A Substrate-Driven Approach to Determine Reactivities of α,β-Unsaturated Carboxylic Esters Towards Asymmetric Bioreduction

The degree of C?C bond activation in the asymmetric bioreduction of α,β-unsaturated carboxylic esters by ene-reductases was studied, and general recommendations to render these "borderline-substrates" more reactive towards enzymatic reduction are proposed. The concept of "supported substrate activation" was developed. In general, an additional α-halogenated substituent proved to be beneficial for enzymatic activity, whereas β-alkyl or β-aryl substituents were detrimental for the reactivity of nonhalogenated substrates, and α-cyano groups showed little effect. The alcohol moiety of the ester functionality was found to have a strong influence on the reaction rate. Overall, activities were determined by both steric and electronic effects.     

Ultrasound and microwave assisted synthesis of isoindolo-1,2-diazine: a comparative study

A comparative study, ultrasound (US) versus microwave (MW) versus conventional thermal heating (TH), for synthesis of isoindolo-1,2-diazine is described. The reaction pathway is fast, efficient and straight applicable, involving a Huisgen [3+2] dipolar cycloaddition of cycloimmonium ylides to 1,4-naphthoquinone. A feasible reaction mechanism for the obtaining of the fully aromatized tetra- and penta- cyclic isoindolo-1,2-diazine is presented. Under US irradiation the yields are much higher (sometimes substantially, by almost double), the reaction time decreases substantially, the reaction conditions are milder. The use of a generator with a higher nominal power induces higher yields and short reaction times. Overall the use of US it proved to be more efficient than MW or TH. A feasible explication for US efficiency is presented.     

A facile synthesis of pyridazinone derivatives under ultrasonic irradiation

A new, efficient and general method for preparation of N-substituted-pyridazinones using ultrasound irradiation is reported. Under ultrasound the reaction time decreases substantially, the yields are high and the reaction conditions are mild. It was noticed that substituents at the 3-(6)-position of pyridazone heterocycle have a substantial influence on the reactivity, while the effect of the substituents at the 1-(2)-position seems to be of minor importance. A comparative study of the reactions performed under ultrasound conditions versus at room temperature has been done.     

EPR study of interaction of vanadium pentoxide with H-ZSM-5 zeolite

The EPR spectroscopy has been used to study the interaction of vanadium pentoxide with H-ZSM-5 zeolite, supplied by ICITPR-Ploiesti, Romania, occurring at the calcination of the mixture at temperatures from 100 to 1000°C, in air, followed by evacuation at 20°C, as well as for those rehydrated at room temperature. The EPR method is also used to follow the thermal stability of the investigated zeolite, the framework alteration and the nature of VO²⁺ complexes formed during solid phase reaction. At least two types VO²⁺ centres were identified in H-ZSM-5 zeolite. One of them being ascribed to dehydrated VO²⁺ complexes in cationic sites, with C_4v ligand field symmetry and the second one corresponding to the hydrated complex with a square bipyramidal symmetry, D_4h. Increasing covalent character, for the in-plane σ-bond and increasing ionic character for the out-of-plane π bond, have been found on rehydrated samples. For a low content of oxide ( 5% wt V₂O₅) in a mixture the thermal stability, T = 1070°C, of the zeolite phase is not affected by the presence of the reaction products.     

The study of non-isothermal degradation of acrylic ion-exchange resins

Summary The thermal behavior under non-isothermal conditions of some low-acidity carboxylic cationites with acrylic-divinylbenzene (DVB) matrix was investigated in air and nitrogen atmosphere up to 600°C. Thermal analysis (TG/DTG) combined with Mass Spectrometry (MS) and Fourier Transform Infrared Spectroscopy (FTIR) was used to characterize the resins decomposition steps and the degradation products. Five decomposition steps were observed. The first step is due to elimination of osmotic and bound water. The second decomposition step is due to dehydration of neighboring (-COOH) groups. The third and fourth mass-loss steps correspond to decarboxylation of the polyanhydrides and to some depolymerization of the polymeric matrix. The last decomposition step is associated with total degradation of the polymeric matrix. The shape and the temperature for each decomposition steps depend on the experimental conditions (heating rate and degradation atmosphere) and on the sample properties (i.e. granulation, cross-linking degree, porosity and physical form).     

Photoinduced energy- and electron-transfer processes in dinuclear Ru(II)-Os(II), Ru(II)-Os(III), and Ru(III)-Os(II) trisbipyridine complexes containing a shape-persistent macrocyclic spacer.

The PF6- salt of the dinuclear [(bpy)2Ru(1)Os(bpy)2]4+ complex, where 1 is a phenylacetylene macrocycle which incorporates two 2,2'-bipyridine (bpy) chelating units in opposite sites of its shape-persistent structure, was prepared. In acetonitrile solution, the Ru- and Os-based units display their characteristic absorption spectra and electrochemical properties as in the parent homodinuclear compounds. The luminescence spectrum, however, shows that the emission band of the Ru(II) unit is almost completely quenched with concomitant sensitization of the emission of the Os(II) unit. Electronic energy transfer from the Ru(II) to the Os(II) unit takes place by two distinct processes (k(en) = 2.0x10(8) and 2.2x10(7) s(-1) at 298 K). Oxidation of the Os(II) unit of [(bpy)2Ru(1)Os(bpy)2]4+ by Ce(IV) or nitric acid leads quantitatively to the [(bpy)2Ru(II)(1)Os(III)(bpy)2]5+ complex which exhibits a bpy-to-Os(III) charge-transfer band at 720 nm (epsilon(max) = 250 M(-1) cm(-1)). Light excitation of the Ru(II) unit of [(bpy)2Ru(II)(1)Os(III)(bpy)2]5+ is followed by electron transfer from the Ru(II) to the Os(III) unit (k(el,f) = 1.6x10(8) and 2.7x10(7) s(-1)), resulting in the transient formation of the [(bpy)2Ru(III)(1)Os(II)(bpy)2]5+ complex. The latter species relaxes to the [(bpy)2Ru(II)(1)Os(III)(bpy)2]5+ one by back electron transfer (k(el,b) = 9.1x10(7) and 1.2x10(7) s(-1)). The biexponential decays of the [(bpy)2*Ru(II)(1)Os(II)(bpy)2]4+, [(bpy)2*Ru(II)(1)Os(III)(bpy)2]5+, and [(bpy)2Ru(III)(1)Os(II)(bpy)2]5+ species are related to the presence of two conformers, as expected because of the steric hindrance between hydrogen atoms of the pyridine and phenyl rings. Comparison of the results obtained with those previously reported for other Ru-Os polypyridine complexes shows that the macrocyclic ligand 1 is a relatively poor conducting bridge.     

Static dielectric constant,viscosity, and structure of pure isomeric pentanols

Static dielectric constants, viscosities, densitites and refractive indices of 2-pentanol, 3-pentanol, 2-methyl-1-butanol, 3-methyl-1-butanol and 2-methyl-2-butanol were measured at 15, 25, 35 and 45°C. These results together with the previous data on n-pentanol have been analyzed in terms of the Kirkwood correlation factor g _k and of the energy of activiation for viscous flow. With the exception of 2-methyl-2-butanol, g _k was found to be greater than unity. These results show that the monomeric units of isomeric pentanols interact by means of hydrogen bonding to form dynamic structures essentially of two types: linear chains where co-association raises the total polarizability and cyclic dimers with nearly zero net dipole moment. Energies of activation for viscous flow as well as Kirkwood correlation factors correlate with the molecular parameters (i.e. position of OH group in the molecules, steric hindrance of alkyl chain etc.) of the alcohols. The implication of these parameters on the molecular association of the isomeric pentanols are discussed.     

Charge exchange in tandem mass spectrometry: dissociative single electron capture by doubly-charged toluene cations

Single electron capture by doubly-charged toluene cations upon collision with various target gases has been investigated by sector tandem mass spectrometry. Both non-dissociative and dissociative charge transfer reactions leading to C(7)H(7)(+) + H and to C(5)H(5)(+) + [C(2),H(3)] are detected. Seven atomic or molecular target gases have been used with ionisation energies ranging from 8.8 eV to 14 eV. The branching ratios between the different non-dissociative and dissociative exit channels have been determined as well as the translational energy release on the dissociation products. The experimental data are compared to the predictions of a two-state semi-classical theoretical model that takes into account the non-adiabatic transition responsible for the charge transfer reaction. A wide reaction window shows up but the internal energies of the C(7)H(8)(+) cations produced by single electron capture are observed to be larger than expected. We assign this effect partly to the influence of the large density of vibrational states and to the multichannel nature of the process. Excited states of the dication are also most probably involved in the charge exchange reaction.