Quantitative estimation of trans-1,4 and cis-1,4 isomers in polychloroprene by high-resolution NMR

In the ¹H-NMR spectrum of polychloroprene dissolved in C₆D₆, the ?CH proton signal was separated into two triplet peaks. These triplet signals were assigned to the ?CH proton in the trans-1,4 and cis-1,4 isomers by measurement of ¹H-NMR spectra of 3-chloro-1-butene and a mixture of trans- and cis-2-chloro-2-butene as model compounds for the 1,2, trans-1,4 and cis-1,4 isomers. In ¹H-NMR spectra (220 Mcps) of polychloroprene dissolved in C₆D₆, two triplet signals were separated completely from which the relative concentrations of trans-1,4 and cis-1,4 isomers could be obtained quantitatively.     

Structure of polypiperylene chain segments according to high-resolution 13C NMR spectra

The microstructure of stereoregular 1,4-trans-and 1,4-cis-polypiperylenes, as well as polymers prepared from the trans-and cis-piperylene isomers via cationic polymerization in the presence of TiCl₄, was studied by high-resolution ¹³C NMR spectroscopy. Polypiperylene synthesized through the cationic polymerization of the cis isomer had a more diversified morphology of the macromolecular chain, i.e., had higher relative amounts of 1,2-cis-units and combinations of irregular-addition 1,4-trans-units. It was shown that ¹³C NMR spectra give the most comprehensive and independent information on the details of structure of the piperylene macromolecular chain.     

Polymerization of butadiene catalyzed by a cobalt-containing catalyst in aliphatic media

The polymerization of butadiene catalyzed by the preformed Co(2-ethylhexanoate)₂-Et₃Al₂Cl₃-isoprene (a molar ratio of 1: 20: 20) catalytic system in toluene, cyclohexane, and hexane has been studied. In toluene, the catalyst demonstrates high activity and stereospecificity and yields a high-molecular-mass polymer. In cyclohexane and hexane, the catalyst has a high activity but affords polymers with lower contents of 1,4-cis-units and reduced intrinsic viscosities. The addition of EtAlCl₂ and/or a decrease in the polymerization temperature make it possible to obtain polybutadiene containing more than 96% 1,4-cis-units and an acceptable intrinsic viscosity (2.0–2.8 dl/g) in the nonaromatic solvents as well.     

Polymerization of butadiene by neodymium catalysts on oxide supports

The suspension polymerization of butadiene in hexane and toluene in the presence of catalysts composed of organoaluminum compounds and neodymium versatate applied on oxides (Al₂O₃, aluminum silicate A-14, Aerosil A-300, and white soot BS-200) has been studied. Triisobutylaluminum and its combination with ethylaluminum sesquichloride are used as organoaluminum compounds. The activity and stereospecificity of the catalysts has been found to depend strongly on the nature of supports. Catalysts based on Aerosil and white soot appear to be the most active. With the use of these catalysts, polybutadiene containing up to 97.5% 1,4-cis-units has been synthesized.     

C(3)-alkylation and cyclization of pyrazino[1,2-b]isoquinolin-4-ones

To avoid the epimerization of the C(11a)-stereocenter previously observed in 6,11a-cis-pyrazino[1,2-b]isoquinolin-1,4-diones, we present in this paper the C(3)-alkylation of 1-methoxy-pyrazino[1,2-b]isoquinolin-4-ones to obtain all-cis derivatives through a very reliable protocol. The success of the acid-promoted cyclization to get pentacyclic (R₃=arylmethyl) or tetracyclic (R₃=2-bromo-2-propenyl) compounds is dependent on the nature of the C(3)-unsaturated chain and of the N-substituent, but these limitations have been overcome by using trifluoromethanesulfonic as a superacid catalyst. The C-(3)-alkylation of pyrazino[1,2-b]isoquinolin-4-one is also studied.     

Stereospecific functionalization of the heterocyclic ring systems of flavan-3-OL and [4,8]-biflavan-3-OL derivatives with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)

Efficient stereospecific 4-methoxylation of both 2,3-trans- and 2,3-cis-flavan-3-ol methyl ethers with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) in CHCl₃-MeOH solution is of both synthetic and degradative significance in oligomeric flavanoid chemistry.     

RuO4-catalyzed oxidative polycyclization of squalene. Determination of the configuration of the penta-tetrahydrofuranyl diol product

The configuration of the penta-tetrahydrofuranyl diol (penta-THF) product obtained by a single-step, RuO₄-catalyzed oxidative polycyclization of squalene, has been determined as cis-threo-cis-threo-trans-threo-trans-threo-trans. The cis-cis-trans-trans-trans sequence for the five contiguous THF rings has been established through extensive 2D-NMR spectroscopic studies carried out both on the intact molecule and on some of its derivatives, including the oxidative cleavage products obtained by degradation of the penta-THF with PCC/AcOH. Four different chemical approaches were devised to determine each of the four threo relationships within each carbon pair connecting adjacent THF rings in the molecule. To this aim, studies have been carried out either on some intermediates of the process leading to penta-THF, obtained by stopping the oxidation of squalene prior to completion, or on a degradation product of the penta-THF, obtained from the latter through a bidirectional double oxidative degradation with PCC.     

Quantum-chemical study of the effect of butadiene interaction with anionic active sites on the polymer microstructure

The electronic and geometric structure of the models for prereaction complexes of the anionic active sites for polymerization of butadiene have been calculated using a modified CNDO method: C₄H₄Li-cis-C₄H₆(I), C₄H₇Li-trans-C₄H₆(II), (C₄H₇Li)₂-cis-C₄H₆(III) and (C₄H₇Li)₂-trans-C₄H₆. The configuration of complexes I and II resulting from the total energy minimization points to the preferential C₄H₆ attack on the α-C atom of the monomeric active site (AS) leading to 1,4-units in polybutadiene. A more pronounced complexation effect observed with I as compared to II was taken into account when interpreting data on the preferential formation of cis-1,4-structure within macromolecules. The structure of models III and IV and also a decrease in the difference of the energy of interaction with C₄H₆ incorporated in these models, as compared to models I and II, indicate a decrease in the 1,4-cis-units content with increasing initiator concentration. Based on results of the present study, an evaluation was also made of the effect of the interaction between the living macromolecule aggregates and diene on the dissociation processes.     

Nature of active centers and stereospecificity of tetravalent titanium benzyl derivatives in polymerization of butadiene

Stereospecificity of tetrabenzyltitanium and its halogeno-derivatives in the polymerization of butadiene has been investigated. The content of 1,2-units decreases while the content of 1,4-cis-units increases in the resulting polybutadiene for the series (C₆H₅CH₂)₄Ti, (C₆H₅CH₂)₃TiCl, (C₆H₅CH₂)₃TiBr, (C₆H₅CH₂)₃Til. Tribenzyltitanium iodide exhibits high stereospecificity for the formation of 1,4-cis-units and their content reaches 94–97%. By determining the number of benzyl groups linked with titanium at different degrees of conversion, it has been shown that the active centre formed from tetrabenzyltitanium contains three benzyl groups and one polymer chain. Two benzyl groups, one iodine atom and one polymer chain are attached to a titanium atom in the active centre for the case of tribenzyltitanium iodide. Electron donors sharply change the stereospecificity of tribenzyltitanium iodide: the content of 1,2-units in the polymer rises to 68%.     

Co(II), Ni(II) and Cu(II) complexes of new [1+1] and [2+2] macrocyclic ligands derived from 1,4-bis(2′-formylphenyl)-1,4-dioxabutane and cis-1,2-diaminocyclohexane

Two new macrocyclic ligands, L¹ (14-membered N₂O₂) and L² (28-membered N₄O₄) from [1+1] and [2+2] condensation, respectively, have been obtained in a one-pot synthesis starting from 1,4-bis(2′-formylphenyl)-1,4-dioxabutane and cis-1,2-diaminocyclohexane.     

Isoprene polymerization using a neodymium phenolate pre-catalyst combined to aluminum based co-catalysts

The discrete phenolate Nd(2,6-di-tert-butyl-OC₆H₃)₃ has been assessed as pre-catalyst for the polymerization of isoprene using various aluminum based co-catalysts. In combination with MAO and MMAO in toluene, the reaction is quantitative in 1 h at 60 °C, yielding activities comparable to neodymium versatate and isopropylate in similar conditions. In contrast with the abovementioned systems, the phenolate leads to a single site catalyst species in combination with MAO and MMAO. Increasing amounts of MAO in the reactive medium lead to a decrease of the number–average molecular weight of polyisoprene, highlighting the occurrence of chain transfer between neodymium and aluminum. The polymerization is modestly 1,4-trans selective, in the range 60–75%. Using pentane as a solvent and MMAO, ultra high activities up to 3000 kg PI/mol Nd/h can be reached at room temperature, as well as 85% 1,4-cis selectivity. Ternary systems with co-catalysts based on the chlorinated AlEt₂Cl in combination with AlⁱBu₃ afford finally a 90–98% 1,4-cis stereoselective polymerization depending on the solvent used for the reaction. The SEC traces are in agreement with the presence of several active species for the latter systems.     

A single-helix coordination polymer with mixed ligands [Zn2(phen)2(e,a-cis-1,4-chdc)2(H2O)2]n (phen=1,10-phenanthroline; chdc=cyclohexanedicarboxylate)

A coordination polymer with mixed ligands [Zn₂(phen)₂(e,a-cis-1,4-chdc)₂(H₂O)₂]_n (chdc=cyclohexanedicarboxylic acid; phen=1,10-phenanthroline) was prepared under hydrothermal conditions and characterized by elemental analyses, IR spectra, TG analysis, and single-crystal X-ray diffraction analysis. X-ray crystal structural analyses reveal that 1 and 2 are isomorphic and belong to the monoclinic system. C₄₀H₃₆Zn₂N₄O₁₀, P2₁/c, a=10.084(2) Å, b=8.9072(18) Å, c=20.276(4) Å, β=99.92(3)°, V=1793.9(6) Å³, Z=1. In the structures of 1, the 1,4-chdc ligand possesses only one kind of e,a-cis-conformation although there are both cis- and trans-conformations in the raw material. Two oxygen atoms of one carboxyl in 1,4-chdc ligand and another oxygen atom of contraposition carboxyl link adjacent Zn atoms into an infinite 1D zigzag chain. The most attractive structural feature of 1 is that it exhibits an infinite chiral chain-like structure with 2₁ helices along the b-axis. In addition, the right- and left-handed chains are alternate. Meanwhile, the adjacent chains of 1 is linked via hydrogen bonds into 2D network structures, which further form 3D frameworks via π-π interactions of 1,10-phen.     

Effect of the particle size of the TiCl<Subscript>4</Subscript>-Al<Emphasis Type="Italic">i</Emphasis>-Bu<Subscript>3</Subscript> catalyst on the contribution from mono- and bimetallic active sites to the polymerization of isoprene

The interaction between the coordinatively unsaturated surface of ß-TiCl₃ particles and a liquid phase in the TiCl₄-Ali-Bu₃ catalyst is responsible for the final particle size and the regularities of isoprene polymerization. The correlations of the catalyst activity and the molecular characteristics of polyisoprene with catalyst particle size in the course of catalyst formation and reactivation are indicative of the occurrence of two groups of active sites. “Surface” active sites correspond to the monometallic Cossee model, and they are characterized by low activity and low 1,4-cis specificity in the polymerization of isoprene. “Colloid” active sites have a bimetallic structure and produce polyisoprene at a high rate; the concentration of 1,4-cis units in the resulting polyisoprene is as high as 97%. The contribution from the colloid active sites to the polymerization of isoprene increases with the particle size of ß-TiCl₃.     

Enhanced formation of quercetin by combined use of gamma ray and H2O2 from cyanidin-3-O-xylosylrutinoside

Cyanidin-3-O-xylosylrutinoside (cya-3-O-xylrut), a major pigment in Schizandra chinensis Baillon, was effectively removed by gamma irradiation of greater than 2 kGy, whereas quercetin, the most abundant of the flavonoids and has anti-inflammatory and anti-allergic effects, could be generated by degradation of cya-3-O-xylrut. In the present study, we investigated the effect of combination treatment of gamma irradiation and hydrogen peroxide (H₂O₂) on the formation of quercetin through the degradation of cya-3-O-xylrut. Cya-3-O-xylrut was significantly degraded (～93%) by gamma irradiation at 2 kGy and it was completely removed by a combination treatment (0.2% H₂O₂ and 2 kGy gamma ray). The formation of quercetin was significantly appeared at 2 kGy of gamma ray, together with disappearance of cya-3-O-xylrut. The quercetin formation by gamma ray is 3.2 μg/ml and combination treatment is 7.7 μg/ml. Therefore, the combination treatment of H₂O₂ and gamma ray is more effective to convert cya-3-O-xylrut into quercetin than gamma irradiation only. In conclusion, gamma ray combined with H₂O₂ would be a promising tool for bio-conversion of organic compounds.     

Synthesis of some new chiral bifunctional o-hydroxyarylphosphonodiamides and their application as ligands in Ti(IV) complex catalyzed asymmetric silylcyanation of aromatic aldehydes

Some new chiral bifunctional o-hydroxyarylphosphonodiamides were synthesized starting from (+)-cis-1,2,2-trimethylcyclopentane-1,3-diamine and the absolute configuration of the phosphorus atom was determined by X-ray diffraction analysis. Excellent enantioselectivity (up to 98% ee) was achieved in asymmetric silylcyanation of aromatic aldehydes using a chiral titanium complex formed in situ from Ti(OⁱPr)₄ and o-hydroxyarylphosphonodiamide as the catalyst.     

Me3SiCl-promoted intramolecular cyclization of aromatic compounds tethered with N,O-acetals leading to the facile preparation of 1,4-benzodiazepine skeletons

The Me₃SiCl-promoted intramolecular aminomethylation of a novel type of N,O-acetals, which were prepared via a facile three-step synthesis from N-alkylaniline derivatives and N-alkyl-2-oxazolidinones that leads to the production of pharmaceutically useful 1,4-benzodiazepine skeletons with a variety of functional groups is described. This method was successfully applied to the facile preparation of both tricyclic benzodiazepine derivatives and a 1,4-benzoxazepine derivative via 7-exo-trig cyclization.     

Unusual ring contraction by substitution of 4-O-activated-pentono-1,5-lactams with cyanide. Stereospecific synthesis of 6-amino-1,4,5,6-tetradeoxy-1,4-imino-hexitols

Reaction of 4-O-sulfonylated 2,3-O-isopropylidene-d-ribo- or -d-lyxo-1,5-lactams with tetrabutylammonium cyanide gave 4-amino-5-C-cyano-4,5-dideoxy-2,3-O-isopropylidene-l-lyxo-5 or -l-ribo-15-1,4-lactams, respectively. A stereospecific ring contraction with inversion at C-4 had taken place in each case. Reduction of the cyano-lactams with LiAlH₄ gave 6-amino-1,4,5,6-tetradeoxy-1,4-imino-l-lyxo-6 or -l-ribo-16-hexitol, respectively. The 6-amino-1,4,5,6-tetradeoxy-1,4-imino-l-ribo-hexitol 16 was found to be a moderate inhibitor of α-l-fucosidase with a K_i of 110 μM.     

Conformational isomorphism in crystalline 1,4-cis-polyisoprene

The possibility that the statistical structure of 1,4-cis-polyisoprene (natural rubber), proposed by Nyburg, could be interpreted with a statistical succession of different conformations along the polymer chain has been investigated. The X-ray data can be interpreted on the basis of a statistical succession of conformations S₊ TS_? or S_? TS₊ of each triplet of consecutive single bonds along the chain.     

Study of chain branching in natural rubber using size-exclusion chromatography coupled with a multi-angle light scattering detector (SEC-MALS)

The different rheological behaviour of natural rubber (NR) compared to industrial synthetic poly(cis-1,4-isoprene) (SR) has been attributed to the gel phase and long-chain branching. Previous studies on branching in NR were carried out using the fractionation technique by precipitation to obtain narrow molar mass distribution. In this study, chain branching of poly(cis-1,4-isoprene) in NR was characterised by size-exclusion chromatography coupled with an online multi-angle light scattering detector (SEC-MALS). The nanoaggregates adsorbed on the column packing interfered with branching characterisation for short and medium chains (M_w < 1000 kg/mol). Using a master curve of linear standard poly(cis-1,4-isoprenes), SEC-MALS revealed no or very little branching in the higher chains (1000 < M_w < 10,000 kg/mol) of natural rubber contrary to previous studies. This study showed that the soluble portion of NR samples was composed of almost linear poly(cis-1,4-isoprene) and nanoaggregates with rather compact structures.     

Effect of capping and particle size on Raman laser-induced degradation of γ-Fe2O3 nanoparticles

Diol capped γ-Fe₂O₃ nanoparticles are prepared from ferric nitrate by refluxing in 1,4-butanediol (9.5 nm) and 1,5-pentanediol (15 nm) and uncapped particles are prepared by refluxing in 1,2-propanediol followed by sintering the alkoxide formed. X-ray diffraction (XRD) shows that all the samples have the spinel phase. Raman spectroscopy shows that the samples prepared in 1,4-butanediol and 1,5-pentanediol and 1,2-propanediol (sintered at 573 and 673 K) are γ-Fe₂O₃ and the 773 K-sintered sample is Fe₃O₄. Raman laser studies carried out at various laser powers show that all the samples undergo laser-induced degradation to α-Fe₂O₃ at higher laser power. The capped samples are however, found more stable to degradation than the uncapped samples. The stability of γ-Fe₂O₃ sample with large particle size (15.4 nm) is more than the sample with small particle size (10.2 nm). Fe₃O₄ having a particle size of 48 nm is however less stable than the smaller γ-Fe₂O₃ nanoparticles.