The interaction between polymerization initiators and inhibitors in solution—IV: The reaction of benzoyl peroxide with hydroquinone in solution

The reaction between benzoyl peroxide and hydroquinone in a wide variety of solvents has been investigated by isolation and identification of the products. Benzoic acid, p-benzoquinone and benzoyloxy derivatives of p-benzoquinone are obtained under all conditions. Their relative amounts are largely determined by the molar ratios of the reactants and the nature of the solvent. In strongly polar solvents of high solvation power, p-benzoquinone is formed in preference to its derivatives. Nevertheless, the yield of 2,5-dibenzoyloxy p-benzoquinone reaches a maximum in acrylonitrile. Only in this solvent, at equal molar ratios of reactants, is full substitution in the hydroquinone nucleus achieved with the formation of tetrabenzoyloxy hydroquinone. These facts, together with the partial polymerization of acrylonitrile at room temperature at slightly higher peroxide/hydroquinone ratios and the complete suppression of the polymerization when perchloric acid is added, could be explained by a heterolytic mechanism involving the formation and controlled separation of ion pairs derived from the reactants.     

Presimplifiable and Cyclic Group Rings

Let R be any commutative ring with identity, and let C be a (finite or infinite) cyclic group. We show that the group ring R(C) is presimplifiable if and only if its augmentation ideal I(C) is presimplifiable. We conjecture that the group rings R(C _n ) are presimplifiable if and only if n = p ^m , p ∈ J(R), p is prime, and R is presimplifiable. We show the necessity of n = p ^m , and we prove the sufficiency when n = 2, 3, 4. These results were made possible by a new formula derived herein for the circulant determinantal coefficients.     

A triptycene-based polymer of intrinsic microposity that displays enhanced surface area and hydrogen adsorption

A novel triptycene-based polymer of intrinsic microporosity (Trip-PIM) displays enhanced surface area (1065 m2 g(-1)) and reversibly adsorbs 1.65% hydrogen by mass at 1 bar/77 K and 2.71% at 10 bar/77 K.     

Optimizing dirhodium(II) tetrakiscarboxylates as chiral NMR auxiliaries

Thirteen enantiopure paddlewheel-shaped dirhodium(II) tetrakiscarboxylate complexes have been checked for their efficiency in the dirhodium method (differentiation of enantiomers by NMR spectroscopy); six of them are new. Their diastereomeric dispersion effects were studied and compared via so-called key numbers KN. Adducts of each complex were tested with five different test ligands representing all relevant donor properties from strong (phosphane) to very weak (ether). Only one of them, the dirhodium complex with four axial (S)-N-2,3-naphthalenedicarboxyl-tert-leucinate groups (N23tL), showed results significantly better for all ligands than the conventional complex Rh* [Rh(II)(2)[(R)-(+)-MTPA](4); MTPA = methoxytrifluoromethylphenylacetate]. On the basis of (1)H{(1)H} NOE spectroscopy and X-ray diffraction, a combination of favourable anisotropic group orientation and conformational flexibility is held responsible for the high efficiency of N23tL in enantiodifferentiation. Both complexes, Rh* and N23tL, are recommended as chiral auxiliaries for the dirhodium experiment.     

Separation of Isomers on Nematic Liquid Crystal Stationary Phases in Gas Chromatography: A Review

Understanding the composition of complex hydrocarbon mixtures is important for environmental studies in diverse fields, but many prevalent compounds cannot be confidently identified using traditional gas chromatography (GC) techniques. Increasing requirements on analyses of isomeric compounds and the problems encountered in their separation demand a study of more efficient systems which exhibit a high selectivity. Kelker and Fresenius first used nematic liquid crystals as stereospecific stationary phases in GC. Nematic liquid crystal has shown this particular selectivity and sensitivity as stationary phases for the separation of isomers having similar volatilities. Because of their unique selectivity towards rigid solute isomers, liquid crystal stationary phases were considered at one time to be a very promising class of materials that give gas chromatographic separations very different from those that can be obtained with any other stationary phase. Since then, a great deal of attention has been paid to the separation properties of this relatively wide group of substances. Liquid crystal can be used to separate a variety of compounds including isomer mixtures which cannot be separated on conventional stationary phases. This paper aims to review all specific experimental results and presents a comparative analytical study of monomeric nematic liquid crystal stationary phases used in GC. A further contribution of this review is in the field of isomeric compounds separation.     

Applications of enzymatic and non-enzymatic methods to access enantiomerically pure compounds using kinetic resolution and racemisation

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Different methods to access enantiomerically pure compounds.     

Coarse graining the dynamics of coupled oscillators

We present an equation-free computational approach to the study of the coarse-grained dynamics of finite assemblies of nonidentical coupled oscillators at and near full synchronization. We use coarse-grained observables which account for the (rapidly developing) correlations between phase angles and natural frequencies. Exploiting short bursts of appropriately initialized detailed simulations, we circumvent the derivation of closures for the long-term dynamics of the assembly statistics.     

Resolution of chiral phosphate, phosphonate, and phosphinate esters by an enantioselective enzyme library

An array of 16 enantiomeric pairs of chiral phosphate, phosphonate, and phosphinate esters was used to establish the breadth of the stereoselective discrimination inherent within the bacterial phosphotriesterase and 15 mutant enzymes. For each substrate, the leaving group was 4-hydroxyacetophenone while the other two groups attached to the phosphorus core consisted of an asymmetric mixture of methyl, methoxy, ethyl, ethoxy, isopropoxy, phenyl, phenoxy, cyclohexyl, and cyclohexoxy substituents. For the wild-type enzyme, the relative rates of hydrolysis for the two enantiomers ranged from 3 to 5.4 x 10(5). Various combinations of site-specific mutations within the active site were used to create modified enzymes with alterations in their enantioselective properties. For the single-site mutant enzyme, G60A, the stereoselectivity is enhanced relative to that of the wild-type enzyme by 1-3 orders of magnitude. Additional mutants were obtained where the stereoselectivity is inverted relative to the wild-type enzyme for 13 of the 16 pairs of enantiomers tested for this investigation. The most dramatic example was obtained for the hydrolysis of 4-acetylphenyl methyl phenyl phosphate. The G60A mutant preferentially hydrolyzes the SP-enantiomer by a factor of 3.7 x 10(5). The I106G/F132G/H257Y mutant preferentially hydrolyzes the RP-enantiomer by a factor of 9.7 x 10(2). This represents an enantioselective discrimination of 3.6 x 10(8) between these two mutants, with a total of only four amino acid changes. The rate differential between the two enantiomers for any given mutant enzyme is postulated to be governed by the degree of nonproductive binding within the enzyme active site and stabilization of the transition state. This hypothesis is supported by computational docking of the high-energy, pentavalent form of the substrates to modeled structures of the mutant enzyme; the energies of the docked transition-state analogues qualitatively capture the enantiomeric preferences of the various mutants for the different substrates. These results demonstrate that the catalytic properties of the wild-type phosphotriesterase can be exploited for the kinetic resolution of a wide range of phosphate, phosphonate, and phosphinate esters and that the active site of this enzyme is remarkably amenable to structural perturbations via amino acid substitution.     

Immobilized versus coated amylose tris(3,5-dimethylphenylcarbamate) chiral stationary phases for the enantioselective separation of cyclopropane derivatives by liquid chromatography

The solvent versatility of Chiralpak IA, a new chiral stationary phase (CSP) containing amylose tris(3,5-dimethylphenylcarabamate) immobilized onto silica gel, is investigated for the enantioselective separation of a set of cyclopropane derivatives using ethyl acetate or dichloromethane (DCM) as non-standard mobile phase eluent and diluent, respectively in high-performance liquid chromatography (HPLC). A comparison of the separation of cyclopropanes on both immobilized and coated amylose tris(3,5-dimethylphenylcarbamate) chiral stationary phases (Chiralpak IA and Chiralpak AD, respectively) in HPLC using a mixture of n-hexane/2-propanol (90/10 and 99/1, v/v) as mobile phase with a flow rate of 0.5 ml/min and UV detection at 254 nm, is demonstrated. The optimized method of separation is used for an online HPLC monitoring for the Rh(II)-catalyzed asymmetric intermolecular cyclopropanations in dichloromethane. Direct analysis techniques without further purification, workup or removal of dichloromethane were summarized. The method provides an easy and direct determination of the enantiomeric excess of the cyclopropanes and selectivity of the catalyst used without any further work up.     

Covalent Modification of Glassy Carbon Surfaces by Using Electrochemical and Solid‐Phase Synthetic Methodologies: Application to Bi‐ and Trifunctionalisation with Different Redox Centres

Glassy carbon electrodes functionalised with two redox centres have been prepared by using electrochemical and solid‐phase synthetic methodologies. Initially the individual coupling of anthraquinone, nitrobenzene and dihydroxybenzene to a glassy carbon electrode bearing an ethylenediamine linker was optimised by using different coupling agents and conditions. Bifunctionalisation was then carried out, either simultaneously, with a mixture of nitrobenzene and dihydroxybenzene, or sequentially, with anthraquinone then nitrobenzene and with anthraquinone then dihydroxybenzene. Characterisation of these electrodes by cyclic voltammetry and differential pulse voltammetry clearly proved the attachment of the pairs of redox centres to the glassy carbon electrode. Their partial surface coverages can be controlled by varying the coupling agent or by controlling the substrate concentration during the solid‐phase coupling process. Trifunctionalisation was also realised according to this methodology.