Intensities of IR stretching bands as a criterion of polarization and initial chemical activation of adsorbed molecules in acid catalysis. Ethane adsorption and dehydrogenation by zinc ions in ZnZSM-5 zeolite

A DRIFT study of ethane adsorbed by zinc cations in ZnZSM-5 prepared by chemical reaction of the hydrogen form of the zeolite with zinc vapor at 770 K, or by wet ion exchange, reveals unusual spectra of adsorbed C2H6 species. In addition to the weakly perturbed narrow bands in the region of C-H stretching vibrations, these spectra exhibit a very intense broad IR band with a frequency that is more than 200 cm(-1) lower than those of the C-H stretching vibrations of gaseous or physically adsorbed ethane. The very high relative intensity of this band indicates a very strong polarizability of the corresponding vibrational mode. It is concluded that these strongly polarized vibrations are closely connected with the subsequent heterolytic dissociation of ethane at moderately elevated temperatures, resulting in the formation of acidic hydroxyl groups and zinc ethyl fragments. At higher temperatures, the zinc ethyl fragments decomposed, resulting in the formation of zinc hydrides and ethylene. The unusual DRIFT spectrum of the molecular form of ethane adsorption by zinc cations represents a first example of anisotropy of polarizability of adsorbed molecules, which may provide a new explanation for selectivity of the acid-catalyzed reactions. In this connection, it is suggested to use the relative intensities of IR bands of adsorbed molecules as a reactivity index that is directly connected with chemical activation of adsorbed molecules via their polarization by the active sites.     

Use of the IR spectra of adsorbed ethane and propane molecules to characterize the strength of active sites in zeolites and to analyze the activation of C-H bonds in these paraffins

The adsorption and activation of ethane and propane on the hydrogen and cationic forms of mordenite, zeolite ZSM-5, and zeolite Y were studied by diffuse-reflectance IR spectroscopy. The effect of the polarization of these molecules by adsorption sites on the intensities and shifts of absorption bands due to C-H stretching vibrations were studied. It was found that weak adsorption species were formed on the hydrogen forms of the above zeolites. In this case, both the intensity distributions and the positions of absorption bands due to C-H stretching vibrations were almost independent of the nature of zeolites. However, both absorption band maximum positions and relative intensity distributions changed upon paraffin adsorption on the cationic forms. It was also found that relative intensity distributions and shifts of absorption bands due to C-H stretching vibrations strongly depended on the nature of cations and zeolites. In this case, the initially totally symmetrical C-H vibrations were found most strongly disturbed. The low-frequency shifts and relative intensities of absorption bands due to these vibrations for various cations and zeolites were found to increase in the following orders: H < Na < Ca < Mg < Zn and zeolite Y < Mord ≈ ZSM-5. The experimental results suggest that ethane and propane molecules can be used as molecular probes for acquiring information on the nature and properties of acidbase sites in zeolites. In this case, both the low-frequency shifts and the relative intensities of absorption bands due to C-H stretching vibrations can be used as measures to characterize the nature of cations and zeolites. However, the latter was found to be much more sensitive to the nature of active sites.     

IR absorption intensities as a new spectral criterion for the activation of adsorbed molecules in heterogeneous acid catalysis

The criterion conventionally used to judge bond activation in molecules adsorbed on active sites of heterogeneous catalysts is the bathochromic shift of the corresponding IR absorption bands. The intensity of these bands, which characterizes bond polariability, can be used as an additional bond activation criterion. This new spectral criterion is particularly promising for acid and acid-base catalysis, in which the activation of adsorbed molecules is due to their polarization by active sites. Examples are provided here to illustrate the fruitfulness of the new approach. These examples include judging the strength of Brønsted acid sites from the intensity of the OH stretching band and an analysis of the dissociative adsorption and dehydrogenation of light paraffins on metal cation forms of zeolites and of proton transfer from Brønsted acid sites to adsorbed paraffin molecules.     

State and properties of ion-exchanged cations in zeolites: 1. IR spectra and chemical activation of adsorbed molecular hydrogen

Adsorption isotherms and of adsorbed molecular hydrogen indicate that H₂ is weakly adsorbed by alkali-metal forms of faujasites, mordenite, and high-silica zeolite ZSM-5. The alkaline-earth forms of the same zeolites adsorb hydrogen somewhat more strongly; nevertheless, the hydrogen molecules adsorbed by the barium form of mordenite are in the hindered rotation state. Molecular hydrogen is most strongly adsorbed by the zinc and cadmium forms of the high-silica zeolite. In this case, molecular hydrogen is strongly polarized and undergoes heterolytic dissociative adsorption, yielding acidic hydroxyl groups and cation-bound hydride ions.     

"Bent bonds" between bismuth and carbon atoms as a result of C-H activation in Mo-Bi complexes

The reaction of molybdocenedihydride with two equivalents of [Bi(OtBu)(3)] proceeds via alcohol elimination and provides the compound [Cp(2)Mo{Bi(OtBu)(2)}(2)] (1), which contains two Mo--Bi metal bonds, in good yields. If the two reagents are employed in a 1:1 ratio continuative condensation reactions occur. These initially lead to [{Cp(2)Mo}(2){mu-Bi(OtBu)}(2)] (2), which, however, is very unstable in solution and decomposes via additional alcohol elimination: Complex-induced proximity effects facilitate the cleavage of C--H bonds within the cyclopentadienyl ligands by the residual alkoxide ligands, so that spontaneously two further equivalents of alcohol are released, thereby yielding two isomeric compounds 3 and 4 with Cp ligands bridging Mo--Bi metal bonds: The first isomer (3) contains two mu(2)-eta(5):eta(1)-C(5)H(4) ligands, the second isomer (4) contains one bridging mu(3)-eta(5):eta(1):eta(1)-C(5)H(3) ligand. The binding of these ligands to molybdenum and bismuth atoms at the same time is made possible through "bent bonds" between the bismuth and certain carbon centres. These unusual bonding situations were analysed by means of calculations based on density functional theory (DFT), the atoms in molecules (AIM) theory, natural bond order (NBO) considerations and the electron localisation function (ELF). According to the results the bonds can be understood in terms of carbanionic centres interacting with bismuth cations (i.e. closed-shell interactions). The formation of these bonds and the thermodynamics/kinetics involved on going from 2 to 3 and 4 were also studied by theoretical methods, so that the product formation is rationalised. The crystal structures of all four new compounds were determined. These structures but also the properties and mechanisms of formation are discussed against the background of the corresponding results obtained while studying the system [(Me)Cp(2)MoH(2)]/[Bi(OtBu)(3)].     

Copper-catalyzed oxidative amination of benzoxazoles via C-H and C-N bond activation: a new strategy for using tertiary amines as nitrogen group sources

An efficient and conceptually new method for oxidative amination of azoles with tertiary amines via copper-catalyzed C-H and C-N bond activation has been developed. This protocol can be performed in the absence of external base and only requires atmospheric oxygen as oxidant. The catalyst system is very simple and efficient, which opens a new way for using tertiary amines as nitrogen group sources for C-N bond formation reactions.     

Formation of a bridging planar trimethylenemethane dianion from a neopentyl precursor via sequential beta-alkyl elimination and C-H activation

The reaction of neopentyllithium, Me3CCH2Li, with [(C5Me5)2Sm][(mu-Ph)2BPh2], 1, was investigated as a route to the unsolvated alkyl, [(C5Me5)2Sm(CH2CMe3)]x, and found to generate the first f element trimethylenemethane dianion complex, [(C5Me5)2Sm]2[mu-eta3:eta3-C(CH2)3], 2. Formation of the [C4H6]2- trimethylenemethane ligand from the [C5H11]1- neopentyl precursor can be explained by a combination of a beta-methyl elimination reaction to form isobutene and [(C5Me5)2SmMe]3, 3, with subsequent C-H activation reactions. This sequence has been modeled in several ways, including the synthesis of 2 from reactions of 3 with CH2=CMe2 and 3 with the 2-methylallyl complex, (C5Me5)2Sm[CH2C(Me)CH2], 4.     

Selective degradation of chemical bonds: from single-source molecular precursors to metallic Ag and semiconducting Ag2S nanocrystals via instant thermal activation

Selective formation of metallic Ag and semiconducting Ag(2)S nanocrystals has been achieved via a modified hot-injection process from a single-source precursor molecule, Ag(SCOPh), which can potentially generate both [Ag] and [AgS] fragments simultaneously. When the precursor molecules are injected into a preheated reaction system at 160 degrees C, spherical Ag(2)S nanocrystals are directly obtained even without a molecular activator, such as alkylamines. Mixtures of Ag and Ag(2)S or pure metallic Ag nanocrystals are obtained if the precursor molecules are injected at lower than 160 degrees C or room temperature. These results are attributed to the direct transfer of thermal energies to precursor molecules, which are enough to dissociate S-C as well as Ag-S bonds simultaneously. Detailed characterizations about the produced nanocrystals have been performed using powder X-ray diffraction (XRD), transmission electron microscopy (TEM), as well as energy-dispersive X-ray (EDX) spectrum.     

Effective oxidation of benzylic and alkane C-H bonds catalyzed by sodium o-iodobenzenesulfonate with Oxone as a terminal oxidant under phase-transfer conditions

Catalytic oxidation of benzylic C-H bonds could be efficiently realized using IBS as a catalyst which was generated in situ from the oxidation of sodium 2-iodobenzenesulfonate (1b) by Oxone in the presence of a phase-transfer catalyst, tetra-n-butylammonium hydrogen sulfate, in anhydrous acetonitrile at 60 °C. Various alkylbenzenes, including toluenes and ethylbenzenes, several oxygen-containing functionalities substituted alkylbenzenes, and a cyclic benzyl ether could be efficiently oxidized. And, the same reagent system of cat. 1b/Oxone/cat. n-Bu(4)NHSO(4) could be applied to the effective oxidation of alkanes as well.     

Structural determination and complete NMR spectral assignments of a new diterpenoid obtained from triptonide by biotransformation

A new diterpenoid, 12β,13α‐dihydroxytriptonide, was obtained from the transformed culture of triptonide by Catharanthus roseus cell suspension cultures. The complete ¹H and ¹³C NMR assignments of the compound were carried out by using DEPT, COSY, HSQC, g‐HMBC and NOESY techniques. Copyright © 2003 John Wiley & Sons, Ltd.     

Synthesis of new 2-vinylation products of indole via a Michael-type addition reaction with dimethyl acetylenedicarboxylate and their Diels-Alder reactivity as precursors of new carbazoles

Reaction of 4,7-dihydroindole and dimethyl acetylenedicarboxylate provided a convenient route to functionalized 2-vinylindoles. Diels-Alder reactions of the 2-vinylindoles with naphthoquinone, p-benzoquinone, 1,2-dicyano-4,5-dichloroquinone, N-phenyltriazolinedione, and tetracyanoethylene were investigated to give [c]annelated 1,2-dihydro, 1,2,3,4-tetrahydro, and fully aromatized carbazoles. The structure and formation mechanism of both 2-vinylindoles and their cycloadduct are discussed.     

Aromatic vs aliphatic C-H bond activation by rhodium(I) as a function of agostic interactions: catalytic H/D exchange between olefins and methanol or water

The aryl-PC type ligand 3, benzyl(di-tert-butyl)phosphane, reacts with [Rh(coe)(2)(solv)(n)()]BF(4) (coe = cyclooctene, solv = solvent), producing the C-H activated complexes 4a-c (solv = (a). acetone, (b). THF, (c). methanol). Complexes 4a-c undergo reversible arene C-H activation (observed by NMR spin saturation transfer experiments, SST) and H/D exchange into the hydride and aryl ortho-H with ROD (R = D, Me). They also promote catalytic H/D exchange into the vinylic C-H bond of olefins, with deuterated methanol or water utilized as D-donors. Unexpectedly, complex 2, based on the benzyl-PC type ligand 1 (analogous to 3), di-tert-butyl(2,4,6-trimethylbenzyl)phosphane, shows a very different reversible C-H activation pattern as observed by SST. It is not active in H/D exchange with ROD and in catalytic H/D exchange with olefins. To clarify our observations regarding C-H activation/reductive elimination in both PC-Rh systems, density functional theory (DFT) calculations were performed. Both nucleophilic (oxidative addition) and electrophilic (H/D exchange) C-H activation proceed through eta(2)-C,H agostic intermediates. In the aryl-PC system the agostic interaction causes C-H bond acidity sufficient for the H/D exchange with water or methanol, which is not the case in the benzyl PC-Rh system. In the latter system the C-H coordination pattern of the methyl controls the reversible C-H oxidative addition leading to energetically different C-H activation processes, in accordance with the experimental observations.     

Synthesis of new tridentate chiral aminoalcohols by a multicomponent reaction and their evaluation as ligands for catalytic asymmetric Strecker reaction

A one-step, multicomponent Mannich-type reaction between phenols, paraformaldehyde, and β-aminoalcohols in the presence of LiCl afforded N-2-hydroxybenzyloxazolidines with high ortho-selectivity. Hydrolytic or reductive ring opening of the oxazolidines provided a series of N-salicyl-β-aminoalcohols in 84-92% overall yield. The synthesized compounds were evaluated as ligands for a titanium-catalyzed catalytic asymmetric Strecker reaction. The reaction employing 10 mol % of catalyst provided the Strecker products in excellent yields and up to 98% ee.     

Comparison of electron and chemical ionization modes by validation of a quantitative gas chromatographic-mass spectrometric assay of new generation antidepressants and their active metabolites in plasma

A gas chromatographic-mass spectrometric method (GC-MS) for the simultaneous determination of the 'new' antidepressants (mirtazapine, viloxazine, venlafaxine, trazodone, citalopram, mianserin, reboxetine, fluoxetine, fluvoxamine, sertraline, maprotiline, melitracen, paroxetine) and their active metabolites (desmethylmirtazapine, O-desmethylvenlafaxine, m-chlorophenylpiperazine, desmethylcitalopram, didesmethylcitalopram, desmethylmianserin, desmethylfluoxetine, desmethylsertraline, desmethylmaprotiline) in plasma using different ionization modes was developed and validated. Sample preparation consisted of a strong cation exchange mechanism and derivatisation with heptafluorobutyrylimidazole. The GC separation was performed in 24.8 min. Identification and quantification were based on selected ion monitoring in electron (EI) and chemical ionization (CI) modes. Calibration by linear and quadratic regression for electron and chemical ionization, respectively, utilized deuterated internal standards and a weighing factor 1/x(2). Limits of quantitation were established between 5 and 12.5 ng/ml in EI and positive ionization CI (PICI), and 1 and 6.25 ng/ml in negative ionization CI (NICI). During validation stability, sensitivity, precision, accuracy, recovery, and selectivity were evaluated for each ionization mode and were demonstrated to be acceptable for most compounds. While it is clear that not all compounds can be quantitated either due to chromatographic (trazodone) or derivatisation problems (O-desmethylvenlafaxine), this method can quantitate most new antidepressants (ADs) in the therapeutic range using EI. PICI and NICI lead to higher selectivity. Moreover, NICI is of interest for small sample volumes and high sensitivity requirements. This paper draws the attention to the pros and cons of the different ionization modes in the GC-MS analysis of these antidepressants in plasma.     

Extraction of neurotransmitters from rat brain using graphene as a solid-phase sorbent, and their fluorescent detection by HPLC

A simple, cost-effective and efficient method was developed for the determination of glycine, gamma-aminobutyrate and taurine in rat brain using graphene as a sorbent for solid-phase extraction. The analytes were eluted from a graphene-packed solid-phase extraction cartridge with methanol, derivatized at their amino groups with the fluorescent label 4-carboxy-2,6-dimethylquinoline N-hydroxysuccinimide ester, and then separated and fluorescently detected by HPLC. The type and volume of eluent, sample pH, extraction time and sample volume were optimized with respect to sensitivity and precision. Under optimal conditions, linear response is obtained in the concentration range from 0.1 to 50?μg?g?1, with correlation coefficients of >0.990. The limits of detection are 23.4?ng?g^?1 (gamma-aminobutyrate), 45.3?ng?g^?1 (glycine) and 67.5?ng?g^?1 (taurine) (S/N?=?3). The results reveal the potential of graphene as a sorbent in the analysis of biological samples.

Significant acceleration of 6 pi-azaelectrocyclization resulting from a remarkable substituent effect and formal synthesis of the ocular age pigment A2-E by a new method for substituted pyridine synthesis

The remarkable acceleration of 6 pi-azaelectrocyclization due to the combination of the C4-carbonyl and the C6-alkenyl or phenyl substituents in 1-azatrienes was found. This observation was rationalized by considering the remarkable orbital interaction between the HOMO and LUMO of 1-azatrienes, which were obtained by molecular orbital calculations. The formal synthesis of the unusual retinal metabolite, A2-E, was achieved by two types of the new one-pot synthesis of substituted pyridines by utilizing the obtained facile 6 pi-azaelectrocyclization, one of which is compatible with the proposed metabolic pathway of A2-E.     

Determination of proteins at nanogram levels by their quenching effect on the chemiluminscence reaction between luminol and hydrogen peroxide with manganese-tetrasulfonatophthalocyanine as a new catalyst

The manganese-tetrasulfonatophthalocyanine (MnTSPc) catalyzed luminol-hydrogen peroxide chemiluminescence (CL) systems can be quenched in the presence of proteins. A highly sensitive CL quenching method has been developed for the determination of proteins. Under optimum conditions, the linear ranges of the calibration curves were 0.1-20 microg/mL for human serum albumin (HSA), 0.2-20 microg/mL for human gamma-IgG, and 0.5-50 microg/mL for the bovine serum albumin (BSA) with the corresponding detection limits were 1.9 ng/mL, 2.7 ng/mL, and 3.4 ng/mL. The method has been applied to the analysis of total proteins in human serum samples and the results were in good agreement with clinical data provided.     

Synthesis, spectroscopic studies and electrochemistry of palladium (II) macrocyclic complexes derived from a new tetraazahalogen substituted ligands by template method and their antimicrobial and pesticidal activities

A new series of Pd(II) macrocyclic complexes have been synthesized by template condensation of bis(benzil)4-chloro 1,2-phenylenediamine (ML1) and bis(benzil)4-fluro 1,2-phenylenediamine (ML2) respectively, with appropriate diamine i.e. 1,2-phenylenediamine, 4-chloro 1,2-phenylenediamine and 4-fluro 1,2-phenylenediamine in the presence of PdCl2 to form complexes of the type [Pd(C40H26N4ClF)]Cl2, [Pd(C40H27N4X)]Cl2 and [Pd(C40H26N4X2)]Cl2, where X=Cl, F. The complexes have been characterized with the help of elemental analysis, IR, 1H NMR, electronic spectra, conductance measurement, magnetic susceptibility, cyclic voltammetry and X-ray powder diffraction studies. On the basis of these studies a square planar geometry has been proposed around the metal ion. The newly synthesized ligands and their complexes have been screened for antimicrobial and pesticidal activities. The results obtained from bioassays indicate that this class of compounds can be utilized for the design of new substance with pesticidal activity and promising antimicrobial activity.