bluuebunny/crossref_metadata_2025 huggingface dataset:
Enamines from formamides. Synthesis of 1,2,3,4-tetrahydro-1-substituted β-carbolines
Authors: Vohra Rahul , Maclean David B.
Published: May 2006

Pyridines and benzopyridines substituted with trimethylsilylmethyl groups, α or γ to the nitrogen atom, have been found to react with formamides by way of a Peterson reaction to form enamines. Trimethylsilylmethylbenzene behaved similarly. The enamines prepared from Nb-formyl-Na,Nb-dimethyltryptamine cyclized readily to β-carbolines. The reaction of formamides with lithiated 4-methylpyridines as a route to enamines was examined but proved to be less general and proceeded in lower yield than the route by way of the Peterson reaction. The nuclear magnetic resonance and mass spectra of the enamines and β-carbolines derived from 3 are discussed.
Reactions of 7-azaindole with niobium and tantalum pentachlorides and coupling to the azaindolyl-azaindolium cation
Authors: Poitras Jacques , Beauchamp André L.
Published: May 2006

The reaction of NbCl5 and TaCl5 with 7-azaindole (Haza) at room temperature in benzene or dichloromethane yielded MCl5(Haza) addition compounds. Under more severe conditions, the same compound was obtained with TaCl5. For NbCl5, some reduction to Nb(IV) was observed and NbCl5(Haza), NbCl4(Haza)2, and the (H2aza)+ ion were identified in the reaction mixture by infrared spectroscopy. Oxidative coupling of two azaindole units via N7—C6 also took place during the reaction, since the 7-(azaindol-6-yl)azaindolium cation was found as counter-ion in the crystal structures of two complex salts. In the crystals of (H2aza-aza)[NbOCl4(Haza)]•0.5CH2Cl2([Formula: see text]a = 7.255 Å, b = 12.412 Å, c = 14.277 Å, α = 89.03°, β = 85.60°, γ = 76.66°, Z = 2, R = 0.062), the anion is the roughly octahedral [NbOCl4(azaindole)]− species containing a neutral N7-coordinated azaindole trans to the Nb=O bond. The [NbOCl5]2− salt ([Formula: see text]a = 7.527 Å, b = 10.168 Å, c = 10.467 Å, α = 66.41°, β = 84.07°, γ = 85.51°, Z = 1, R = 0.037) contains the distorted octahedral [NbOCl5]2− ion disordered over two orientations. The infrared spectra suggest monomeric octahedral structures for the MCl5(Haza) and NbCl4(Haza)2 complexes. 1H NMR spectroscopy shows that NbCl5(Haza) is not dissociated in CD2Cl2.
Ditopic thiacyclophanes containing the S(CH2)2X(CH2)2S (X = O, S) linkage. Synthesis and structures of 2,5,8,17,20,23-hexathia[9](1,2)[9](4,5)cyclophane and 5,20-dioxa-2,8,17,23-tetrathia[9](1,3)[9](4,6)-2,5-bis(ethoxy)cyclophane
Authors: Loeb Stephen J. , Shimizu George K.H.
Published: May 2006

The Cs+/DMF mediated reactions of 1,2,4,5-tetra(bromomethyl)benzene and 1,2,4,5-tetra(bromomethyl)3,6-bis(ethoxy)benzene with dithiols HSCH2CH2XCH2CH2SH (X = S,O) were explored in order in order to ascertain the effect of structural variations on the regioselectivity of double ring-closure. Although a structure with bis(ortho) geometry was the target molecule, the major products are those with a bis(meta) structure. Two crystal structures, one with a bis(ortho) structure, 2,5,8,17,20,23-hexathia[9](1,2)[9](4,5)cyclophane, 1, and one with a bis(meta) structure, 5,20-dioxa-2,8,17,23-tetrathia[9](1,3)[9](4,6)−2,5-bis(ethoxy)cyclophane, 7, are reported. 1 crystallized in the triclinic space group [Formula: see text] with a = 8.583(3), b = 11.806(3), c = 5.448(2) Å, α = 97.18(2), β = 107.18(2), γ = 99.40(2)°, V = 511.6(6) Å3, and Z = 1. The structure refined to R = 6.48% and Rw = 7.13% for 1020 reflections with Fo2 > 3σ(Fo2). 7 crystallized in the triclinic space group [Formula: see text] with a = 9.568(2), b = 9.577(2), c = 8.312(2) Å, α = 113.53(2), β = 94.58(2), γ = 114.94(2)°, V = 604.3(7) Å3 and Z = 1. The structure refined to R = 3.86% and Rw = 4.20% for 1129 reflections with Fo2 > 3σ(Fo2). Both compounds sit on a crystallographic centre of symmetry and have the thioether linkages oriented on opposite sides of the central aromatic ring.
Nickel(II) and palladium(II) chelates of dehydroacetic acid Schiff bases derived from thiosemicarbazide and hydrazinecarbodithioate
Authors: Kubaisi Abdulla Al , Ismail Kamal Z.
Published: May 2006

Dehydroacetic acid Schiff bases DAE (1) and DATS (2) were isolated from the reaction of dehydroacetic acid (2-acetyl-5-hydroxy-3-oxo-4-hexenoic acid-δ-lactone) with hydrazine-S-methylcarbodithioate and the corresponding acid amide, thiosemicarbazide, respectively. Reaction of such Schiff bases with nickel(II) and palladium(II) ions afforded the dimeric monoligand chelates [M(DAE-2H)]2 and [M(DATS-2H)]2; M = Ni(II) or Pd(II). In the presence of monodentate Lewis bases, B, square-planar chelates [M(DAE-2H)B] and [M(DATS-2H)B], in which B = pyridine or triphenylphosphine, were isolated. The stoichiometry and structure of the isolated organic ligands and their chelates were confirmed through elemental analyses, molecular weight determination, and infrared, ultraviolet–visible, and 1H and 13C nuclear magnetic resonance spectroscopy.
Excess volumes and excess viscosities of binary mixtures of 2-chloro-2-methylpropane with isomeric butanols at 298.15 K
Authors: Cea Pilar , Lafuente Carlos , Artigas Hector , Royo Felix M. , Urieta Jose S.
Published: May 2006

Excess volumes VE, excess viscosities ηE, and excess free energies of activation of flow G*E of binary mixtures of 2-chloro-2-methylpropane with isomeric butanols have been calculated from experimental measurements of density and viscosity at 298.15 K. The results are correlated by means of a Redlich–Kister type equation and explained in terms of molecular interactions.
Crystal structure – reactivity correlations in the solid state photochemistry of N-(tert-butyl)succinimide
Authors: Fu Tai Y. , Scheffer John R. , Trotter James
Published: May 2006

In order to provide experimental evidence on the stereoelectronic requirements for intramolecular hydrogen atom abstraction by carbonyl oxygen, N-(tert-butyl)succinimide, 1, was investigated by the crystal structure – reactivity correlation method. In this method, the success or failure of a given reaction in the crystalline state is correlated with the geometric parameters associated with the process as determined by X-ray crystallography. In the case of N-(tert-butyl)succinimide, the crystal and molecular structure shows that the molecule adopts a conformation in which there are very close contacts (d = 2.17–2.24 Å) between the carbonyl oxygen atoms and certain γ-hydrogen atoms on the tert-butyl group. In accord with these close contacts, ultraviolet irradiation of N-(tert-butyl)succinimide in the crystalline state leads to transfer of one of the γ-hydrogen atoms from carbon to oxygen. This produces a 1,4-biradical which closes to form a cyclobutanol, and the cyclobutanol undergoes ring opening to afford the final stable product, tetrahydro-1H-azepine-2,5-dione, 2. The crystal and molecular structure of photoproduct 2 is also reported. In contrast to many solid state transformations, there was no discernable melting of the crystals during reaction, and complete conversions of 1 into 2 could be achieved. The isolated chemical yield of photoproduct 2 from the solid state reaction (79%) is in fact considerably higher than that reported by Kanaoka and Hatanaka for the corresponding solution phase process (49%). This permitted the solid state reaction to be followed to 100% conversion by X-ray powder diffractometry, which showed that the process is a single crystal-to-polycrystalline transformation involving a regular and progressive conversion of 1 into 2 with no obvious diffraction peaks that might be attributable to the intermediate cyclobutanol.
Crystal chemistry of tetraradial species. Part 6. Embarras de richesses: lithium cation coordinated by eight O—H … phenyl bonds
Authors: Bakshi Pradip K. , Sereda Sergey V. , Knop Osvald , Falk Michael
Published: May 2006

Lithium tetraphenylborate tetrahydrate, [Li(OH2)4]BPh4 (LiTBw; tetragonal, I41/a, a = 27.566(2) Å, c = 12.228(2) Å, Z = 16) is remarkable in that the four H2O molecules coordinating the Li+ ion all form O—H …π hydrogen bonds to the phenyl groups of the anion. LiTBw thus appears to be the first reported example of such an exhaustive O—H …π coordination and can be described as a 3-dimensional, completely H-bonded polymer in which all the H2O hydrogens are bonded to phenyl groups and all the phenyl groups are involved in O—H … phenyl bonds. Six of the O—H … phenyl bonds are essentially normal and two are highly bent, possibly bifurcated. The existence of the H-bonds has been corroborated from variable-temperature FT-ir spectra of weakly deuterated LiTBw. The O—Li—O angles in the LiO4 coordination tetrahedron (of symmetry C1) exhibit large departures from the tetrahedral angle, two of the angles bisected by a quasi-S4 axis being 118.3° and 122.5°, respectively. An ab initio (6-31G*) investigation of the Li(OH2)4+ and Be(OH2)42+ cations has shown that such large O—M—O angles are to be expected even in the free ions and are thus not necessarily the result of packing effects. A detailed comparison with several Li(OH2)4+ and Be(OH2)42+ salts provides a rationale for the observed M(OH2)4n+ (M = Li, Be) geometries.
Rate constants for reactions of horseradish peroxidase compounds I and II with 4-substituted arylboronic acids
Authors: Sun Weimei , Ji Xiaoying , Kricka Larry J. , Dunford H. Brian
Published: May 2006

The rate constants for the reactions of horseradish peroxidase compound I (k1) and compound II (k2) with three 4-substituted arylboronic acids, which enhance chemiluminescence in the horseradish peroxidase catalyzed oxidation of luminol by hydrogen peroxide, were determined at pH 8.6, total ionic strength 0.11 M, using stopped-flow kinetic measurements. For comparison, the rate constants of the reactions of 4-iodophenol with compounds I and II were also determined under the same experimental conditions. The three arylboronic acid derivatives and their rate constants are: 4-biphenylboronic acid, k1 = (1.21 ± 0.08) × 106 M−1 s−1, k2 = (4.6 ± 0.2) × 105 M−1 s−1; 4-bromophenylboronic acid, k1 = (5.5 ± 0.2) × 104 M−1 s−1, k2 = (3.6 ± 0.2) × 104 M−1 s−1; and 4-iodophenylboronic acid, k1 = (1.1 ± 0.2) × 105 M−1 s−1, k2 = (1.3 ± 0.1) × 104 M−1 s−1. 4-Biphenylboronic acid, which shows comparable luminescent enhancement to 4-iodophenol, has the highest reactivity in the reduction of both compounds I and II among the three arylboronic acid derivatives tested.
Electron inductive perturbation(s) of heteronuclear metal–metal bonds. Isocyanide and indenyl derivatives of the mixed metal dimers [(η5–C5H5)Fe(CO)2Re(CO)5] and [MnRe(CO)10]
Authors: Harvey Pierre D. , Johnston Peter , Coville Neil J.
Published: May 2006

The properties of the heteronuclear metal–metal bonds in the unbridged [η5–C5H5)Fe(CO)2Re(CO)5−n(CNR)n] and [(η5–C9H7)Fe(CO)2Re(CO)5−n(CNR)n complexes (n = 0, 1, 2; R = tert-butyl (tBu) and 2,6-dimethylphenyl (Xy)) and the two equatorially substituted isomers of [MnRe(CO)8(CN-tBu)2] have been investigated theoretically by Extended Hückel Molecular Orbital calculations (EHMO) and experimentally by UV–visible spectroscopy, electrochemistry, and by microRaman or FT-Raman spectroscopy. The expected dσ* orbital is the lowest unoccupied molecular orbital (LUMO), mainly fabricated by interactions of the metal [Formula: see text] orbital, but the dπ* and dσ orbitals are the highest occupied molecular orbital (HOMO) and HOMO-1, respectively, as demonstrated experimentally from the UV–visible spectra. The EHMO computations demonstrate the mixing between these two dπ* (and dδ) and dσ MO orbitals. The influence of substituent effects on the spectroscopic and electrochemical properties is complicated and is interpreted in terms of inductive effects and relative destabilisation of the dσ, dπ*, and dσ* molecular orbitals. Finally, the metal–metal stretching frequencies for four mixed metal dimers (MnRe and FeRe) are reported.
Chemoenzymatic enantioselective synthesis of baclofen
Authors: Chênevert Robert , Desjardins Michel
Published: May 2006

We report two different chemoenzymatic enantioselective syntheses of baclofen based on the distinction between enantiotopic ester groups in compounds bearing a prochiral centre. In the first approach, the key step is the highly stereoselective enzymatic hydrolysis of dimethyl 3-(4-chlorophenyl)glutarate by chymotrypsin in an aqueous medium. In the second approach, the key step is the enzyme-catalyzed esterification of 2-(4-chloropheny 1)-1,3-propanediol by acetic anhydride in the presence of a lipase in an organic medium.
Hydration equilibria of 9-acridinecarboxaldehyde
Authors: McClelland Robert A. , Sukhai Pratima , Engell Karen M. , Sorensen Poul E.
Published: May 2006

Hydration rate constants and equilibrium constants have been obtained for 9-acridinecarboxaldehyde in aqueous solution. Under acidic conditions where the acridine is protonated, signals for the hydrate and free aldehyde forms can be observed as separate species in the 1H NMR spectrum. Integration provides the hydration equilibrium constant[Formula: see text] Using an apparent acidity constant obtained from a spectroscopic titration curve, the rate–pH profile was fitted to provide the hydration constant for the equilibration of the neutral acridines, KH = 0.07. This analysis also provides the acidity constants for the two acridinium ions, the aldehyde with pK = 3.78 and the hydrate with pK = 5.36. A comparison with the 4-pyridinecarboxaldehdye system reveals that the [Formula: see text] ratios for the acridine and pyridine are the same within experimental error, but that the acridine and acridinium aldehydes are 20-fold less hydrated than their pyridine analogs. A comparison with benzaldehydes reveals that, in their reactivities, the two heterocyclic aldehydes behave in a similar manner. Thus, for example, plots of log kH for the acid-catalyzed dehydration and hydrations versus log Kh for the equilibrium hydration show single correlation lines including the points for the benzaldehydes and heterocyclic aldehydes (but not the aliphatic aldehydes).
Control of regioselectivity in reactions of dialkylstannylene acetals. Part II. NMR results and mechanistic analysis
Authors: Kong Xianqi , Grindley T. Bruce
Published: May 2006

The 119Sn NMR spectra of dialkylstannylene acetals derived from a number of carbohydrate-based terminal 1,2-diols have been recorded. The spectra of chloroform-d solutions at room temperature of most of these dialkylstannylene acetals were consistent with the presence of single symmetric dimers only although some did contain small proportions of higher oligomers. A general mechanistic scheme is presented that explains the regioselectivity of p-toluenesulfonation reactions of dialkylstannylene acetals presented in the preceding paper. In the scheme, the reaction intermediates are the dialkylstannylene acetal dimers. For p-toluenesulfonation reactions, equilibration between the three possible dimers is faster than reaction with p-toluenesulfonyl chloride. Reaction regioselectivity depends both on the position of the dimer equilibrium and on the rates of reaction of individual dicoordinate oxygen atoms in the dimers. The general scheme was also used to explain the regioselectivity obtained for the very fast oxidation reactions of dialkylstannylene acetals with bromine or N-bromosuccinimide. In these kinetically controlled reactions, regiochemistry is determined by the initial reaction of dicoordinate oxygen atoms in the most populated dimer(s) with the electrophilic oxidizing reagent.
Porphyrin chemistry pertaining to the design of anti-cancer drugs; part 2, the synthesis and in vitro tests of water-soluble porphyrins containing, in the meso positions, the functional groups: 4-methylpyridinium, or 4-sulfonatophenyl, in combination with phenyl, 4-pyridyl, 4-nitrophenyl, or 4-aminophenyl
Authors: Meng Guangzhen G. , James Brian R. , Skov Kirsten A. , Korbelik Mladen
Published: May 2006

Water-soluble, meso-substituted porphyrins are synthesized by sulfonation or methylation of the phenyl or pyridyl groups, respectively, of some non-water-soluble functionalized porphyrins that we have recently synthesized (Can. J. Chem. 72, 1894 (1994)). Twelve anionic sulfonated and seven cationic methylpyridinium porphyrins, usually containing other functional groups (phenyl, pyridyl, nitro- and amino-phenyl), are reported on. Twelve of these porphyrins are new, and all nineteen porphyrins are characterized by 1H NMR. UV–VIS data are presented for the isolated porphyrins, while mass spectral data are noted for selected compounds. Partition coefficients of the porphyrins in octanol–water, and the accumulation of the porphyrins in HT-29 cells of a human-cancer tumor, were measured and both were inversely related to the charges of the porphyrins. The 11 porphyrins soluble in biological buffer solutions are basically nontoxic to Chinese hamster ovary (CHO) cells in oxic or hypoxic conditions or to aerobic HT-29 cells at the 100 µM concentration used (which is sometimes limited by solubility). No significant X-ray radiosensitization was observed for selected porphyrins toward CHO cells at the 100 µM concentrations used, but some photosensitized cell kill (using 630 nm light) was observed using the disodium salt of 5,10-bis(4-pyridyl)-15,20-bis(4-sulfonatophenyl)porphyrin (24).
Gold-acetylide chemistry: the structure of the complex [Au2(μ-dppm) (C≡C—CMe3)2]
Authors: Payne Nicholas C. , Ramachandran Ravindranath , Puddephatt Richard J.
Published: May 2006

The complex [Au2(μ-dppm)(C≡Ct-Bu)2], 1, dppm=Ph2PCH2PPh2, is formed by reaction of [AuC≡Ct-Bu] with dppm. Compound 1, as an acetone solvate, crystallizes in the monoclinic space group P21/c with cell dimensions a = 14.047(1), b = 16.124(1), c = 17.140(3) Å, β = 103.17(2)° and four formula units per cell. The crystal and molecular structures of this salt were solved by X-ray diffraction techniques using 2634 counter data with I > 2σ(I). Refinement of 278 variables by full-matrix least squares converged at an agreement factor R = 0.023. Thermolysis of complex 1 in refluxing toluene led to intermolecular elimination of t-BuC≡CH, by combination of a t-BuC≡C ligand of one molecule of 1 with a proton from the CH2P2 group of a second, and formation of the stable tetragold(I) complex [Au4(μ3-Ph2PCHPPh2)(μ-dppm)(C≡Ct-Bu)3], 2, which was characterized by its spectroscopic properties. Keywords: acetylide complexes, gold complexes, X-ray crystallography, nmr spectroscopy, bis(diphenylphosphino)methane.
Étude de la stéréochimie de la réaction de cycloaddition dipolaire-1,3 de quelques 5-phényl-3,6-dihydro-2H-1,4-oxazin-2-ones avec les N-méthyl et N-phénylmaléimides
Authors: Monnier Karin , Schmitt Gérard , Laude Bernard , Mercier Marie-France , Kubicki Marek M. , Jannin Michel
Published: May 2006

Six derivatives of 5-phenyl-3,4-dihydro-2H-1,4-oxazin-2-ones 1 were synthetized from α-amino acids. These compounds are precursors of six-membered cyclic azomethine ylids involving one stereocenter. The 1,3-dipolar species react with N-methyl and N-phenylmaleimides, leading diastereospecifically to cycloadducts where the approach of the dipolarophile occurs from the less hindered side of the 1,3-dipole and with the maleimido moiety in an exo position. Unfortunately, racemization was observed during the cycloaddition reaction, presumably owing to reversible imine–enamine transformation. Keywords: α-amino acids, cyclic azomethine ylids, diastereospecificity.
Studies on the mechanism of chelate degradation in iron-based, liquid redox H2S removal processes
Authors: Chen Dian , Martell Arthur E. , McManus Derek
Published: May 2006

Available data on the oxidation of nitrilotriacetic acid (NTA), ethylenedinitrilotetraacetic acid (EDTA), N-hydroxyethylethylenediaminetriacetic acid (HEDTA), and other aminopolycarboxylate chelating agents are reviewed and the intermediates and products of the oxidative degradation of each chelating agent are described and compared. The oxidation of these chelating agents occurs during the reoxidation of the ferrous chelate to the ferric chelate, during which a Fenton type side reaction occurs in which hydrogen peroxide is formed and which in turn generates the hydroxyl radical by reaction with ferrous ion. The site of oxidative attack by the hydroxyl radical on these ligands seems to be the -CH2- groups α to the carboxylate groups, as well as the -CH2- groups in the ethylene bridges between the nitrogens. The evidence for the implication of the hydroxyl radical as the active oxidant is discussed, and the use of phenyl-tert-butylnitrone (PBN) as the trapping agent for the hydroxyl radical is described. The use of chelating agents as Fe3+/Fe2+redox catalysts for the oxidation of H2S to sulfur that are less reactive toward the hydroxyl radical is recommended. Keywords: chelate degradation, H2S oxidation, nitrilotriacetic acid (NTA), ethylenedinitrilotetraacetic acid (EDTA), N-hydroxyethylethylenediaminetriacetic acid (HEDTA).
Design and synthesis of near-infrared absorbing pigments. I. Use of Pariser–Parr–Pople molecular orbital calculations for the identification of near-infrared absorbing pigment candidates
Authors: Désilets Denis , Kazmaier Peter M. , Burt Richard A.
Published: May 2006

The usefulness of the Pariser–Parr–Pople molecular orbital method for the identification of near-infrared absorbing pigment candidates for photogenerator applications is outlined. Several pigments based on the well-known class of photogenerators N,N′-dialkyl-3,4,9,10-perylenetetracarboxylic diimides were investigated and pigments of the dibenzoperylene, bisanthene, and zethrene classes were identified as the most promising candidates of the series. On the basis of the predictions, 7,8,15,16-dibenzo[a, j]perylenetetracarboxylic diimide was prepared and the validity of the calculations was verified. Keywords: photoconductor, near-infrared, Pariser–Parr–Pople calculations, polycyclic aromatic hydrocarbon, aceanthrene green.
Dicyclohexylethyleneglycol, -diethyleneglycol, -triethyleneglycol, and related monosubstituted cyclohexanes. Conformational analysis using low-temperature 13C and 1H NMR spectroscopy
Authors: Buchanan G.W. , Moghimi A. , Reynolds V.M. , Bourque K.
Published: May 2006

Two conformations of each of the title molecules have been detected by 100 MHz 13C NMR at 210 K. For the dicyclohexyl systems, the conformations are related via a single ring inversion. In each case the equatorial–axial conformer is 4.7 ± 0.4 kJ/mol less stable than the diequatorially substituted form in CD2Cl2 solution. For monosubstituted models, the conformational free energy (−ΔG0) values of the -O-CH2-CH2-OCH3, -OCH2-CH2-O-CH2-CH3, and -O-CH2-CH2-O-CH-(CH3)2 groups have been determined to be 5.4,6.1, and 6.1 ± 0.2 kJ/mol, respectively. In methanol, the latter equilibria are slightly more biased towards the axially substituted conformers. Keywords: substituted ethylene glycols, conformational equilibria.
Long range antiferromagnetic exchange in dinuclear copper(II) complexes containing 1,4-dicyanamidobenzene dianion bridging ligands
Authors: Cheruiyot Linnus L. , Crutchley Robert J. , Thompson Laurence K. , Greedan J.E. , Liu Guo
Published: May 2006

Six Cu(II)dimers, [{Cu(dien)}2(μ-dicyd)][CF3SO3]2 (1), [{Cu(dien)}2(μ-Me2dicyd)][CF3SO3]2 (2), [{Cu(dien)}2(μ-Cl2dicyd)][CF3SO3]2 (3), [{Cu(L)}2(μ-dicyd)] (4), [{Cu(L)}2(μ-Me2dicyd)] (5), and [{Cu(L)}2(μ-Cl2dicyd)] (6), where dicyd2−, Me2dicyd2− and Cl2dicyd2− are unsubstituted, 2,5-dimethyl- and 2,5-dichloro-1,4-dicyanamidobenzene dianions, respectively, dien is diethylenetriamine, and L− = 1,3-bis(2-pyridylimino)isoindolinato, have been synthesized and characterized by elemental analysis, IR, UV–vis, and EPR spectroscopy, and magnetic studies. Temperature-dependent magnetic susceptibility measurements of the complexes 1–6 from 5 to 300 K are reported. The data for 2, 3, 5, and 6 have been fitted to a dimer model with a modified Bleaney–Bowers expression which derived antiferromagnetic exchange constants −J = 10.6, 4.5, 5.2, and 3.0 cm−1, respectively (where the Hamiltonian is of the form [Formula: see text]) For 4, an approach to a maximum in χm with decreasing temperature gave an estimated −J < 3.5 cm−1. Only complex 1 showed Curie–Weiss behavior. This is far weaker antiferromagnetic exchange compared to that observed for dinuclear Ru(III) complexes incorporating the dicyd2− bridging ligands (J. Am. Chem Soc. 114, 5130 (1992)) and is attributed to a symmetry and energy mismatch between Cu(II) σ* magnetic orbitals and the πnb molecular orbitals of the bridging ligand which are important for superexchange. Keywords: superexchange, copper dimer, dicyanamidobenzene.
Studies on the interaction of selenite and selenium with sulfur donors. Part 3. Sulfite
Authors: Ball Sheila , Milne John
Published: May 2006

Elemental selenium dissolves in sulfite solution to form selenosulfate ion: Se + SO32− = SeSO32−.The formation constants for this equilibrium at temperatures from 0 to 35 °C are reported for the first time. The isomeric thioselenate anion, SSeO32−, is not, however, produced by the reaction of sulfur with selenite nor is the selenoselenate ion, Se2O32−, formed from selenium and selenite. Selenotrithionate is formed rapidly from the reaction of selenous acid with sulfite and hydrogen sulfite according to: HSeO3− + 3 HSO3− = Se(SO3)22− + SO42− + 2H2O.Two isomers of the selenotrithionate ion are observed by Se-77 NMR and Raman spectroscopy, one with O-bonded Se, Se(OSO2)22−, and the other with S-bonded Se, Se(SO3)22−. Both isomers are formed in reactions with hydrogen sulfite but only the O-bonded isomer is formed in sulfite solutions at ambient temperatures. The Raman and Se-77 NMR spectra of the various sulphur–selenium anions formed are given and the parallel with the reactions of selenous acid and thiols is discussed. Keywords: selenium, sulfite, selenosulfate, selenotrithionate, Se-77 NMR, Raman spectroscopy, equilibria, aqueous solutions.
Metal ion – biomolecule interactions. Part 16. C(2)-H isotopic exchange in Co(III)-coordinated imidazoles
Authors: Buncel Erwin , Yang Fan , Moir Robert Y. , Onyido Ikenna
Published: May 2006

Transition-metal-bound imidazoles are suitable models for evaluating the roles of metal ions in biomolecules having the imidazole moiety and similar heterocyclic residues as part of their structure. Such studies provide useful insights into metal–biomolecule interactions in biological systems, especially when the lability of the metal–ligand bond is substantially reduced, such that the identity of the metal–ligand complex is preserved during the course of the reaction under investigation. The present paper reports on a kinetic study of tritium exchange from the C(2) position of the imidazole moiety in the substitution-inert complex cations [Co(NH3)5[2-3H]-imidazole]3+ (1) and [Co(NH3)5-1-methyl-[2-3H]-imidazole]3+ (2). Rate–pH profiles have been determined in aqueous solution at 60 °C. Both substrates are believed to react through rate-determining attack of hydroxide ion (kM+ pathway) at C(2)-T. Dissection of the kinetic data reveals an additional pathway for 1 consequent upon deprotonation of its pyrrole-like N-H(T) to yield 3, which is then attacked by hydroxide at C(2) (kM pathway). The ratio kM+/kM = 103 that is obtained is in accord with the expected reduced reactivity of 3. Comparison of the present data with those reported for a variety of heterocyclic substrates shows that the order of reactivity, protonated [Formula: see text] metal ion coordinated [Formula: see text] neutral form of substrates, prevails. The superiority of the proton over metal ions in catalyzing isotopic hydrogen exchange is attributed to its larger ground state acidifying effect coupled with the greater transition state stabilization it affords, relative to metal ions. The exchange reaction of 3 via the kM pathway is the first example of a reactive anionic species in which the negative charge is located α to the exchanging C-H. Keywords: tritium exchange, cobalt (III)-coordinated imidazoles.
The influence of surfactants on the electrocatalytic hydrogenation of organic compounds in micellar, emulsified, and hydroorganic solutions at Raney nickel electrodes
Authors: Chambrion Philippe , Roger Laurence , Lessard Jean , Béraud Valérie , Mailhot Josée , Thomalla Marc
Published: May 2006

The electrocatalytic hydrogenation (ECH) of limonene (1) and carvone (4) at a Raney nickel cathode was studied both in hydroorganic medium (methanol–water) and aqueous micellar and emulsified solutions. In methanol–water, the ECH of 1 stopped at p-menthene (2) and proceeded with low current efficiencies (18–24%). In micellar solutions containing a cationic surfactant (CTAB), the ECH of 1 was more efficient, giving p-menthane (3) as the main product with current efficiencies up to 30%. The pH of the solution (hydroorganic and micellar) had little influence. With anionic (SDS) and nonionic (Brij 35) surfactants, some p-menthane (3) was formed but the conversion rate and current efficiency were much lower. The ECH of carvone (4) was slightly less efficient in methanol–water than in micellar and emulsified solutions. The saturated alcohols neocarvomenthol (8), carvomenthol (9), isocarvomenthol (10), and neoisocarvomenthol (11) were the main products with current efficiencies of 75% (hydroorganic medium) to 90% (micellar solutions). The effect of surfactant concentration and pH on the efficiency and stereochemistry of the ECH of carvone (4) was also studied. Keywords: electrocatalytic hydrogenation, limonene, carvone, Raney nickel, micellar medium.
Synthesis of acetylenedicarboxylic acid esters and asymmetric Diels–Alder reactions of the bis(methyl (S)-lactyl) ester
Authors: Charlton James L. , Chee Gaiklean , McColeman Heather
Published: May 2006

A general method has been developed for the preparation of acetylenedicarboxylic acid esters and applied to the synthesis of both achiral and chiral esters. Diels–Alder reactions of one of the chiral esters, the bis(methyl (S)-lactyl) ester, with various types of dienes, such as phenylbutadiene, orthoquinodimethanes, and isobenzofuran, have been investigated. Moderate asymmetric induction was observed in reactions with dienes bearing an α-hydroxyl group. In one case, an unusual solvent-induced reversal of asymmetric induction was observed. Keywords: acetylenedicarboxylic acid esters, phenylbutadiene, orthoquinodimethanes, isobenzofurans, Diels–Alder, asymmetric.
Preparation of ortho-hydroxyalkylanilines as ortho-quinone methide imine precursors via an aminoborane complex
Authors: Migneault David , Bernstein Michael A. , Lau Cheuk K.
Published: May 2006

A general process for ortho-specific hydroxyalkylation of secondary anilines has been developed. N-Methylanilinochlorophenylboranes react readily with aryl aldehydes, and with α,β-unsaturated or saturated aliphatic aldehydes to give the corresponding ortho-hydroxyalkylanilines in good yield. Thermolysis of the latter generated the corresponding ortho-quinone methide imine, which can participate in an electrocyclic or intramolecular [4 + 2] cycloaddition reaction to give various dihydro- and tetrahydroquinoline derivatives. Application of the method to the syntheses of 5-aza analogs of hexahydrocannabinol is demonstrated. Keywords: quinone methide imine, ortho-hydroxyalkylaniline, azacannabinoids.
1-Aminocyclohexene-2,4-dicarbonitrile derivatives. Syntheses and structural study
Authors: Lorente Antonio , Galan Carmen , Fonseca Isabel , Sanz-Aparicio Juliana
Published: May 2006

Substituted 1-aminocyclohexene-2,4-dicarbonitriles were obtained by reaction of α,β-unsaturated nitriles (two equivalents) with benzyl cyanide. By recrystallization from ethanol one diastereomeric racemate was isolated in each case. The cyclohexene structures were established from spectroscopic data (IR, MS, and one- and two-dimensional NMR). Relative stereochemical configurations and conformational preferences in the solid state of cyclohexenes 2 and 3 and propanedicarbonitrile 1 were established from X-ray crystallography. Crystals of 1 (C17H14N2) belong to the orthorhombic space group Pbca. Cell dimensions are a = 17.168(7), b = 21.612(5), c = 7.508(2) Å, V = 2785(7) Å3. Final R = 0.078 and Rw = 0.091; 1363 reflections were observed. The compound 2 (C26H21N3) crystallizes in the monoclinic space group P21/n with Z = 4. The crystal data for 2 are a = 7.743(1), b = 24.420(1), c = 11.164(1) Å, β = 102.65(1)°, V = 2059.7(3) Å3. Final R = 0.046 and Rw = 0.059; 2701 reflections were observed. Crystals of 3 (C16H17N3) belong to the triclinic space group [Formula: see text] Cell dimensions are a = 14.582(1), b = 7.283(1), c = 7.116(1) Å, α = 110.84(0)°, β = 85.92(0)°, γ = 97.81(1)°, V = 696.9(2) Å3. Final R = 0.047 and Rw = 0.052; 2227 reflections were observed. The cyclohexene ring in 2 and 3 exists as a sofa rather then as a half-chair conformation. Keywords: 1-aminocyclohexenes, α,β-unsaturated nitriles, benzyl cyanide.
A study of the Reformatsky reaction on formylcoumarins
Authors: Dutta Lakshmi Narayan , Bhattacharyya Manju , Sarkar Asoke Kumar
Published: May 2006

The reactions of bromozinc enolate reagents on formylcoumarins (1, 2) afforded spiro-γ-lactones (15–18), furan derivatives (13, 14), and coumarinylcarbinols (3, 4) as highly unusual products along with the expected β-hydroxyesters (5–12). Stereochemical features of some of these products turned out to be highly informative and were unambiguously corroborated. Keywords: formylcoumarins, unusual Reformatsky products, stereochemistry.
1,3-Dioxan-5-ones: synthesis, deprotonation, and reactions of their lithium enolates
Authors: Majewski Marek , Gleave D. Mark , Nowak Pawel
Published: May 2006

A general synthetic route to 2-alkyl- and 2,2-dialkyl-1,3-dioxan-5-ones, using tris(hydroxymethyl)-nitromethane as the starting material, is described. Deprotonation of these compounds was studied. It was established that these dioxanones could be deprotonated with LDA; however, the reduction of the carbonyl group via a hydride transfer from LDA, giving the corresponding dioxanols, often competed with deprotonation. The reduction could be minimized by using Corey's internal quench procedure to form silyl enol ethers and was less pronounced in 2,2-dialkyldioxanones (ketals) than in 2-alkyldioxanones (acetals). Self-aldol products were observed when dioxanone lithium enolates were quenched with H2O. Addition reactions of lithium enolates of dioxanones to aldehydes were threo-selective as predicted by the Zimmerman–Traxler model. Dioxanones having two different alkyl groups at the 2-position were deprotonated enantioselectively by chiral lithium amide bases with enantiomeric excess (ee) of up to 70%. Keywords: 1,3-dioxan-5-ones, enantioselective deprotonation, chiral lithium amides.
Base-catalyzed ring openings of benzocyclobutenones and -ols
Authors: Bradley J.C. , Durst T.
Published: May 2006

The base-catalyzed ring opening of a number of isomeric E-and Z-benzylidenebenzocyclobutenones and -ols has been studied in both protic and aprotic solvents. Cleavage of the C1—C2 bond results in the formation of stilbenes with mainly, and at times exclusively, retained stereochemistry. For the alcohols, these results point to an oxyanion-induced carbon–carbon bond cleavage leading to a vinyl anion that is protonated with retention of configuration in the protic solvents rather than to an electrocyclic ring opening to an alkoxy o-quinodimethane. Reaction of the Z isomer of benzylidenebenzocyclobutenol with methyllithium in THF at 20 °C causes isomerization to the E isomer, cleavage of the C1—C2 bond, and recyclization of the resultant isomerized vinyl anion. Keywords: benzylidenebenzocyclobutenones, base-catalyzed ring opening; benzylidenebenzocyclobutenols, base-catalyzed ring opening.
Studies on the transannular Diels–Alder reaction of 15-membered macrocyclic trienes containing a trans-trans diene. Part II: Evidence for unsymmetrical transition states in the formation of A.B.C.[6.6.7] tricyclic products
Authors: Hall Dennis G. , Müller Renate , Deslongchamps Pierre
Published: May 2006

In the preceding paper (Part I) we described the preparation of 10 model TTT and TTC cyclopentadecatrienes. Their conversion to A.B.C.[6.6.7] tricyclic products via a transannular Diels–Alder (TADA) reaction and the analysis of the resulting diastereoselection along with proofs of structures are described in the present article (Part II). The required temperature of reaction was significantly higher (~100 °C) in comparison to that required for the 14-membered lower homologs, indicating that proximity effects are playing a key role in the TADA reaction. The presence of ester groups in the chain was found to favor either of the two possible transition states (endo or exo) on the basis of steric effects, thus controlling the TST/CSC ratio of tricycles from TTC macrocycles, and the TAC/CAT ratio from TTT macrocycles. A formyl substituent on the dienophile can further influence the diastereoselection through the hypothesis of unsymmetrical transition states and allowed, for the first time, the use of Lewis acids as catalysts in the TADA reaction. This investigation demonstrates a general methodology for the stereocontrolled synthesis of A.B.C.[6.6.7] tricycle structures related to several polycyclic natural products. Keywords: transannular, Diels–Alder, 15-membered ring, A.B.C.[6.6.7] tricycles, diterpene.
Pyridine hydrobromide perbromide mediated regioselective heterocyclization of ortho-cyclohexenyl phenols
Authors: Majumdar Krishna C. , Kundu Anup K.
Published: May 2006

A number of 3,4-benzo-9-bromo-2-oxabicyclo[3.3.1]nonanes (9a–h) are synthesized in almost quantitative yields by regioselective heterocyclization of ortho-cyclohexenyl phenols (8a–h). The structures of 3,4-benzo-9-bromo-2-oxabicyclo[3.3.1]nonane 9 have been unambiguously settled by the synthesis of 9-chloromercuri-3,4-(4′-methoxybenzo)-2-oxabicyclo[3.3.1]nonane 13, which has an X-ray determined structure, and by converting 13 to product 9c with bromine in sunlight in 45% yield. The 6-chloro-2-cyclohexenenyl phenol 8i and 2-cyclohexenyl-4-methyl phenol 8j failed to give any cyclic product but instead furnished the ring-brominated products 10i and 10j, respectively. Keywords: pyridine hydrobromide perbromide, regioselective heterocyclization, ortho-cyclohexenyl phenol, oxabicyclo[3.3.1] nonane, 6-endo trig.
A stabilized and persistent tetraketene: 1,4-bis(1′,4′-dioxo-3′-trimethylsilyl-2′-buta-1′,3′-dienyl)benzene
Authors: Liu Ronghua , Tidwell Thomas T.
Published: May 2006

Reaction of 1,4-bis(trimethylsilylethynyl)benzene with CCl3COCl and Zn gave 1,4-bis(4′,4′-dichloro-2′-trimethylsilyl-3′-oxo-1′-cyclobutenyl)benzene (7), which reacted with concentrated H2SO4 at 87 °C to give 1,4-bis(2′-trimethylsilyl-3′,4′-dioxo-1′-cyclobutenyl)benzene (8). Photolysis of 8 in CDCl3 with 350 nm light gave the bisketene 9, which on further photolysis formed the title tetraketene 10, as identified by its spectral properties. Upon heating of 10 the successive closure of the two bisketene moieties, first to give the mono(cyclobutenedione) 9 and then to reform 8 in a slower step, was observed by NMR and UV spectroscopy. Reaction of 10 with MeOH gave the tetraester 11, which was assigned the erythro configuration at each of the succinate moieties. Keywords: ketenes, photolysis, cyclobutenediones, esterification, silicon.
Solvent clustering, polar network density, and thermodynamic interactions in coordination polymers: an inverse gas chromatography study of metal-ion-containing polyesters of poly(diethylene glycol-co-succinic acid)
Authors: Vancso G. Julius , Tan Zhanjie
Published: May 2006

The spatial distribution of toluene and THF in the polyester poly(diethylene glycol-co-succinic acid) and its Mg2+-containing "telechelic" derivative was described by Kirkwood–Buff–Zimm (KBZ) cluster integrals. The values of KBZ integrals as a function of the volume fraction of solvent in the polymer–solvent systems were obtained from finite concentration inverse gas chromatography measurements utilizing the elution-on-a-plateau technique. The results show that toluene has a higher self-affinity to form clusters in the pure polyester than THF, which is more homogeneously distributed in the polymer. Data for preferential solvation indicate that a segregation of parts of the polymer chains is present in the toluene–polyester system. When metal ions are introduced, the self-affinity of the solvent molecules to gather increases, whereas solvent clusters of toluene form in the free volume, and the gathering of the THF is likely to take place close to the metal ions. Based on the electron donacity values of the various donor groups present in the metal-ion-containing polymer it was assumed that THF in the Mg2+-containing polyester will be bound to the central metal ion while the apolar toluene will participate in the solvation of the apolar parts of the polymer and will be "repelled" from the ion-containing regions. The concentration of these ionic centres, which act as effective cross-links in the coordination polymer, was determined from measurements of thermodynamic activity. A comparison of the experimental and estimated effective cross-link density values indicates a nearly atomic dispersion of Mg2+ in the metal-ion-containing polyesters. Keywords: inverse gas chromatography, cluster integrals, solvent partition in polymers, metal-ion-containing polyesters, Flory–Huggins interaction parameter.
Protected building blocks for luminescent and redox-active dendritic metal complexes. Excited state properties and electrochemical behaviour
Authors: Juris Alberto , Venturi Margherita , Pontoni Luca , Resino Immaculada Resino , Balzani Vincenzo , Serroni Scolastica , Campagna Sebastiano , Denti Gianfranco
Published: May 2006

The mononuclear complexes [Ru(2,3-Medpp)3]5+, [Ru(2,3-Medpp)2(bpy)]4+, and [Ru(2,3-Medpp)(bpy)2]3+ and the dinuclear complex [(2,3-Medpp)2Ru(μ-2,3-dpp)Ru(2,3-Medpp)2]8+ (bpy = 2,2′-bipyridine; 2,3-dpp = 2,3-bis(2-pyridyl)pyrazine; 2,3-Medpp+ = monomethylated 2,3-bis(2-pyridyl)pyrazine) have been synthesized. Their absorption spectra, luminescence properties, and electrochemical behavior have been investigated and compared with those of the mononuclear [Ru(bpy)3]2+, [Ru(2,3-dpp)3]2+, [Ru(2,3-dpp)2(bpy)]2+, and [Ru(2,3-dpp)(bpy)2]2+ and the dinuclear [(bpy)2Ru(μ-2,3-dpp)Ru(bpy)2]4+ parent compounds. The complexes examined are fundamental units in the synthesis of polynuclear metal complexes of dendrimeric structure and nanometer size. Keywords: luminescence, ruthenium complexes, dendrimers, electrochemistry, photochemistry.
Solid-state photochemistry of π-conjugated poly(3-alkylthiophenes)
Authors: Abdou Mohamed S.A. , Holdcroft Steven
Published: May 2006

Films of poly(3-alkylthiophenes) undergo simultaneous photobleaching, cross-linking, and chain scission when irradiated in air with UV or visible light. The quantum yields of the latter two processes were determined using Chalesby–Pinner theory. Under the irradiation conditions employed the cross-linking density is ~ 4.5 times greater than the fracture density (chain scission). This corroborates the observation that poly(3-alkylthiophenes) are negative-type photoresists. Although the quantum yield of cross-linking is relatively low, the polymer maintains a relatively high degree of π-conjugation following irradiation. The mechanism of cross-linking and insolubilization proceeds via a classical photooxidation route that is initiated by photolysis of residual iron impurities. Photobleaching involves photosensitization, and reaction of singlet oxygen. 1O2 undergoes a 1,4 Diels–Alder addition to thienyl units which results in a loss of π-conjugation, a blue shift in the absorption spectrum of the polymer, and a decrease in the optical density. Photobleaching, cross-linking, and chain scission are considerably reduced in the absence of oxygen. Keywords: photochemistry, polymers, thin films, degradation, cross-linking.
Cocrystallization in random copolymers of poly(β-hydroxybutyrate-co-β-hydroxyvalerate) and its effect on crystalline morphology
Authors: Orts William J. , VanderHart David L. , Bluhm Terry L. , Marchessault Robert H.
Published: May 2006

Small-angle X-ray scattering (SAXS) and solid state CPMAS l3C NMR were used to describe the crystalline morphology of a series of bacterially produced poly(β-hydroxybutyrate-co-β-hydroxyvalerate) copolymers containing random distributions of 3-hydroxybutyrate (3HB) and 3-hydroxyvalerate (3HV). Modeling of SAXS results showed that the morphology of this isodimorphic system is better described by two phases, crystalline and amorphous, having defects within each domain. This is in contrast to a model with a large interfacial region between phases. For the 3HV composition range 0–27 mol%, the polymer crystallizes in a poly(β-hydroxybutyrate)-type crystalline lattice. Solid state NMR results showed that there is significant incorporation of the 3HV minor component into the poly(β-hydroxybutyrate) crystalline phase over this composition range. The ratio of the 3HV content in the crystalline phase relative to the overall 3HV content is not linear, but increases with increasing 3HV. For the 21 and 27% 3HV samples, the 3HV content in the crystalline phase is as much as 2/3 of the overall 3HV content. Inclusion of 3HV is correlated to an increase in crystalline disorder (as measured by SAXS), implying that it is easier to accommodate the bulkier 3HV comonomer into a crystalline region that already contains defects. Keywords: bacterial polyesters, poly(3-hydroxyalkanates), small-angle X-ray scattering, solid state NMR, cocrystallization.
Synthesis of steroidal [4,6-b,c]-benzothiazepines, a new class of aromatase inhibitor
Authors: Holland Herbert L. , Kumaresan Sudalaiyandi , Lakshmaiah Gingipalli
Published: May 2006

Several steroidal [4,6-b,c]benzothiazepines have been prepared via base-catalyzed Michael addition of 2-aminothiophenol to 3β,17β-diacetoxyandrost-4-en-6-one. The product of this reaction has the 4β,5α stereochemistry, but cyclizes to a benzothiazepine with the steroidal 4β,5β configuration, confirmed by X-ray crystallographic analysis. 2-Aminothiophenol reacts with 3β,17β-diacetoxyandrost-4-en-6-one under acidic conditions to give a steroidal 6-spiro-benzothiazole. An androst-4-ene-3,17-dione-based [4,6]benzothiazepine has been shown to be a moderate competitive inhibitor of the human placental aromatase enzyme with IC50 = 42.3 µM. Keywords: steroid, aromatase, benzothiazepine.
Synthesis of the carbocyclic analog of uridine 5′-(α-D-galactopyranosyl diphosphate) (UDP-Gal) as an inhibitor of β(1 → 4)-galactosyltransferase
Authors: Yuasa Hideya , Palcic Monica M. , Hindsgaul Ole
Published: May 2006

Uridine 5′-(5a-carba-α-D-galactopyranosyl diphosphate), the carbocyclic analog of UDP-galactose where the ring oxygen of the galactose residue is replaced by a methylene group, was chemically synthesized as a potential inhibitor of galactosyltransferases. It was found to be a competitive inhibitor of UDP-galactose: N-acetylglucosamine β(1 → 4)-galactosyltransferase from bovine milk (EC 2.4.1.22) with a Kivalue of 58 µM, similar to the Kmvalue for UDP-galactose (25 µM). Keywords: carbocycles, UDP-5a-carbagalactose, galactosyltransferase, glycosyltransferase inhibitor.
Taxamycins: a new enediyne family with synthetic and biological potential
Authors: Lu Yee-Fung , Harwig Curtis W. , Fallis Alex G.
Published: May 2006

The Pd(0) based synthesis of two disilyl synthons (Z-1-trimethylsilyl-6-tert-butyldiphenylsilylhex-3-ene-1,5-diyne (18) and Z-1-trimethylsilyl-6-triisopropylsilylhex-3-ene-1,5-diyne (19)) and the selective removal of the trimethylsilyl group (K2CO3, MeOH) to afford 24 and 25 is described. These building blocks are employed in the construction of the taxamycin-12 compound 38 (16,17,18-trimethyl-2-methyloxymethoxy-9-hydroxybicyclo[9.3.1]pentadec-5-ene-3,7-diyne). The final ring closure to the 12-membered ring utilizes an intramolecular Cr–Ni mediated condensation of the iodoalkyne 37 (1,3,3-trimethyl-2-(2-oxoethyl)-4-(Z-1-methyloxymethoxy-7-iodohept-4-ene-2,6-diynyl)cyclohexene). Keyword: cancer, enediynes, synthesis, Pd(0) coupling.