bluuebunny/crossref_metadata_2025 huggingface dataset:
Computational determination of the relative stabilities of some nitro carbocations
Authors: Redfern Paul C. , Murray Jane S. , Politzer Peter
Published: May 2006

The relative stabilities of a group of nitro carbocations (derivatives of the methyl, cyclopropyl, and cyclopropenyl cations) are determined by means of abinitio SCF/3-21G calculations, and compared to the corresponding results for other substituent groups, both electron donating and withdrawing. The α-nitro carbocations are generally destabilized relative to the parent cation, but often to a lesser extent than anticipated from the strongly electron-withdrawing nature of NO2. The optimized structures indicate that this is due to the stabilizing formation of an intramolecular ring involving the nitro group; however, this requires the proximity of a sufficiently positive carbon. Keywords: carbocations, abinitio SCF calculations, relative stabilities, nitro derivatives.
Synthesis, multinuclear magnetic resonance spectra, and chemistry of some complexes [(μ-SR)6(MX)4]2− (R = alkyl; M = Zn or Cd; X = Cl, Br, or I) and the X-ray structural analysis of (Et4N)2[(μ-SPri)6(CdBr)4]
Authors: Dean Philip A. W. , Vittal Jagadese J. , Wu Yuyang
Published: May 2006

The self-assembly method has been used to prepare a wide range of new adamantane-like anions of the type [(μ-SR)6(MX)4]2− (M = Zn or Cd; R = alkyl or benzyl; X = Cl, Br, or I) as their Et4N+ salts. Metal (111 or 113Cd) NMR data have been measured for the cadmium complexes, and also for many of the possible mixed-metal complexes [(μ-SR)6(CdX)n(ZnX)4−n]2−. In the complexes with mixed metals, the effects of Zn substitution on the metal chemical shifts are generally larger for the alkyl- and benzyl-thiolate complexes than for related benzenethiolate complexes. However, for [(μ-SPri)6(CdX)n(CdX′)4−n]2− (X/X′ = Cl/Br, Cl/I, or Br/I), substituent effects are no larger than found in some PhS−-bridged complexes. The reaction of [(μ-SPri)6(CdX)4]2− (X = Cl, Br, or I) with E′8 (E′ = S or Se) in CH2Cl2 gives [Cd4(μ4-E′) (μ-SPri)12(CdX)4]2− as the major species detectable by metal, 13C, and, where applicable, 77Se NMR. However, the reaction does not occur stoichiometrically. The compound (Et4N)2[(μ-SPri)6(CdBr)4] crystallizes in the monoclinic space group P21/c with cell dimensions a = 24.079(3) Å, b = 11.365(2) Å, c = 22.561(3) Å, β = 113.89(1), and Z = 4. The structure was refined to R(Rw) = 0.0663(0.0703) with the use of 2669 unique data with I > 2.5σ(I). The anion contains an adamantane-type (μ2-S)6Cd4 cage composed of a nearly regular Cd4 tetrahedron and a distorted S6 octahedron, the irregularity of which is caused by 1,3-interactions of the substituent groups on the sulfur atoms. The axial or equatorial dispositions of the six alkyl groups in the four fused M3S3 rings are [aae, aae, aee, aee]. For the zinc-group elements, this is the first example of an adamantane-like (μ-Salkyl)6M4 cage that has been characterized crystallographically. Keywords: cadmium complex, cadmium-111/113 NMR, thiolate complex, X-ray analysis, zinc complex.
The structure of the pyranosyl nucleoside 1-(2′,3′-dideoxy-(β-D-erythro-hex-2′-enopyranosyl)thymine and a comparison with AZT
Authors: Tari Leslie W. , Secco Anthony S.
Published: May 2006

Pyranosyl nucleosides are among the most recent modifications of natural nucleosides to receive attention as potential HIV inhibitors. As part of our studies involving modified nucleosides, the crystal structure of 1-(2′,3′-dideoxy-β-D-erythro-hex-2′-enopyranosyl)thymine was determined and its molecular structure and conformation compared with AZT in an effort to assess the title compound's potential activity against HIV. The compound crystallizes in the monoclinic space group P21 with cell dimensions a = 7.966(3), b = 18.004(3), c = 8.346(3) Å, β = 91.29(3)°. The crystal structure, solved by direct methods and refined to R = 0.032 and Rw = 0.041 for 1285 observed data, contains two independent molecules in the asymmetric unit, each displaying different torsion angles, χ (64.5(6)° and 72.7(6)°) and γ (62.0(7)° and 178.6(5)°). Molecular conformations, described as slightly distorted chairs with O5′ endo and C5′ exo and with equatorial substituents, are stabilized by a network of hydrogen bonds throughout the crystal. Despite the substantial difference in χ values with respect to AZT in the solid state, molecules appear to exhibit sufficient structural overlap with AZT to expect a similar capacity for binding to the active site of phosphorylation; the barrier to rotation about the N-glycosidic bond is only slightly greater for the title compound. The significant difference, on comparison with AZT, appears at the primary hydroxyl, which is so oriented as to probably preclude activity of this pyranosyl compound as an anti-HIV agent. The conformational parameters given are in accordance with the IUPAC–IUC Joint Commmission on Biochemical Nomenclature (Pure Appl Chem. 55, 1273 (1983)). Keywords: crystal structure, modified nucleoside, pyranosyl nucleoside, anti-HIV drugs.
The electronic spectra of acetaldehyde-h4 and -d4
Authors: Yakovlev N. N. , Godunov I. A.
Published: May 2006

The [Formula: see text] electronic absorption spectra of acetaldehyde-h4 and -d4 were recorded in the vapour phase at room temperature. The major experimental requirement was a high pressure × path length (650 Torr × 140 m). The vibrational structure of these electronic transitions was interpreted in terms of the torsional modes [Formula: see text] and [Formula: see text] attached to the [Formula: see text] out-of-plane bending mode. The main CH3CHO results agreed with those obtained earlier (Moule and Ng); the values of the [Formula: see text] transition and torsional barrier height were 27240.1 and 590 cm−1 respectively. Three inversion (out-of-plane bending) energy levels in the excited [Formula: see text] state were found and the inversion potential function was determined with a barrier height of 1110 cm−1. The CD3CDO spectrum confirmed the CH3CHO analysis. The values of the [Formula: see text] transition and torsional barrier height were equal to 27270 and 610 cm−1. Keywords: vibronic spectrum, acetaldehyde, molecular structure.
A group equivalents scheme for free energies of formation of organic compounds in aqueous solution
Authors: Guthrie J. Peter
Published: May 2006

Group contributions have been determined allowing the calculation of free energies of formation in aqueous solution for organic compounds containing carbon, hydrogen, and oxygen. The system works well for monofunctional compounds. The available literature data for 198 compounds can be accommodated using 79 parameters with an rms deviation of 0.74 kcal/mol.
Isolation and structure of cribrostatins 1 and 2 from the blue marine sponge Cribrochalinasp
Authors: Pettit George R. , Collins Jeremiah C. , Herald Delbert L. , Doubek Dennis L. , Boyd Michael R. , Schmidt Jean M. , Hooper John N. A. , Tackett Larry P.
Published: May 2006

The blue marine sponge cribrostatins 1 (1, 8.8 × 10−6% yield) and 2 (2, 3.1 × 10−6% yield) active against the P388 lymphocytic leukemia cell line (ED50 1.58 and 2.73 μg/mL respectively) have been isolated. The previously known isoquinolinequinones mimosamycin (3), renierone (4), and O-demethylrenierone (5) were also isolated and shown to display activity against the P388 cell line (ED50 0.73, 2.75, and 8.36 μg/mL, respectively). Importantly, both cribrostatin 1 and mimosamycin showed selective activity against all of the nine human melanoma cell lines employed by the U.S. National Cancer Institute. Structural determinations were accomplished utilizing high field nuclear magnetic resonance (400 MHz) and mass spectral studies. Confirmation of the cribrostatin 1 structure was achieved by X-ray crystallographic techniques.
Natural products. Antitubulin effect of congeners of N-acetylcolchinyl methyl ether: synthesis of optically active 5-acetamidodeaminocolchinyl methyl ether and of demethoxy analogues of deaminocolchinyl methyl ether
Authors: Boyé O. , Brossi A. , Yeh H. J. C. , Hamel E. , Wegrzynski B. , Toome V.
Published: May 2006

Trimethoxy-substituted dihydrodibenzocycloheptenes 4–7, required for a structure–activity study measuring the inhibition of tubulin polymerization in vitro, were synthesized by four different routes: (1) Synthesis of 4 was achieved from 2,3-dimethoxybenzaldehyde via biphenyl aldehyde 17, chain lengthening to propionic acid 20, acid-catalyzed cyclization toward ketone 21, and removal of the carbonyl group. (2) Compound 5 was obtained by eliminating the sterically most hindered methoxy group in 25 or 26 by metal reduction in alcohol. (3) Compound 6 was prepared from biphenyl aldehyde 34 obtained by Grignard reaction on oxazoline 32. (4) Compound 7 was obtained by reductive deoxygenation of the tetrazolyl ether derivative of N-acetylcolchinol 41. The key role of the aromatic oxygen atoms in colchicine and allo congeners as points of interaction with the colchicine binding site on tubulin was demonstrated by the lack of inhibitory activity of compounds 4–7. Optically active 5-acetamide 8a,b isomers of N-acetylcolchinyl methyl ether 2 were obtained after chemical resolution of amine 47. The absolute configuration of the optical isomers 47a,b and 8a,b was determined by 1H NMR and CD measurements. These compounds were found inactive as inhibitors of tubulin polymerization.
The synthesis of β-keto lactones via cyclization of β-keto ester dianions or the cyclization of Meldrum's acid derivatives
Authors: Lermer Leonard , Neeland Edward G. , Ounsworth James P. , Sims Russell J. , Tischler Samuel A. , Weiler Larry
Published: May 2006

Two new methods to synthesize macrocyclic β-keto lactones have been developed. The first involves the synthesis of ω-halo-β-keto esters and an intramolecular alkylation of the dianions to these compounds. The reaction is complicated by elimination in the small and medium ring systems and by difficulties in purifying the final products. However, it is possible to obtain modest yields of the desired β-keto lactones. This procedure was used to synthesize the 25- and 27-membered ring β-hydroxy lactones that are the constituents of termite defense compounds. The second method involves the thermolysis of acylated Meldrum's acid derivatives, which leads directly to β-keto lactones. This process gives modest yields of macrocyclic systems and good yield of the unsubstituted 3-oxopentan-5-olide (25). The 14-mem-[Formula: see text]bered macrocyclic β-keto lactone 9j has a complex 1H NMR spectrum, which has been interpreted in terms of multiple conformations. The temperature dependence of the NMR spectrum of 9j is consistent with entropic, rather than enthalpic, control of the equilibrium. Quasiharmonic entropy calculations are consistent with this model.
Reactions of 2-phenyl-4H-1,3,2-benzodioxaborin, a stable ortho-quinone methide precursor
Authors: Chambers Jeffrey D. , Crawford Jason , Williams Haydn W. R. , Dufresne Claude , Scheigetz John , Bernstein Michael A. , Lau Cheuk K.
Published: May 2006

Thermolysis of 1-phenyl-4H-1,3,2-benzodioxaborins generated the corresponding ortho-quinone methides, which were found to undergo intermolecular cycloaddition reactions with ethyl vinyl ether, dihydropyran, β-methylstyrene, cyclohexene, and 1-ethoxy-1-z-trimethylsiloxy-1-propenes to give various substituted chromans. Intramolecular trapping of the quinone methides with an olefin led to the syntheses of several analogs of tetrahydrocannabinols. ortho-Quinone methides, generated by treatment of the 2-phenyl-4H-1,3,2-benzodioxaborins with a Lewis acid, react with various nucleophiles to give the corresponding 1,4-addition products. Thus, alkyl and aryl thiols, alcohols, amine, hydride, allyl trimethylsilane, acetophenone, and diethylmalonate as well as some aryl compounds react with the quinone methide to give various 2-substituted phenols. Intramolecular reaction of the quinone methide with an aryl group led to the preparation of some 4-phenylchromans and tetralins.
Hydrogen bonding and internal rotation barriers of glycine and its zwitterions (hypothetical) in the gas phase
Authors: Yu Dake , Armstrong David A. , Rauk Arvi
Published: May 2006

The structures of the major glycine conformers, and several transition state structures, were optimized at HF/6-31G* and HF/6-31 + G* levels of theory and the correlation energies were calculated at MP2/6-31G* and MP2/6-31 + G** levels on these geometries, respectively. The heat of formation of gas phase glycine is calculated to be[Formula: see text]. Thermodynamic properties, [Formula: see text] etc. are calculated as functions of temperature using standard statistical thermodynamic methods. MP2/6-31 + G* optimization was carried out on the zwitterion, found by HF/6-31 + G* optimization to be a local minimum. The results suggest that glycine in the zwitterionic form in the gas phase is not a stationary point. Zwitterionic glycine does not exist in the gas phase.
Sirosterol and dehydroazasirosterol, unusual steroidal adducts from a Sirococcus species
Authors: Ayer William A. , Ma Yu-Ting
Published: May 2006

Two unusual compounds, which are made up of a sterol portion fused at C-5 and C-6 to an atrovenetin-like part, have been isolated from a fungus that causes Sirococcus shoot blight of spruce. The structures of these compounds, sirosterol (4) and dehydroazasirosterol (13), were determined by spectroscopic methods and comparison with appropriate model compounds. Sirosterol is an adduct of ergosterol and atrovenetin (5) and dehydroazasirosterol is an adduct of 9(11)-dehydroergosterol and an azaatrovenetin. Both compounds give a monomethyl ether when treated with diazomethane and both have been transformed to the corresponding 3-ketosteroids. The preparation of the triol 6 resulting from the selective hydroxylation of the 5,6-double bond in ergosterol is described. A possible biogenesis of sirosterol from ergosterol endoperoxide and a desoxyatrovenetin is discussed.
Computational and ultraviolet photoelectron spectroscopic evidence that (Z)-2-methyl-1,3-pentadiene prefers twisted s-cis conformers in the gas phase
Authors: Werstiuk Nick Henry , Timmins George , Ma Jiangong , Wildman Timothy A.
Published: May 2006

A redetermination of the ultraviolet photoelectron (pe) spectrum of (Z)-2-methyl-1,3-pentadiene has led to a correction of the published spectrum. By studying (Z)-2-methyl-1,3,-pentadiene (1a) and (E)-2-methyl-1,3-pentadiene (1b) with MMX, MNDO, AM1, and abinitio MO computational methods and pe spectroscopy, we have shown that a combination of these methods provides useful insights on the conformational behaviour of methyl-substituted 1,3-dienes in the gas phase. Synthetic pe spectra, derived from the computed potential energy surfaces and angle-dependent orbital energies, are in good agreement with experiment. Thus, the E isomer prefers the s-trans conformer but the Z isomer prefers twisted s-cis conformations in the gas phase.
Reactions of unactivated olefins with Vilsmeier reagents
Authors: Katritzky Alan R. , Shcherbakova Irina V. , Tack Robert D. , Steel Peter J.
Published: May 2006

Reactions of N-formylmorpholine/POCl3 with several unactivated olefins yield the corresponding α,β-unsaturated aldehydes. Norbornene affords with dimethylformamide/POCl3 a product formed by a novel inverse-electron demand Diels–Alder reaction of an acyclic 2-azoniodiene. The structure of this product was confirmed by X-ray analysis. Crystals are monoclinic: a = 8.025(3), b = 11.670(3), c = 9.309(4) Å, β = 108.03(3)°, P21, Z = 2; the structure was refined to R = 0.044 for 1379 independent observed reflections.
Complexes of Cu(II) with D-aldonic and D-alduronic acids in aqueous solution
Authors: Escandar Graciela M. , Sala Luis F.
Published: May 2006

A quantitative study of the equilibria involved in the interaction of Cu(II) ion with D-glucoheptonic, D-gluconic, D-galactonic, D-ribonic, D-glucuronic, and D-galacturonic acids in aqueous medium has been carried out by means of potentiometric measurements of hydrogen ion concentrations at a constant temperature of 20 °C and an ionic strength of 0.10 (NaNO3). The equilibrium data were processed with the FORTRAN computer program BEST. It was found that Cu(II) interacts with D-aldonic acids to form 1:1, 2:2, and 1:2 (M/L) complexes. The copper(II) complexes with D-galacturonic and D-glucuronic acids are considerably weaker than those of D-aldonic acid. Possible structures of the complex species formed are discussed.
Preparation and NMR spectra of PF4(N-N)+ and SiF4(N-N), where N-N = 2,2′-bipyridine, 4-fluoro-2,2′-bipyridine, and 1,10-phenanthroline
Authors: Nguyen Tuan Q. , Qu Fanqi , Huang Xiaoling , Janzen Alexander F.
Published: May 2006

Cationic complexes PF4(N-N)+, where N-N = 2,2′-bipyridine (bpy), 4-fluoro-2,2′-bipyridine (fbpy), and 1,10-phenanthroline (phen), were prepared in modest yield and identified by their A2B2 and A2BC fluorine NMR spectra, which are similar to those of the isoelectronic SiF4(N-N) adducts. The ligands bpy, fbpy, and phen may be useful for monitoring reactions of HF and H2O, or other fluorinated compounds, with glass apparatus because any liberated SiF4 is then trapped as SiF4(N-N), which can be readily detected by NMR spectroscopy.
Basicité de liaison hydrogène de formamidines substituées sur l'azote imino
Authors: Raczyńska Ewa D. , Laurence Christian , Berthelot Michel
Published: May 2006

The basicity of the hydrogen bonds of formamidines 1–19 was measured by means of the formation constant KHB of their complexes with p-fluorophenol and the frequency shift Δν(OH) of methanol hydrogen-bonded to 1–19. The study of the ν(C=N) band shows that hydrogen bonding takes place with the imino nitrogen atom. On the hydrogen-bonding basicity scale, the formamidines appear to be more basic than the corresponding amides and pyridines, and as basic as the imidazoles. The field effect of electron-withdrawing substituents and the steric effect of bulky alkyl groups on the imino nitrogen atom markedly decrease the hydrogen-bonding basicity.
Calculated electrostatic potentials and local surface ionization energies of para-substituted anilines as measures of substituent effects
Authors: Haeberlein Markus , Murray Jane S. , Brinck Tore , Politzer Peter
Published: May 2006

Using an abinitio self-consistent-field molecular orbital approach, we computed 3-21G//STO-3G* and STO-5G//STO-3G* electrostatic potentials and average local ionization energies for 17 para-substituted anilines. Our results demonstrate that the most negative potentials (Vmin) and the local surface ionization energy minima (ĪS,min) associated with the amine nitrogen lone pairs are highly sensitive indicators of the electron-donating and electron-attracting tendencies of the para substituents. We find excellent linear relationships between the 3-21G//STO-3G* amine nitrogen Vmin and ĪS,min and the σp0 Hammett constants of the substituent X; the correlation coefficients are 0.99. Correlations of slightly lesser quality are shown to exist between Vmin, ĪS,min, and σp−, σp, and pKa. Estimates of previously unknown σp0 and pKa values are given. The presence of ring carbon ĪS,minmeta to the substituent X also provides a predictive capability for determining σm values.
Open-chain nitrogen compounds. Part XV. A kinetic study of the hydrolysis of 1-aryl-3-aryloxymethyl-3-methyltriazenes and related triazenes
Authors: Vaughan Keith , Hooper Donald L. , Merrin Marcus P.
Published: May 2006

The kinetics of hydyrolysis of a series of 1-aryl-3-aryloxymethyl-3-methyltriazenes, Ar-N=N-NMe-CH2OAr′, was studied over the pH range 2–7.5. Reactions were followed by the change in UV absorbance spectra of the triazenes. The aryloxymethyltriazenes decompose more slowly at pH 7.5 than the hydroxymethyltriazenes, Ar-N=NMe-CH2OH; the hydrolysis is favoured by the presence of an electron-withdrawing group in Ar′. A mixed isopropanol/buffer system was developed in order to improve solubility of the aryloxymethyl triazenes. Lowering the pH caused an increase in the rate of hydrolysis and under strongly acidic conditions an electron-withdrawing group in Ar′ actually slows down the reaction. A Hammett plot of the pseudo-first-order rate constant, kobs, is curved, indicating that two or more mechanisms operate simultaneously and that the contribution of each mechanism is substituent-dependent. A plot of kobs vs. [buffer] is linear; the slope of the plot affords the rate constant, kb for the buffer-catalyzed reaction for each substituent. A Hammett plot of kb vs. σ is linear with ρ = +0.55, suggesting that the buffer-catalyzed reaction involves nucleophilic displacement of the phenoxy group by the buffer anion. Further analysis afforded the specific acid-catalyzed rate constants, [Formula: see text], for each substituent; this component of the reaction has a negative ρ, consistent with a mechanism involving protonation at the ether oxygen. The postulation that specific acid catalysis is a component of the reaction mechanism was confirmed by the observation of a solvent deuterium isotope effect, 2.28 > kH/kD > 1.60. Only the p-NO2 and p-CN phenyloxymethyltriazenes showed any spontaneous decomposition.
The acidity of polyhalogenated silanes and silyl radicals
Authors: Rodriquez C. F. , Hopkinson A. C.
Published: May 2006

The results of abinitio molecular orbital calculations at the MP4SDTQ/6-31++G(d,p)//HF/6-31++G(d,p) level have been used to calculate acidities of fluoro- and chloro-substituted silanes and silyl radicals. The radicals are more acidic than the silanes and substituent effects are also slightly larger in the radicals. For the gas phase deprotonation of fluorosilanes at 298 K, ΔHr (kcal/mol) values are SiH4, 378.5; SiH3F, 374.5; SiH2F, 366.7, and SiHF3, 351.0, i.e., interaction between fluorine atoms leads to increased enhancement of acidity. For chlorosilanes substituent effects are larger but strictly additive (13 kcal/mol for each Cl atom) with ΔHr values SiH3Cl, 365.4; SiH2Cl2 352.5, and SiHCl3 339.4. The electron affinities of silyl radicals calculated using isogyric reactions at the MP4SDTQ/6-31++G(d,p) level are too low by ~0.3 eV, but at the MP4SDTQ/6-311++G(2df,p) level the calculated electron affinity of SiH3 is 1.39 eV, compared with an experimental value of 1.44 ± 0.03 eV. This higher level of theory gives calculated electron affinities of 1.53 eV for SiH2F and 1.92 eV for SiH2Cl. Heats of formation obtained by using isogyric reactions to calculate atomization energies at the MP4SDTQ/6-311++G(2df,p) level are within 3 kcal/mol of experimental values except for SiH2F (where the "experimental" value was obtained from linear interpolation between SiH3 and SiF3). [Formula: see text] (kcal/mol) calculated for the anions are SiH3−, 14.4; SiH2F−, −78.0; and SiH2Cl−, −37.6.
The conformational behavior of isobutylbenzene in CS2/C6D12 solution at 300 K
Authors: Schaefer Ted , Lee Lina B.-L.
Published: May 2006

The 1H nuclear magnetic resonance spectrum of isobutylbenzene as a dilute solution in CS2/C6D12 at 300 K is analyzed. The gauche conformer (statistical weight 2) predominates, as was previously shown for a jet-cooled stream of this compound. The apparent twofold barrier to rotation about the Csp2—Csp3 bond is 10.5 ± 0.5 kJ/mol, as must be the case for the analogous gauche conformer of n-propylbenzene. The internal barriers are compared for ethylbenzene, gauche isobutylbenzene, and neopentylbenzene. AM1 and STO-3G MO computations of the conformer stabilities and barriers are reported, the latter being in better agreement with experiment.
Boron-11, boron-10, and nitrogen-14 NMR relaxation rates and quadrupole coupling constants in BH3NH3
Authors: Penner Glenn H. , Daleman Stephen I. , Custodio Angela R.
Published: May 2006

The 11B, 10B, and 14N spin–lattice relaxation times (T1) for aqueous solutions of BH3NH3 were measured by NMR spectroscopy. The results of this investigation are consistent with the nuclear quadrupolar coupling constants reported in previous nuclear quadrupolar resonance and microwave studies. The activation energy associated with rotational reorientation of BH3NH3 in aqueous solution is 11.7 ± 0.6 kJ/mol. Electric field gradients were calculated at various levels of abinitio molecular orbital theory, in order to obtain theoretical 14N and 11B quadrupolar coupling constants. At the highest level of calculation (CI(SD)/6-31G**//MP2/6-31G**), these are in agreement with recently reported microwave results but not with previously reported NQR experiments.
Studies of the role of water on some imine–acid complexes by means of Raman spectroscopy
Authors: Turcotte P. , Alex S. , Vocelle D.
Published: May 2006

The percentages of protonation were determined for a conjugated Schiff base trans, trans-2,4-heptadienylidene tert-butylamine in the presence of 3-chloropropionic acid (CPR) and dichloroacetic acid (DCA) in three solvents of different polarities. In dioxane, a solvent of low polarity, protonation is only important for the strong acid DCA (50–60%). By using solvents of higher polarities, protonation is seen to increase and is almost complete for DCA in ethanol. When water molecules are added to these systems, hydrolysis of the Schiff base, measured inside the time span of the experiments (10 min), occurs readily in dioxane, is very slow in a chloroform–dioxane mixture (9:1), and is totally absent in a mixture of ethanol–dioxane (1:9). Results indicate that water does not mediate protonation in all three sets of solvent combinations. The results are also discussed in terms of the possible role that water molecules could have in the visual and bacterial pigments.
Additions covalentes de nucléophiles sur les pyrido-as-triazines
Authors: Plé Nelly , Turck Alain , Queguiner Guy
Published: May 2006

Covalent addition of various nucleophiles to pyrido-as-triazines was studied and the σ-adduct structures were determined by NMR. A remarkable reactivity toward carbon nucleophiles has been highlighted for pyrido-as-triazines bearing a pyridinic nitrogen in the β position: only in the pteridine series has a similar reactivity been encountered. Complete regioselectivity of the nucleophilic attack on the pyridine nucleus peri to the triazine was observed.
Preparation and structure of chloro-copper(II) complexes of 7-azaindole
Authors: Poitras Jacques , Beauchamp André L.
Published: May 2006

The light green trans-CuCl2(Haza)2 adduct precipitates immediately from mixtures of CuCl2•2H2O with excess 7-azaindole (Haza) in methanol. Upon standing in azaindole-rich solution, this solid is replaced by a deep green Cu2Cl4(Haza)6 compound, whose crystal structure (orthorbombic, Fdd2, a = 26.558, b = 23.750, c = 12.727 Ǻ, Z = 8, R = 0.068, Rw = 0.066) shows the presence of square-planar [Cu(Haza)4]2+ and trans-[CuCl2(Haza)2] units connected by bridging Cl− ions, making each Cu atom (4 + 2)-coordinated. Azaindole is bonded to the metal via the pyridine N7 site, whereas the five-membered ring retains its N1—H proton. The azaindole ligands are disordered over two orientations in the [CuCl2(Haza)2] unit. The square-planar Cu species and apical Cl− ions lie on a twofold axis and define along c an infinite chain based on the [Formula: see text] pattern. Equimolar mixtures of azaindole and CuCl2•2H2O yield a brown Cu4OCl6(Haza)4 compound. The crystal structure of its ethyl acetate solvate (monoclinic, P21/c, a = 11.661, b = 22.415, c = 15.077 Ǻ, β = 109.85°, Z = 4, R = 0.048, Rw = 0.054) shows the presence of a tetranuclear cluster consisting of a μ4-oxide ion surrounded by a tetrahedron of Cu(II) atoms bridged in pairs by chlorine atoms. The trigonal-bipyramidal coordination of each copper atom is completed by a monodentate N7-bonded azaindole molecule hydrogen bonded to bridging chlorines via N1—H. The influence of complexation on the infrared spectrum of azaindole is discussed.
Static dielectric constants of the N,N-dimethylformamide/2-methoxyethanol solvent system at various temperatures
Authors: Corradini Fulvio , Marcheselli Luigi , Tassi Lorenzo , Tosi Giuseppe
Published: May 2006

Measurements of static dielectric constants (ε) have been made for binary liquid mixtures of N,N-dimethylformamide (DMF)/2-methoxyethanol (ME) at 19 temperatures ranging from −10 to +80 °C. Some empirical equations of the type ε = ε(T), ε = ε(X1), and ε = ε(T,X1) have been applied to check their validity. The εE values, which refer to the deviation of the dielectric constants of the binaries from the values arising from mole fraction mixture law, have been calculated. Deviations from ideal behaviour have been found to be positive at all temperatures. The positive εE values are attributed to a specific interaction between unlike molecules, which leads to the formation of complexes between DMF and ME in the liquid state. These nDMF•mME complex moieties were found to have the stoichiometric ratios 2:1, 1:1, and 1:2 in the temperature range of −10 to +80 °C.
Mechanism of atomization in a Massmann-type graphite furnace using the basic principles of thermodynamics
Authors: Cathum S.J.
Published: May 2006

A method has been developed to determine the enthalpy of atom formation in a graphite furnace without invoking the order of reaction. The method is based on the equation In (AT) = −ΔH0/RT + C, where A is the peak-height absorbance, T is the absolute temperature, ΔH0 is the enthalpy of atom formation which, for highly endothermic reactions, is considered to be equal to the activation energy, R and C are the universal gas constant and the constant of integration, respectively. The plot of In (AT) vs. 1/T is similar to an Arrhenius-type plot and should yield the activation energy. Atomization mechanisms for eight elements, Cu, Co, Ga, Au, Ni, Fe, Ag, and Al, are proposed by correlating the experimental enthalpy with a very limited set of possible primary reactions. One significant result of this work is that the experimentally observed enthalpy of reaction correlates well with the activation energy, this means no excess energy other than the enthalpy of atomization is involved in the reaction.
Transition metal complexes of 6-amino 2-thiouracil; crystal structure of bis(6-amino-2-thiouracilato)aquazinc(II) dihydrate
Authors: Romero M. Angustias , Sánchez M. Purificación , Quirós Miguel , Sánchez Francisco , Salas Juan Manuel , Moreno Miguel N. , Faure René
Published: May 2006

Complexes of 6-amino-2-thiouracil (H2ATU) with the metal ions Ni(II), Co(II), Zn(II), Cd(II), Cu(I), Ag(I), and Hg(II) have been isolated and characterized by means of elemental analysis, infrared, NMR, and visible spectroscopies, and magnetic measurements. The crystal structure of [Zn(HATU)2(H2O)] • 2H2O has been solved by X-ray diffraction showing that the metal atom is five-coordinated by a water molecule and two ligands that bind through the sulphur and N1 atoms, closing a four-member chelate ring. Diffuse reflectance spectra and magnetic measurements suggest that [Co(HATU)2(H2O)] • 2H2O has a similar structure to that of the zinc complex, and that the nickel complexes [Ni(HATU)2] and [Ni(HATU)2(H2O)2] have, respectively, a tetrahedral and a D4h-distorted octahedral environment.
Nucleic acid related compounds. 77. 2′,3′-Didehydro-2′,3′-dideoxy-2′(and 3′)-methylnucleosides via [3,3]-sigmatropic rearrangements of 2′(and 3′)-methylene-3′(and 2′)-O-thiocarbonyl derivatives and radical reduction of a 2′-chloro-3′-methylene analogue
Authors: Samano Vincente , Robins Morris J.
Published: May 2006

Treatment of 5′-O-(tert-butyldiphenylsilyl)-2′(and 3′)-deoxy-2′(and 3′)-methyleneuridine (and adenosine) derivatives with phenyl chlorothionocarbonate gave the 3′(and 2′)-O-phenoxythiocarbonyl intermediates, which underwent spontaneous [3,3]-sigmatropic rearrangement to give the 2′,3′-didehydro-2′,3′-dideoxy-2′(and 3′)-(phenoxycarbonylthio)methyl analogues. These allylic thioesters were subjected to tributylstannane-mediated hydrodesulfurization and deprotection to give 2′,3′-didehydro-2′,3′-dideoxy-2′(and 3′)-methyluridine (and adenosine). Tributylstannane-mediated hydrodehalogenation of a 2′-chloro-2′,3′-dideoxy-3′-methyleneuridine derivative afforded the 2′,3′-didehydro-2′,3′-dideoxy-3′-methyl product of allylic transposition exclusively.
Stereoselective 1,3-debromination reactions
Authors: Liu Yingchun , Sorensen Ted S. , Sun Fang
Published: May 2006

The metallate, PPN+Cr(CO)4NO−, was used for the 1,3-debrominative reductive cyclization of the (±) and meso isomers of dimethyl 2,4-dibromoglutarate and dimethyl 2,4-dibromo-2,4-dimethylglutarate. In both (±) isomers, the reaction is stereospecific in giving the trans cyclopropane product. In the meso case, the reaction is unselective in the first case, but distinctly favors the cis cyclopropane isomer in the second set of compounds. This (±) and meso pair thus represent the first example of near stereospecificity in the debrominations of both 1,3-dibromo diastereomers. Using an enantiomerically enriched dimethyl 2,4-dibromoglutarate, and determining the absolute stereochemistry of the cyclopropane product, it was found that a strict double inversion [Formula: see text] mechanism is involved in the (±) debromination reaction and presumably also in the near-stereospecific meso case reported above.
Processus élémentaires en solution: Apports et limites de la théorie du complexe activé dans le cas d'un prééquilibre rapide d'échange protonique entre réactifs
Authors: Solastiouk B. , Merlin A. , Deglise X.
Published: May 2006

The kinetic study of N-chlorination reactions in the aqueous phase of different nitrogenized molecules (secondary amines, imides, cyanuric acid, etc… ) shows that the correct chlorination mechanism has not yet been determined because fast proton exchange causes weak acido-basic behaviour of the reagents in aqueous solution. We extended this result to reagents of any charge capable of fast proton exchange. This generalization demonstrates that the indeterminate nature of the mechanism is imposed by the hypotheses of transition state theory.
Silylation de l'isoquinoléine: influence des conditions opératoires sur l'obtention de nouveaux hétérocycles siliciés
Authors: Grignon-Dubois Micheline , Fialeix Michelle , Leger Jean-Michel
Published: May 2006

Silylation of isoquinoline with Me3SiCl/Li/THF or Me3SiCl/Mg/THF reagents has been studied. The choice of the metal and the experimental conditions govern the course of the silylation reaction and lead to new trimethylsilyl heterocyclic compounds.
Spiro-fused β-lactam oxadiazolines by formal [2+2] cycloaddition of 2-imino-Δ3-1,3,4-oxadiazolines to ketenes
Authors: Zoghbi Michel , Warkentin John
Published: May 2006

Sixteen spiro-fused β-lactam oxadiazolines were prepared by treatment of 2-imino-Δ3-1,3,4-oxadiazolines with carboxylic acid chlorides in the presence of triethylamine. Two others were prepared by methylation of β-lactam oxadiazoline anion equivalents with methyl iodide. Isomer ratios, spectra, stereochemistry, and relative thermodynamic stabilities of a pair of diastereomeric β-lactam oxadiazolines are reported. Three cases of failure of the approach are noted, to define some of the limitations. The oxadiazolines are known to undergo thermolysis, in solution, to afford N2, a ketone, and a β-lactam-4-ylidene. Phenyl substitution at C5 of the oxadiazoline ring serves to decrease the thermolysis temperature by about 50 °C but it also leads to the onset of a side reaction that lowers the yield of ylidene. This observation indicates a limitation of the oxadiazoline approach to the generation of β-lactam-4-ylidenes.
A new annulation method. Total syntheses of the sesquiterpenoids (±)-chiloscyphone and (±)-6-epi-chiloscyphone
Authors: Piers Edward , Allan Tse Hoi Lun
Published: May 2006

The efficacy of a new annulation method, developed for the construction of functionalized bicyclic compounds, is illustrated by conversion of the β-keto esters 18 and 19 into the bicyclo[4.3.0]non-1-enes 33 and 34, respectively, and by transformation of 2-methoxycarbonylcyclopentanone (20) into the bicyclo[4.3.0]non-6-ene 38 and the bicyclo[5.3.0]dec-7-enes 48 and 51. Application of the method to total syntheses of the structurally unusual sesquiterpenoids (±)-chiloscyphone (16) and (±)-6-epi-chiloscyphone (17) is described.
Dissociation constants of some alkanolamines
Authors: Xu S. , Otto F.D. , Mather A.E.
Published: May 2006

The dissociation constants of piperidine (PD), 1-piperidineethanol (1–PE), N-methylmorpholine (MML), 4-(2-hydroxyethyl)morpholine (4-EML), and tropine (TP) have been determined at 15.0, 25.0, 35.0, 45.0, and 60.0 °C. The standard state enthalpy changes of the dissociation reaction for these amines have been obtained from the dependence of the dissociation constants upon temperature.
Investigation of the mechanism of alkyl radical elimination from ionised pentenyl methyl and hexenyl methyl ethers by analysis of the collision-induced dissociation mass spectra of C4H7O+ and C5H9O+ ions
Authors: Wright Andrew D. , Bowen Richard D.
Published: May 2006

Collision-induced dissociation (CID) mass spectra are reported for C4H7O+ and C5H9O+ ions generated by loss of an alkyl radical from 11 isomers of C5H9OCH3+• and 8 isomers of C6H11OCH3+• produced by ionisation of alkenyl methyl ethers derived from stable alkenols. The oxonium product ions have acyclic structures (CH=CHCH=O+CH3 for C4H7O+; CH2=CH(CH3)C=O+CH3, CH3CH=CHCH=O+CH3, or CH2=(CH3)CCH=O+CH3 in the case of C5H9O+). Elimination of a methyl radical does not always occur by simple α-cleavage. Expulsion of an alkyl substituent attached to a carbon atom at either end of the C=C double bond also takes place readily; this process sometimes competes with or pre-empts α-cleavage, as is shown by 2H-labelling experiments. Plausible mechanisms for this σ′-cleavage are considered. A route involving a 1,2-H shift to the radical centre of a distonic ion, followed by γ-cleavage of the resultant ionised enol ether, is shown to provide the most accurate unifying description of this unusual fragmentation. The mechanistic significance of this interpretation of the σ′-cleavage is discussed by analysing the reverse reaction (addition of an alkyl radical to a methyl cationated enal) in frontier molecular orbital terms. A comparison is made between the mechanisms by which an alkyl radical is lost from ionised alkenyl methyl ethers by σ′-cleavage and the parallel process starting from ionised carboxylic acids or isomeric distonic ions derived from these CnH2n+1CO2H+• species. Both classes of fragmentation are best understood to occur via γ-cleavage of a distonic ion of general structure R1CH2CH•C+(X)OR2 (R1 = alkyl; X = OH, R2 = H; or X = H, R2 = CH3), thus yielding (R′)• and CH2 = CHC+(X)OR2.
Stereocontrolled construction of 1,7-dimethyl A.B.C.[6.6.6] tricycles. Part II. Transannular Diels–Alder reaction of 14-membered macrocycles containing cis-dienophiles
Authors: Xu Yao-Chang , Roughton Andrew L. , Soucy Pierre , Goldstein Solo , Deslongchamps Pierre
Published: May 2006

The synthesis and transannular Diels–Alder reactions of 14-membered macrocyclic trienes containing a methyl-substituted diene and a methyl-substituted cis-dienophile moiety are described. As a result of the dienophile cis geometry the 1,7-dimethyl A.B.C.[6.6.6] tricycles obtained from the Diels–Alder reaction have a different stereochemistry at the four chiral centers than similar tricycles that were the subject of the preceding paper in this series. Thus trans-syn-cis (TSC) tricycle 30 was the sole product obtained from the trans-cis-cis (TCC) macrocycle 1b. In a similar fashion TCC macrocycle 1d afforded only trans-syn-cis (TSC) tricycle 31. On the contrary, the transannular Diels–Alder reaction in cis-trans-cis (CTC) macrocycle 1a and trans-trans-cis (TTC) macrocycle 1c led to a mixture of the same four tricyclics (30, 32–34) but in different ratio. The above experimental results are rationalized by taking into consideration the interconversion of macrocyclic trienes 1a, 1b, and 1c. Pathways for these interconversions are also proposed.
Solvent effect on the reactivity of monosubstituted phenols towards singlet molecular oxygen (O2(1Δg)) in alkaline media
Authors: Luiz Marta , Gutiérrez María I. , Bocco Graciela , García Norman A.
Published: May 2006

The influence of solvent polarity on the dye-sensitized photooxidation (singlet molecular oxygen (O2(1Δg)) mediated) of a series of para-substituted phenolates was studied. Kinetic evidence obtained shows that the overall and the pure chemical interactions, phenolate–O2(1Δg), depend on the solvent polarity in a different way. This is clearly reflected in the efficiency of O2(1Δg) photooxidation of the substrates: surprisingly, the photooxidation quantum yield increases as the overall quenching rate constant decreases. The substrate photooxidation quantum yields obtained ranged from 0.05 to 0.15, the upper limit corresponding to a moderately polar medium (a benzene–methanol mixture) and the lower to an aqueous medium. We conclude that a high solvent polarity favours only the obtainment of the encounter complex (O2(1Δg)–phenolate), whereas the reactive step is affected in much the same way as those processes where charges are neutralized along the reaction pathway. The results obtained are discussed in terms of a partly polar excited state complex between O2(1Δg) and the phenolates. The rate constant for the reactive pathway in both organic and aqueous media is correlated with the Hammet-type substituent constant R−.
Thermodynamic behaviour of some electrolytes in ethane-1,2-diol from −10 to +80 °C
Authors: Corradini Fulvio , Marcheselli Luigi , Tassi Lorenzo , Tosi Giuseppe , Fanali Salvatore
Published: May 2006

Conductivities of the electrolytes NaBr, NaPi, HPi, NaBPh4, and Ph4PBr in ethane- 1,2-diol were determined in the −10 ≤ t ≤ +80 °C temperature range. The experimental data were analyzed by the Fuoss–Hsia equation, which provides further informative parameters such as the dissociation constant (K) of the ion pairs formed in solution, the limiting equivalent conductivity (Λ0), and the ion-size parameter (å). Thermodynamic behaviour of these electrolytes was derived from analysis of the K values. Single-ion conductivities were evaluated on the basis of the assumption of Ph4PBPh4 as reference electrolyte.
5-Nitroimidazoles. II: Unexpected reactivity of ronidazole and dimetridazole with thiols
Authors: Girard Michel , Clairmont François , Maneckjee Aspi , Mousseau Nicole , Dawson Brian A. , Whitehouse Larry W.
Published: May 2006

Ronidazole and dimetridazole, two important veterinary drugs, were found to react readily in the presence of cysteine, under neutral aqueous conditions, leading to the formation of 5-S-cysteinyl-1-methylimidazole-2-methanol carbamate and 5-S-cysteinyl-1,2-dimethylimidazole respectively through nitro displacement. The reaction products were identified by spectroscopic techniques. The rate of reaction was accelerated by increasing the pH of the medium and was accompanied by a slight change in the product distribution. The reaction was also observed, albeit at a slower rate than that of cysteine, with glutathione, another ubiquitous thiol substrate found in biological systems. While this type of nucleophilic reaction has previously been observed for suitably substituted nitrobenzene derivatives, to the best of our knowledge its occurrence with nitro-heteroaromatics has never been reported. The ready reaction of the parent nitro drugs under such mild aqueous conditions suggests that this may be an alternative path for the formation of nonextractable bound residues in tissues.
Thiophosgene monomer–dimer equilibria in perfluoroalkane solutions: spectroscopic and photophysical implications of self-association in the ground state
Authors: Maciejewski Andrzej , Szymanski Marian , Steer Ronald P.
Published: May 2006

The S2–S0 absorption, emission, and emission excitation spectra of thiophosgene in a perfluoroalkane solvent have been measured as a function of the analytical concentration, C, of the solute. Strong evidence of the formation of van der Waals dimers at 10−4 M < C < 10−1 M and of higher oligomers at C > 10−1 M has been found. Variations in the absorption spectra at C < 10−1 M have been analyzed successfully in terms of a single [Formula: see text] equilibrium, for which the value of K is (5.0 ± 0.4) × 102 M−1 at 295 K. The mole fractions of monomer and dimer have been calculated at each C, and have been used to construct the absorption spectrum of the dimer. Both the monomer and dimer have similar S0 → S1, and S0 → S2 absorption spectra, but the dimer has an oscillator strength that is about twice that of the monomer. The effects of self-aggregation on the S2–S0 emission and excitation spectra have been analyzed. Absorption by the dimer in the ultraviolet also leads to emission, but with a different quantum efficency compared with direct excitation of the monomer. The implications of these observations on the interpretation of the spectroscopy, photophysics, and photochemistry of thiophosgene in solution are assessed.
The negative ion BN−and its role in determining the ground state of BN via photodetachment. An MRDCI ab initio study
Authors: Mawhinney Robert C. , Bruna Pablo J. , Grein Friedrich
Published: May 2006

Extensive multireference CI calculations using a contracted 6s5p3d1f Gaussian basis set on the B and N atoms indicate that the X2∑+, A2Π, and B2∑+ states of BN− are bound with respect to the electron attachment BN + e−. The 14∑+, 14Δ, 14∑−, and 14Π states of BN− are resonant states, all autodetaching into neutral BN. The electron affinity X3Π → X2∑+ is predicted to be 2.84 eV (with a best estimate of 3.10 eV). Other features studied for BN− include the infrared spectrum of the X2∑+ ground state as well as the electronic radiative properties involving the excited states A2Π and B2∑+. The present study supports a X3Π ground state for BN, with a1∑+ lying about 0.03 eV higher. As an alternative to optical spectroscopy on BN, experimental studies of the photodetachment spectrum of BN− are proposed in order to find the ordering and energetic separation of the lowest 3Π and 1∑+ states of BN.
Ab initio predictions of the zero field splittings and the singlet–triplet transition strengths for the n → π* transition of selenoformaldehyde
Authors: Moule D.C. , Chantranupong L. , Judge R.H. , Clouthier D.J.
Published: May 2006

The energy levels of the lower valence and Rydberg states of selenoformaldehyde, CH2Se, have been calculated by the SCF/CI method. Wavefunctions for the ROHF (restricted open shell Hartree–Fock) states were obtained with the Binnings–Curtis double-ζ basis set, augmented with Rydberg and polarization functions. Configuration interaction was applied to the parent configurations, PCMO (parent configuration molecular orbital). Oscillator strengths were evaluated for the allowed electric dipole transitions by the RPA (random phase approximation), and SOPPA (second-order polarization propagator approximation) methods. The spin-orbit contribution to the zero field splitting of the first triplet state, 3A2(n,π*) as well as the oscillator strengths to the three spin components were calculated by perturbation theory. These calculations predict that the Sx, Sy, and Sz components are shifted by −96.091,−96.707, and + 29.167 cm−1, respectively, from their unperturbed position. The oscillator strengths for the three components fx, fy, and fz of the 3A2(n,π*) ← 1A1(g.s.) transition were calculated to be 3.45 × 10−7, 1.15 × 10−7, and 173.0 × 10−7.
Accurate ab initio relativistic Dirac–Fock–Coulomb calculations on heavy atoms using universal Gaussian basis set
Authors: Silva A.B.F. Da , Malli G.L. , Ishikawa Yasuyuki
Published: May 2006

A universal Gaussian basis set (UGBSII) is reported for ab initio relativistic Dirac–Fock calculations on atoms and molecules. The Dirac–Fock–Coulomb energies obtained with this universal Gaussian basis set are presented for the neutral heavy atoms Yb (Z = 70), W (Z = 74), Pb (Z = 82), Ra (Z = 88), and Pu (Z = 94). The total Dirac–Fock–Coulomb energies computed with the UGBSII, for all the systems, are in better agreement with the corresponding numerical finite-difference results than those obtained with our previous universal Gaussian basis set (UGBSI). The UGBSII represents a very accurate Gaussian basis set for relativistic Dirac–Fock atomic calculations.