High-pressure [2 + 2]cyloaddition of acyl isocyanates to glycals

Abstract

Isocyanates derived from uretanes 4, s and benzamides 6, 7 react with di-O-benzyl-D-arabinal 3 under 12 kbar pressure to afford the corresponding cycloadducts with high stereoselectivity. Alkoxycarbonyl isocyanates 4 and 5 from [2 + 2]cycloadducts almost exclusively, where benzoyl iSocyanates produce 1:1 mixtures of [2 + 21 and [4 + 2]cycloadducts.     

Quenching of Pyrene Fluorescence by Calix[4]arene and Calix[4]resorcinarenes

Interactions involving calixarene and its derivatives are of major importance due to their widespread applications as unique hosts. Fluorescence from a common probe pyrene is used to study interactions involving calix[4]resorcinarene [1a] and its tetra-morpholine derivative [1b] in 1 M aqueous NaOH. These compounds efficiently quench the pyrene fluorescence. A comparison with the fluorescence quenching behavior of N-methylmorpholine clearly indicates the presence of long-range interactions involving 1a and 1b; the interactions are specific to the calixarene molecular framework. This is not the case for a tetra-nitro-substituted calix[4]arene [2b], an electron/charge acceptor quencher, as p-nitrophenol also shows similar interactions with pyrene. Effectiveness of cesium as the quencher of pyrene fluorescence is reduced in the presence of electron/charge donating 1b; fluorescence enhancement is observed upon addition of cesium as the concentration of 1b is increased in the solution. The role of calixarene framework in interactions involving such compounds is established.     

A New Benzothiazin-functionalized Calix[4]arene-based Fluorescent Chemosensor for the Selective Detection of Co2+ Ion

Calixarenes, which have a great place in supramolecular chemistry, have become the most prominent macrocyclic compounds in synthetic organic chemistry due to their easy synthesis and functionalization. In this study, p-tert-butyl calix[4]arene dihydrazide derivative was synthesized and then reacted with 3-oxo-3,4-dihydro-2 H-benzo[b][1,4] thiazin-2-ylideneacetyl chloride to prepare new calixarene based chromophore compound 4. The structure of the synthesized compound was elucidated by spectroscopic methods such as ¹H NMR ¹³C NMR and FT-IR spectroscopy. Chromogenic and fluorescence properties of compound 4 were evaluated. It was observed from both studies that compound 4 was Co²⁺ selective and shows fluorescence Switched-off behavior. Stoichiometry, binding constant and the detection limit were calculated. The stoichiometry between compound 4 and Co²⁺ was found to be 1:1. The binding constant value (K) was calculated as 666.67 M^??1 using Benesi–Hildebrand equation, while the detection limit for Co²⁺ ion was calculated as 0.0465 µM.

     

The spin-crossover complex [Fe(tpa)(NCS) 2 ]

The temperature-induced spin crossover of iron(II) in the [ Fe ( tpa )( NCS ) ₂ ] complex has been investigated by nuclear forward scattering (NFS), nuclear inelastic scattering (NIS), extended X-ray absorption fine structure (EXAFS) spectroscopy, conventional M?ssbauer spectroscopy (MS) and by measurements of the magnetic susceptibility (SQUID). The various measurements consistently show that the transition is complete and abrupt and exhibits a hysteresis between 102 and 110 K. The dependence of the hyperfine parameters of the high-spin (HS) and of the low-spin (LS) phase on temperature is gradual while the effective thickness (determined by the Lamb-M?ssbauer factor f _LM ) shows a step at the transition temperature. This step could be identified clearly because the effective thickness is measured directly by NFS. The Lamb-M?ssbauer factor, the Debye temperature and the mean-square displacement of iron(II) could be determined for the HS and for the LS phase. When comparing the NIS data with the results from density functional theory (DFT), the Fe-N stretching vibrations of both LS and HS phases could be unambiguously identified and the f _LM could be factorized for both phases into a lattice and a molecular part. The structural information from EXAFS and DFT geometry optimization are in reasonable agreement. Received 19 June 2001     

Analytical Evaluation of Cu<Superscript>2+</Superscript> Selective Behavior of calix[4]arene Derivative

The present article describes the solvatochromic effect including solvent system selection, time study and a detailed complexation study along with exploration of extraction properties of 5,11,17,23-tetrakis[(diethylamino)methyl]-25,26,27,28-tetrahydroxycalix[4]arene (4) that bears nitrogen atom as a donor group available for chelating metal ions. Complexation properties of 4 toward selected transition metal ions have been investigated by UV-visible and fluorescence spectroscopies. The% efficiency of 4 toward selected transition metal ions was found in order Cu²⁺> Ni²⁺> Hg²⁺> Zn²⁺> Co²⁺> Cd²⁺> Pb²⁺. It has been noticed that 4 is not only proved to be an efficient Cu²⁺ selective chromoionophore but also possesses an effective extraction property for transferring Cu²⁺ ions from an aqueous to dichloromethane layer. The FT-IR spectroscopic method has also been applied for further confirmation of the complexation phenomenon of 4 with Cu²⁺ ion and found adequate.     

Unitary renormalization of [φ4]2+1

The two-space dimensional ⁴ interaction is renormalized by unitary transformation. A sequence of unitary operators is defined which transform a sequence of cut-off Hamiltonians, arranged in order of increasing cut-off energy, to a sequence of operators converging strongly on a dense set of states. The proof is outlined, calculations leading to requiredL ² estimates on the kernels of a finite number of diagrams are not here detailed.This work was supported in part by NSF grant GP 17523.     

Magnetic order in LaEuSrCuO studied [2pt] by Fe Mössbauer spectroscopy

⁵⁷Fe M?ssbauer effect studies of La_1.65Eu_0.20Sr_0.15CuO₄ doped with 0.5 at% ⁵⁷Fe performed in the temperature region 300 K > T > 4.2 K give an onset temperature for magnetic ordering of K. This temperature practically is the same as that found in Nd doped La_2-xSr_xCuO₄. It indicates that the magnetic ordering temperature in the LTT phase of rare earth (RE) doped La_2-xSr_xCuO₄ is independent of the RE moment. The direction of the ⁵⁷Fe magnetic moment in the magnetically ordered state is within the CuO₂ plane, while it has been found to be parallel to the c-axis in Nd doped La_2-xSr_xCuO₄. Received: 23 June 1998 / Accepted: 14 July 1998     

Solid-phase synthesis of 2-substituted 4-amino-7-oxo-5,6,7,8-tetrahydropyrido[2,3-d]pyrimidines: An example of cyclization-assisted cleavage

An efficient solid-phase synthesis of 2-substituted 4-aminopyrido[2,3-d]pyrimidines 12 by cyclization-assisted cleavage from resin is reported. The procedure starts by solid supporting an ,-unsaturated acid 8 to the Wang resin 13 by using DCC and 4-DMAP in THF. The resulting ,-unsaturated ester 14 is converted to the Michael adduct by treatment with malononitrile in NaOMe/THF. Such Michael addition constitutes the first example of a Michael reaction with malononitrile in solid-phase. Finally, the Michael adducts 15 are treated with an amidine system in MeOH to yield the corresponding pyridopyrimidines 12. Compounds 12 present three diversity centers R1, R2 and G. Having validated the chemistry on solid support, a 40-membered combinatorial library was obtained using this protocol.     

The[g 9 2/2 ]8+ state in96Zr

An 8⁺ state has been identified at 4390 keV in the doubly semi-magic nucleus⁹⁶Zr through the study of angular correlations among the γ radiation which is emitted in the β^? decay of the 10 s isomer of⁹⁶Y. This state probably has the configuration πg ₉ ^2/2 and is populated through a GT decay from the 8⁺ member of the [πg_9/2?νg_7/2] multiplet in⁹⁶Y. The deexcitation indicates that it belongs to a band based on the 0⁺ state in⁹⁶Zr.     

Magnetic quantum oscillations in the organic superconductor κ-(BEDT-TTF)2Cu[N(CN)2]Br

The interlayer magnetoresistance of the organic superconductor κ-(BEDT-TTF)₂Cu[N(CN)₂)Br is measured at ambient pressure and under pressures of up to 12.5 kbar. In addition to the slow Shubnikov-de Haas oscillations with a frequency of ≃150 T observed at P⩾5 kbar, rapid oscillations attributed to the magnetic breakdown orbit enveloping an area equal to 100% of the area of the Brillouin zone are found to emerge above B=20 T. The latter oscillations are observed at ambient pressure as well as under pressures of up to 9 kbar. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 3, 190–194 (10 August 1997) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Modulated structure of the superconducting phases in Pb-modified Bi[sbnd]Sr[sbnd]Ca[sbnd]Cu[sbnd]O

Abstract

High resolution electron microscopy has shown that a modulated structure occurs in the Bi[sbnd]Sr[sbnd]Ca[sbnd]Cu[sbnd]O system. Electron diffraction and x-ray diffraction observations show that this modulation produces extra reflections. Diffraction patterns of the Pb-free material have superstructure reflections with wave vectors [0, 1/4.7, 1] and [0, ?1/4.7, 1]. These two waves vectors can generate all of the superstructure relections associated with each fundamental reflection. When PbO is added to the BSCCO system a new wave vector [0. 1/8.5, 0] is introduced. Observations show that within a single grain the two wave vectors coexist for a Pb and Bi ratio of about 0.25. This produces a non-uniform modulated structure which may be due to a superposition of the two wave vectors. The intensity of the [0, 1/4.7, 1] wave vector is substantially reduced in the Pb-modified material. The b' component of both wave vectors in incommensurate with the lattice period. The same superstructures occurs in both the low-T_c phase and the high-T_c phase.     

Anomalous magnetic behavior of superconducting thin films in small magnetic fields [4] close to the transition temperature

Using a specially designed SQUID magnetometer we measured the temperature dependence of the critical current density in a ring patterned thin film for magnetic fields parallel to the c-axis. In addition, the temporal relaxation of the remanent state as prepared by field cooling in an external field of 100 Oe at different temperatures is determined. The j c ( T ) data show a field-dependent anomalous kink close to T_c pointing to reduced dissipation with increasing temperature allowing to construct a corresponding H-T borderline. A similar behavior is observed for the normalized relaxation rate S ( T ) as extracted from the temporal behavior of the remanent state, which, at low temperatures, exhibits the expected increase for increasing T-values, while an anomalous decrease of S ( T ) is found for temperatures above 85 K. While the low-T regime is attributed to creep of 2D pinned single vortex lines, the high-T behavior is suggested to be dominated by collective motion with a more sluggish dynamics. This change in dynamics is also reflected by the activation barriers for flux creep U ( j ), which show a corresponding crossover in μ from 0.06 to 0.99. An additional scaling analysis of the E-j characteristics for according to vortex glass theory reveals quasi-2D collective creep behavior with . Received: 8 April 1998 / Revised: 15 July 1998 / Accepted: 2 September 1998     

Experience with the [13C2]Oxalate Absorption Test

Abstract

Hyperoxaluria is the most important risk factor for a formation of calcium oxalate-urinary stones. Usually, the bulk of oxalate will be formed in the human body, but in many patients the oxalate from food plays the decisive role. Conventionally, in urine the endogenous oxalate can not be distinguished from food derived oxalate. We have developed a standardized oxalate-absorption test, applying a physiological dose (50 mg disodium salt of [¹³C₂]oxalic acid) of labelled oxalate. The assay has been published. Now we report on the first extensive applications of this test in 86 volunteers and 135 patients from different groups with calcium oxalate stones or an increased risk of the formation of such stones. In one-third of the patients with calcium oxalate-urinary stones an oxalate hyperabsorption was diagnosed. For these patients, a dietetic stone prophylaxis and/or therapy is indicated.     

High accuracy Dirac-finite-element (FEM) calculations for H 2 + and Th 2 179 +

A two-dimensional, fully numerical approach to the four-component first-order Dirac-differential-equation utilizing the Finite-Element-Method (FEM) is employed for H ₂ ⁺ and Th ₂ ^{179 +} . Using elliptic-hyperbolic coordinates and further one-dimensional singular transformations, scaling transformations and extrapolation techniques (geometrical over iteration steps and logarithmic over grid points) we achieve for the molecules H ₂ ⁺ and Th ₂ ^{179 +} relative accuracies better than 10^-12 for 1(1/2) g energies. Received 16 February 2001 and Received in final form 12 July 2001     

Theoretical study of the [Si,C, P,O] potential energy surface

The structures, energetics, spectroscopies and stability of the doublet [Si, C, P, O] radical are explored at the density functional theory and ab initio levels. Eighteen isomers connected by 22 interconversion transition states are located at the DFT/B3LYP/6-311G(d) level. The structures of the kinetically stable isomers and the relevant transition states are further optimized at the QCISD/6-311G(d) level followed by CCSD(T)/6-311 + G(2df) single-point energy calculations. At the QCISD/6-311G(d) level, the lowest-lying isomer is the cyclic O-cCSiP 8 (0.0 kcal/mol) with considerable kinetic stability of 22.0 kcal/mol. In addition, two bent isomers OSiCP 1 (7.3 kcal/mol) and SiCPO 3 (34.7 kcal/mol) also possess considerable kinetic stability (more than 10.0 kcal/mol). As a result, three isomers 1, 3 and 8 are predicted to be possible candidates for future experimental and astrophysical detection. The bonding nature of the three isomers is analysed. The calculated results are compared with those of the analogous radical [Si, C, N, O]. Implications in the laboratory and interstellar space are also discussed. The predicted structures and spectroscopic properties are expected to be informative for the identification of [Si, C, P, O] in the laboratory and space.     

Determination of Hypochlorite via Fluorescence Change from Blue to Green Based on 4-(1 H-imidazo [4,5-f] [1,10]-phenanthrolin-2-yl) Benzaldehyde Oxime

The new design strategy will provide the possibility for preparing a dynamic sensor by employing the inhibition of C?=?N isomerization. In this work, the functional probe 4-(1 H-imidazo [4,5-f] [1,10]-phenanthrolin-2-yl) benzaldehyde oxime (compound 4) has been synthesized and such molecule gives rise to blue emission. Due to the incorporation of hypochlorite, the oxime group can be oxidized to the structure of aldehyde. As a result, the molecular motif exhibits sharp emission change from blue to green due to the addition of hypochlorite with enough sensitivity and selectivity (detection limit?=?53 nM, linear range 0.5-8.0 µM). It has also been used for monitoring ClO^? by employing solution color change and the absorption signal difference could effectively rule out the effects of interference species. To our knowledge, it will be the first case of a highly selective hypochlorite sensor derived from oxime isomerization reaction based on phenanthroline backbone.

     

High pressure 17O NMR study of solvent exchange on square planar complexes: Water exchange on [Pd(dien)H2O]2+

Abstract

The water exchange reaction of [Pd(dien)H₂O]²⁺ (dien = diethylenetriamine) was studied as function of temperature (268-308 K) and pressure 0.1-197 MPa) using ₁₇O NMR techniques. The rate and activation parameters are: k_cx = 5100 s^?1 at 298 K; ΔH^# =38 kJ mol^?1; ΔS^# = -47 JK^?1 mol^?1; ΔV^# = -2.8 cm³ mol^?1 at 296 K. The results are discussed in reference to solvent exchange data for other Pt(II) and Pd(II) complexes, and are interpreted in terms of an associatively activated substitution process.     

Optical,structural characterization and dielectric relaxation of [C2H5NH3]2ZnCl4 compound

ABSTRACT

The new organic-inorganic compound [C₂H₅NH₃]₂ZnCl₄ has been grown by the slow evaporation at room temperature. The zero-dimensional (0-D) structure for this compound was determined by the single X-ray diffraction. It crystallizes at room temperature in the non-centrosymmetric space group Pna2₁ and consists of ethylammonium cations [C₂H₅NH₃]⁺ and [ZnCl₄]^2? tetrahedra anions. That is interconnected by means of hydrogen bonding contacts N-H···Cl. The molecular geometry and vibrational frequencies of [ZnCl₄]^2? and [C₂H₅NH₃]⁺ in the ground state was calculated using density functional method (B3LYP) with 6–31G(d) and 6–311G (d,p) basis set. The optimized geometric bond lengths and bond angles, obtained by using B3LYP/6–311G (d,p), show the best agreement with the experimental data. The optical absorbance was measured in order to deduce the absorption coefficient α, optical band gap E_g. The optical band gap is determined by extrapolating the plotted graph of (αhυ)^1/2 vs. (hυ). The large value of indirect optical band gap energy indicates the insulating nature of this material. Moreover, the extinction coefficient, refractive index and the dielectric permittivity of [C₂H₅NH₃]₂ZnCl₄ compound were calculated and the results are discussed. The evolution of the dielectric loss as a function of frequency revealed a distribution of relaxation times, probably ascribed to the reorientational dynamics of alkyl chains in this compound, and then analyzed with the Cole–Cole formalism.     

Parallel and automated library synthesis of 2-long alkyl chain benzoazoles and azole[4,5-<Emphasis Type="BoldItalic">b</Emphasis>]pyridines under microwave irradiation

Summary A versatile route to 40-membered library of 2-long alkyl chain substituted benzoazoles (1 and 2) and azole[4,5-b]pyridines (3 and 4) via microwave-assisted combinatorial synthesis was developed. The reactions were carried out in both monomode and multimode microwave oven. With the latter, all reactions were performed in high-throughput experimental settings consisting of an 8×5 combinatorial library designed to synthesize 40 compounds. Each step, from the addition of reagents to the recovery of final products, was automated. The microwave-assisted N-long chain alkylation reactions of 2-alkyl-1H-benzimidazole (1) and 2-alkyl-1H-benzimidazole[4,5-b] pyridines (3) were also studied.     

A diversity oriented, microwave assisted synthesis of N-substituted 2-hydro-4-amino-pyrido[2,3-d]pyrimidin-7(8H)-ones

A protocol for the synthesis of N-substituted 2-hydro-4-amino-pyrido[2,3-d]pyrimidin-7(8H)-ones (11) is described. Thus, the formylation of a 2-aminopyridone 12 in 85% formic acid/Ac₂O, proceeding via in situ cyclization to the intermediate formamide 13, affords the corresponding 2-hydro-4-oxo-pyridopyrimidine 14, which is converted to a 4-chloro-pyridopyrimidine 15 upon treatment with POCl₃. The subsequent transformation to the title compounds is carried by treatment with the corresponding amine in MeOH under microwave irradiation conditions.