxides under supercritical conditions. Supporting Bu₄NBr on soluble polymer PEG6000 enhances the catalytic activity. The workup procedure is straightforward, and the catalyst can be reused over five times with no appreciable loss of catalytic activity and selectivity.     

Enzyme-mediated enantioselective hydrolysis of poly(ethylene glycol)-supported carbonates

Kinetic resolution of poly(ethylene glycol)(PEG)-supported carbonates by enzymatic hydrolysis is discussed. Water-soluble carbonates are prepared by immobilization of racemic secondary alcohols onto low-molecular weight monomethoxy PEG (MPEG) through a carbonate linker. Porcine pancreas lipase (PPL) enantioselectively catalyzes the hydrolysis of the substrates to give optically active compounds. In this system, the separation of the resulting alcohols and the remaining substrates is achieved by an extraction process without laborious column chromatography. The carbonates are easily hydrolyzed with K₂CO₃ to afford the corresponding alcohols.     

Kinetic resolution of poly(ethylene glycol)-supported carbonates by enzymatic hydrolysis

The enzyme-mediated enantioselective hydrolysis of poly(ethylene glycol) (PEG)-supported carbonates is disclosed. The water-soluble carbonates were prepared by immobilization of a racemic secondary alcohol (4-benzyloxy-2-butanol) onto low-molecular weight (av MW 550 and 750) monomethoxy PEG through a carbonate linker. For the screening of the hydrolytic enzymes, the substrate was enantioselectively hydrolyzed by commercially available lipase from porcine pancreas (PPL; Type II, Sigma) to afford the optically active compounds. In this system, the separation of the remaining (S)-substrate and the resulting (R)-alcohol was achieved by an extraction process without a laborious column chromatography. The (S)-carbonate was easily hydrolyzed with K₂CO₃ to afford the corresponding (S)-alcohol. Other MPEG-supported substrates were also hydrolyzed to afford the corresponding optically active alcohols.     

Synthesis of poly(ethylene glycol)-supported manganese porphyrins: efficient,recoverable and recyclable catalysts for epoxidation of alkenes

Two new poly(ethylene glycol) supported manganese porphyrins have been prepared and their catalytic activity and recyclability were investigated for the epoxidation of alkenes using H2O2 and PhIO as stoichiometric oxidants.     

Immobilization of catalysts derived from Cinchona alkaloids on modified poly(ethylene glycol)

The straightforward immobilization of some derivatives of Cinchona alkaloids on modified poly(ethylene glycol)s is reported. The compounds, obtained by simple reactions exploiting different sites for the attachment of the alkaloids to the polymer, were tested as catalysts in the enantioselective benzylation of the benzophenone imine of glycine t-butyl ester (ee up to 64%) and in the conjugate addition of thiophenol to cyclohexenone (ee 22%). The observed stereoselectivities were compared to those obtained either with the unsupported catalysts or with the catalysts immobilized on different polymeric matrixes. The influence of the poly(ethylene glycol) moieties on the catalytic activity is discussed.     

Poly(ethylene glycol)-supported α,α,α-trifluoroacetophenone in dioxirane mediated alkene epoxidation reactions

Poly(ethylene glycol) (PEG) was used for the immobilization of α,α,α-trifluoroacetophenone and the utility of this supported ketone has been examined in dioxirane mediated epoxidation of alkenes. The PEG-ketone reagent was found to be an effective homogeneous catalyst for the epoxidation of a variety of alkenes in the presence of Oxone^® and was readily recovered from the reaction mixtures and reused.     

Theoretical investigation on mechanism of asymmetric Michael addition of malononitrile to chalcones catalyzed by Cinchona alkaloid aluminium(III) complex

The mechanism of Michael addition of malononitrile to chalcones catalyzed by Cinchona alkaloid aluminium(III) complex has been investigated by DFT and ONIOM methods. Calculations indicate that the reaction proceeds through a dual activation mechanism, in which Al(III) acts as a Lewis acid to activate the electrophile α,β-unsaturated carbonyl substrate while the tertiary amine in the Cinchona alkaloid works as a Lewis base to promote the activation of the malononitrile and deprotonation. A stepwise pathway involving C-C bond formation followed by proton transfer from the catalyst to the carbonyl substrate is adopted, and latter step is predicted to be the rate-determining-step in the reaction with an energy barrier of 12.4 kcal mol(-1). In the absence of the Al(III)-complex, a Cinchona alkaloid activates the carbonyl substrate by a hydrogen bonding of the hydroxyl group, involving a higher energy barrier of 30.4 kcal mol(-1). The steric repulsion between the phenyl group attached to the carbonyl group in the chalcone and isopropoxyl groups of the Al(III)-complex may play an important role in the control of stereoselectivity. The π-π stacking effect between the quinuclidine ring of the quinine and the phenyl group of the chalcones may also help the stabilization of the preferred molecular complex. These results are in agreement with experimental observations.     

Aerobic oxidation of alcohols to carbonyl compounds mediated by poly(ethylene glycol)-supported TEMPO radicals

Two poly(ethylene glycol)-supported TEMPO (PEG-TEMPO) has been successfully applied as soluble, recyclable catalysts in the chemoselective oxidation of primary and benzylic alcohols with molecular oxygen in the presence of Co(NO₃)₂ and Mn(NO₃)₂ as co-catalysts (Minisci's conditions). Under those conditions, secondary alcohols are also oxidized to ketones, although usually in lower yields. The insertion of a spacer between the PEG moiety and TEMPO has beneficial effects on both the activity and ease of recovery of the supported catalyst.     

In situ formation and characterization of poly(ethylene glycol)-supported lipid bilayers on gold surfaces

Inclusion of a polymer cushion between a lipid bilayer membrane and a solid surface has been suggested as a means to provide a soft, deformable layer that will allow for transmembrane protein insertion and mobility. In this study, mobile, tethered lipid bilayers were formed on a poly(ethylene glycol) (PEG) support via a two-step adsorption process. The PEG films were prepared by coadsorbing a heterofunctional, telechelic PEG lipopolymer (1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-poly(ethylene glycol)-2000-N-[3-(2-(pyridyldithio)propionate]) (DSPE-PEG-PDP) and a nonlipid functionalized PEG-PDP from an ethanol/water mixture, as described in a previous paper (Munro, J. C.; Frank, C. W. Langmuir 2004, 20, 3339-3349). Then a two-step lipid adsorption strategy was used. First, lipids were adsorbed onto the PEG support from a hexane solution. Second, vesicles were adsorbed and fused on the surface to create a bilayer in an aqueous environment. Fluorescence recovery after photobleaching experiments show that this process results in mobile bilayers with diffusion coefficients on the order of 2 microm2/s. The mobility of the bilayers is decreased slightly by increasing the density of tethered lipids. The formation of bilayers, and not multilayer structures, is also confirmed by surface plasmon resonance, which was used to determine in situ film thickness, and by fluorimetry, which was used to determine quantitatively the fluorescence intensity for each 18 by 18 mm sample. Unfortunately, fluorescence microscopy also shows that there are large defects on the samples, which limits the utility of this system.     

Oxidation chemistry of poly(ethylene glycol)-supported carbonylruthenium(II) and dioxoruthenium(VI) meso-tetrakis(pentafluorophenyl)porphyrin

[Ru(II)(F(20)-tpp)(CO)] (1, F(20)-tpp=meso-tetrakis(pentafluorophenyl)porphyrinato dianion) was covalently attached to poly(ethylene glycol) (PEG) through the reaction of 1 with PEG and sodium hydride in DMF. The water-soluble PEG-supported ruthenium porphyrin (PEG-1) is an efficient catalyst for 2,6-Cl(2)pyNO oxidation and PhI==NTs aziridination/amidation of hydrocarbons, and intramolecular amidation of sulfamate esters with PhI(OAc)(2). Oxidation of PEG-1 by m-CPBA in CH(2)Cl(2), dioxane, or water afforded a water-soluble PEG-supported dioxoruthenium(VI) porphyrin (PEG-2), which could react with hydrocarbons to give oxidation products in up to 80 % yield. The behavior of the two PEG-supported ruthenium porphyrin complexes in water was probed by NMR spectroscopy and dynamic light-scattering measurements. PEG-2 is remarkably stable to water. The second-order rate constants (k(2)) for the oxidation of styrene and ethylbenzene by PEG-2 in dioxane-water increase with water content, and the k(2) values at a water content of 70 % or 80 % are up to 188 times that obtained in ClCH(2)CH(2)Cl.     

Segmented block copolymers of poly(ethylene glycol) and poly(ethylene terephthalate)

A new series of segmented copolymers were synthesized from poly(ethylene terephthalate) (PET) oligomers and poly(ethylene glycol) (PEG) by a two‐step solution polymerization reaction. PET oligomers were obtained by glycolysis depolymerization. Structural features were defined by infrared and nuclear magnetic resonance (NMR) spectroscopy. The copolymer composition was calculated via ¹H NMR spectroscopy. The content of soft PEG segments was higher than that of hard PET segments. A single glass‐transition temperature was detected for all the synthesized segmented copolymers. This observation was found to be independent of the initial PET‐to‐PEG molar ratio. The molar masses of the copolymers were determined by gel permeation chromatography (GPC). © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4448–4457, 2004     

Self-assembly of medium-chain alkyl monoglucosides in ammonium sulfate solutions with poly(ethylene glycol)

We study the phase behavior and microstructure of alkyl-beta-monoglucosides with intermediate chain lengths (octyl- and nonyl-beta-glucoside) in aqueous solutions containing ammonium sulfate and poly(ethylene glycol) (PEG). When the glucoside surfactants are mixed with PEG of molecular weight 3350 or larger, two different phase transitions are observed in the temperature range 0-100 degrees C, with lower and upper miscibility gaps separated by a one-phase isotropic region. Isothermal titration calorimetry is used to quantify the effect of PEG on the micellization properties of the alkyl monoglucosides, whereas small-angle neutron scattering gives insight into the microstructure of the surfactant/polymer mixtures near the liquid-liquid phase boundary. Results show that the range and the strength of the interactions in these solutions are highly affected by the presence of PEG. Solutions with nonyl-beta-glucoside contain larger micelles than those with octyl-beta-glucoside, and the intermicellar interactions are much stronger and longer ranged. The relevance of these findings for membrane protein crystallization is discussed.     

Asymmetric epoxidation catalyzed by novel azacrown ether-type chiral quaternary ammonium salts under phase-transfer catalytic conditions

Asymmetric epoxidation of (E)-chalcone with alkaline hydrogen peroxide by novel chiral phase-transfer catalysts (chiral PTCs) with quaternary ammonium salts of azacrown ether proceeded in high yield and good enantioselectivity. Remarkably, this reaction depended on the length of the carbon chain on the nitrogen atom with the chiral PTCs and on the bulk of the base.     

Surface chemical studies on the competitive adsorption of poly(ethylene glycol) and ammonium poly(methacrylate) onto alumina

The adsorption of poly(ethylene glycol) (PEG) and ammonium poly(methacrylate) (APMA) onto alumina has been examined both individually and in combination. The adsorption density of APMA was found to be higher than that of PEG onto alumina. The adsorption isotherms of PEG and APMA for alumina exhibited a Langmuirian behavior. The adsorption density of PEG was significantly reduced in the presence of APMA, but the reverse was not true. About 60% desorption of PEG from alumina was achieved, while in the case of APMA the amount desorbed was only 10% in the pH range of 3-6. The zeta potential values of alumina were decreased and the isoelectric point (i.e.p.) values were shifted toward acidic pH values, proportional to the concentration of APMA added. However, such changes in the electrokinetic behavior were not observed by the addition of PEG. The dispersion behavior of alumina in the combined presence of PEG and APMA essentially followed the trends obtained for the alumina-APMA system, corroborating the electrokinetic measurements. Coprecipitation tests confirmed complexation between aluminum species and APMA in the bulk solution, but not with PEG. The interaction between alumina and PEG is primarily governed by hydrogen-bonding forces, while both hydrogen bonding and chemical interaction are involved in the case of the alumina-APMA system. FTIR spectroscopic studies provided evidence in support of the interaction mechanisms proposed.     

Poly（ethylene glycol）-supported Liquid-phase Parallel Synthesis of Di（aryloxyacetyl）thiosemicarbazides

An efficient poly(ethylene glycol) (PEG)-supported liquid-phase parallel approach to di(aryloxyacetyl)thiosemicarbazides is described. PEG-bound phenol reacted with chloroacetic acid to afford PEG-bound phenyloxyacetic acid, which was readily converted into corresponding phenyloxyacetyl chloride. Subsequent nucleophilic substitution with ammonium thiocyanate followed by addition of aryloxyacetic acid hydrazides gave PEG-bound di(aryloxyacetyl)thiosemicarbazides, which were easily cleaved to give the resulting library of 1-aryloxyacetyl-4-(4‘-methoxylcarbonylphenyloxyacetyl)thiosemicarbazides in good to high yield and high purity.     

Synthesis and characterization of poly(ethylene glycol) methyl ether endcapped poly(ethylene terephthalate)

Linear and branched poly(ethylene terephthalate) (PET) copolymers with polyethylene glycol) (PEG) methyl ether (700 or 2000 g/mol) end groups were synthesized using conventional melt polymerization. DSC analysis demonstrated that low levels of PEG end groups accelerated PET crystallization. The incorporated PEG end groups also decreased the crystallization temperature of PET dramatically, and copolymers with a high content of PEG (>17.6 wt%) were able to crystallize at room temperature. Rheological analysis demonstrated that the presence of PEG end groups effectively decreased the melt viscosities and facilitated melt processing. XPS and ATR-FTIR revealed that the PEG end groups tended to aggregate on the surface, and the surface of compression molded films containing 34.0 wt% PEG were PEG rich (85 wt% PEG). PEG end-capped PET (34.0 wt% PEG) and PET films were immersed into a fibrinogen solution (0.7 mg/mL BSA) for 72 h to investigate the propensity for protein adhesion. XPS demonstrated that the concentration of nitrogen (1.05%) on the surface of PEG endcapped PET film was statistically lower than PET (7.67%). SEM analysis was consistent with XPS results, and revealed the presence of adsorbed protein on the surface of PET films.     

Catalytic epoxidation of cyclic vinylsilanes by ruthenium(II) complexes under aerobic conditions

A new methodology has been developed for the catalytic epoxidation of cyclic vinylsilanes using a ruthenium(II) bisoxazoline complex 2 with molecular oxygen. An attempt has been made to understand the role of -SiMe₃ group on the rate of epoxidation process.     

Surface modification of polyethersulfone membranes by blending triblock copolymers of methoxyl poly(ethylene glycol)-polyurethane-methoxyl poly(ethylene glycol)

The surface of polyethersulfone (PES) membrane was modified by blending triblock copolymers of methoxyl poly(ethylene glycol)-polyurethane-methoxyl poly(ethylene glycol) (mPEG-PU-mPEG), which were synthesized through solution polymerization with mPEG Mns of 500 and 2000, respectively. The PES and PES/mPEG-PU-mPEG blended membranes were prepared through spin coating coupled with liquid-liquid phase separation. FTIR and (1)H NMR analysis confirmed that the triblock copolymers were successfully synthesized. The functional groups and morphologies of the membranes were studied by ATR-FTIR and SEM, respectively. It was found that the triblock copolymers were blended into PES membranes successfully, and the morphologies of the blended membranes were somewhat different from PES membrane. The water contact angles and platelet adhesion were decreased after blending mPEG-PU-mPEG into PES membranes. Meanwhile, the activated partial thromboplastin time (APTT) for the blended membranes increased. The anti-protein-fouling property and permeation property of the blended membranes improved obviously. SEM observation and 3-(4, 5-Dimethylthiazol-2-yl)-2, 5-diphenyl tetrazolium bromide (MTT) assay proved the surfaces of the blended membranes promoted human hepatocytes adhesion and proliferation better than PES membrane.