Acceleration of the DABCO-promoted Baylis-Hillman reaction using a recoverable H-bonding organocatalyst

It has been shown that catalytic amounts (20-40 mol %) of bis-aryl (thio)ureas greatly accelerate the DABCO-promoted Baylis-Hillman reaction between a range of aromatic aldehydes and methyl acrylate in the absence of solvent. These robust organocatalysts are superior mole per mole promoters of the reaction than either methanol or water and are recoverable in high yield after the reaction by column chromatography.     

An Infrared Spectroscopic Study on the Hofmann- and en-Td-type Clathrates: Mn(NH3)2Zn(CN)4 .2benzene and Mn(ethylenediamine)Zn(CN)4.2benzene

Infrared spectra of Mn(NH₃)₂Zn(CN)₄.2C₆H₆ andMn(ethylenediamine)Zn(CN)₄.2C₆H₆ are reported. The spectral data suggest that these compounds are similar in structure toHofmann-T_d-type and the en-T_d-type clathrates,respectively. There is good evidence for H-bonding from ammoniaN–H to benzene as a to H-bond.     

Optically clear simultaneous interpenetrating polymer networks based on poly(ethylene glycol) diacrylate and epoxy. I. Preparation and characterization

Poly(ethylene glycol) diacrylate was synthesized from poly(ethylene glycol) of molecular weight 600 with acryloyl chloride in a molar ratio of 1:2. Poly(ethylene glycol) diacrylate (PEGDA) was then blended with diglycidyl ether of bisphenol A (DGEBA) in various ratios, followed by curing with 2,2′-azobisisobutyronitrile (AIBN) and isophronediamine (IPDA) simultaneously. Viscosity changes before and during IPN formation were examined with a Brookfield viscometer. Formation of H-bonding and functional group changes were investigated with FTIR. Exothermic curing thermograms were recorded with dynamic DSC. Optically clear IPNs thus obtained were characterized with rheometric dynamic spectroscopy (RDS) and scanning electron microscopy (SEM) to check possible compatibility of the two networks. Experimental results revealed that during IPN formation hydrogen bonds between PEGDA and DGEBA and interlock of networks had profound effect on viscosity change and pot-life. Complete compatibility of the IPNs was found as DGEBA content was higher than 50% by weight. The compatibility between PEGDA and DGEBA networks was evidenced from inner shift of a single damping peak in RDS. In the meantime, SEM micrographs confirmed the coincidence with the result of RDS © 1992 John Wiley &Sons, Inc.     

聚(4-偶氮磺酸苯乙烯-co-4-乙烯基吡啶)与本征态聚苯胺的氢键自组装及其光电转换性能

合成了一种新的共聚体——聚(4-偶氮磺酸苯乙烯-co-4-乙烯基吡啶), 它含有吡啶环, 能作为氢受体与本征态聚苯胺进行氢键自组装. 在紫外光照下, 组装膜通过偶氮磺酸基的光解, 形成稳定的共价交联结构, 在电解质水溶液中也不被破坏, 可用作光电转换膜, 并能在盐水溶液中直接测定它的光电流. 结果表明含有本征态聚苯胺的自组装膜是一种良好的光电转换材料.     

Crystal structure and single crystal EPR of (NH_4)_2(15-crown-5)_3[Cu(mnt)_2] and (NH_4)_2(benzo-15-crown-5)_4[Cu(mnt)_2]·0.5H_2O

The X-ray crystal structures of (NH4)2(15-crown-5)3[Cu(mnt)2] (1) and (NH4)2(benzo-15-crown-5)4- [Cu(mnt)2]·0.5H2O (2) were determined. Two single crystals are composed of distinct structures of ammonium-crown ether supramolecular cation and [Cu(mnt)2]2? anion. The triple-decker dication in complex 1 and a sandwich dimmer in complex 2 were observed. X-Band EPR studies on the single crystals of both complex 1 and complex 2 have been carried out at room temperature, which revealed that complex 2 showed a perfect hyperfine structure of Cu whereas that of complex 1 could not be observed. The principal values and direction cosines of the principal axes of the g and A tensors were computed by a least-squares fitting procedure. The spin density of Cu(II) was estimated according to the principal values of the A tensors and compared well with the results calculated based on DFT method.     

Vibrational spectroscopic detection of H-bonded β- and γ-turns in cyclic peptides and glycopeptides

The conformation of N-glycoproteins and N-glycopeptides has been the subject of many spectroscopic studies over the past decades. However, except for some preliminary data, no detailed study on the vibrational spectroscopy of glycosylated peptides has been published until recently.

This paper reports FTIR spectroscopic properties in DMSO and TFE of the N-glycosylated cyclic peptides cyclo[Gly-Pro-Xxx(GlcNAc)-Gly-δ-Ava] 3a and 3b in comparison with data on the non-glycosylated parent peptides cyclo(Gly-Pro-Xxx-Gly-δ-Ava) 2a and 2b [a, Xxx = Asn; b, Xxx = Gln; δ-Ava = NH-(CH₂)₄-CO] and N-acetyl 2-acetamido-2-deoxy-β- -gluco pyranosylamine (GlcNAc-NHAc, 4). The assignment of amide I band frequencies to conformation is based on ROESY experiments and determination of the temperature coefficients in DMSO-d₆ solution. (For the synthesis and NMR characterization of 2a and 3a see Ref. [19].)

Cyclic peptides are expected to adopt folded (β- and/or γ-turn) conformations which may be fixed by intramolecular H-bonding(s). A comparison of the temperature coefficients of the NH protons and amide I band frequencies and intensities suggests that in DMSO there is no significant difference in the backbone conformation and H-bond system of the N-glycosylated models and their parent cyclic peptides. The common feature of the backbone conformation of models 2 and 3 is the predominance of a 1 ← 4 (C₁₀) H-bonded type II β-turn encompassing Pro-Xxx or Pro-Xxx(GlcNAc), respectively. The ROESY connectivities in the Asn(GlcNAc) model (3a) have not been found to reflect intramolecular H-bondings between the peptide and the sugar.

The unique feature of the FTIR spectra in DMSO of the cyclic models is the lack or weakness of low-frequency (< 1640 cm⁻¹) amide I component bands. In TFE the amide I region of the FTIR spectra shows an increased number of components below 1650 cm⁻¹ reflecting a mixture of open and H-bonded β- and γ-turn conformers.

Because of its destabilizing effect upon γ-turns and other weakly H-bonded structures, DMSO decreases the number of backbone conformers. DMSO also destroys side-chain-backbone H-bondings of type C₇, C₆ or C₈. Possible ‘glyco’ C₇ H-bondings in GlcNAc-NHAc (4) or in glycopeptides 3a and 3b cannot resist the effect of DMSO either.

The FTIR data in TFE of models 2–4 suggest that the acceptor amide group of strong C₇ H-bondings in peptides and glycopeptides absorbs at 1630 ± 5 cm⁻¹ and that of bifurcated H-bondings between 1600–1620 cm⁻¹.     

Adsorption of 2,4-Dinitrotoluene on Dickite: The Role of H-Bonding

The adsorption of 2,4-dinitrotoluene (DNT) on the tetrahedral and octahedral surfaces of dickite (a clay mineral of the kaolinite group with a 1:1 dioctahedral structure characterized by the common chemical formula Al₂Si₂O₅(OH)₄) was studied using small representative mineral models. The calculations have been performed at the HF/3-21G level of theory. The orientation of DNT on the tetrahedral surface of dickite was found to be coplanar with the surface plane. In the case of the adsorption on the octahedral surface the DNT molecule is placed with an inclination about 30^∘ to the surface plane. This type of adsorption results in an electron density redistribution of DNT on the surface of the mineral that is more significant in the case of the adsorption on the tetrahedral surface. The interaction energies of DNT with the octahedral and tetrahedral surfaces corrected by the BSSE energy were found. The adsorption energy of a DNT-tetrahedral fragment amounts to −6.5 kcal/mol, and the interaction energy of the DNT-octahedral fragment system is −17.7 kcal/mol.     

Synthesis,binding and fluorescence studies of a new neutral H-bonding receptor for anions based on 3,5-bis(trifluoromethyl)phenylurea

The synthesis and anion binding studies of the new neutral receptor 1,1′-(2,2′-(4,10-dimethyl-1,4,7,10-tetraazacyclododecane-1,7-diyl)bis(2-oxoethane-2,1-diyl))bis(3-(3,5-bis(trifluoromethyl)phenyl)urea) (L1) are reported. L1 is a macrocyclic ligand containing the 3,5-trifluoromethylphenylureido-binding fragment attached as a side arm on the tetraazacyclododecane. L1 is soluble in numerous organic solvents; the binding properties of L1 towards several simple anions (G) were investigated by NMR, UV–vis and fluorescence techniques in DMSO and CH₃CN solutions. L1 is able to bind F^? , Cl^?  and AcO^?  in both solvents; in addition, it binds Br^?  in CH₃CN. Fluoride shows the highest constant values in the halide series (F^?  > Cl^?  > Br^? ) and AcO^?  is the most strongly bound among all the anions investigated. L1 is able to signal the presence of the anions in solution by fluorescence change; in the case of acetate, this occurs in the visible range.     

四溴代对苯二甲酸及2，2′-联吡啶构筑的铜配位聚合物的合成、晶体结构及热稳定性

以四溴代对苯二甲酸(H2TBTA)与2,2′-联吡啶(2,2′-bipy)为配体,与硝酸铜在水和甲醇混合溶剂中合成了1个一维铜配合物{[Cu(2,2′-bipy)(TBTA)(H2O)].H2O}n,并通过元素分析、红外分析、热重分析、X-射线粉末衍射和X-射线单晶衍射对其进行了表征。该配合物属于正交晶系,空间群为Pna21,a=1.399 9(11)nm,b=0.929 3(7)nm,c=1.557 9(11)nm,V=2.027(3)nm3,Z=4,R1=0.053 5。中心Cu(Ⅱ)离子处于一个五配位环境中,且在配合物中存在着π-π相互作用和氢键作用。热重分析表明该配合物在203℃开始发生分解。