摘要:

The electrocatalytic hydrogenation (ECH) of &bgr;-O-4 lignin model compounds has been carried out at constant current at Raney nickel and palladium-based cathodes in aqueous sodium hydroxide solution at temperatures ranging from 25 to 75°C. It was found that the hydrogenolysis of phenolic &bgr;-arylethyl-aryl ethers with one &bgr;-O-4 linkage takes place at both types of electrodes to give a mixture of phenolic compounds. The ECH of 1-(4-hydroxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)-1-ethanol (model1) at Raney nickel gives guaiacol (9), acetovanillone (11b), alpha-methylvanillyl alcohol (12b), and products derived from their further hydrogenation. The &bgr;-O-4 bond is more difficult to cleave upon replacing one hydrogen of the methylene group of1by a hydroxymethyl group (see2). However, model4with two &bgr;-O-4 linkages obtained by the introduction of an &agr;-methylvanillyl group at the phenolic position of1is hydrogenolysed at the same rate as1at Raney nickel. Only the C-O aryl bond attached to the hydroxyethylphenol moiety is cleaved. At palladium, the hydrogenolysis of1requires only half of the theoretical amount of electricity due to the generation of chemisorbed hydrogen by dehydrogenation of1followed by hydrogenolysis of the resulting ketone. A general reaction scheme is proposed for the ECH of model1.Key words: Lignins, lignin models, electrocatalytic hydrogenation, Raney nickel electrodes, palladium electrodes

ISSN:0008-4042    

DOI:10.1139/v00-009

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

The title complex is asymmetric in the crystal due to the spatial orientation of the NCS function. The space group has been determined to beP21witha= 9.496(3),b= 8.736(3),c= 9.676(3) Å, &bgr; = 117.859(5)°, andZ= 2. The solid state13C NMR spectrum is consistent with the lack of symmetry in the crystal and there is little evidence for large amplitude motion in the macrocycle as determined from the dipolar dephased spectrum.Key words: macrocyclic crown ether, lithium comple

ISSN:0008-4042    

DOI:10.1139/v00-008

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

Bis(hexafluoroacetylacetonate)cobalt(II), Co(hfac)2, reacted with the nitronyl nitroxide radical 2-(4-methylphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, NITPhMe, to yield an octahedral adduct of formula Co(hfac)2(NITPhMe)2. The X-ray crystal structure shows that it crystallizes in the space groupP1, witha= 9.837(3),b= 11.618(3),c= 20.502(4) Å, &agr; = 81.22(2), &bgr; = 87.93(2), &ggr; = 72.24(3)º,V= 2205(1) Å3,Z= 2. The mechanism of ferromagnetic exchange between cobalt(II) and the radical can be qualitatively understood in terms of a charge-transfer interaction (Jbeing 25.7 cm-1, the Hamiltonian is written in the formH= -2J(SRad1·SCo+SCo·SRad2)).Key words: crystal structure, cobalt(II) complex, nitronyl nitroxide, magnetic proper

ISSN:0008-4042    

DOI:10.1139/v00-019

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

The nature of the hydrogen transferred in the CH3/CH4+, CH3/CH4·, and CH3/CH4-systems is investigated by analyzing the topology of the contracted intracule and extracule electron-pair densities and their respective Laplacians. The CH3/CH4+, CH3/CH4·, and CH3/CH4-systems are taken as simple models for the study of hydride (H-), hydrogen (H·), and proton (H+) transfer reactions, respectively, under a constrained C-C distance. The study is focused on the comparison of the intracule and extracule densities at the intermediate structures for the three H-transfer reactions, complementing a previous investigation of the same model reactions based on the analysis of one-electron densities. The results obtained by analyzing the contracted electron-pair densities are consistent with those obtained from the analysis of one-electron densities. The electronic nature of the H atom being transferred in the three systems can be differentiated by the topologies of the corresponding intracule and extracule densities. However, the analysis underlies also the difficulties to interpretation of the topologies of contracted electron-pair densities, as different electron-electron interactions may contribute to the same point in the intracule or extracule spaces. In particular, for the systems studied, the contribution of the electron-electron interaction associated to the probability of having two electrons on the H being transferred is not reflected separately neither in the intracule nor in the extracule distributions. Nevertheless, the nature of the H being transferred can still be studied by comparing the importance of the electron-electron interactions associated to the probability of having one electron in C and one in the transferring H. The effects of inclusion of electron correlation are also discussed by means of (HF-CISD//HF) intracule and extracule density difference maps.Key words: hydrogen transfer, electron-pair density, intracule density, extracule density, topological density analyisis.

ISSN:0008-4042    

DOI:10.1139/v00-016

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

he metabolism of analogs of the cruciferous phytoalexin brassinin by the phytopathogenic fungiPhoma lingam(Tode ex Fr.) "virulent" and "avirulent" groups (sexual stageLeptosphaeria maculans(Desm.) Ces. et de Not.) andAlternaria brassicae(Berk.) Sacc. is reported. It was established that each pathogen detoxified methyl tryptamine dithiocarbamate, although yielding different metabolic products. While the biotransformation by virulentP. lingamproceeded to yield methyl 3a-hydroxy-3,3a,8,8a-tetrahydropyrrolo[2,3-b]indol-1(2H)-yl carbodithioate and indole-3-acetic acid as final products, avirulentP. lingamyielded indole-3-carboxylic acid viaNb-acetyltryptamine, andA. brassicaeyieldedNb-acetyltryptamine, as final products. Furthermore, to establish the importance of the dithiocarbamate group in antifungal activity againstP. lingamandA. brassicae, carbamates were compared with dithiocarbamates and it was established that carbamates were devoid of antifungal activity. These products, contrary to methyl tryptamine dithiocarbamate, showed no inhibitory activity against either pathogen.Key words:Alternaria brassicae, brassinin, carbamates, detoxification, dithiocarbamates,Leptosphaeria maculans,Phoma lingam, phytoalexins.

ISSN:0008-4042    

DOI:10.1139/v00-024

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

A compound isolated from a synthesis directed toward 42-crown-14 (42C14) has been purified and characterized by means of IR, HPLC, NMR, MS, and DSC techniques. Its infrared spectrum contains no OH, tosylate, or unsaturated unit signals. The HPLC retention time of the compound differs from that of "42C14" and other crown ethers examined. The chemical shifts of the singlets in the1H and13C NMR spectra differ from those of poly(ethylene oxide)s (PEOs), and crown ethers. The FAB mass spectra contain signals attributable to a compound with twice the mass of 42C14 and the fragmentation pattern is consistent with [2]catenane9based on interlocked 42C14 units, because the next major signals generally correspond to 42C14 and its adducts. The DSC results showed that the compound, a colorless wax at room temperature, was amorphous with a glass transition temperature which was 80° higher than those of aliphatic crown ethers, which are crystalline compounds with melting points >40°C. Considering the high yield of the compound (8% in the reaction mixture as determined by NMR, 1.7% isolated), it is believed that sodium cation-crown ether-linear ether complexation was involved in templating its formation. Evidence for such complexation was also observed in the FAB MS.Key words: catenanes, crown compounds, macrocycles, template synthesis, rotaxane

ISSN:0008-4042    

DOI:10.1139/v00-017

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

2-Benzyloxy-2-methoxy-5,5-dimethyl-&Dgr;3-1,3,4-oxadiazolines, including the parent as well asp-substituted analogues, undergo thermolysis at 100°C in benzene to afford a mixture of products. Two primary fragmentations of the oxadiazolines were identified. The major pathway involves 1,3-dipolar cycloreversion to N2and the corresponding carbonyl ylides. The latter dissociate to acetone and the corresponding benzyloxy(methoxy)carbenes, which undergo fragmentation to ArCH2and MeOCO radical pairs that recombine to afford methyl arylacetates. Carbene dimers were not observed, showing that the fragmentation process is faster than carbene dimerization. A second fragmentation pathway observed for the oxadiazolines is an alternative cycloreversion to the corresponding benzyl methyl carbonate and 2-diazopropane. Products from diazopropane included acetone azine and, in some instances, traces of propene.Key words: benzyloxy(methoxy)carbene, carbene, radical pair, rearrangement

ISSN:0008-4042    

DOI:10.1139/v00-021

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

Organosulfur compounds in garlic, like allyl methyl disulfide, have been found to be involved in antimutagenic, anticarcinogenic, antithrombotic, and lipid-lowering activities, and it has also been found to act as an antioxidant. Ab initio molecular computations were performed on dihydrogen disulfide (1) with respect to torsional angle &tgr;1= &tgr;(H·S-S·H), hydrogen methyl disulfide (2) with respect to torsional angle &tgr;1= &tgr;(H·S-S·CH3), and allyl methyl disulfide (3) with respect to torsional angles &tgr;1= &tgr;(H3C2·CH2·S-S·CH3), &tgr;2= &tgr;(H3C2·CH2-S·S·CH3), and &tgr;3= &tgr;(H3C2-CH2·S·S·CH3). Potential energy curves (PEC) were obtained from1and2, i.e.,E=E(&tgr;1), from which optimized structures were obtained at the HF/6-31G* level of theory. These optimized structures were used to investigate the potential energy hypersurface surface (PEHS) of3, i.e.,E=E(&tgr;1,&tgr;2,&tgr;3). One-dimensional scans along &tgr;2and &tgr;3(where &tgr;1= ±90°; &tgr;1= 180°) were performed at the HF/3-21G* level of theory. From these scans, six lower energy pairs of enantiomeric minima (i.e., [g+g+g+| g-g-g-], [g+ag-|g-ag+], [g+g-g+| g-g+g-], [g+g+g-| g-g-g+], [g+ag-| g-ag+], and [g+g-g-| g-g+g+]) as well as 3 higher energy minima (i.e., [g+g+s | g-g-s], [g+as | g-as], and [g+g-s | g-g+s]) were optimized at &tgr;1= ±90° at the HF/6-31G* and B3LYP/6-31G* levels of theory. The global minimum was determined to be the [g+g-g+|g-g+g-] enantiomeric pair of conformers, and the fully symmetricalanti-anti-anti[a a a] structure was determined to be a second-order saddle point on the PEHS of3. Although there are no stereocentres in3, there is chirality in the conformational twist with respect to the [a a a] conformation through &tgr;1= &tgr;2= &tgr;3= 180°. Based on the energies and MO diagrams of the HUMO and LUMO +1 of3, the anticarcinogenic and cholesterol lowering activity mechanism of3is presented.Key words: ab initio MO computations, allyl methyl disulfide, multidimensional conformational analysis (MDCA), anticarcinogenic, cholesterol lower

ISSN:0008-4042    

DOI:10.1139/v00-020

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

Reduction of [(η-C5Me5)MoCl(O)]2(&mgr;-O) or (η-C5Me5)MoCl2(O) with sodium or magnesium amalgam, magnesium turnings, or tributyltin hydride produced [(η-C5Me5)Mo]4O7, with [(η-C5Me5)Mo(O)(&mgr;-O)]2as a co-product. [(η-C5Me5)Mo]4O7was characterized by X-ray diffraction, mass spectrometry,1H NMR and IR spectroscopies, and magnetism. Crystals of [(η-C5Me5)Mo]4O7contained a tetrahedral [(η-C5Me5)Mo]4unit (Mo-Mo = 2.909 (3) Å) with the Mo4O7core having the structure Mo4(&mgr;2-O(b))3(µ2-O(c))3(µ3-O(a)) (3). Microcrystalline samples of [(η-C5Me5)Mo]4O7were paramagnetic over the temperature range 2-300 K, with an effective moment of 1.26 &mgr;Bat 300 K. [(η-C5Me5)Mo]4O7was also paramagnetic in chloroform solution, over the temperature range 223-298 K, with an effective moment of 1.43 µBat 298 K. The1H NMR spectrum showed a broad resonance at 16.3 ppm (&Dgr;&ngr;1/2= 113 Hz) and two narrow resonances at 1.89 ppm and 1.69 ppm (&Dgr;&ngr;1/2= 5 Hz). The magnetism and NMR spectra showed that [(η-C5Me5)Mo]4O7existed in two forms which were in equilibrium in solution. One form was paramagnetic (S= 1), with the Mo4O7core having the geometry3, and the other was diamagnetic (S= 0), with the Mo4O7core having the geometry4.Key words: cluster, cyclopentadienyl, molybdenum, oxide, paramagnetism

ISSN:0008-4042    

DOI:10.1139/v00-030

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

Selenocysteine is expected to have 9 × 9 = 81 conformations [3 × 3 = 9 backbone: ψ (g+,a,g-) × φ (g+,a,g-) and 3 × 3 = 9 side-chain: χ1(g+,a,g-) × χ2(g+,a,g-)]. In the present study, all the torsional modes of the side-chain (χ1: rotation about the C&agr;-C&bgr;and χ2: rotation about the C&bgr;-Se bonds) were investigated in the relaxed &ggr;Lbackbone [(φ,ψ); (g-,g+)] conformation. Seven out of the nine expected minima were found at the RHF/3-21G level of theory forN-formyl-L-selenocysteinamide (For-L-Sec-NH2) andN-acetyl-L-selenocysteine-N-methylamide (Ac-L-Sec-NHMe). The stabilization energy exerted by the -CH2-SeH side-chain has been compared with that of -CH2-SH and -CH2-OH. Relative energies of the various conformers were also obtained via single point calculations at the B3LYP/6-31G(d,p) level of theory. Topological analysis of the electron density has been performed by Bader's Atoms in Molecule (AIM) approach using the results. The structures were also optimized at the B3LYP/6-31+G(d,p) level of theory.Key words: selenocysteine side-chain conformations, ab initio MO study, Multidimensional Conformational Analysis (MDCA), Atoms in Molecules (AIM), Bader's electron density anal

ISSN:0008-4042    

DOI:10.1139/v00-029

出版商:NRC Research Press    

年代:2000

数据来源: NRC