Crosslinking investigation of polybutadiene thermal degradation by carbon-13 nuclear magnetic resonance

Thermal degradation of polybutadiene (PBD) in anaerobic atmosphere at 250 °C had been studied by carbon-13 nuclear magnetic resonance spectroscopy (¹³C NMR) before complete crosslinking. In this investigation four types of low molecular weight PBD with different 1,2-vinyl isomer content had been chosen, then pure and mixed samples of PBD were heated in different time periods. ¹³C NMR spectra showed that two kinds of crosslinking mechanisms occur that both of them produce methyl groups. The first mechanism is a reaction between 1,2-vinyl isomers of two PBD chains, and the second one occurs between 1,2-vinyl isomer of one chain via methylene carbon of cis or trans isomer in another chain. Also ¹³C NMR results showed that the presence of 1,2-vinyl isomer in the PBD structure is necessary and without it none of the mentioned reactions will occur. Furthermore isomers sequence is another important parameter which affects crosslinking. Results show that cis or trans isomer which is not adjacent to 1,2-vinyl isomer does not take part in crosslinking reaction. Moreover such cis or trans isomer can take part in second mechanism of crosslinking that 1,2-vinyl isomer was attached from head to cis or trans isomer, thus in this arrangement of isomers second mechanism of crosslinking will become dominant rather than first mechanism of crosslinking.     

四甲基二硅桥连二（环己基环戊二烯基）四羰基二铁的合成、结构及热量排反应研究

1，2－二（环己基环戊二烯基）四甲基二硅烷与Fe(CO)_5在二甲苯中加热回流 生成二铁化合物(Me_2SiSiMe_2)[(c-C_6H_(11)-C_5H_3)Fe(CO)]_2(μ-CO)_2 （ 2）。通过柱层析分离到2的顺反两种异构体2c和2t，并分别进行热重排反应，发现 顺式底物2c重排生成反式重排产物[Me_2Si(c-C_6H_(11)C_5H_3)Fe(CO)_2]_2 （ 3t），而反式底物2t重排则生成顺式重排产物3c。这表明重排反应是立体专一性的 。通过X射线衍射分析测定了化合物2c和3t的晶体结构。     

Selection of the cis and trans phosph(III)azane macrocycles [{P(mu-NtBu)}2(1-Y-2-NH-C6H4)]2(Y=O, S)

The 1 : 1 reactions of [ClP(mu-NtBu)]2 with the difunctional aromatic amines 1,2-1-YH-2-NH2-C6H4 in the presence of Et3N give the dimeric phosph(III)azane macrocycles [{P(mu-NtBu)2(1-Y-2-HN-C6H4)]2, predominantly as the cis isomer in the case of Y=O (1.cis) and as the trans isomer for Y=S (2.trans). Model M.O. calculations suggest that the selection of the cis and trans isomers is not thermodynamically controlled. The alternative isomers 1.trans and 2.cis are generated exclusively by the deprotonation of the model intermediates [(1-Y-2-NH2-C6H4)P(mu-NtBu)]2[Y=O (3), S (4)] with nBuLi followed by cyclisation with [ClP(mu-NtBu)]2. The solid-state structures of 1.cis/trans(50 : 50), 2.cis, 3 and 4 are reported.     

UV-, VIS- and IR-light-induced isomerization of HSNO in a low-temperature matrix

An infrared spectroscopic investigation has been performed on the trans and cis isomers of thionitrous acid (HSNO) and their D- and ¹⁵NO-isotopic modifications in argon matrices at 12 K. The substances were prepared photolytically from thionylimide (HNSO) isotopes in the matrix. With UV (250 nm), VIS (585 nm), and IR irradiation the cis → trans or the trans → cis isomerization of HSNO was induced, allowing an unequivocal distinction between the closely resembling IR spectra of the trans and cis isomers. Complete sets of fundamental frequencies of both rotamers were obtained and assigned by normal coordinate analysis using the transferable valence force field (TVFF) approach. Parallel to this analysis ab initio calculations on the SCF- and CI-levels were performed to predict energy, geometry, and barrier of internal rotation for the two HSNO rotamers.     

Thermal rearrangement study of low molecular weight polybutadiene

Various polybutadienes (PBDs) of low molecular weight were heated below complete crosslinking at 250 °C under anaerobic nonpyrolytic conditions, and the structural changes were investigated. The predominant crosslinking reactions arise from the presence of 1,2-vinyl isomer and the most important one is intermolecular reaction accompanied with methyl group formation. The analysis showed that two crosslinking types as well as two types of methyl groups have been produced in which one was the result of 1,2-vinyl isomer of one chain crosslinked via methylene carbon of another chain of cis or trans isomer, and the second methyl group was the product of the reaction between 1,2-vinyl isomers of two PBD chains. Chain scission also occurred in two pathways due to the presence of 1,2-vinyl isomer, scission at two adjacent 1,2-vinyl isomer and scission at adjacent 1,2-vinyl with cis or trans isomer giving rise to methyl carbons.     

Porphyrins 27. Synthesis of metal complexes of cis-1,2-bis(octaethyl-5-porphyrinyl)ethylene and their transformation into the trans isomers

The synthesis of the Cu, Ni, and Zn complexes of cis-1,2-bis(octaethyl-5-porphyrinyl)ethylene has been carried out and their transformation into the corresponding trans isomers has been investigated under various conditions. The crystal and molecular structure of the Ni complex of trans-1,2-bis(octaethyl-5-porphyrinyl)ethylene was determined by x-ray structural analysis. It was shown that the Zn and Ni complexes of the cis isomer were readily converted into the trans isomers in chloroform. A stepwise introduction of Ni atoms into the trans dimer was discovered on heating it with Ni(OAc)₂ in DMF. The conversion of the Zn complex of the cis dimer into the trans dimer was facilitated on adding pyridine.For Communication 26 see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1339–1344, October, 1993.The author is sincerely grateful to Professor Y. Sakata for making this investigation possible, to Professor Y. Kai for the XSA of complex (Ib), to Dr. Y. Takai for plotting the PMR spectra, and to Professor K. M. Smith for his interest in the work.     

Chromatographic analysis of cis/trans carotenoid isomers.

Cis/trans configurations of carotenoids are known to effect the biochemistry of carotenoids in certain situations. Methodology for separating carotenoid cis/trans isomers is of importance to nutritionists and food scientists because cis isomers of provitamin A carotenoids have lower provitamin A activities than the all-trans form. Traditional food processing and preservation methods, especially thermal treatments, induce the formation of cis isomeric forms. However, many challenges, are apparent for identifying and analyzing cis/trans isomers present in foods and other biological tissues. The development of current chromatographic methods for the separation of carotenoid cis/trans isomers is reviewed. For the separation of beta-carotene isomers, most procedures employ either Ca(OH)2 or Vydac C18 columns. In general, polymeric C18 columns allow for the detection of cis carotenes, while monomeric C18 columns provide for some separation of certain xanthophylls. The main cis isomers detected in foods are the 13-cis and 9-cis forms, although other forms have also been found (mainly 15-cis and various di-cis isomers). More studies involving the metabolism and physiological consequences of cis/trans isomers in the diet are needed. However, due to limitations in current techniques, further method development in the area of separation, detection and quantitation of cis/trans carotenoid isomers will be required.     

The importance of lone pair delocalizations: theoretical investigations on the stability of cis and trans isomers in 1,2-halodiazenes

The relative and thermodynamic stabilities of cis and trans isomers of 1,2-dihalodiazenes (XN=NX; X = F, Cl, or Br) were examined using high level ab initio and density functional theory (DFT) calculations. For 1,2-dihalodiazenes, it was found that the cis isomers were more stable than the corresponding trans isomers, despite the existence of several cis destabilizing mechanisms, such as steric exchange between halogen lone pairs and dipole-dipole electrostatic repulsions (Delta(trans-cis) = 3.15, 7.04, and 8.19 kcal mol(-1), respectively, at BP86/6-311++G(3df,3pd)//B3LYP /6-311++G(3df,3pd) level). Their origin of the cis-preferred difference in energy was investigated with natural bond orbital (NBO) analysis to show that the "cis effect" came mainly from antiperiplanar interactions (AP effect) between the nitrogen lone pair and the neighboring antibonding orbital of the N-X bond (n(N) --> sigma(N'X'*)). The delocalization of halogen lone-pair into the antibonding orbital of the N=N bonds (the LP effects) was also found to enhance the cis preference by 1.20 to 6.58 kcal mol(-1), depending on the substituted halogen atom. The total amount of the AP effect increased as the halogen atom became larger, and the increased AP effect promoted the triple-bond-like nature of the N=N bond (shorter N=N bond length and wider NNX angle). The greater AP effect also made the N'-X' bond easier to cleave (longer N-X bond length), and a higher energy level than that of the nitrogen lone pair was found in the N-Br bonding orbital in 1,2-dibromodiazenes, thus indicating the significant instability of this molecule. The degradability of the N-Cl bond in 1,2-dichlorodiazenes and the fair stability of the N-F bond in 1,2-fluorodiazenes were also confirmed theoretically, and were found to be consistent with the previous experimental and theoretical reports. These results clearly indicate the dominance of lone-pair-related hyperconjugations on the basic electronic structure and energetic natures of 1,2-dihalodiazene systems.     

手性铜(Ⅱ)-席夫碱配合物催化苯乙烯不对称环丙烷化反应

Twelve chiral copper(Ⅱ) Schiff base complexes, derived from (R) (+) 2 amino 1,1 diaryl 1 propanol with substituted salicylaldehydes, were examined as a catalyst for asymmetric cyclopropanation of styrene with ethyl diazoacetate. It was found that the substituents at 3 and 5 positions of salicylaldehyde in the ligands had great effects on catalytic activity and enantioselectivity of the catalyst. The complex with strong electron withdrawing group (NO 2) at 5 position and the smallest stereo hinder (H) at 3 position of salicylaldehyde showed highly catalytic activity and enantioselectivity, up to ee =87 4% for trans and ee =82 8% for cis isomers respectively, and the ratio 39/61 of cis to trans isomers was obtained at 40 ℃ with 1,2 dichloroethane as solvent.     

Total synthesis of amiclenomycin, an inhibitor of biotin biosynthesis

We describe the first synthesis of amiclenomycin, a natural product that has been found to inhibit biotin biosynthesis and, as a consequence, to exhibit antibiotic properties. Structure 1, with a trans relationship between the ring substituents. had previously been proposed for amiclenomycin on the basis of its 1H NMR spectrum. We have prepared the trans and cis isomers 1 and 2 by unequivocal routes and we conclude that the natural product is in fact the cis isomer 2. The properly substituted cyclohexadienyl rings were constructed first. A cycloaddition reaction between 1,2-di(phenylsulfonyl)ethylene and the N-allyloxycarbonyl diene 13, followed by reductive elimination of the phenylsulfinyl groups, gave the cis isomer 15. To obtain the trans isomer, the O-trimethylsilyl diene was used to give the cis hydroxylated Diels-Alder adduct 33, which was transformed into the corresponding trans amino derivative by means of a Mitsunobu reaction. The L-alpha-amino acid functionality was introduced by means of a Strecker reaction on the aldehydes 16 and 42, followed by enzymatic hydrolysis with immobilised pronase.     

Hyperaromatic stabilization of arenium ions: acid-catalyzed dehydration of 2-substituted 1,2-dihydro-1-naphthols

Rate constants for acid-catalyzed dehydration of cis-2-substituted 1,2-dihydro-naphthols are well correlated by the Taft relationship log k = -0.49 - 8.8σ(I), with minor negative deviations for OH and OMe. By contrast the trans substituents show a poor correlation with σ(I) and in most cases react more slowly than their cis isomers. The behavior is consistent with rate-determining formation of a 2-substituted carbocation (naphthalenium ion) intermediate that for cis reactants possesses a 2-C-H bond suitably oriented for hyperconjugation with the charge center. For the trans isomers the 2-substituent itself is oriented for hyperconjugation in the initially formed conformation of the cation. It is argued that k(cis)/k(trans) rate ratios for substituents (Me, 8.4; Bu(t), 12.7; Ph, 3.8; NH(3)(+), 160; OH, 440) reflect their hyperconjugating ability relative to hydrogen. Faster reactions of trans isomers are observed for substitutents known (RS, N(3)) or suspected (EtSO, EtSO(2)) of stabilizing the cation by a π or σ neighboring group effect. The good Taft correlation is taken to indicate that cis substuents are reacting normally, differentiated only by their inductive effects. The slower reactions of the trans isomers are the judged to be "abnormal". This is confirmed by comparing effects of cis and trans β-OH substituents on the reactivities of dihydro phenols, naphthols, and phenanthrols. Whereas k(H)/k(OH) for cis substituents varies by less than 8-fold and is consistent with the influence of an inductive effect of the OH group (k(H)/k(OH) ≈ 2000), k(H)/k(OH) for the trans substituents varies by 3 orders of magnitude, reflecting the additional influence of the lesser hyperconjugating ability of a C-OH bond compared to a C-H bond. The magnitude and variation of this difference is consistent with C-H hyperconjugation conferring aromatic character on the arenium ions.     

Esters of 2,5-multisubstituted-1,3-dioxane-2-carboxylic acid: their conformational analysis and selective hydrolysis

The carbomethoxy group at the C2 position of the 2,5-multisubstituted 1,3-dioxanes prefers the axial conformation rather than the equatorial one due to an anomeric effect. The trans isomers of the 5-monosubstituted compounds are more selectively hydrolyzed than the cis isomers. Based on the calculated results, hydrolysis to the trans isomers is attributed to the larger carbonyl charges of the trans than those of the cis isomers. The anomeric and homoanomeric effects will explain the axial preference of the carbomethoxy group and selective hydrolysis to the trans isomers. Furthermore, the calculated stability between the cis and trans isomers is in good agreement with the experimental results in the equilibrium state.     

1.1](3,3')-azobenzenophane: novel crystal structure and cis-trans isomerization of distorted azobenzene

[formula: see text] [1.1](3,3')-Azobenzenophane, in which two azobenzenes are cyclically connected by -CH2- chains at the meta positions, has been synthesized. The crystal structures of all isomers have been revealed. This is the first report on the crystal structure of the cis isomer of macrocyclic azobenzenes. The trans,trans isomer was slightly distorted, the trans,cis isomer highly deformed, and the cis,cis isomer unstrained. The thermal stability of cis isomers in solutions are deducible from the crystal structures.     

Reappraisal of cis effect in 1,2-dihaloethenes: an improved virtual orbital multireference approach

Computed relative stabilities for isomers of 1,2-difluoroethene and 1,2-dichloroethene isomers are compared with predictions based on chemical hardness (eta) and electrophilicity (omega) using the principles of maximum hardness and minimum electrophilicity. The chemical hardness and electrophilicity deduced either from improved virtual orbital (IVO) energies or from correlated treatments correctly predict that cis 1,2-difluoroethene and 1,2-dichloroethene are energetically more stable than the corresponding trans isomers, and the ground state energies from multireference perturbation theory with IVO orbitals agree with these predictions. However, when the same quantities are computed using Hartree-Fock orbitals, serious inconsistencies between the two approaches emerge in predicting the stability of the isomers of the 1,2-dihaloethenes. The present study clearly demonstrates that the IVO energies are appropriate for the computation of hardness related parameters, notably the chemical hardness and electrophilicity. Moreover, the IVO methods also provide smooth potential energy curves for the cis-trans isomerization of the two 1,2-dihaloethenes.     

Separation of cis/trans geometrical fatty acid isomers by silver-exchanged zeolite Y

The separation of cis and trans isomers is relevant for biological and nutritional applications, silver-exchanged zeolite Y was prepared and applied for the treatment of mixtures of cis and trans geometrical isomers of mono- and polyunsaturated fatty acid methyl esters (FAMEs). cis FAMEs were adsorbed into zeolite with a high degree of selectivity (cis/trans ratio in the range of 1.9-3.2). The effectiveness was due to the synergism of the π-complexation between the silver ion and the double bonds and the different FAME structures trapped into the zeolite cages. Some indication of the complex stabilities came from theoretical studies using unsaturated lipids. A prototype cartridge was also designed for application in the fractionation of cis/trans FAME mixtures.     

Molecular structure of the trans and cis isomers of metal-free phthalocyanine studied by gas-phase electron diffraction and high-level quantum chemical calculations: NH tautomerization and calculated vibrational frequencies

The molecular structure of the trans isomer of metal-free phthalocyanine (H2Pc) is determined using the gas electron diffraction (GED) method and high-level quantum chemical calculations. B3LYP calculations employing the basis sets 6-31G**, 6-311++G**, and cc-pVTZ give two tautomeric isomers for the inner H atoms, a trans isomer having D2h symmetry and a cis isomer having C2v symmetry. The trans isomer is calculated to be 41.6 (B3LYP/6-311++G**, zero-point corrected) and 37.3 kJ/mol (B3LYP/cc-pVTZ, not zero-point corrected) more stable than the cis isomer. However, Hartree-Fock (HF) calculations using different basis sets predict that cis is preferred and that trans does not exist as a stable form of the molecule. The equilibrium composition in the gas phase at 471 degrees C (the temperature of the GED experiment) calculated at the B3LYP/6-311++G** level is 99.8% trans and 0.2% cis. This is in very good agreement with the GED data, which indicate that the mole fraction of the cis isomer is close to zero. The transition states for two mechanisms of the NH tautomerization have been characterized. A concerted mechanism where the two H atoms move simultaneously yields a transition state of D2h symmetry and an energy barrier of 95.8 kJ/mol. A two-step mechanism where a trans isomer is converted to a cis isomer, which is converted into another trans isomer, proceeds via two transition states of C(s) symmetry and an energy barrier of 64.2 kJ/mol according to the B3LYP/6-311++G** calculation. The molecular geometry determined from GED is in very good agreement with the geometry obtained from the quantum chemical calculations. Vibrational frequencies, IR, and Raman intensities have been calculated using B3LYP/6-311++G**. These calculations indicate that the molecule is rather flexible with six vibrational frequencies in the range of 20-84 cm(-1) for the trans isomer. The cis isomer might be detected by infrared matrix spectroscopy since the N-H stretching frequencies are very different for the two isomers.     

顺反异构键参数拓扑指数及其在相关顺反异构分子性质中的应用

顺反异构键参数拓扑指数及其在相关顺反异构分子性质中的应用张宏光，辛厚文（中国科技大学近代化学系，合肥２３００２６）关键词：顺反异构分子，键参数拓扑指数分子的拓扑指数是近几十年来发展起来的一种重要的分子图不变量。自１９４７年Ｗｉｎｎｅｒ提出第一个分子拓...     

四氢2,4-二取代-1,5-苯并二氮杂草的高效薄层色谱

随着药物化学的进展,苯并含氮或含硫的七员杂环化合物受到越来越多的关注^[1.2]。这类化合物具有一定的生理活性,它们的立体化学及反应过程都是很有意思的。     

Darstellung und Konformationsuntersuchungen von cis- und trans-Isomeren des 1,2,4,5-Tetramethyl-3,6-diphenoxy-3,6-dithioxo-1,2,4,5-tetraaza-3,6-diphosphacyclohexans

Synthesis and Conformational Analysis of Cis- and Trans-Isomers of 1,2,4,5-Tetramethyl-3,6-diphenoxy-3,6-dithioxo-1,2,4,5-tetraaza-3,6-diphosphacyclohexane A mixture of isomers of the title compound could be prepared in much better yields (up to ca. 25%) by the reaction of bis(1,2-dimethylhydrazido) thiophosphoric acid O-phenylester with phenoxythiophosphoryl dichloride in the presence of triethylamine and aluminum chloride as catalyst. Separation of cis- and trans-isomers was achieved by sorting of crystals or in low yields by fractional crystallization. ³¹P- and mainly ¹H-NMR investigations at different temperatures show, that in both isomers the saturated sixmembered ring has a twist-conformation in which sulfur and phenoxy substituents located cis or trans respectively adopt the isoclinal positions. The isomers reveal remarkably different barriers of activation for the interconversion of two enantiomeric twist forms (cis ΔG^# = 63 ± 1 kJ/mol; trans 47 ± 1 kJ/mol).     

Peroxide linkage N2O4 molecule: Prediction of new ONOONO isomers

Ab initio and density functional theory (DFT) have been applied to predict the existence of the ONOONO molecules. Two new isomers, cis‐perp‐cis and cis‐perp‐trans ONOONO, are found to be minima on the potential energy hypersurface calculated at MP2 level with 6‐311G(d) basis set and Becke3P86 and Becke3LYP levels with the cc‐PVDZ basis set. Vibrational frequencies are calculated at the optimized structures. Calculations at both ab initio and DFT methods show that cis‐perp‐cis is energetically favored in all three O O bonded ONOONO isomers. © 1999 John Wiley & Sons, Inc. Int J Quant Chem 76: 77–82, 2000