bluuebunny/crossref_metadata_2025 huggingface dataset:
Kinetics and mechanism of the hydrolysis of a (5,6)-spirophosphorane. Thermodynamics of the hydrolysis of cyclic five-membered and six-membered phosphonium ions
Authors: McGall Glenn H. , McClelland Robert A.
Published: May 2006

The cyclic five-membered phosphonium ion 2b (2-(2′-hydroxyethoxy)-2-phenyl-1,3,2-dioxaphospholan-2-ylium) derived from ring-opening of the (5,5)-spirophosphorane 1b (5-phenyl-1,4,6,9-tetraoxa-5-phosphaspiro[4,4]nonane) has been observed in neat CF3SO3H and at >85% H2SO4. The cation undergoes hydrolysis in the latter solutions, and an extrapolation has been carried out to obtain an estimate for reactivity in 100% water. Hydrolysis rate constants for phenyltrialkoxyphosphonium ions in water are 107, 100, and 5 × 10−3 s−1 for cyclic five-membered, cyclic six-membered, and acyclic derivatives respectively; these show an excellent correlation with rate constants for a similar series of phosphate esters. An investigation of the hydrolysis of the (5,6)-spirophosphorane 5 (5-phenyl-8,8-dimethyl-1,4,6,10-tetraoxa-5-phosphaspiro[4,5]decane) provides a clue as to the origins of these rate differences. This phosphorane can in principle hydrolyze via two isomeric cyclic phosphonium ions, the six-membered 14 and the five-membered 15. The former is thermodynamically more stable, being the only cation observed under equilibrating conditions of strong acid. However, the hydrolysis of the spirophosphorane, as well as the hydrolysis of fully formed 14, channels through the cyclic five-membered 15. A thermodynamic breakdown reveals that the 9.5 kcal mol−1 difference in activation free energy for the hydrolysis of five- and six-membered cyclic phosphonium ions is due to a combination of a higher free energy (2.5–4.5 kcal mol−1) for the five-membered cation, and a lower free energy (7–5 kcal mol−1) for the pentacoordinate transition state with the five-membered ring. This analysis also shows that a (5,6)-spirophosphorane is 6–8 kcal mol−1 more stable than a (6,6)-spirophosphorane. Thus, a five-membered ring has a significant stabilizing effect on a pentacoordinated phosphorus structure. The accelerated hydrolysis of cyclic phosphonium ions and phosphate esters with five-membered rings is caused by a combination of this stabilizing effect in the transition state and a destabilizing effect in the ground state associated with ring strain. Key words: phosphorane, hydrolysis, phosphate, phosphonium.
Gas-phase heteroaromatic substitution. 14. Attack of dimethylfluoronium ion on 2- and 3-methyl-pyrroles, -furans, and -thiophenes
Authors: Cecchi Patrizio , Pizzabiocca Adriano , Renzi Gabriele , Sparapani Cinzia , Speranza Maurizio
Published: May 2006

The gas-phase methylation of 2- and 3-methyl-pyrroles (2P and 3P), -furans (2F and 3F), and -thiophenes (2T and 3T) by (CH3)2F+ ions, from γ radiolysis of CH3F, has been investigated at pressures ranging from 50 to 760 Torr, in the presence of a thermal radical scavenger (O2) and variable concentrations of an added base (NMe3: 0–10 Torr). The mechanism of the methylation process is discussed and the intrinsic positional selectivity of the (CH3)2F+ ions evaluated in the framework of the Charge and Frontier Orbital Control concept. Owing to the very large energy gap between the LUMO of (CH3)2F+ and the HOMOs of the selected heteroaromatic substrates, their gas-phase methylation is characterized by a distinct affinity of the ionic electrophile toward those substrate positions with the highest net negative charge, i.e., the C3 in 2P (100%), the C4 in 3P (100%), the heteroatom of 2F and 3F (>80%), the C5 of 2T (32%), and the C2 of 3T (47%). Analysis of the methylated product distribution from 2F and 3F as a function of the experimental conditions reveals that the interaction of the (CH3)2F+ with the heteroatom of furans gives rise to the reversible formation of two sets of electrostatic adducts, i.e., a "chelate" adduct (III) and a single proton-bonded adduct (IV), the first rapidly evolving to the α-substituted heteroarenium intermediate by proximity effects and the latter slowly rearranging to the chelate structure III. Formation of these categories of electrostatic adducts from furans, which is much less extensive in the case of thiophenes and absent in pyrroles, accounts for the apparent pronounced affinity of gaseous alkylating electrophiles, irrespective of their LUMO energy, for the α carbons of furans. Key words: gas-phase ion chemistry, electrophilic aromatic substitutions, radiolysis, dimethylfluoronium ions, methylated heteroarenes.
Radical reductions of Cu(II)–penicillamine complexes: Evidence for a Cu(II)– disulphide anion intermediate.
Authors: Mezyk Stephen P. , Armstrong David A.
Published: May 2006

not available
Reactions of dimethyl polysulfides with organomagnesium reagents
Authors: Clark Peter D. , Mann Russell S. , Lesage Kevin L.
Published: May 2006

Reactions of a mixture of dimethyl polysulfides (DMPS, CH3SxCH3, x = 3 – 8) with methyl- and phenylmagnesium halides are described. The type of product obtained was dependent on the molar ratio of DMPS to Grignard reagent. When a 6:1 methyl-Grignard to DMPS ratio was used, methanethiol and dimethyl sulfide were the major products obtained after acidification of the reaction mixture. Lesser quantities of methyl-Grignard favored the formation of dimethyl sulfide, dimethyl disulfide, and H2S. Experiments with a 6:1 phenylmagnesium bromide to DMPS ratio produced benzenethiol and phenylmethyl sulfide as major products after acidification. No methanethiol was observed in these experiments. Mixtures of phenylmethyl mono-, di-, and trisulfides and H2S were obtained with a 3:1 Grignard/DMPS molar ratio. From a mechanistic viewpoint, product distributions obtained from reaction of Grignard reagents with DMPS can be explained by the formation of magnesium thiolates that are most readily stabilized by adjacent structures. Experiments using phenyl Grignard reagent in limited supply suggested that the internal sulfur atoms of the polysulfide chains were most reactive. Keywords: organic polysulfides, Grignard reagents.
A MNDO study of planar thioacyl-substituted carbocations
Authors: Ginsburg Jack Leon , Langler Richard Francis , Raheja Raj Kumar , Precedo Laura
Published: May 2006

Relative gas phase stabilities of pairs of planar regioisomeric thioacyl-substituted carbocations have been calculated using the MNDO method. These systems are stabilized by good π donors. The role of S π-electron population as a gauge of substituent effects is examined. Similar results were obtained using isodesmic reactions to gauge substituent effects. Results of this and earlier studies are consistent with the resonance saturation effect. Keywords: semiempirical calculations, carbonium ions.
Electron paramagnetic resonance study of the reactions of the spin trap 2,4,6-tribromonitrosobenzene in benzene solutions
Authors: Smith Peter , Maples Kirk Randall , Lau Ramona Lillian
Published: May 2006

By means of static-sample studies involving the thermal decomposition of symmetric azo compounds and the oxidation of substrates with nickel peroxide, we have characterized by electron paramagnetic resonance 12 spin adducts of 2,4,6-tribromonitrosobenzene, 1, in benzene at 25 °C. In addition, we have used electron paramagnetic resonance spectroscopy to investigate benzene solutions of 1 both by itself and in the presence of olefins, and both in the presence and absence of light. Although 1 by itself did not give evidence for any free radical formation, when 1 was mixed with any one of the 11 olefins tested we could detect nitroxide formation. The presence of light did not affect our results. These observations support the "ene" reaction mechanism of nitroxide formation. Keywords: 2,4,6-tribromonitrosobenzene, spin trapping, nickel peroxide.
The structure of 2,2-dibutyl-1,3,2-dioxastannane in the solid state and in solution
Authors: Grindley T. Bruce , Thangarasa Rasiah , Bakshi Pradip K. , Cameron T. Stanley
Published: May 2006

Crystals of 2,2-dibutyl-1,3,2-dioxastannane (1) are orthorhombic, of space group Pnma, with a = 7.663(3), b = 18.437(2), c = 9.277(4) Å, Z = 4, R = 0.0568 (Rw = 0.0551) for 1183 independent reflections with I > 3σ(I). Compound 1 is a polymer in which each monomer unit is joined to the next by a four-membered (SnO)2 ring. The Sn—O bond lengths inside the monomer units average 2.04 Å while those between monomers average 2.57 Å. The mirror plane of the crystal contains the atoms in the four-membered rings and the other oxygen atoms. Two of the three remaining carbon atoms in the six-membered rings of the monomer units are close to the mirror plane. The other carbon atom is disordered above and below the plane. It was shown by 119Sn NMR spectroscopy that solutions of 1 contain mixtures of oligomers that consist mainly of dimers, trimers, and tetramers in chloroform-d. ΔG0 values for dimmer–trimer equilibria and dimmer–tetramer equilibria of −2.5 and −1.5 kcal mol−1 were obtained from integration of low temperature 119Sn NMR spectra. These values favour the higher oligomers slightly less than those for 2,2-dibutyl-1,3,2-dioxastannolane. Keywords: 1,3,2-dioxastannanes, stannylene acetals. X-ray crystallography, 119Sn NMR spectroscopy.
Polarizabilities of trans and cis polyacetylene and interactions among chains in crystalline polyacetylene
Authors: Huang Zhonghao , Jelski Daniel A. , Wang Rongshun , Xie Demin , Zhao Chenda , Xia Xinfu , George Thomas F.
Published: May 2006

In this paper, we use the semi-empirical Hartree–Fock method introduced within the finite electric field perturbation approach, to calculate the molecular and π-system polarizabilities of trans and cis polyene H(-CH=CH-)nH (n = 3, 4, 5, …, 18). The result is that for a given n, the order of both molecular and π-system polarizabilities of polyene is trans > cis. This result is consistent with the order of experimental values of trans and cis hexatriene. When the n increases from 3 to 18, the contribution of π-system polarizability to the molecular polarizability increases from 45 to 70%. Using these results, we have extrapolated the polarizabilities of both trans and cis polyacetylene. The order is trans > cis. These results and crystal parameters lead to an estimate of the dispersion energies among polyacetylene chains. The result is also Edisp(trans) > Edisp(cis). Keywords: polarizability, polyacetylene, cis/trans, CNDO, dispersion energy.
Abinitio calculations and the chemical reaction molecular dynamics simulation
Authors: Nagaoka Masataka , Okuno Yoshishige , Yamabe Tokio , Fukui Kenichi
Published: May 2006

To clarify the mechanism of chemical reactions in solution, each solvent molecule was classified as either a reactive-solvent molecule or a medium-solvent molecule in accordance with its role in the reaction. The proton transfer reaction of formamidine in water was treated as a model reaction. Abinitio molecular orbital calculations were performed for the complex made up of a formamidine, a reactive-water molecule, and a medium-water molecule. The optimized geometries and the solvation energies were obtained and the orbital interaction analysis carried out along the intrinsic reaction coordinate (IRC). It was found that the medium-water molecule influences the reaction dynamically rather than energetically. There was no energy change in the potential barrier under the influence of the medium-water molecule, in contrast to the remarkable barrier-reducing effect of the reactive-water molecule. The situation is different from that in chemical reactions involving ionic states. Chemical reaction molecular dynamics (CRMD) simulation for this system was performed in order to investigate the energy relaxation mechanism. It was found that just after the reaction finishes, a relative translational motion is first induced between the super molecule, which consists of a formamidine and a reactive-water molecule, and the medium-water molecule, and is then followed by a rotational motion of the medium-water molecule. Keywords: formamidine, chemical reaction molecular dynamics method, reactive-solvent molecule, medium-solvent molecule, energy relaxation mechanism.
Statistical theory of electron densities at nonzero temperatures
Authors: Hoffman Gary G. , Harris Robert A. , Pratt Lawrence R.
Published: May 2006

This paper derives the finite temperature optimized Thomas–Fermi theory applicable to the electronic structure of atoms, molecules, and ions under conditions where both the bulk electron density and the electron temperature are substantial. The derivation provides a simple rule for transcribing the finite temperature case from the previous ground-state statistical electron-density functional theories. Keywords: plasma simulation, plasma chemistry, statistical theory of electron densities, Thomas–Fermi theory, optimized Thomas–Fermi theory.
Ion chemistry of aluminum and boron in methane–oxygen flames
Authors: Crovisier Pierre N. , Horton J. Hugh , Hassanali Carl S. , Goodings John M.
Published: May 2006

The ion chemistry of aluminum and boron, primarily in the +3 oxidation state, was studied by doping premixed, methane–oxygen flames of both fuel-rich and fuel-lean (oxygen-rich) composition with 1 × 10−6 mol fraction of AlCl3 and 5 × 10−4 mol fraction of B(OC2H5)3. Ions were observed by sampling the flames at atmospheric pressure through a nozzle into a mass spectrometer. At this low concentration level, aluminum exhibits two cation series: (a) AlO+•nH2O (n = 2, 3) formed by proton transfer to AlO(OH) and Al(OH)3 in which hydration reactions are involved; and (b) Al+•nH2O (n = 0, 1) by protonation of AlOH with hydration/dehydration steps. At the higher concentration of the boron additive, four cation series were observed: (a) B(OC2H5)3H+•nH2O (n = 0, 1) based on proton transfer to the B(OC2H5)3 additive; (b) BO+•nH2O (n = 1–4) similar to aluminum; (c) HB2O3+•nH2O (n = 1–3) involving the protonated dimer of metaboric acid, BO(OH); and (d) B3O4+•nH2O (n = 2, 3) involving the protonated trimer of BO(OH) whose structures might be cyclic or linear. Other series members are formed by subsequent hydration or dehydration of the parent cations. The anions BO2− and BO− previously studied by D. E. Jensen were also observed. The formation chemistry and probable structures of these ions are discussed, and compared with similar results obtained previously for flames doped with transition metals; notably Sc, Ti, V, and Cr in the +3 oxidation state. Keywords: aluminum, boron, ions, flame, gas phase.
Transformation of 1-methyluracils and uridine to respective, 4-substituted pyrimdin-2(1H)-ones via pyridinium salts
Authors: Wenska Grazyna , Skalski Bohdan , Gdaniec Zofia
Published: May 2006

1-Methyluracil and 1-methylthymine are converted into N-(1-methylpyrimidin-2-one-4-yl)- and N-(1-dimethylpyrimidin-2-one-4-yl)-pyridinium chlorides, respectively. The pyrimidinyl-pyridinium salts were then reacted with various nucleophiles under very mild conditions to give high yields of the desired 4-substituted pyrimidinones. In a similar manner 2′,3′,5′-tri-O-acetyluridine is efficiently converted into new 4-substituted pyrimidinone nucleosides.
Characterization, by two-dimensional NMR spectroscopy, of a complex tetrasaccharide glycoside isolated from Ipomoeastans
Authors: Enriquez Raul G. , Leon Ismael , Perez Fidel , Walls Fernando , Carpenter Katherine A. , Puzzuoli Frank V. , Reynolds William F.
Published: May 2006

A mixture of glycosidic acids, each of which consisted of a tetrasaccharide unit linked by an anomeric carbon to 11-hydroxypalmitic acid and containing short-chain fatty acids ester-linked to the sugar units, was isolated from Ipomoeastans. Basic hydrolysis allowed isolation of a single compound whose structure was determined by two-dimensional NMR methods to be (S)-11-hydroxypalmitic acid 11-O-β-quinovopyranosyl-(1-4)-α-rhamnopyranosyl-(1,2)-β-glycopyranosyl-(1-2)-β-quinovopyranoside. Further HPLC separation of the original mixture allowed separation of a compound that was shown, by 2D NMR, to have 2-mefhylbutanoic acid molecules linked to C-4 of the terminal quinovose and C-2 of rhamnose, 3-hydroxy-2-methyl butanoic acid linked to C-6 of glucose, and with the palmitic acid forming a lactone at C-3 of rhamnose. Closer inspection revealed that this compound was actually a mixture of diastereomers involving different ((2R,3R) and (2S,3S)) enantiomers of 3-hydroxy-2-methylbutanoic acid bonded to the chiral tetrasaccharide core. Keywords: tetrasaccharide, two-dimensional NMR.
Curve crossing analysis of LFER data in Michael addition reactions
Authors: Gross Zeev , Hoz Shmaryahu
Published: May 2006

Rate constants for the Michael addition of CN− in water, aqueous sulfolane, and DMSO, and MeO− in MeOH to eight mono- and disubstituted 1,1-diaryl-2,2-dinitroethylenes were measured. The ρ values are in the range 0.7 (CN−/H2O) –1.6 (MeO−/MeOH). For each of the four systems a correlation was found between log k and the 13C chemical shift of the carbon that undergoes the nucleophilic attack. However, the quality of the correlation as well as the slope decreases with decreasing ρ values. Analysis of the electronic configuration of the transition state using the curve crossing model shows that the traditional interpretation of the location of the transition state should be reversed. This analysis provides a rationale for the observed dependence of the chemical shift – rate correlation on the ρ value.
Silicon cation and anion chemistry in a fuel-rich, methane–oxygen flame
Authors: Horton J. Hugh , Goodings John M.
Published: May 2006

Silicon cations and anions in a fuel-rich, premixed, methane–oxygen flame at atmospheric pressure doped with 0.01 mol% of trimethylsilane were observed by sampling the flame through a nozzle into a mass spectrometer. Twelve cations were observed which can be grouped into five series: SiOH+.nH2O (n = 0–2); SiOCH3+.nH2O (n = 0–2); Si(OH)3+.nH2O (n = 0–2); cations by nucleophilic substitution (e.g., Si(OH)(CH3)2(H2O)+); and carbonaceous aromatic cations (c-HSiCH=CH+ and c-HSiCH=CCH3+). Similarly, five anions were observed as members of two series: HxSiO3− (x = 0, 1) and HxSiO4− (x = 1–3). The chemical ionization reactions for the formation of these ions are discussed in detail, including proton transfer and also methyl cation transfer, three-body addition, nucleophilic substitution (SN2) of both the ions themselves and also their neutral silicon precursors, and H-atom abstraction reactions. The neutral silicon chemistry in the flame is dominated by SiO, but evidence was obtained from both the cation and the anion chemistry for the presence of HSiO(OH), silanoic acid; SiO(OH)2, metasilicic acid; and Si(OH)4, orthosilicic acid. The silicon ion chemistry differs markedly from the normal carbon ion chemistry that occurs naturally in the undoped methane–oxygen flame; the silicon ions show a strong tendency towards Si—O bond formation. Consideration is given to the probable structures of the various silicon cations and anions observed.
Synthesis of non-enolizable α- and β-peltatins
Authors: Durst Tony , Behnia Leslie Donald
Published: May 2006

The preparation of non-enolizable derivatives of both α- and β-peltatins via reaction of the lactone enolates of the corresponding TBDMS derivatives with CH3I and hexachloroethane is described. These derivatives are considerably less toxic than the precursor peltatins but show, at best, only marginal activity in in vivo tests against P 388 leukemia.
Phosphoranes et thio-oxazaphospholidines d'aldohexoses protégés. Synthèses et études stéréochimiques
Authors: Bounja Zouhair , Houalla Douraid , Revel Monique , Wolf Robert
Published: May 2006

The alkoxyphosphoranes 1–5 have been obtained by the reaction between the bicyclophosphane 6 and the five following alcohols: 1,2,3,4-di-O-isopropylidene-α-D-galactopyranose 7, 1,2,5,6-di-O-isopropylidene-α-D-glucofuranose 8, methanol, isopropanol, and tertiobutanol. The sulfuration of 1–5 yields to the corresponding thio-oxazaphospholidines: 1(P=S)–5(P=S). In each case the intermediate tautomeric form 1(P=S)*–5(P=S)*, with an eight-membered ring, is observed. A detailed stereochemical study, based on 1H, 13C, and 31P NMR results is presented for eight compounds: 1–4 and 1(P=S)–4(P=S). In solution we observed the quantitative transformation of 4(P=S) into the corresponding oxo derivative 4(P=O) by the trifluoroacetic anhydride method.
Isolation and characterization of culmorin derivatives produced by Fusariumculmorum CMI 14764
Authors: Kasitu Gertrude C. , ApSimon John W. , Blackwell Barbara A. , Fielder David A. , Greenhalgh Roy , Miller J. David
Published: May 2006

Liquid culture fermentations of Fusariumculmorum yield a wide variety of secondary metabolites. In addition to known isoprenoids, such as the trichothecenes and modified trichothecenes, four new derivatives of culmorin have been isolated and characterized by spectral analysis. These compounds are 15-hydroxyculmorone (7), and 5-hydroxy- (8), 12-hydroxy- (10), and 15-hydroxyculmorin (9). The structure of culmorone is confirmed. The biosynthesis of culmorin from farnesyl pyrophosphate is substantiated through the use of 13C-acetate and the oxidation of these derivatives is discussed.
Identification of 2-deoxy-D-ribono-1,4-lactone at the site of benzophenone photosensitized release of guanine in 2′-deoxyguanosine and thymidylyl-(3′-5′)-2′-deoxyguanosine
Authors: Buchko Garry W. , Cadet Jean
Published: May 2006

Benzophenone-mediated photosensitization of 2′-deoxyguanosine (dG) and thymidylyl-(3′-5′)-2′-deoxyguanosine (d(TpG)) at 350 nm in oxygen-saturated aqueous solution produces 2-deoxy-D-ribono-1,4-lactone (dL) and thymidylyl-(3′-5′)-2-deoxy-D-ribono-1,4-lactone (d(TpL)), respectively. These photoproducts have been isolated by reverse phase high-performance liquid chromatography and characterized by proton magnetic resonance spectroscopy and fast atom bombardment mass spectroscopy. Conformational analysis of d(TpG) and d(TpL) in aqueous solution using coupling constant data suggests no major conformational differences between the two dinucleoside monophosphates.
Volumes of mixing of ternary alkane mixtures with branched chain components
Authors: Weiguo Shen , Williamson Arthur G.
Published: May 2006

Measurements at 25 °C are reported for the volumes of mixing of binary mixtures of n-dodecane + 2 methyl pentane and of ternary mixtures created by mixing an equimolar mixture of (n-decane + n-tetradecane) with each of the isomers of hexane. The results are compared with the work of Hamam etal. for binary mixtures of n-dodecane with the hexane isomers and shown to be in accord with the principle of congruence within the limits of experimental accuracy of the measurements.
Hydroxythioéthers éthyléniques: synthèse et réarrangement spontané
Authors: Martin Ghislaine , Sauleau Jean , David Michèle , Sauleau Armelle , Sinbandhit Sourisak
Published: May 2006

Reactions of vinyloxiranes and thiophénols or phenylthiotrimethyl silane with ZnI2 or n BuLi, at room temperature, were studied. These condensations proceed regio and stereospecifically to afford, in good yields, three families of hydroxy aryl ethylenic sulfides. The regiochemistry is determined by the stereochemistry, the substitution of the oxiranes, and the reaction conditions. During the condensation, 1,3 phenylthio migration occurs. Light initiates the rearrangement by bringing about cleavage of the allyl sulfur bond — a radical chain mechanism — of the hydroxy allylphenyl sulfides. Scope, rate constants, and product composition of the rearrangement were studied.
The solution structures of chiral Ti4+ alkoxides. II. The roles of diolate basicity and side-chain binding group polarity
Authors: Bianchet Stephen , Potvin Pierre G.
Published: May 2006

With a view to reproduce the structure in solution of the Katsuki–Sharpless catalyst (the dimeric complex formed by homochiral diisopropyl tartrate with Ti(OiPr)4), five C2-symmetric chiral 2,3-butanediols, related to L-threitol but bearing N-, S-, and P-containing groups at the 1- and 4-positions, were prepared and their reactions with Ti(OiPr)4 were examined by nuclear magnetic resonance spectroscopy. The 1,4-dithioether formed a non-fluxional 2:2 tricyclic complex entirely analogous to those formed by alkylated sugar derivatives. The related disulfone formed a similar complex, but only at low temperatures, probably because of oligomerization at higher temperatures. A symmetric 2:1 complex also formed with excess Ti(OiPr)4, but incompletely and in equilibrium with the 2:2 species. No well-defined complexes were formed with pyrazole or phosphine oxide functionalities. However, in the presence of trifluoroacetic acid (TFA), a 1:2:2 L:Ti:TFA complex was identified with the dimethylpyrazole-containing diol and this resembled the 2:3 complexes formed by tartramides. Generally, the degree of spectral asymmetry and the coupling constant between diolate protons were instrumental in assigning structures and were used to classify all chiral diolate complexes into two groups according to whether or not they exhibited metal chelation and diolate bridging. By comparison of the effects of complexation on the diolate nuclei, a correlation was found between 13C and 1H nmr signal positions and between the pairs of signals in the spectra of asymmetric complexes, except in four cases where metal-bound side-chain groups induced a shielding of the bridging diolate proton. This could serve as an additional indication of metal chelation and diolate bridging and confirmed that nuclei at bridging positions appeared upfield of terminal ones. Noting that all the diols that readily chelated and bridged possessed electron-releasing side chains, while tartrates and tartramides that did not chelate possessed electron-withdrawing side chains, the basicity of the diolate oxygens is considered the most important determinant of structure. Side-chain polarity was instead found to influence whether or not a well-defined complex could form.
A concise synthesis of (+)-muscarine
Authors: Chan T. H. , Li C. J.
Published: May 2006

(+)-Muscarine was synthesized from S-(−)-ethyl lactate in five steps with the application of a zinc-mediated allylation reaction in aqueous media. Reversal of chelation-control stereoselectivity was observed in the allylations of chiral α-alkoxy aldehydes in aqueous media.
Dinuclear cobalt(II) complexes of a polyhydroimidazole ligand. Dinuclear (2:1) non-metallocyclic derivatives and dinuclear (1:1) metallocyclic complexes with a large, unoccupied central cavity
Authors: Tandon Santokh S. , Thompson Laurence K. , Bridson John N. , Dewan John C.
Published: May 2006

The ligand BTIM (1,2,4,5-tetrakis(4,5-dihydro-imidazol-2-yl)benzene) reacts with cobalt(II) salts to form two series of complexes. The 1:1, dinuclear, metallocyclic derivatives [Co2(BTIM)2X2]X2 (X = Cl (I), Br (II)) involve two bis-dentate ligands in a metallocyclic structure with a large unoccupied cavity. The 2:1, binuclear derivatives [Co2(BTIM)X4] (X = Cl (III), Br (IV)) involve two metals bound to a single, bis-bidentate ligand. The crystal and molecular structures of II and III are reported. Compound II crystallized in the monoclinic system, space group P21/c, with a = 13.642(6) Å, b = 11.560(3) Å, c = 18.406(7) Å, β = 101.73(3)° and four formula units per unit cell. Refinement by full-matrix least squares gave final residuals of R = 0.060 and Rw = 0.062. Compound III crystallized in the triclinic system, space group [Formula: see text], with a = 8.367(2) Å, b = 14.254(3) Å, c = 7.649(2) Å, α = 100.99(2)°, β = 101.44(2)°, γ = 106.85(1)° and one formula per unit cell. Refinement by full-matrix least squares gave final residuals of R = 0.052 and Rw = 0.045. In the metallocyclic structure (II) the square-pyramidal cobalt(II) centres are separated by 7.599(4) Å, while in the 2:1 derivative the two tetrahedral cobalt(II) centres have a much larger separation (8.736(3) Å).
Structural studies of organoboron compounds. LVI. 1-Benzyl-7-methyl-3,5-diphenyl-2,4,6-trioxa-1-azonia-3-bora-5-boratabicyclo[3.3.0]octane and 1,4,6,9-tetramethyl-2,7-diphenyl-3,8,11,12-tetraoxa-1,6-diazonia-2,7-diboratatricyclo[5.3.1.12,6]dodecane
Authors: Kliegel Wolfgang , Lubkowitz Gottfried , Rettig Steven J. , Trotter James
Published: May 2006

The preparation of the N-(2-hydroxypropyl)-N-alkylhydroxylamines, 6a (R = CH3) and 6b (R = CH2Ph), and their reactions with phenylboronic acid are described. Regardless of the molar ratios of reactants employed, the reaction with 6b leads to the 1:2 condensate 1-benzyl-7-methyl-3,5-diphenyl-2,4,6-trioxa-1-azonia-3-bora-5-boratabicyclo[3.3.0]octane, 7, while that with 6a gives rise to the 1:1 condensate 1,4,6,9-tetramethyl-2,7-diphenyl-3,8,11,12-tetraoxa-1,6-diazonia-2,7-diboratatricyclo[5.3.1. 12,6]dodecane, 11 (the cyclic BONBON dimer of 4,6-dimethyl-2-phenyl-1,3-dioxa-4-aza-2-boracyclohexane, 9). Compounds 7 and 11 both crystallize in the triclinic space group [Formula: see text]: for 7; a = 13.126(1), b = 15.337(1), c = 10.9469(5) Å, α = 91.727(5), β = 104.647(5), γ = 72.922(7)°, Z = 4; and for 11; a = 9.0807(4), b = 9.1653(3), c = 6.4876(2) Å, α = 97.708(3), β = 108.830(3), γ = 89.188(4)°, Z = 1. The structures were solved by direct methods and were refined by full-matrix least-squares procedures to R = 0.038 and 0.032 for 5879 and 1827 reflections with I ≥ 3σ(F2), respectively. Compound 7 has the expected bicyclic pyroboronate structure, but represents the first reported N-substituted example of this type of compound. Bond lengths involving boron in 7 are (C) O—B(sp3) = 1.428(2) and 1.420(2), (B)O—B(sp3) = 1.472(2) and 1.468(2), N—B(sp3) = 1.737(2) and 1.762(2), C(phenyl)—B(sp3) = 1.588(2) and 1.584(2), (N)O—B(sp2) = 1.402(2) and 1.404(2), (B)O—B(sp2) = 1.331(2) and 1.329(2), C(phenyl)—B(sp2) = 1.555(3) and 1.553(2) Å. The X-ray analysis establishes a centrosymmetric, twofold N → B coordinated, dimeric structure in the solid state for 11 in which each B—O—N segment of a central six-membered BONBON ring is bridged by an O—C—C moiety. Compound 11 represents the first fully characterized example of a new type of "BONBON" compound. Bond distances involving the boron atom are (N)O—B = 1.465(1), (C)O—B = 1.428(1), N—B = 1.695(2), and C(phenyl)—B = 1.607(2) Å. Spectroscopic evidence indicates that in solution and in the gas phase this material exists predominantly as the monomer 9.
Basicity of substituted 2-pyridyl-1,1,3,3-tetramethylguanidines and aminopyridines in acetonitrile and water solvents
Authors: Foroughifar Naser , Leffek Kenneth T. , Lee Yong Gu
Published: May 2006

A series of substituted 2-pyridyl-1,1,3,3-tetramethylguanidines has been synthesized and the pKa of each has been measured in acetonitrile and water solvents, together with the pKa of the corresponding aminopyridine. A relationship between the pKa's in these two solvents has been derived that allows the estimation of a pKa in acetonitrile from that in water. The results were fitted to the Hammett equation, giving ρ = 2.31 ± 0.14 and 1.64 ± 0.11 for the pyridylguanidines in acetonitrile and water, respectively, and 6.16 ± 0.4 and 5.24 ± 0.2 for the aminopyridines in the same solvents.
A physicochemical study on organic eutectics and addition compound; benzidine–pyrogallol system
Authors: Rai U. S. , George Santhi
Published: May 2006

The phase diagrams of the binary organic system of benzidine–pyrogallol was determined by the thaw–melt method. The solidification behaviour of the pure components, their eutectics, and the addition compound were studied by measuring the movement of growth front in a capillary. From the data on X-ray diffraction, thermal and microscopic investigations it can be inferred that the eutectics are not simple mechanical mixtures of the components involved. The IR and NMR spectral investigations were carried out to throw light on the nature of bonding between the two components forming the addition compound.
Influence of metal–metal interaction on magnetic and redox properties in different series of heterodinuclear copper(II), nickel(II), and zinc(II) complexes
Authors: Srinivas Bhamidi , Zacharias Panthappally S.
Published: May 2006

Different series of heterodinuclear complexes, viz.,|CuNiL](ClO4)2•nH2O,[CuZnL](ClO4)2•nH2O,[NiCuL](ClO4)2•nH2O, and [NiZnL](ClO4)2•nH2O complexes where L is a dinucleating Schiff base ligand derived from the condensation of 1 mole of 2-hydroxy-5-methylbenzene-1,3-dicarbaldehyde, 1 mole of 1,3-diaminopropane, and 2 moles of ortho- or para-substituted aniline, have been synthesized and investigated. The μeff values of the CuNiL and NiCuL series of complexes suggest presence of weak antiferromagnetic coupling in them. In MeCN and DMF the CuNiL and NiCuL series of complexes exhibit two reversible one-electron redox couples at two different potentials. The large separation between the two redox couples is attributed to the antiferromagnetic interaction between the metal centres in the complexes. In the NiCuL series of complexes, the observed shift of 100–150 mV in E1 to more negative values in complexes with potential donor groups such as OMe, OEt, SMe, etc. in the ortho position of the phenyl group of L is explained on the basis of weak bonding interactions of these groups with the copper centre. The CuNiL series of complexes do not exhibit such shift in E1 values since there is no significant change in the square planar geometry of the copper centre. The CuZnL and NiZnL series of complexes exhibit only single one-electron redox couple as could be anticipated.
Cobalt(II) catalyzed oxidation of 2-substituted 1,3-dioxolanes with molecular oxygen
Authors: Li Pei , Alper Howard
Published: May 2006

Cobalt(II) chloride catalyzed oxidation of 2-substituted 1,3-dioxolanes in 1,2-dimethoxyethane afforded formate esters and acids in high yields. It was found that the presence of catalytic amounts of ZnCl2 increased the rate of oxidation. A free-radical mechanism is proposed, involving participation of superoxocobalt, and the esterification of the alcohol and acid.
Ab initio studies on the reaction SN+ + SNS+ → S3N22+
Authors: Grein F.
Published: May 2006

The reaction SN+ + SNS+ → S3N22+ has been studied by ab initio SCF methods. Geometry optimizations with the 3-21G(*) basis set were followed by 6-31G* single-point calculations. The barrier of S3N22+ to dissociation is 10.9 kcal/mol with 3-21G(*), and 19.7 kcal/mol with the (single-point) 6-31G* basis set. The transition structure is cyclic (Cs), and is similar to the optimized C2v structure of S3N22+. The relative orientation of the two approaching cations is governed by electrostatic forces rather than electronic (frontier orbital) effects. The product lies 99.3 (3-21G(*)) and 95.2 (6-3IG*) kcal/mol higher than the reactants.
High-pressure Raman studies of triquinacene and dodecahedrane
Authors: Bertz S. H. , Kourouklis G. A. , Jayaraman A. , Lannoye G. , Cook J. M.
Published: May 2006

The dimerization of triquinacene (C10H10) to dodecahedrane (C20H20) has been a goal of organic chemists for more than three decades. An attempted synthesis of dodecahedrane from triquinacene using high pressure and near-UV radiation was not successful, but both hydrocarbons survived the 15 GPa treatment. There is no evidence for a pressure-induced transition in either of the materials. The results of high-pressure Raman spectroscopic investigations to 20 GPa on triquinacene and dodecahedrane are reported. Liquid triquinacene crystallizes near 0.2 GPa at 295 K and the Raman peaks associated with the molecular modes sharpen strikingly. In the crystal the external mode peaks exhibit a large pressure dependence, as expected, but the surprising fact is that the C—H stretching modes behave likewise. In dodecahedrane a similar effect is observed for the C—H stretching modes, but no external mode peaks are observed. The Raman data on dodecahedrane are compared with earlier results and calculations, which support the Ih symmetry for the molecule and an fcc lattice for the crystal.
Secondary metabolites of the aspen fungus Stachybotrys cylindrospora
Authors: Ayer William A. , Miao Shichang
Published: May 2006

The secondary metabolites produced by the fungus Stachybotrys cylindrospora, which is known to be strongly antagonistic to the blue-stain fungus Ceratocystiopsis crassivaginata, have been examined. The compounds responsible for the antifungal activity are trichodermin (5) and trichodermol (6), two previously known mycotoxins belonging to the trichothecene family. The other metabolites, stachybotrydial (1), stachybotramide (8), and 6,8-dihydroxy-3,5,7-trimethylisochroman (7), although not active against C. crassivaginata, are new natural products. The structures of the new compounds were established by spectroscopic techniques.
Determination of Al, P, and Si in fruit juices stored in glass and aluminized containers
Authors: Soliman K. , Zikovsky L. , Dallaire C.
Published: May 2006

The concentration of Al, P, and Si in 142 samples of five fruit juices, measured with neutron activation analysis and atomic absorption spectroscopy, varies from 0 to 1.44, 31 to 201, and 1.5 to 35 µg/mL, respectively. The mean concentration of Al in apple, grape, grapefruit, orange, and pineapple juices is 0.54, 0.94, 0.24, 0.29, and 0.62 µg/mL, respectively. The mean concentration of P in the same juices is 102, 115, 124, 127, and 76 µg/mL. The storage of juice in aluminized containers with internal plastic coating does not increase the concentration of Al in the juice.
1,2-Nucleophilic additions of organolithium reagents to chiral oxime ethers
Authors: Dieter R. Karl , Datar Ravindra
Published: May 2006

Chiral oxime ethers are readily prepared from chiral alkoxyamines derived from ephedrine and norephedrine. These chiral oxime ethers of isobutyraldehyde and benzaldehyde undergo 1,2-nucleophilic addition reactions with alkyllithium reagents (n-BuLi, PhLi and n-BuLi, tert-butyllithium, respectively) to afford the chiral alkoxyamines with a diastereomeric excess of 64–88%. Reduction of the alkoxyamines with LiAlH4 gave the corresponding chiral amines. The diastereoselectivity of the reaction appears to directly mirror the E:Z ratio of the starting oxime ethers.
Anomalous stabilizing and destabilizing effects in some cyclic π-electron systems
Authors: Politzer Peter , Grice M. Edward , Murray Jane S. , Seminario Jorge M.
Published: May 2006

Ab initio computational studies have been carried out for three molecules that are commonly classed as antiaromatic: cyclobutadiene (1), 1,3-diazacyclobutadiene (7), and 1,4-dihydropyrazine (6). Their dinitro and diamino derivatives were also investigated. Stabilizing or destabilizing energetic effects were quantified by means of the isodesmic reaction procedure at the MP2/6-31G*//HF/3-21G level, and calculated molecular electrostatic potentials (HF/STO-5G//HF/3-21G) were used as a probe of electron delocalization. Our results do not show extensive delocalization in the π systems of any one of the three parent molecules. The destabilization found for 1 and 7 is attributed primarily to strain and to repulsion between the localized π electrons in the C=C and C=N bonds, respectively. However, 6 is significantly stabilized, presumably due to limited delocalization of the nitrogen lone pairs. NH2 groups are highly stabilizing, apparently because of lone pair delocalization. NO2 is neither uniformly stabilizing nor destabilizing.
Solubility and activity coefficients of lead chloride in potassium nitrate solutions at 25 °C and at boiling. Calculation of the supersaturation
Authors: Torrent-Burgués Joan , Veintemillas-Verdaguer Sabino
Published: May 2006

The solubility and activity coefficients of lead chloride in aqueous potassium nitrate solutions, at 25 °C and at boiling, are reported. The activity product constant of lead chloride at boiling has been determined. The Pitzer equations have been applied to the system and have allowed us to determine unknown ionic interaction parameters at 25 °C. The calculation of the lead chloride supersaturation is illustrated using the activities instead of the concentrations.
Solvent influence on the conformation of cyclosporin. An FT-IR study
Authors: Shaw R.A. , Mantsch H.H. , Chowdhry B.Z.
Published: May 2006

Infrared spectra of the cyclic peptide cyclosporin A and three analogues have been measured in a number of organic solvents (CCl4, CDCl3, acetonitrile, DMSO, and 50:50 acetonitrile:D2O). Seven of the eleven amide groups of cyclosporin A are methylated, the remaining four N-H protons forming strong intramolecular hydrogen bonds in the crystal and in CDCl3 solution. These hydrogen bonds give rise to amide I (C=O stretching) bands at positions characteristic of β-turns, γ-turns, and β-sheet domains in proteins and model polypeptides. Increasing the polarity of the solvent eliminates some of these features; however, the spectra in DMSO and acetonitrile–D2O retain strong amide I absorptions characteristic of hydrogen-bonded carbonyl groups. The conformation of cyclosporin A in water cannot be observed directly due to low solubility; these findings suggest that the structure likely retains strong intramolecular hydrogen bonding. Spectra of the three analogues are consistent with this interpretation. Implications respecting the mechanism of pharmacological action of cyclosporin A are discussed.
Molecular structure and tautomerism of 3,5-bis(4-methylpyrazol-1-yl)-4-methylpyrazole
Authors: Perrin Monique , Thozet Alain , Cabildo Pilar , Claramunt Rosa Ma , Valenti Eduard , Elguero José
Published: May 2006

The title compound C12N6H14, 1, crystallizes in the space group P21/n (a = 8.222(2) Å, b = 27.336(8) Å, c = 5.574(2) Å, α = 90.00°, β = 100.97(4)°, γ = 90.00°), Z = 4, d = 1.308 g cm−3. The conformation about the N—C bonds linking the pyrazole rings can be defined as EZ, with "pyridine-like" nitrogen atoms in an anti disposition [Formula: see text] and "pyridine-like" and "pyrrole-like" nitrogen atoms in a syn disposition [Formula: see text] with regard to the central pyrazole. Intermolecular hydrogen bonds between the central and the terminal pyrazole ring of configuration Z form centrosymmetric dimers. They pack in sheets nearly parallel to the (−2 3 1) plane. Its tautomerization barrier has been determined in methanol-d4; the value, 11.9 kcal mol−1, is similar to those of 3,5-dimethyl-4-chloropyrazole (12.8 kcal mol−1) and 3,5-dimethyl-4-nitropyrazole (12.1 kcalmol−1). These values together with the shape of the conformational potential surface (calculated using the AM1 Hamiltonian) suggest that, in compound 1, prototropy and rotation about the N—C bonds linking the three pyrazole rings take place simultaneously.
Electron ionization mass spectrometry of aryl- and fluoroalkyl-substituted nickel(II) β-diketonates and monothio-β-diketonates
Authors: Reimer Mark L.J. , Westmore John B. , Das Manoranjan
Published: May 2006

Electron ionization positive-ion mass spectra are reported for 18 substituted nickel(II) β-diketonates and monothio-β-diketonates, NiII(RCXCHCOR′)2, where X = O or S; R = phenyl, 2-thienyl, 5-methyl-2-thienyl, or 2-naphthyl; and R′ = difluoromethyl, trifluoromethyl, pentafluoroethyl, or n-heptafluoropropyl. Each mass spectrum contains a prominent molecular ion, [NiL2]+, which, for β-diketonates, fragments mainly by elimination of the odd-electron R′• species; the resulting ion decomposes by losses of one or more even-electron neutral species to yield ions containing nickel(II). In contrast, the major fragmentation of the molecular ion of monothio-β-diketonates is loss of NiIL to yield an abundant L+ ion. Furthermore, while most ions contain nickel(II), some prominent ions contain nickel(I); among these are [NiISC(R) = CH]+ and [NiI(HCCR)]+. Interesting ions are formed by hydrogen transfer processes. The [NiHL]+ ion is favored in the spectra of β-diketonates having a phenyl substituent, consistent with hydrogen transfer to the metal from the aryl group. On the other hand, the [Ni(L–H)]+ ion, favored for the monothio-β-diketonates, could be formed by hydrogen transfer from the central position of the chelate ring or from an aryl substituent. Alternative mechanisms of formation are discussed. Some minor ions are formed by fluorine transfer to nickel. The trends in their abundances are influenced by the hardness of nickel as an acid in its different oxidation states, under the influence of the ligand donor atoms, and by the hardness of the carbon atoms of the perfluoroalkyl substituents.
Tetrakis[(trimethylsilyl)ethynyl] Group 14 metal derivatives: an examination of the electronic interaction between two Group 14 metals connected by an acetylene wire
Authors: Dallaire Carol , Brook Michael A. , Bain Alex D. , Frampton Christopher S. , Britten James F.
Published: May 2006

The tetrakis(trimethylsilyl)ethynyl derivatives of Si, Ge, Sn, and Pb were prepared and examined spectroscopically. NMR and Mössbauer spectroscopy and X-ray crystal structure data clearly demonstrate an electronic interaction in the ground state between the distal SiMe3 groups and the central metal atom, leading to a strong shielding of the central metal atom; the respective nmr signals of the central metal are among the most shielded examples reported for a tetrahedral Group 14 metal centre with four carbon ligands. The nature of these electronic interactions is discussed.
Raman spectral studies of solutions at elevated temperatures and pressures.13. Sodium formate – water
Authors: Bartholomew Richard J. , Irish Donald E.
Published: May 2006

Raman spectra of the formate anion in water (H2O and D2O) have been measured for four concentrations under ambient conditions and for two concentrations at temperatures ranging from 49 to 239 °C and a pressure of 10 MPa. Five of the six fundamental modes of vibration are polarized. This result is inconsistent with C2ν symmetry. The Fermi resonance doublet clearly results from the interaction of 2ν5 and ν1. The latter mode decreases in frequency as the temperature rises, thus increasing the coupling and hence the intensity of the 2ν5 component. The coupling constant, W, and the positions of the unperturbed bands [Formula: see text] and [Formula: see text] have been calculated. No evidence to support a bifurcated structure for the solvated anion was found.
Conformational preferences and barriers to internal rotation in fluorothioanisoles from long-range spin–spin couplings, photoelectron spectroscopy, and semiempirical molecular orbital calculations
Authors: Chmielewski Dietmar , Werstiuk Nick Henry , Wildman Timothy A.
Published: May 2006

The conformational preferences and barriers to internal rotation about the S—C(phenyl) bond have been investigated for thioanisole and its 2-fluoro, 2,6- and 3,5-difluoro, and 2,3,5,6-tetrafluoro derivatives. Measurements of long-range spin–spin couplings between the methyl carbon and the para ring proton indicate that the 2-fluoro and 3,5-difluoro compounds prefer conformations with all heavy atoms coplanar. The 2,6-difluoro and 2,3,5,6-tetrafluoro compounds prefer conformations in which the methyl carbon lies in or near the plane perpendicular to the aromatic ring. Semiempirical molecular orbital calculations with the MNDO method indicate that all of the molecules prefer perpendicular conformations while similar calculations with AM1 indicate that all prefer planar conformations. Apparently the conformational behaviour can be quite sensitive to subtle changes in intramolecular interactions, which may indicate improvements to these semiempirical methods. The NMR results have been used to derive an internally consistent set of rotational potentials. Synthetic photoelectron spectra derived from these potentials and the AM1 orbital energies are in good agreement with the experimental spectra.