The absolute configuration of trans-1,2-dimethylcyclopropane

By application of the newly discovered fact that 1° alkyl chlorides with γ-hydrogen form cyclopropanes on treatment with sodium metal, (−)-trans-1,2-dimethylcyclopropane has been prepared from (+)-1-chloro-2-methylbutane. Rigorous assignment of configuration to the isomeric cis- and trans-dimethylcyclopropanes is thereby achieved. The absolute configuration, (1R : 2R) trans-1,2-dimethylcyclopropane, can be assigned to the (−) enantiomer by several lines of argument and agrees with that calculated by Fitts.     

Syntheses, crystal structures and electronic properties of a series of copper(II) complexes with 3,5-halogen-substituted Schiff base ligands and their solutions

We have systematically investigated the structural features, electronic properties, thermally-induced structural phase transitions and absorption spectra depending on the solvent for ten Cu(II) complexes with 3,5-halogen-substituted Schiff base ligands. Structural characterization of two new complexes, bis(N-R-1-phenylethyl- and N-R,S-2-butyl-5-bromosalicydenaminato-κ²N,O)copper(II), reveals that they afford a compressed tetrahedral trans-[CuN₂O₂] coordination geometry with trans-N–Cu–N = 159.4(2)° and trans-O–Cu–O = 151.7(3)° for the 1-phenylethyl complex and trans-N–Cu–N = 157.9(3)° and trans-O–Cu–O = 151.0(3)° for the 2-butyl one. All the complexes exhibit a structural phase transition by heating in the solid state regardless of their structures at room temperature. The absorption spectra of a series of ten complexes exhibit a slight shift of the d–d band at 16 000–20 000 cm⁻¹ and remarkable shift of the π–π* band at 24 000–28 000 cm⁻¹, which suggests that the dipole moment of the solvents presumably affects the conformation of the π-conjugated moieties of the ligands rather than the coordination environment. We have also attempted ‘photochromic solute-induced solvatochromism’ by a system of bis(N-R-1-phenylethyl-3,5-dichlorosalicydenaminato-κ²N,O)copper(II) and photochromic 4-hydroxyazobenzene in chloroform solution. We successfully observed a change of the d–d and π–π* bands of the complex in the absorption spectra caused by cis–trans photoisomerization of 4-hydroxyazobenzene.     

Degradation study on the thermally stable nickel phthalocyanine sheet polymer

Poly (nickel phthalocyanine) which has exceptional thermal stability is synthesized. Knowledge of modes of degradation of this polymer is found to be necessary for its high temperature applications. This polymer showed very high thermal stability with maximum polymer decomposition temperatures (PDT_max) of 500 °C in air and 890 °C in N₂, with char yield 93% at 800 °C. Because of its excellent thermal stability, degradation study with MS as well as GC–MS techniques were found to be very difficult. The present publication deals with MS and GC–MS studies of nickel phthalocyanine sheet polymer at high temperatures ranging from 700 to 1000 °C. Tentative mechanisms are proposed for its modes of fragmentations and based on GC–MS studies, the most probable degradation products are identified.     

Thermal degradation studies of methacrylonitrile polymers and copolymers—3. Polymers capable of chain end-initiated colouration

Polymethacrylonitrile (PMAN) prepared by free-radical polymerisation with the initiator 4,4′-azo-bis(4-cyanovaleric acid) (ACVA) has chain ends from the initiator fragment which incorporate carboxyl groups. These ends are found to act as reactive sites for initiation of colouration in the polymer at temperatures from 140 °C. Several polymers, with different chain-end concentrations, have been made and the thermal degradation behaviour has been studied under programmed heating in the absence of air to 500 °C, with full product characterisation. In contrast with pure PMAN prepared with 2,2′-azo-bis(isobutyronitrile) (AIBN) as initiator, which degrades quantitatively to monomer, these ACVA-initiated polymers give very much reduced monomer yields, an important tar/wax fraction and a substantial amount of residue, amounting to 32–48% of the sample weight, dependent on the initial molecular weight of the polymer. The infrared spectral changes at the onset of the colouration reaction have been examined at 160 °C. Mechanisms are suggested to account for the observed products.     

Upper and lower critical solution temperatures in polybutadiene-alkane systems : Effect of the surface-to-volume ratio of the polymer

Cloud point curves (CPC) have been measured for 1–4 cis polybutadiene (M_r = 40,000–830,000) in n-hexane, 2-methyl-hexane, 2,2,3 trimethylbutane and 2,2,4 trimethylpentane. These four systems show upper and lower critical solubility temperatures which approach one another as the molecular weight increases. An hourglass-shaped CPC is found for two systems. The CPC are found to be much more dependent on polymer concentration than is usual. Calculated (Prigogine, Patterson, Flory theories) and experimental critical temperatures, critical volumes and shapes of the CPC are in much better agreement for these systems than for other systems involving polystyrene, polyisobutylene or polybutene-1. Analysis of these results with those for other systems in the literature indicates the importance of the surface-to-volume ratio s of the polymers. A small value of s = s₂/s₁ (0·5–0·8) for thick polymer chains improves the calculated value of the critical temperature while s = 1 is quite good for thin polymer chains such as polybutadiene and polyethylene.     

The effect of heat ageing on fracture toughness in uPVC pressure pipe

This paper reports a study of the effect of accelerated heat ageing in air on the fracture toughness of two uPVC pipes. The pipes were extruded at 179°C and 195°C respectively and curved fracture toughness specimens were aged at temperatures between 100 and 140°C. After testing at a crosshead rate of 1 mm/min, a similar activation energy of between 104 and 115 kJ/mol was calculated, for both pipes, from the variation of fracture toughness with ageing time and temperature. Extrapolation of the elevated-temperature data down to 20°C showed thermo-oxidative degradation to have a small effect on the fracture toughness during the expected service lifetime of 50–100 years.     

Thermal cis—trans isomerization of bis(diamine) cobalt(III) and chromium(III) complexes in a solid phase

Thermal cis—trans isomerization of the simple bis(diamine) complexes [MX₂(aa or bb)₂]X · HX · n H₂O and the mixed bis(diamine) complexes [MX₂(aa)(bb)]X · HX · n H₂O was investigated in a solid phase, where M = Co(III) or Cr(III) ion, X = Cl or Br, aa and bb are one of the diamines selected from ethylenediamine (en), d, l-1,2-propane-diamine (pn), d,l-2,3-butanediamine (dl-bn), d,l-1,2-cyclohexanediamine (chxn), 1,3-propanediamine (ln) and d,l-2,4-pentanediamine (ptn), and n = 0–2. The information obtained may be summarized as follows. (1) The features of isomerization are considerably dependent upon the kind of metal ions, halide ions and diamines contained in the complexes. (2) Trans-cis isomerization was identified in the simple bis(diamine) complexes containing en, pn, dl-bn or chxn which can form five-membered chelate rings with metal ions, whereas cis-trans isomerization was detected in the simple bis(diamine) complexes containing tn or ptn which forms six-membered rings; all the mixed bis(diamine) complexes isomerize from trans to cis even when they have a combination of five- and six-membered chelate rings. (3) The cobalt(III) complexes isomerize in a temperature range of dehydration and/or dehydrohalogenation with activation energies of about 100 kJ mole⁻¹, whereas the chromium(III) complexes usually isomerize in the anhydrous state and the activation energies amount to as much as 150–190 kJ mole⁻¹. (4) “Aquation-anation” and “bond rupture” were proposed for the isomerization of the cobalt(III) and the chromium(III) complexes, respectively.     

The effect of melt-processing on the degradation of selected polyhydroxyacids: polylactides, polyhydroxybutyrate, and polyhydroxybutyrate-co-valerates

A series of resorbable polyhydroxyacids were studied by size exclusion chromatography (SEC), viscosity measurements, and differential scanning calorimetry (DSC) for the effect of the injection moulding process on the molecular weight and thermal properties of the polymers. The polymers studied were polylactides (PLA), polyhydroxybutyrate (PHB) and polyhydroxybutyrate-co-valerate (PHB/VA) (5–22% VA content). All polylactides underwent extensive degradation, in the range of 50–88%, and an increase in molecular weight distribution (MWD) following injection moulding at temperatures ranging from 130 to 215°C. In contrast to the polylactides, the decrease in molecular weight of the PHB and PHB/VA polymers after injection moulding at temperatures from 135 to 160°C was less dramatic, in the range of 4–53%. This was accompanied by a decrease of the MWD. No evidence for low molecular weight non-volatile degradation products was observed. Injection moulding led to a decrease in the melting temperature (T_m) and the heat of melting (ΔH_m) of PLA. Conversely, the moulding process did not significantly affect the melting temperature and heat of melting of polyhydroxybutyrate/valerates.     

Hydrolysis of amide bond in histidine-containing peptides promoted by chelated amino acid palladium(II) complexes: dependence of hydrolytic pathway on the coordination modes of the peptides

Hydrolytic reactions between cis-[Pd( -Ala-N,O)Cl₂]⁻ and cis-[Pd( -Ala-N,O)(H₂O)₂]⁺, in which -Ala is alanine coordinated through N and O atoms, and N-acetylated peptides -histidylglycine (MeCO-His-Gly), glycyl- -histidine (MeCO-Gly-His), glycylglycyl- -histidine (MeCO-Gly-Gly-His) and glycyl- -histidylglycine (MeCO-Gly-His-Gly) were studied by ¹H NMR spectroscopy. All reactions were carried out in the pH range 2.0–2.5 and two different temperatures, 22 and 60°C. In the reactions of these two palladium(II) complexes with MeCO-His-Gly, complete hydrolysis of the amide bond involving carboxylic group of histidine occurs in less than 24 h. The cleavage is regioselective. With peptides containing free a carboxylic group of histidine, MeCO-Gly-His and MeCO-Gly-Gly-His, palladium(II) complex promote the cleavage of the MeCO–Gly and Gly–Gly amide bonds. No cleavage of the Gly–His amide bond was observed. The mechanism of these hydrolytic reactions involves release of -Ala ligand and aquation of the palladium(II) complex chelated to the substrate through the imidazole N-3 atom and deprotonated nitrogen atom of the amide bond involving amino group of histidine. This aqua complex represents a catalytically active form different from the initially added catalytically inactive complex. In the reactions of palladium(II) complexes with tripeptide MeCO-Gly-His-Gly, two amide bonds, MeCO–Gly and His–Gly, were cleaved. The mechanism of the cleavage of these amide bonds is correlated with two different palladium(II)–substrate catalytically active forms. These findings contribute to the better understanding of selective cleavage of peptides and proteins and must be taken into consideration in designing new reagents for this purpose.     

A comparative ab initio study of the geometry and force field of thionformic acid with formic and thiolformic acids

The energy and force field for the planar cis and trans conformers of thionformic acid have been calculated using the 4–31 G basis set, augmented by a complete set of d-functions on the sulfur atom, with full geometry optimization. Extensive comparisons are made between the changes in geometry and selected force constants in going from cis- (chain) to the trans- (ring) structures of thionformic, thiolformic and formic acid. These changes are discussed in terms of a hydrogen bonding type of interaction in the O---HS, S---HO and O---HO structural units respectively. Of the thioacid conformers, the trans-thiol is found to be the most stable; the trans-thion and cis-thiol both about 10 kJ mol⁻¹ less stable; and the cis-thion the least stable by about 38 kJ mol⁻¹.     

Base induced C-5 epimerisation of 4-methyl-5-phenyl oxazolidinones: Chiral auxiliaries derived from norephedrine and norpseudoephedrine.

Treatment of cis- and trans-4-methyl-5-phenyl oxazolidinones 2 and 3 with excess butyllithium at 0°C results in C-5 epimerisation, via a common intermediate N,C-5-dianion, generating after protonation a 1:4 mixture of 2:3.     

Sul-containing fluorinated polyimides for optical waveguide device

Novel sul-containing fluorinated polyimides have been synthesized by the reaction of 2,2′-bis-(trifluoromethyl)-4,4′-diaminodiphenyl sulfide (TFDAS) with 1,4-bis-(3,4-dicarboxyphenoxy)benzene dianhydride (HQDPA), 2,2′-bis-(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA), 4,4′-oxydiphthalicanhydride (ODPA) or 3,4,3′,4′-biphenyl-tetracarboxylic acid dianhydride (s-BPDA). The fluorinated polyimides, prepared by a one-step polycondensation procedure, have good solubility in many solvents, such as N-methyl-2-pyrrolidinone (NMP), dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), cyclohexanone, tetrahydrofuran (THF) and m-cresol. The molecular weights (M_n's) and polydispersities (M_n/M_w's) of polyimides were in the range of 1.24 × 10⁵ to 3.21 × 10⁵ and 1.59–2.20, respectively. The polymers exhibit excellent thermal stabilities, with glass-transition temperatures (T_g) at 221–275 °C and the 5% weight-loss temperature are above 531 °C. After crosslinking, these polymers show higher thermal stability. The films of polymers have high optical transparency. The novel sul-containing fluorinated polyimides also have low absorption at both 1310 and 1550 nm wavelength windows. Rib-type optical waveguide device was fabricated using the fluorinated polyimides and the near-field mode pattern of the waveguide was demonstrated.     

Organometallic complexes for nonlinear optics: Part 32: Synthesis, optical spectroscopy and theoretical studies of some osmium alkynyl complexes

The complexes trans-[Os(CCPh)Cl(dppe)₂] (1), trans-[Os(4-CCC₆H₄CCPh)Cl(dppe)₂] (2), and 1,3,5-{trans-[OsCl(dppe)₂(4-CCC₆H₄CC)]}₃C₆H₃ (3) have been prepared. Cyclic voltammetric studies reveal a quasi-reversible oxidation process for each complex at 0.36–0.39 V (with respect to the ferrocene/ferrocenium couple at 0.56 V), assigned to the Os^II/III couple. In situ oxidation of 1–3 using an optically transparent thin-layer electrochemical (OTTLE) cell affords the UV–Vis–NIR spectra of the corresponding cationic complexes 1⁺–3⁺; a low-energy band is observed in the near-IR region (11 000–14 000 cm⁻¹) in each case, in contrast to the neutral complexes 1–3 which are optically transparent below 20 000 cm⁻¹. Density functional theory calculations on the model compounds trans-[Os(CCPh)Cl(PH₃)₄] and trans-[Os(4-CCC₆H₄CCPh)Cl(PH₃)₄] have been used to rationalize the observed optical spectra and suggest that the low-energy bands in the spectra of the cationic complexes can be assigned to transitions involving orbitals delocalized over the metal, chloro and alkynyl ligands. These intense bands have potential utility in switching nonlinear optical response, of interest in optical technology.     

Enantioselective catalytic epoxidation of nonfunctionalized prochiral olefins by dissymmetric chiral Schiff base complexes of Mn(III) and Ru(III) metal ions II

A series of new dissymmetric chiral Schiff base complexes has been obtained by a systematic condensation of (1S,2S)(+)-diaminocyclohexane and 3-acetyl-4-hydroxy-6-methyl-2-pyrone with salicylaldehyde, 5-chloro-, 5-methoxy-and 5-nitrosalicylaldehyde and by subsequent metallation with manganese and ruthenium. The characterization of the complexes 1–8 was accomplished by physico chemical studies viz. microanalysis, IR-, UV/VIS-, and CD spectral studies, optical rotation, molar conductance measurements and cyclic voltammetry. Enantioselective epoxidation of non functionalised olefins, viz. cis-stilbene, trans-3-nonene and trans-4-octene with iodosyl benzene as oxidant was demonstrated in the presence of catalytic amounts of chiral Mn(III) and Ru(III) dissymmetric Schiff base complexes. Good optical yields of epoxides were obtained for the catalyst 4 with the substrates trans-3-nonene and cis-stilbene.     

Selectivity for the methoxycarbonylation of ethylene versus CO---ethylene copolymerization with catalysts based on C4-bridged bidentate phosphines and phospholes

The C₄-bridged phospholes 11,12-bis(2,3,4,5-tetramethylphospholylmethyl)-9,10-dihydro-9,10-ethano-anthracene (cis, 4a; trans, 4b) and diphosphines 11,12-bis(diphenylphosphinomethyl)-9,10-dihydro-9,10-ethano-anthracene (cis, 5a; trans, 5b) and their corresponding palladium complexes [(P---P)PdCl₂] (6a–d) have been prepared and characterized. Single-crystal X-ray analyses of 6a–d have been undertaken and they reveal that 4a and b and 5a and b coordinate in a bidentate manner forming seven-membered chelate rings with natural bite angles between 98.62 and 100.30°. The palladium-catalyzed carbonylation of ethylene has been studied using 4a and b and 5a and b. Catalyst mixtures generated from 4a and b, palladium acetate and methanesulfonic acid are selective for the copolymerization of ethylene with carbon monoxide, generating low molecular weight polymers. Surprisingly, the activity of catalyst systems based on cis-(4a) is markedly higher than that based on its trans-isomer, 4b. The marked influence of the nature of the four-carbon tether is highlighted by comparative catalyst testing with [{1,4-bis(2,3,4,5-tetramethylphospholyl)butane}Pd(OAc)₂], which rapidly decomposes under the conditions used for copolymerization. In contrast, under identical conditions catalyst mixtures formed from 5a and b show a marked dependence of the selectivity on the stereochemistry of the ethano-anthracene tether, the former generating a low molecular weight copolymer while the latter generate mainly methyl propanoate. Interestingly, polyketone generated from catalysts based on bisphosphole 4a has a markedly higher average molecular weight than that formed using its diphenylphosphino counterpart, 5a.     

LDPE protected by secondary amines of dehydroabietic methyl ester derivatives

The thermal degradation of low density polyethylene in the presence of several esters—methyl-12N-(phenyl)amino-14-amino-dehydroabietate (A 8), methyl-12N-(2-methoxyphenyl)amino-14-amino-dehydroabietate (A 9), methyl-12N-(4-methoxyphenyl)amino-14-amino-dehydroabietate (A 11) and methyl-12N-(phenyl)amino-14-nitro-dehydroabietate (A 12)—was investigated by chemiluminescence at three temperatures: 180, 200, and 220 °C. Kinetic parameters that depict the stability of polymers—temporal characteristics (oxidation induction times, half-period of oxidation and maximum oxidation time), oxidation rates and activation energies—were calculated from the dependencies of chemiluminescence intensity on thermal degradation time. The stabilization effectiveness of the compounds tested and other antioxidants, namely 4010 (amine compound) and Irganox 1010 and 1076 (hindered phenols), is presented for comparison. Experimental data have demonstrated that two members of the studied series, A 8 and A 11, are more efficient than good commercial hindered phenols.     

Steric effects of the 2-(diphenylphosphino)pyridine bridging ligand in the synthesis of binuclear palladium(II) complexes

Treatment of [Pd{CH₂C(CH₃)CH₂}(Ph₂PPy)Cl] (Ph₂PPy = 2-(diphenylphosphino)pyridine) with cis-[Pd(^tBuNC)₂Cl₂] in dichloromethane affords the mixed isocyanide-tertiary phosphine complex cis-[Pd(^tBuNC)Ph₂PPy)Cl₂], in which the Ph₂PPy is a monodentate P-donor, and [{Pd[CH₂C(CH₃)CH₂]Cl}₂]. The steric effects of the Ph₂PPy bridging ligand in determining the reaction course is discussed. The complex cis-[Pd(^tBuNC)(Ph₂PPy)Cl₂] was crystallographically characterized: P2₁/n, a = 15.143(2), b = 9.527(1), c = 17.517(4) Å, β = 113.96(1)°, V= 2309.4(7) ų, Z = 4. The final R value was 0.044, R^w= 0.046 for the 3078 reflections with I > 3σ(I).     

Synthesis and crystal structure of 9,10-dihydro-9,10-etheno-1,8-dichloro-11-diphenylphosphinyl-12-(diphenylphosphinylethynyl)anthracene

The title compound, 9,10-dihydro-9,10-etheno-1,8-dichloro-11-diphenylphosphinyl-12-(diphenylphosphinylethynyl)anthracene (1), has been synthesized and its crystal structure has been determined. The compound 1 crystallized into the triclinic space group P-1 with =74.837(4)°, β=88.156(4)°, γ=65.398(4)°, Z=2, D_c=1.352 gcm⁻³. In the crystal structure of 1a, one chloroform molecule was included by the compound 1 with a 1:1 ratio and the existence of non-classical intermolecular C–HO hydrogen bonds, intramolecular C–HCl and C–HO hydrogen bonds and π–π stacking were observed.     

Molecular determinants for drug-receptors : Part 11. The preferred conformation of N-(p-anisoyl)pyrrolidin-2-one (“Aniracetam”) in the solid and solution states as indicated by X-ray crystal structure analysis, dipole moment and theoretical calculations

N-(p-anisoyl)pyrrolidin-2-one in the crystalline state exhibites a cis—rans conrotatory conformation with N---CO and CO---C_ar rotational angles of 33.5° and 38.5° respectively, and the p-methoxy group situated cis to the central carbonyl bond, as shown by X-ray structure analysis. As suggested by dipole moment analysis and MMP2 molecular mechanics calculations, in solution similar conrotatory models hold for both c- and t-subconformers having the p-methoxy group cis or trans to the central carbonyl bond. INDO calculations were also carried out, indicating that both subconformers are equally stable.     

The synthesis and liquid crystal transition temperatures of some weakly polar nematic trans-4-substituted-cyclohexyl (E)-alk-2-enoates

The effect on the liquid crystal transition temperatures of introducing various groups (for example incorporating C=C, O, CO₂ and CO) into the terminal alkyl chain of a weakly polar model compound 1-[trans-4-(trans-4-propylcyclohexyl)cyclohexyl]pentane has been investigated systematically. Only the compound containing both an ester function and a trans-carbon-carbon double bond exhibited a wide-range nematic mesophase at elevated temperatures. Therefore, a wide variety of trans-4-substituted-cyclohexyl (E)-alk-2-enoates incorporating a carbon double bond with a trans-configuration (E) in the terminal alkyl chain has been synthesized. Nearly all the two-ring esters prepared exhibit a nematic phase over a wide temperature range (≤ 100°C) at elevated temperatures (≤ 200°C). The tendency to form smectic mesophases is often low. Comparisons with the corresponding derivatives incorporating either just a carboxy group (COO) or just a carbon-carbon double bond (C=C) in the same positions indicate that synergetic effects lead to broader nematic phases than would otherwise have been expected. The new compounds are easily prepared from known starting materials.