bluuebunny/crossref_metadata_2025 huggingface dataset:
Tripodal trisamides based on nicotinic and picolinic acid derivatives
Authors: Hoveyda H R , Karunaratne Veranja , Nichols Christopher J , Rettig Steven J , Stephens Ashley KW , Orvig Chris
Published: April 2011

A number of polydentate arylamide ligands have been prepared by coupling various acyclic tripodal or linear polyamines with derivatives of nicotinic and picolinic acids. Two synthetic procedures were utilized; tris{[(2-hydroxynicotinyl)carbonyl]-2-aminoethyl}amine (H3NICTREN) was prepared by Method A, the HOSu/DCC method, and the other arylamides in this study were prepared by Method B, the CDI method. Method A involved the reaction of N-hydroxysuccinimide with 2-hydroxynicotinic acid (in the presence of dicyclohexylcarbodiimide (DCC) as a dehydrative coupling reagent) to form the succinimide ester, followed by reaction with TREN to yield H3NICTREN. Method B involved reaction of a carboxylic acid (2-hydroxynicotinic, 3-hydroxypicolinic, nicotinic, or picolinic acids) with carbonyldiimidazole (CDI) to form the N-acylimidazolide, followed by reaction with the amine (TREN, TAME, spermidine, or TRPN) to yield the desired arylamide. The X-ray structure of 1,1,1-tris{[(3-hydroxypicolinyl)carbonyl]-2-aminomethyl}ethane (H3PICTAME) was determined; crystals of H3PICTAME are monoclinic, a = 10.257(2), b = 15.572(3), c = 15.208(2) Å, β = 96.124(15)°, Z = 4, space group P21/a. The structure was solved by direct methods and refined by full-matrix least-squares procedures to R = 0.041 and Rw = 0.038 for 2506 reflections with I >= 3 sigma (I). In the solid state, H3PICTAME contains an extensive hydrogen-bonding network, with eight intra- and one intermolecular H-bonds per molecule; the ligand is partially preorganized for metal ion chelation. The acid dissociation constants of H3NICTREN and those of 1,1,1-tris{[(2-hydroxynicotinyl)carbonyl]-2-

aminomethyl}ethane (H3NICTAME) have been determined; pKa1 = 11.2 (10.68), pKa2 = 10.7 (10.58), pKa3 = 10.0 (9.71), and pKa4 = 6.25 for H3NICTREN (H3NICTAME); the high phenolic pKa's are consistent with the hydrogen bonding observed in the solid state.Key words: arylamide, hydrogen bonding, preorganization.
Article
Authors: Zhao Yixing , Freeman Gordon R
Published: April 2011

The energy and asymmetry of the optical absorption spectrum of solvated electrons, es- , change in a nonlinear fashion on changing the solvent through the series HOH, CH3OH, CH3CH3OH, (CH3)2CHOH, (CH3)3COH. The ultimate, quantum-statistical mechanical, interpretation of solvated electron spectra is needed to describe the solvent dependence. The previously reported optical spectrum of es- in tert-butanol was somewhat inaccurate, due to a small amount of water in the alcohol and to limitations of the infrared light detector. The present note records the remeasured spectrum and its temperature dependence. The value of the energy at the absorption maximum (EAmax) is 155 zJ (0.97 eV) at 299 K and 112 zJ (0.70 eV) at 338 K; the corresponding values of G epsilon max (10-22 m2 aJ-1) are 1.06 and 0.74. These unusually large changes are attributed to the abnormally rapid decrease of dielectric permittivity of tert-butanol with increasing temperature. The band asymmetry at 299 K is Wb/Wr = 1.8.Key words: optical absorption spectrum, solvated electron, solvent effects, tert-butanol, temperature dependence.
Generalized valence bond study of rotational singlet structures and pi bond energies for systems containing C==C, Si==Si, and C==Si double bonds
Authors: Wang Youliang , Poirier Raymond A
Published: April 2011

Ab initio GVB(6/12)/6-31G** calculations were performed on A2X==YB2 (A, B = H, F; X, Y = C, Si) to obtain the optimized geometries for planar and twisted singlet structures, and to also calculate pi bond energies (rotational barriers). The nature of C-C, Si-Si, and C-Si pi bonds has been investigated. The results show that the C-C pi bond energy (E pi (ethene) = 65.4 kcal/mol) decreases with increasing fluorine substitution. The pyramidalization at the carbon or silicon center for the twisted structures decreases the pi bond energies in the substituted ethenes and their silicon counterparts. The Si-Si (E pi (disilene) = 23.2 kcal/mol) and C-Si (E pi (silaethene) = 31.6 kcal/mol) pi bonds become much weaker. Fluorine substitution stabilizes both the diradical and the dipolar twisted singlet structures.Key words: pi bond energy, ab initio calculations, generalized valence bond, fluorine substitution, disilene, and silaethene.
An unusual ground-state stabilization effect and origins of the alpha-effect in aminolyses of Y-substituted phenyl X-substituted benzoates
Authors: Um Ik-Hwan , Chung Eun-Kyung , Lee So-Mi
Published: April 2011

Second-order rate constants have been measured spectrophotometrically for the reactions of X-C6H4CO2C6H4-Y with a series of primary amines in H2O containing 20 mol% DMSO at 25.0 ± 0.1°C. The reactivity increases as the substituent (X and Y) becomes a stronger electron-withdrawing group. The sigma + constants give better Hammett correlation than sigma constants for the reactions of 4-nitrophenyl X-substituted benzoates with glycylglycine (glygly) and hydrazine (NH2NH2), indicating that the ground-state stabilization effect is unusually significant on the reaction rates. The reactions of X-C6H4CO2C6H4-Y with glygly and NH2NH2 appear to proceed through the same mechanism, but the degree of leaving-group departure and the negative charge developed in the acyl moiety at the rate-determining TS is considered to be more significant for the glygly system than the NH2NH2 system based on ßlg and rho X values. The magnitude of the alpha -effect is observed to be not always dependent on the ßnuc value but dependent on the electronic nature of the substituent X and Y, i.e., an electron-donating substituent increases the alpha -effect, while an electron-withdrawing one decreases the alpha -effect. The present study has led to the conclusion that the ground-state effect is important for the reaction rates but it is not solely responsible for the alpha -effect, and the intramolecular H-bonding interactions (4) are proposed for the cause of the increasing or decreasing alpha -effect trends observed in the present system.Key words: alpha -effect, intramolecular H-bonding interaction, ground-state stabilization effect.
Microwave-enhanced metal- and acid-catalysed hydrogen isotope exchange reactions
Authors: Barthez Jean M , Filikov Anton V , Frederiksen Lottie B , Huguet Marie-Loire , Jones John R , Lu Shui-Yu
Published: April 2011

The benefits of using microwaves to accelerate the rates of hydrogen isotope exchange reactions are illustrated by reference to heterogeneous and homogeneous metal-catalysed reactions as well as to homogeneous acid-catalysed reactions. The results show that good incorporation of deuterium is obtained in very short time intervals, typically <20 min, and that the approach has considerable potential for the labeling of a wide range of compounds with both deuterium and, by implication, tritium.Key words: microwave enhanced deuteriations, hydrogen isotope exchange.
Kinetic effects induced by cellulose on water-catalyzed reactions. Hydrolysis of 2,4-dinitrophenyl cellulose xanthate and some sugar xanthate ester analogues
Authors: Humeres Eduardo , Sequinel Luiz Fernando , Nunes Mauricéa , Oliveira Célia MS , Barrie Patrick J
Published: April 2011

The hydrolysis of 2,4-dinitrophenyl cellulose xanthate (CelXDNP) was studied in 10% v/v aqueous ethanol at 25°C and μ = 0.1 (KCl). The water-catalyzed hydrolysis showed that, as for p-nitrobenzyl cellulose xanthate, it occurs through two parallel reactions with rate constants k'H2O = 4.40 x 10-3 s-1 for the fast hydrolysis, and k''H2O = 6.90 x 10-5 s-1 for the slow hydrolysis. The entropy of activation of the fast hydrolysis was 0.7 ± 1.8 cal K-1 mol-1. External nucleophiles such as hydroxide and simple amines show simple first-order kinetics. The spontaneous hydrolysis of CelXDNP in acetone-water mixtures indicates that the fast reaction does not occur through water polymers and that for water molarity higher than 30 M there are no acetone molecules (or very few) in the highly ordered cybotactic region of cellulose. The spontaneous hydrolysis of methyl 4,6-O-benzylidene- α -D-glucopyranoside 3-(S-p-nitrobenzyl-xanthate) although is faster than the 6-isomer, it is slower than the fast hydrolysis of p-nitrobenzyl cellulose xanthate (CelXNB). Also Δ Sdouble dagger is highly negative (-41.0 cal K-1 mol-1), as it is for alkyl and sugar analogues. Only for the fast hydrolyses of CelXDNP and CelXNB is the entropy of activation almost zero. It is concluded that there is no neighbouring OH effect on the fast hydrolysis of cellulose xanthate esters. Key words: hydrolysis, water catalysis, cellulose xanthate esters, methyl glucose, xanthate esters, neighbouring OH effect.
Article
Authors: Molina Patricia Gabriela , Zón María Alicia , Fernández Héctor
Published: April 2011

The acid dissociation constants for alternariol monomethyl ether (AME), altertoxin-I (ATX-I), and alternariol (AOH), three of the Alternaria alternata genus mycotoxins, are estimated through conventional UV-VIS spectroscopic studies at different pH values. Experimental variations of absorbance as a function of pH at a given wavelength were fitted by using the exact equations that describe the systems studied. Good agreement between the experimental absorbance vs. pH plots and the curves generated by the fitting process was found. A comparison between the acidic constants for the first step of dissociation of these mycotoxins and the value for the acidic constant of phenol is discussed by assuming that phenolic structures are responsible for the acidity of these mycotoxins.Key words: mycotoxins, alternariol monomethyl ether, altertoxin-I, alternariol, acidic constants.
The reaction of S-nitrosothiols with thiols at high thiol concentration
Authors: Dicks Andrew P , Li E , Munro Andrew P , Swift Helen R , Williams D Lyn H.
Published: April 2011

Reactions of S-nitrosothiols (RSNO) with their corresponding thiols (RSH) present in a large excess (>20-fold) proceed readily to give the disulfide. Ammonia is formed together with some nitrite anion, and these constitute >90% of the "nitrogen" products. This is in marked contrast with the reaction at low thiol concentration, where nitric oxide is the major initial "nitrogen" product, which is rapidly converted in the presence of oxygen in water to nitrite anion. Also in marked contrast to the "low thiol concentration" reaction, the reaction at high thiol concentration is not affected by added Cu2+, nor by the metal-ion scavenger EDTA. Kinetically all reactions were excellent first-order processes, and the reactions were also strictly first order in thiol concentration. A large range of nitrosothiols were studied and the generality of the reaction established. Some reactions of RSNO with other thiols (R'SH) were examined and the results readily interpreted in terms of a prior rapid equilibrium transnitrosation. The pH dependence for the reaction of S-nitrosocysteine with cysteine clearly showed that the reactive species is the cysteine thiolate anion. The results are discussed along with those of two other recent reports of these reactions, in terms of thiolate attack initially at the nitroso nitrogen atom, and subsequently at sulfur atoms, eliminating RSSR and yielding hydroxylamine, which is rapidly reduced by thiolate ion to ammonia. The results are also discussed in connection with the release of NO from nitrosothiols and with the important biological consequences, both for the in vivo reactions of NO and for the potential of nitrosothiols as NO-releasing drugs for medical use.Key words: S-nitrosothiols, thiols, nitric oxide, ammonia, nitrous oxide.
The conformational equilibrium ofN,N'-bis[(S)-1-phenylethyl]-thiourea and its solvent dependence, studied by NMR and CD spectra and by X-ray crystallography
Authors: Rang Knut , Sandström Jan , Svensson Christer
Published: April 2011

Favoured conformations of N,N'-bis[(S)-1-phenylethyl]thiourea (1) have been predicted by empirical force-field calculations, and the corresponding rotational strengths for the important transitions in the near UV region have been calculated. Experimental studies by1H NMR, IR, UV, and CD spectroscopy have permitted the assignment of a conformation denoted E,Za to the conformer, which dominates in nonpolar solvents, while hydroxylic solvents were found to favour conformations of Z,Za type. The E,Za conformer is also predicted by force-field calculations to be the most stable one.Key words: circular dichroism, conformation, crystal structure, dynamic NMR spectroscopy, N,N'-bis[(S)-1-phenylethyl]thiourea.
Article
Authors: Jiang Zhi-Hua , Argyropoulos Dimitris S
Published: April 2011

The reactivity of lignin model compounds under Mannich conditions with piperidine and formaldehyde was studied. The piperidinomethyl group was confirmed to be introduced quantitatively at sterically unhindered positions ortho to a phenolic hydroxyl when the substrate was allowed to react under ambient conditions. The sequential application of the Mannich reaction followed by quantitative 31P NMR on a variety of lignin samples allowed the amounts of various aromatic groups bearing free phenolic hydroxyls to be determined. More specifically, the technique allowed the quantification of para-hydroxyl phenols, catechols, guaiacols, and phenols bearing C5 or C6 substituents. The quantitative reliability of the technique was also examined with a series of dissolved kraft lignins, isolated at various degrees of delignification. The data were found to favourably compare to those obtained by permanganate oxidation.Key words: lignin, lignin model compounds, Mannich reaction, nuclear magnetic resonance (NMR), phenols, phosphorus, piperidine, quantitative analysis.
Kinetics of the self-assembly of bold alpha -cyclodextrin [2]pseudorotaxanes with polymethylene threads bearing quaternary ammonium and phosphonium end groups
Authors: Lyon Angela P , Banton Nicola J , Macartney Donal H
Published: April 2011

The kinetics and mechanism of the formation and dissociation of a series of [2]pseudorotaxanes, comprised of α -cyclodextrin (α -CD) as the cyclic component and the ([Me3N(CH2)nNMe3]2+ (n = 8-12), [Me2EtN(CH2)10NEtMe2]2+, and [Me3P(CH2)10PMe3]2+) dications as the threads, were determined by means of 1H and 31P NMR in aqueous solution. The length of the polymethylene chain (n) of the thread, which has a minor effect on the rate constant for pseudorotaxane formation, is important in the kinetics of the dissociation reactions, with the longer, more hydrophobic chains resulting in slower pseudorotaxane dissociation. The replacement of one methyl substitutent by an ethyl group in each of the end groups on the [Me3N(CH2)10NMe3]2+ thread results in a 30-fold decrease in the formation rate constant. Replacements, by ethyls, of two or all of the methyl substitutents prevent the formation of the pseudorotaxane, even after prolonged heating. The pseudorotaxane containing the {Me3P(CH2)10PMe3.; α-CD}2+ thread forms only at elevated temperatures by a slippage mechanism, and the rate constant for its self-assembly at 75°C (8 x 10-5 M-1 s-1) is more than 106 smaller than the rate constant at 75°C (200 M-1 s-1) extrapolated for the corresponding {Me3N(CH2)10NMe3 . α -CD}2+complex. The enthalpies and entropies of activation for the formation and dissociation of the [2]pseudorotaxanes decrease with an increase in the size and hydrophobicity of the end groups, suggesting a reduced role of desolvation of the quaternized atoms in the threading or dethreading processes.Key words: pseudorotaxane, α -cyclodextrin, kinetics, self-assembly, slippage, supramolecular.
Dephosphorylation and aromatic nucleophilic substitution in nonionic micelles. The importance of substrate location
Authors: Bunton Clifford A , Foroudian Houshang J , Gillitt Nicholas D , Whiddon Christy R
Published: April 2011

Reactions of OH-and F-with p-nitrophenyl diphenyl phosphate (pNPDPP) are inhibited by very dilute dodecyl (10) and (23) polyoxyethylene glycol (C12E10and C12E23, respectively), but rate constants become independent of surfactant concentrations at concentrations above the critical micelle concentration. Low charge density anions, e.g., ClO4-, inhibit and low charge density cations, e.g., (n-C7H15)4N+, accelerate reactions, probably by controlling concentrations of nucleophiles in the palisade layer. Diphenyl phosphorofluoridate, generated by attack of F-, is not detected but is rapidly hydrolyzed to phenyl phosphorofluoridate or diphenyl phosphate ion with loss of phenol or F-. The products are different in DMSO containing modest amounts (<15 vol%) of water and no surfactant and are phenyl phosphorofluoridate and difluorophosphate ions generated by attack of F-on the initial phosphorofluoridate. These differences are consistent with the micellar palisade layer being water-rich. Although the nonionic surfactants do not intervene nucleophilically in reactions of pNPDPP, considerable amounts of ether are formed in the reaction of 2,4-dinitrochlorobenzene (DNCB), in C12E10and C12E23at high pH by attack of alkoxide ion with the relatively hydrophilic DNCB located close to the micellar surface. The differences in the chemistries of reactions of pNPDPP and DNCB appear to be associated largely with differences in locations of these substrates in the nonionic micelles.Key words: fluoridates, p-nitrophenyl diphenyl phosphate, 2,4-dinitrochlorobenzene, nonionic micelles, palisade layer.
Heteroleptic gold complexes with 4,5-diselenolate-1,3-dithiole-2-thione (C3C3Se22-, dsit)
Authors: Cerrada Elena , Laguna Mariano
Published: April 2011

The reaction of (NBu4)2[Zn(dsit)2] (dsit = 4,5-diselenolate-1,3-dithiole-2-thione) with AuClL in a 1:4 ratio leads to [Au2(dsit)L2] (L = PPh3 (1), PPh2Me (2)), transferring the diselenolate group. Complex 2 reacts further with equimolar amounts of [Au(OClO3)L], affording the trinuclear complex [Au3(dsit)(PPh2Me)3]ClO4 (3). When the reaction of (NBu4)2[Zn(dsit)2] with [Au2Cl2(P-P)] was carried out in a 1:2 ratio, complexes [Au2(dsit)(P-P)]n (P-P = dppe (1,2-bis(diphenylphosphine)ethane), n = 1 (4); P-P = dppm (bis(diphenylphosphine)methane), n = 2 (5)) are obtained. Anionic derivatives such as (NBu4)[Au(dsit)2] (6) could be obtained by reaction of the zinc complex with (NBu4)[AuBr4] in a 1:1 ratio while other gold complexes require a tin complex as ligand transfer. So [Sn(dsit)Me2] (7), obtained from the zinc complex and [SnCl2Me2], reacts with PPN[Au(C6F5)Cl] (PPN = bis(triphenylphosphine)iminium) or [AuBr2(S2CNR2)] affording [Au(dsit)(S2CNR2)] (R = Me (8); R = Bz (9)) or (PPN)2[Au2(dsit)(C6F5)2] (10), respectively. Electrochemical oxidation of 6 gives (NBu4)0.4[Au(dsit)2] (11) and the reaction with (TTF)3(BF4)2 (TTF = tetrathiafulvalene) affords (TTF)[Au(dsit)2] (12). Electrical conductivities of these complexes at room temperature in compacted pellets are 3 × 10-8 (11) and 5 × 10-3 (12) S cm-1.Key words: diselenolate, dsit, gold complexes, tin complexes.
On the influence of secondary structure on the α-C→H bond dissociation energy of proline residues in proteins: a theoretical study
Authors: Block D A , Yu D , Armstrong D A , Rauk A
Published: April 2011

Ab initio computations (B3LYP/6-31G(D), coupled with isodesmic reactions) were used to predict αC→H bond dissociation energies (BDEs) for proline as a residue in a model peptide, intended to mimic the environment in proteins. The environment was further constrained to mimic common proline positions in turns of different types. The BDEs were found to be very dependent on the structural constraints imposed by the turn type, implying different structure-mediated susceptibilities to free radical damage to proline residues. Unnatural repair of proline (inversion of chirality) was found to be thermodynamically unfavourable. The predicted BDEs for the proline αC→H bond, in kJ mol-1, to an estimated accuracy of ±10 kJ mol-1 are as follows: fully optimized trans rotamer, 368.6; fully optimized cis rotamer, 357.7; ß turn type I, 380.7; ß turn type II, 397.8; ß turn type II', 385.4; ß turn type VIa, 374.0; ß turn type VIb, 355.0. Key words: proline, ß -turns, free radical, bond dissociation energy, molecular structure, oxidative damage.<
Article
Authors: Wentzell Peter D , Wang Ji-Hong , Loucks Leon F , Miller Kelli M
Published: April 2011

A new procedure to extract spectral and concentration information for unknown components in a series of mixture spectra is described. The algorithm, a form of self-modeling curve resolution (SMCR), requires no assumptions about the mixture data except for linear additivity and non-negativity of component spectra, and non-negativity of concentrations. It provides permissible bands for the spectra and concentration profiles of individual components in the mixtures and is applicable to problems in chromatography, equilibrium studies, and kinetics, among others. The new method employs a nonlinear optimization procedure and is substantially more efficient than similar methods that have been previously reported. The SMCR algorithm is demonstrated by applying it to the autocatalytic oxidation of oxalic acid by permanganate in the presence of sulfuric acid. The reaction was monitored spectroscopically between 200 and 1000 nm under four different reaction conditions. Results verified the presence of two reaction intermediates previously proposed in the literature and confirmed their spectral properties. Concentration profiles for the intermediates are described.Key words: curve resolution, kinetics, chemometrics, permanganate, oxalic acid.
Three-dimensional H-bonded supramolecular complexes. Synthesis and crystal structures of [Mn(μ-bpe)(H2O)4](ClO4)2·4(bpe)·2H2O and [M(μ-bpe)(bpe)2(H2O)2](ClO4)2·bpe·H2O, where M = Zn, Ni and bpe = 1,2-bis(4-pyridyl)ethane
Authors: Rochon Fernande D , Andruh Marius , Melanson Robert
Published: April 2011

The reaction of Mn(II), Zn(II), and Ni(II) perchlorate with 1,2-bis(4-pyridyl)ethane (bpe) was studied and the products were characterized by IR spectroscopy and X-ray diffraction methods. The zinc and nickel compounds are isomorphous with a metal-bpe molar ratio of 1:4, while the manganese compound is quite different with a Mn-bpe ratio of 1:5. [Mn(μ-bpe)(H2O)4](ClO4)2·4(bpe)·2H2O (1) is monoclinic, C2/c, with a = 30.141(7), b = 13.993(4), c = 19.765(4) Å, β = 129.41(1)°, and R = 0.059. The Mn atom is located on an inversion centre and forms an infinite chain with the bridged bpe ligand. The four other bpe molecules are hydrogen-bonded to the aqua ligands and form an extensive H-bonded interpenetrating 3D network. The bonds Mn-N = 2.316(3) Å and Mn-OH2 = 2.148(4) and 2.178(3) Å. [Zn(μ-bpe)(bpe)2(H2O)2](ClO4)2·bpe·H2O (2) crystallized in the P21/c space group, with a = 14.609(4), b = 21.164(5), c = 16.482(5) Å, β = 114.69(2)°, and R = 0.058. For [Ni(μ-bpe)(bpe)2(H2O)2](ClO4)2·bpe·H2O (3), a = 14.511(6), b = 21.216(7), c = 16.467(7) Å, β = 114.76(3)°, and R = 0.065. The aqua ligands are located trans to each other. The bridged bpe ligands form an infinite chain with the metal atoms. Two other terminal bpe ligands are coordinated to the metal in trans positions to each other. These ligands form H-bonds with the aqua ligands of other chains resulting in an H-bonded 3D network. The Zn-N distances vary from 2.151(3) to 2.185(4) Å while the Ni-N bonds are between 2.098(10) and 2.126(11) Å. The bonds Zn-OH2 = 2.125(4), 2.161(4) Å and Ni-OH2 = 2.077(11), 2.110(11) Å. Key words: crystal structure, manganese complex, zinc complex, nickel complex, 1,2-bis(4-pyridyl)ethane.
Electrochemical behaviour of silver in aqueous chromate solutions
Authors: Abd El Rehim Sayed S , Ibrahim Magdy AM , Hassan Hamdy H , Amin Mohammed A
Published: April 2011

The electrochemical behaviour of silver was studied under cyclic voltammetry and chronoamperometry conditions in aqueous K2CrO4 solutions. The forward cyclic voltammograms exhibited one oxidation peak, A1, due to the formation of Ag2CrO4. The height of the anodic peak, A1, increases with increasing chromate concentration, temperature, and scan rate. The solid films formed on the anode surface have been examined by X-ray diffractometry. The reverse voltammograms exhibited two reduction peaks, C1 and C2, indicating the formation of two distinct surface layers of Ag2CrO4, an inner compact layer reduced in peak C1 and an outer powdery layer reduced in peak C2.Key words: silver electrode, cyclic voltammetry, K2CrO4 solutions.
Transition metal azolates from metallocenes. Part 3: Polymeric manganese(II) and nickel(II) pyrazolates; synthesis, characterization, and magnetochemistry
Authors: Storr Alan , Summers David A , Thompson Robert C
Published: April 2011

Direct reactions of nickelocene and manganocene with molten pyrazoles in an inert atmosphere or under vacuum have led to the isolation of the following metal pyrazolate compounds: [Ni(4-Xpz)2]x (where X = H, Cl and pz = pyrazolate); [M(4-Xdmpz)2]x (where X = H, CH3, Cl, Br and M = Ni, Mn anddmpz = 3,5-dimethylpyazolate); and [Mn(4-Xpz)2(4-XpzH)]x (where X = Cl, Br and pzH = pyrazole). On the basis of indirect evidence all compounds are considered to have extended chain structures with metal ions linked in chains by double bridging pyrazolates. The two [Ni(4-Xpz)2]x compounds are diamagnetic while all others are paramagnetic. Variable temperature magnetic susceptibility studies on the paramagnetic compounds reveal antiferromagnetic coupling between neighbouring metal ions. Analysis of the magnetic data (Hamiltonian of the form H = -2J sum Si·Sj) yields values of the exchange coupling constant J of -14 to -17 cm -1for the [Ni(4-Xdmpz)2]x compounds, -1.2 to -2.1 cm-1 for the [Mn(4-Xdmpz)2]x compounds, and -0.41 cm-1 for the [Mn(4-Xpz)2(4-XpzH)]x compounds.Key words: nickel, manganese, manganocene, nickelocene, pyrazolates, antiferromagnetic exchange.
Article
Authors: Yuasa Yoshifumi , Yuasa Yoko , Tsuruta Haruki
Published: April 2011

(2S)-3-(tert-Butylsulfonyl)-2-benzylpropionic acid 1, which is used as the N-terminal chiral building block of renin inhibitors, is synthesized from (Z)-2-(tert-butylsulfonylmethyl)-3-phenyl-2-propenoic acid 6 by enantioselective hydrogenation using Ru-BINAP complexes in the presence of triethylamine with 84% ee. The enantioselective hydrogenation of a substrate with sulfone functionality is first reported.Key words: hydrogenation, enantioselectivity, Ru-BINAP, oxidation, renin inhibitor.
Photochemical nucleophile-olefin combination, aromatic substitution (photo-NOCAS) reaction: methanol, beta-myrcene, and 1,4-dicyanobenzene. Intramolecular cyclization of an ene-diene radical cation
Authors: Arnold Donald R , McManus Kimberly A
Published: April 2011

The photochemical nucleophile-olefin combination, aromatic substitution (photo-NOCAS) reaction of methanol, 7-methyl-3-methylene-1,6-octadiene ( β-myrcene, 1), and 1,4-dicyanobenzene yields five 1:1:1 adducts:cis-2-(4-cyanophenyl)-4-(1-methoxy-1-methylethyl)-1-methylenecyclohexane (15), trans-2-(4-cyanophenyl)-4-(1-methoxy-1-methylethyl)-1-methylenecyclohexane (16), 1-(4-cyanophenylmethyl)-4-(1-methoxy-1-methylethyl)cyclohexene (17), 4-[4-methoxy-3,3-dimethylcyclohex-(E)-1-ylidenyl]methylbenzonitrile (18), and 4-(1-vinyl-4-trans-methoxy-3,3-dimethylcyclohexyl)benzonitrile (19). All of these adducts are cyclic; variation in the product ratio as a function of methanol concentration indicates cyclization is occurring, 1,6-endo, with both the initially formed radical cation and with the intermediate β-alkoxyalkyl radicals. Evidence based upon comparison of the ionization and oxidation potential of β-myrcene with model alkenes and with conjugated dienes indicates the initial electron transfer involves the trisubstituted mono alkene moiety; the diene moiety, mono-substituted at a nodal position, has a higher oxidation potential. High-level ab initio molecular orbital calculations (MP2/6-31G*//HF/6-31G*) provide useful information regarding the nature (relative energies and charge and spin distribution) of the intermediate radical cations, which supports the proposed reaction mechanism. Key words: photoinduced electron transfer, radicals, radical cations, β-myrcene, cyclization.
Synthesis and comparative study of three monomeric boronates by spectroscopic methods and X-ray crystallography
Authors: Höpfl Herbert , Sánchez Mario , Farfán Norberto , Barba Victor
Published: April 2011

Three boronates have been synthesized by reaction of tridentate azomethine ligands derived from salicylaldehyde with phenylboronic acid. The [5.4.0], [4.4.0], and [4.3.0]heterobicyclic structures obtained have been characterized by NMR spectroscopy and X-ray crystallography, whereby it could be proved that the [4.4.0]heterobicycle is the most stable one. Unexpectedly the salicylideneaminoethanol derivative does not lead to a macrocyclic structure, although molecular modeling reveals high ring strain for the monomeric product that is actually obtained.Key words: boronates, boron complexes, tridentate azomethine ligands, phenylboronic acid, X-ray crystallography.
Ryanoids and related compounds. Isolation and characterization of four minor ryanoids from the plant Ryania speciosa Vahl: 8,9-didehydroryanodine, 10-O-acetylryanodol, 3-O-benzoylryanodol, and a first natural 4-deoxyryanoid: 4-deoxy-8ax-hydroxy-10-O- methyl-10-epiryanodine (4-deoxyester A)
Authors: Ruest Luc , Berthelette Carl , Dodier Marco , Dubé Laurence , St-Martin David
Published: April 2011

In a search for minor ryanoids from the plant Ryania Speciosa Vahl, we characterized four new members of that family of natural compounds: 8,9-didehydroryanodine (5), 10-O-acetylryanodol (6), 3-O-benzoylryanodol (9), and the first member of the natural 4-deoxyryanoid series: 4-deoxy-8ax-hydroxy-10-O-methyl-10-epiryanodine (10) (4-deoxyester A).Key words: natural ryanoids, ryanodine, deoxyryanoids, ryanodol derivatives.
Quantum molecular similarity. Part 2: The relation between properties in BCP space and bond length
Authors: O'Brien Sean E , Popelier Paul LA
Published: April 2011

In previous work we introduced an abstract space of bond critical point properties, called BCP space, to measure the similarity between molecules. In this contribution we critically examine the dependence of properties in BCP space on equilibrium bond length, a topic which has been extensively investigated in the literature. For that purpose we have designed a data set of 57 molecules yielding 731 bond critical points. We confirm the existence of local linear relationships provided the bonds vary little in their chemical surroundings. Such relationships break down completely for larger subsets of bond critical points encompassing a wider variety of bonds. The patterns observed in the global picture show so little correlation that one may safely conclude that BCP properties cannot be trivially recovered or even predicted by knowledge of bond length alone.Key words: atoms in molecules, molecular similarity, bond critical points.
Preparation of N-aryl-4-oxo-4H-1-benzothiopyran-2-acetamides from trilithiated acetoacetanilides and lithiated methyl thiosalicylate
Authors: Angel April J , Finefrock Anne E , French Kristen L , Hurst Douglas R , Williams Angela R , Rampey Mary E , Studer-Martinez Shannon L , Beam Charles F
Published: April 2011

Acetoacetanilide and substituted acetoacetanilides were trilithiated in excess lithium diisopropylamide, and the polylithiated intermediates were regioselectively condensed with lithiated methyl thiosalicylate, followed by acid cyclization to afford substituted N-aryl-4-oxo-4H-1-benzothiopyran-2-acetamides (thiochromone-2-acetamides).Key words: thiochromones, strong-base multiple anion synthesis, Claisen-type condensation.
Can there be more than a single definition of an atom in a molecule?
Authors: Bader RFW
Published: April 2011

Arguments are presented that the theoretical definition of an atom in a molecule or of a functional grouping of atoms that derive from experimental chemistry must be unique. Definitions based on the orbital model or, as recently proposed, in terms of domains defined by isovalued density envelopes fail for a number of reasons, among them being their failure to enable a quantum mechanical description of the atomic or group properties. Chemistry is concerned with the observation and measurement of properties. Definitions that do not predict the measurable, additive properties found for atoms in molecules fail to recover the essence of the atomic concept and can play no operational or predictive role in chemistry. Atoms exist in real space and their form determines their properties. There is but a single definition for an atom, free or bound, that meets this essential requirement.Key words: atoms in molecules, proper open systems.
Formation and behavior of intramolecular N-(styrylalkyl)aniline exciplexes
Authors: Lewis Frederick D , Wagner-Brennan Jill M , Miller Alan M
Published: April 2011

The photophysical and photochemical behavior of several aniline spacer - styrene molecules in which the spacer is a short polymethylene chain has been investigated. Long wavelength excitation selectively excites the aniline chromophore which undergoes intramolecular quenching by the styrene to yield an intramolecular exciplex. Exciplex fluorescence is observed for both of the tertiary anilines investigated and for one of the three secondary anilines. More rapid exciplex formation in nonpolar vs. polar solvents is attributed to the necessity of desolvating the polar aniline singlet in polar solvents. Intramolecular adduct formation is the major photochemical process for the secondary anilines, but is highly inefficient for the tertiary anilines. The behavior of the intramolecular aniline-styrene exciplexes is compared to that of the styrene-amine exciplexes previously investigated in this laboratory.Key words: intramolecular exciplex, fluorescence, aniline, styrene.
C(2)-H isotopic exchange in coordinated imidazoles revisited. The case of the [Co(NH3)5ImH]3+ ion
Authors: Clark Charles R , Blackman Allan G , Grimmett M Ross , Mobinikhaledi Akbar
Published: April 2011

The temperature dependence of the acid ionization constants of [Co(NH3)5ImH]3+ in H2O (I = 1.0 M (NaClO4)): pKa (°C) = 10.10 ± 0.04 (25.0), 9.92 ± 0.03 (30.0), 9.82 ± 0.02 (35.0), 9.62 ± 0.03 (40.0), and [Co(ND3)5ImD]3+ in D2O (I = 0.35 M (NaClO4)): pKa (°C) = 10.58 ± 0.06 (25.0), 9.46 ± 0.08 (60.0) is reported. Observed first-order rate constants for H/D exchange at C-2 in [Co(ND3)5ImD]3+ over the pD range 8.08-11.20 (60.0°C, I = 0.35 M (NaClO4)) follow an equation of the form: kobs = kODKw/(aD+ + Ka)γ±, with kOD (0.27 ± 0.06 M-1 s-1) corresponding to the rate constant for OD--catalyzed abstraction of H-2 in [Co(ND3)5ImD]3+, and Ka ((2.8 ± 0.7) × 10-10 M, pKa = 9.55 ± 0.13) to the acid ionization constant of this species. No evidence was found for a pathway to H/D exchange in the imidazolate moiety of [Co(ND3)5Im]2+.Key words: kinetics, H/D-exchange, imidazole, metal complex.
Préparation du chlorure mercureux et mesure de sa pression de vapeur
Authors: Awitor Komla Oscar , Bernard Laurent , Bonnin Olivier , Coupat Bernard , Fournier Jean Paul , Verdier Philippe
Published: April 2011

Mercurous chloride (Hg2Cl2), or calomel, was prepared by direct synthesis from commercial mercury and mercuric chloride. The mercuric chloride mass fraction in the synthesized material is particularly small. The mercurous chloride vapor pressure was measured between 353 and 453 K by the Knudsen cell method. In order to obtain the mercurous chloride vapor pressure, we took into account the dissociation in the vapor phase into mercury and mercuric chloride. Our results are compared with those in the literature.Key words: mercurous chloride, calomel, vapor pressure, mass spectrometry, Knudsen cell.
Energétique des liaisons inter- et intramoléculaires dans les trois acides carboxypyridiniques (acides picolinique, nicotinique et isonicotinique)
Authors: Sabbah Raphaël , Ider Safi
Published: April 2011

The present work is concerned with a thermodynamic study of the three carboxypyridinic acids (general formula: C6H5NO2). It was achieved using four techniques: combustion calorimetry of small amounts of substance (a few milligrams), sublimation calorimetry, heat capacity measurements by the drop method, and differential thermal analysis. From this study, it was possible to determine the enthalpies of combustion, sublimation, formation, and fusion of these compounds and their triple point temperatures; to discuss the relative stability of the three molecules and to compare it to those of other pyridine derivatives; to determine the resonance energy and to compare it to those of other pyridine derivatives; to determine the atomization enthalpies of our compounds and an enthalpy value for the bonds of the group (C-N=C); to determine the nature and the enthalpy of the intermolecular bonds.Key words: thermodynamics, calorimetry, differential thermal analysis, carboxypyridinic acids, picolinic acid, nicotinic acid, isonicotinic acid, enthalpy of combustion, of sublimation, of fusion, of formation, of inter- and intramolecular bonds, resonance energy, triple point temperature, reference material.
Facile formation of azines from reactions of singlet methylene and dimethylcarbene with precursor diazirines: theoretical explorations
Authors: Shustov Gennady V , Liu Michael TH , Houk K N
Published: April 2011

The reactions of the singlet methylene (1a) and dimethylcarbene (1b), with their diazirine precursors, diazirine (2a), and dimethyldiazirine (2b), have been studied theoretically using ab initio and density functional theory. The reaction has no activation barriers for the parent system (1a + 2a) and proceeds via a reactive complex and a transition state with a small negative enthalpy of activation Δ Hnot =298 = -1.1 kcal mol-1, ΔSnot =298 = -34.4 cal mol-1 K-1, ΔG°298 = 9.2 kcal mol-1) for the dimethyl derivatives (1b + 2b). The formation of N-methylene diazirinium ylides (3a,b) is exothermic by 64-80 kcal mol-1. The isomer, 1,3-diazabicyclo[1.1.0]butane (4a), is more stable (5-12 kcal mol-1) than isomer 3a, but can neither be formed by direct thermal reaction of 1a with 2a nor undergo the direct rearrangement into formaldazine (5a). The rearrangement of ylides 3a,b into azines 5a,b proceeds by conrotatory C3-N1 ring opening. The predicted activation barrier of ca. 15 kcal mol-1 for the ring opening in ylide 3b is in excellent agreement with experimental data. The formation of pyridinium ylides from carbenes and pyridine is also studied.Key words: diazirinium ylide, ab initio MO (molecular orbital) theory, density functional theory, pyridinium ylide, CIS (singles configuration interaction) transition energies.
Ether synthesis using trifluoromethanesulfonic anhydride or triflates under mild reaction conditions
Authors: Nishiyama Tomihiro , Kameyama Hideaki , Maekawa Hideki , Watanuki Kouhei
Published: April 2011

Reactions of magnesium alkoxides with trifluoromethanesulfonic anhydride or triflates gave corresponding symmetrical or unsymmetrical ethers in moderate to good yields under mild reaction conditions. The scope and limitations of the reaction are discussed.Key words: ethers, trifluoromethanesulfonic anhydride, triflates, magnesium alkoxides.
The adsorption of carbon dioxide by tertiary alkanolamines
Authors: Gomes M. Teresa SR , Barros Cristina MF , Santana-Marques M Graça O. , Oliveira João ABP
Published: April 2011

The quantification of gaseous carbon dioxide, CO2, adsorbed by tertiary alkanolamines was performed using a quartz crystal microbalance. Carbon dioxide was injected over piezoelectric quartz crystals coated with different amounts of N,N,N',N'-tetrakis(2-hydroxyethyl)ethylenediamine (THEED), N,N,N',N'-tetrakis(2-hydroxypropyl)ethyl enediamine (Quadrol), and triethanolamine (TEA), and the frequency decrease of the crystals was recorded. The nature of the interaction of the alkanolamines with CO2 was investigated by nuclear magnetic resonance spectroscopy (NMR), Fourier transform infrared spectroscopy (FTIR), fast atom bombardment mass spectrometry (FABMS), and mass spectrometry/mass spectrometry (MS/MS).Key words: alkanolamine, quartz crystal microbalance, piezoelectric quartz crystal.
Styrene hydration and stilbene isomerization in strong acid media. An excess acidity analysis
Authors: Cox Robin A
Published: April 2011

Rate constants obtained for the hydration of some ring-substituted styrenes, YC6H4CR=CH2 (R = H, CH3, CF3) (1-3), and for the isomerization of some cis-stilbenes substituted in both rings, YC6H4CH=CHC6H4Z (4), in aqueous H2SO4, D2SO4, and HClO4 media at 25°C and, in some cases, at other temperatures, have been subjected to an excess acidity analysis. Data obtained by different groups and in different media for the same substrates agree very well. The m values obtained show that all the substrates react via the A-SE2 mechanism, rate-determining proton transfer to carbon; the transition state is late, with the proton transfer being about three-quarters complete. The reactivities of 1-3 in the aqueous standard state are obtained by extrapolation; they are 1:103:10-7, respectively. The stilbenes are relatively unreactive; log k0 values for compounds with substituents in the ring adjacent to the developing positive charge show a good correlation with σ+ with a large negative ρ+ value, but those for compounds with substituents in the other ring correlate with σ with a small negative ρ. Extrapolated log k0 values for 1 and 3 show a good correlation with σ+, but those for 2 correlate with neither σ+ nor σ, because the α-CH3 group twists the molecule and reduces the resonance interaction of ring substituents with the developing positive charge. On the other hand, this effect is not seen in 3, when the strongly deactivating α-CF3 group is present, meaning that the transition state is forced to be planar in this case. The activation parameters, solvent isotope effects, and excess acidity slope parameters mm* obtained in the analysis are tabulated and discussed. The parameters obtained for the parent styrenes 1 are very similar to those previously obtained for the phenylacetylenes YC6H4Ctriple bondCH, meaning that reactions with vinyl cation intermediates and reactions with benzyl cation intermediates can be quite similar. Key words: styrenes, stilbenes, hydration, excess acidity, LFER, mechanism.
Determination of the configuration of ribitol in the C-polysaccharide of Streptococcus pneumoniae using a synthetic approach
Authors: Qin Huiping , Grindley T Bruce
Published: April 2011

The C-polysaccharide is an antigen common to all known serotypes of Streptococcus pneumoniae bacteria and is a potential target for vaccine preparation. The final uncertainty in the structure of its repeating unit, a pentasaccharide phosphate containing two phosphorylcholine side chains, has been resolved by determining the configuration of ribitol. Assignment of configuration was performed by synthesis of the two trisaccharide phosphate fragments that have either D- or L-ribitol at their centers and comparison of their 1H and 13C NMR spectral data with that of the natural polysaccharide. The syntheses employed common synthons added in different orders to an asymmetrically substituted chiral ribitol derivative to obtain opposite chiralities in the ribitol segments. The data for the trisaccharide containing D-ribitol was almost identical to that of the natural material while that for the trisaccharide containing L-ribitol differed significantly. In particular, the chemical shift differences between the two protons of the primary carbons of ribitol units directly attached to the β-2-acetamido-2-deoxy-galactopyranosyl residue were 0.10, 0.12, and 0.33 ppm, in the natural polysaccharide, the D-ribitol-containing trisaccharide, and the L-ribitol-containing trisaccharide, respectively. The average difference between the 13C NMR chemical shifts of corresponding ribitol carbons from the natural polysaccharide and the D-ribitol-containing trisaccharide phosphate was 0.034 ppm. This evidence indicates that ribitol in the C-polysaccharide has the D-configuration and that a very similar mixture of conformations in the ribitol portions is present for the natural polysaccharide and the D-ribitol-containing trisaccharide phosphate.Key words: C-polysaccharide, Streptococcus pneumoniae, ribitol, dibutylstannylene acetal, determination of configuration.
Solvent isotope effect on the hydroxide-ioncatalyzed hydration of ketenes in aqueous solution
Authors: Andraos J , Chiang Y , Eustace S J , Kresge A J , Paine S W , Popik V V , Sung K
Published: April 2011

Five ketenes, phenyl(ethyl)ketene, phenyl(methylthio)ketene, diphenylketene, pentafluorophenylketene, and 1-naphthylketene, were generated flash photolytically and solvent isotope effects (H2O vs. D2O) on their hydroxide-ion-catalyzed hydration in aqueous solution were determined. The values obtained are all weakly inverse and closely similar (kHO/kDO = 0.76-0.97), as expected for these fast, hydroxide-ion-consuming reactions, known to proceed by nucleophilic attack of hydroxide on the ketene carbonyl group. The characteristic magnitude of these isotope effects should prove useful in identifying new examples of this reaction.Key words: ketenes, flash photolysis, photo-Wolff reaction, solvent isotope effects on hydroxide ion consumption.
Photoinduced electron transfer chemistry of 2,3-dicyanonaphthalene
Authors: Brancaleon Lorenzo , Brousmiche Darryl , Johnston Linda J
Published: April 2011

The electron transfer photochemistry of 2,3-dicyanonaphthalene has been studied by a combination of fluorescence and transient absorption spectroscopy. The singlet excited state has a lifetime of 26 ns in acetonitrile and reacts with aromatic and alkene donors with oxidation potentials less than ~1.8 V with rate constants that are close to the diffusion-controlled limit. Transient absorption measurements demonstrate that the fluorescence quenching leads to efficient formation of free-radical ions. The radical ion yields have been measured for several donors and are compared to those for the more commonly used sensitizer, 1.4-dicyanonaphthalene. In the absence of added donors, direct excitation of 2,3-dicyanonaphthalene provides evidence for photoionization at high laser energy, in addition to triplet formation. The results illustrate the utility of this sensitizer for photoinduced electron transfer reactions.Key words: photoinduced electron transfer, laser flash photolysis, fluorescence, photosensitizers.
Kinetics of oxidation of hydrogen peroxide and ascorbic acid by a tribridged manganese(IV,IV) dimer in feebly acidic media
Authors: Bhattacharya Anup Kumar , Mondal Anath Bondhu , Dash Anadi C , Brahma G S , Banerjee Rupendranath
Published: April 2011

In weakly acidic, aqueous buffer (MeCO2-+ bipy), the complex ion [Mn2IV(μ-O)2(μ-MeCO2)(bipy)2(H2O)2]3+, 1 (bipy = 2,2prime-bipyridine), coexists in rapid equilibrium with its hydrolytic derivatives, [Mn2IV(μ-O)2(bipy)2(H2O)4]4+, 2, and [Mn2IV(μ-O)2(μ-MeCO2)(bipy)(H2O)4]3+, 3. The solution quantitatively oxidizes hydrogen peroxide to oxygen and ascorbic acid to dehydroascorbic acid, itself being reduced to MnII. In the presence of excess reductant, the reactions follow simple first-order kinetics with no evidence for the accumulation of a significant amount of any intermediate manganese complex. The ascorbate anion shows overwhelming kinetic dominance over ascorbic acid, but no evidence is available for deprotonation of hydrogen peroxide. The preferred intimate mechanism for hydrogen peroxide is inner sphere but that for ascorbic acid is uncertain. For both reductants, increased extent of aquation leads to increased kinetic activity in the order: 1 < 2 < 3.Key words: kinetics, manganese, ascorbic acid, hydrogen peroxide, 2,2prime-bipyridine.