bluuebunny/crossref_metadata_2025 huggingface dataset:
Aromatic aldonitrones of 2-(hydroxyamino)benzyl alcohol and their cyclic isomers. Crystal and molecular structures of a 1-hydroxy-1,2-dihydro-4H-3,1-benzoxazine, a boron chelate, and its parent nitrone ligand
Authors: Kliegel Wolfgang , Metge Jörg , Rettig Steven J , Trotter James
Published: April 2011

The synthesis of a series of C-aryl-N-[2-(hydroxymethyl)phenyl]nitrones, 5 (that can also exist under certain conditions as isomeric 1-hydroxy-2-aryl-1,2-dihydro-4H-3,1-benzoxazines, 8), via 2-(hydroxyamino)benzyl alcohol, 4, and their subsequent reactions with oxybis(diphenylborane), (Ph2B)2O, leading to the 5-(arylmethylene)-7,7-diphenyl-6,8-dioxa- 5-azonia-7-borata-5H-6,7,8,9-tetrahydrobenzocyclo-

heptenes 6 are described. Crystals of 1-hydroxy-2- (4-methoxyphenyl)- 1,2-dihydro-4H-3,1-benzoxazine, 8b, are monoclinic, a = 9.379(2), b = 10.699(2), c = 12.9392(7) Å, β = 99.916(2)°, Z = 4 (two independent molecules), space group Pa; those of C-[4-(dimethylamino)phenyl]-N-[(2-hydroxymethyl)phenyl]nitrone, 5c, are monoclinic, a = 7.687(1), b = 7.891(1), c = 11.5053(9) Å, β = 92.781(9)°, Z = 2, space group P21; and those of 5-[4-(dimethylamino)phenylmethylene]-7,7-diphenyl-6,8-dioxa-5-azonia-7-borata-5H-6,7,8,9-

tetrahydro-benzocycloheptene, 6a, are monoclinic, a = 10.771(1), b = 13.1057(9), c = 16.8724(7) Å, β = 90.005(5)°, Z = 4, space group P21/n. The structures were solved by direct methods and refined by full-matrix least-squares procedures to R(F2) = 0.120 (Rw(F2) = 0.135) for all 3149 reflections (R(F) = 0.071, Rw(F) = 0.063 for 1500 reflections with I >3 σ (I)) for 8b and R(F) = 0.035 and 0.036 (Rw(F) = 0.031 and 0.038) for 1071 and 3594 reflections with I >3 σ (I), respectively, for 5c and 6a. Compound 8b is the first structurally characterized 1-hydroxy-1,2-dihydro-4H-3,1-benzoxazine derivative and 6a features a relatively rare seven-membered boron-containing heterocycle.Key words: C-aryl-N-[2-(hydroxymethyl)phenyl]nitrones, 1-hydroxy-2-aryl-1,2-dihydro-4H-3,1-benzoxazines, organoboron compounds, crystal structures
Article
Authors: Miller Jack M , Wails David , Hartman J Stephen , Schebesh Karla , Belelie Jennifer L
Published: April 2011

Novel mesoporous silicas have been prepared via a sol-gel route involving fluoride-catalyzed hydrolysis of tetraethylorthosilicate (TEOS). Incorporation of zinc chloride by sol-gel synthesis gives a range of mesoporous materials with significantly higher catalytic activity than the commercially available catalyst, Clayzic, in a model Friedel-Crafts alkylation reaction. The dependence of catalytic activity and physical structure on the amounts of solvent, water, zinc chloride, and potassium fluoride used in the preparation are explored, and the materials have been further characterized by nitrogen adsorption to determine surface areas, total pore volumes, and pore-size distributions and by 29Si and 19F MAS NMR spectroscopy. The most active catalysts generally have the highest total pore volumes, with pore-size distributions larger than 8 nm.Key words: sol-gel, Friedel-Crafts, Clayzic, silica.
Kinetics and mechanism of the aminolysis of ethyl aryl carbonates in acetonitrile
Authors: Koh Han Joong , Lee Ji-Won , Lee Hai Whang , Lee Ikchoon
Published: April 2011

The aminolysis reactions of ethyl aryl carbonates with benzylamines in acetonitrile at 25.0°C are investigated. The base-catalyzed path, k2, disappears when strong nucleophiles (X = p-CH3O and p-CH3) react with a substrate activated by a strong nucleofuge (Z = p-NO2). The large magnitude of rhox(-1.7 to -2.5), rhoz(3.4 to 4.3), and rhoxz(1.4) values, and relatively large kH/kD(1.6 to 1.8) found for the uncatalyzed path (k1) can be accounted for in terms of a stepwise mechanism with rate-limiting expulsion of the phenoxide leaving group. The catalyzed process (k2) is characterized by the much smaller magnitude of rhox(-1.0 to -1.7), rhoz(0.4 to 0.7), and rhoxz(0.2), the larger kH/kD(2.1 to 2.5) values, and the lower Δ Hdouble daggervalues (1.8-1.9 kcal mol-1) than those of the uncatalyzed process (k1) with large negative Δ Sdouble daggervalues (-65 to -67 cal K-1mol-1). These results are consistent with four- and six-centered transition states for the two processes, k1and k2, respectively.Key words: ethyl aryl carbonates, aminolysis mechanism, base catalysis, stepwise mechanism, cross-interaction constant.
Selective hydroboration of alkenes and alkynes in the presence of aldehydes and ketones
Authors: Kabalka George W , Yu Su , Li Nan-Sheng
Published: April 2011

The reactions of terminal alkenes in the presence of ketones or aldehydes with a variety of borane reagents have been investigated. It was found that the selective hydroboration of a terminal alkene in the presence of a ketone or an aldehyde is most efficient when dicyclohexylborane is used as the hydroborating agent. The hydroboration of olefinic ketones and olefinic aldehydes with dicyclohexylborane generates the corresponding hydroxyaldehydes and hydroxyketones in good yields after oxidation with sodium perborate. The hydroboration of alkynyl ketones and alkynyl aldehydes with dicyclohexylborane yields the corresponding olefinic carbonyl compounds after protonation, or dicarbonyl compounds after oxidation.Key words: hydroboration, reduction, dicyclohexylborane, hydroxyaldehyde, hydroxyketone.
Formation of cis-[Rf(CO)Re(CO)4Hal]Na complexes as an evidence of halogen-metal exchange between [Re(CO)5]Na and polyfluorinated aryl and vinyl halides
Authors: Artamkina Galina A , Sazonov Petr K , Ivushkin Vasily A , Beletskaya Irina P
Published: April 2011

Halo(acyl)rhenate complexes cis-[Rf(CO)Re(CO)4Hal]Na (Hal = Cl, Br; Rf = C6F5, trans-(CF3)3CCF== CF) are the primary products of the reactions of [Re(CO)5]Na with the corresponding aryl or vinyl halides RfHal.Key words: pentacarbonylrhenate anion, carbonylmetallates, halogen-metal exchange, nucleophilic aromatic substitution, nucleophilic vinylic substitution.
Solvent-induced stereospecific isomerization of an allylic alcohol to a homoallylic alcohol catalyzed by a chiral lithium amide
Authors: Arvidsson Per I , Hansson Maria , Khan Agha Zul-Qarnain , Ahlberg Per
Published: April 2011

Deprotonation of cyclohexene oxide, 1, by lithium (S)-2-(1-pyrrolidinylmethyl)pyrrolidide, 2-Li, on changing the solvent from tetrahydrofuran (THF) to, for example, 2,5-dimethyltetrahydrofuran (DMTHF) or diethyl ether (DEE) has been shown to yield, besides the lithium alkoxide of 2-cyclohexene-1-ol, 3-Li, the lithium alkoxide of the homoallylic alcohol 3-cyclohexene-1-ol, 4-Li. It was shown that compound 4-Li is formed from 3-Li. No such rearrangement has been observed in THF. We have now shown that the solvent-induced isomerization of the lithium alkoxide of (S)-3-methyl-2-cyclohexene-1-ol, (S)-5-Li, catalyzed by 2-Li to the lithium alkoxide of (S)-3-methyl-3-cyclohexene-1-ol, (S)-6-Li, is 100% stereospecific. Furthermore, deuterium-labeling experiments suggest that the rearrangement of the proton is close to 100% intramolecular.Key words: 1,3-proton transfer, chiral lithium amide, intramolecular, solvent-induced isomerization, stereospecific.
Identity-reaction proton transfers from nitrogen acids yielding localized vs. delocalized conjugate bases. An ab initio study
Authors: Verth James E. Van , Saunders, Jr. William H , Kermis Thomas W
Published: April 2011

Identity-reaction proton transfers from a series of nitrogen acids to the corresponding conjugate bases have been studied by ab initio methods at the MP2/6-31+G*//MP2/6-31+G* level. The acids are NH4+, H2NNH3+, CH3NH3 +, CH2NH2, OCNH 2+, OCHNH3+, H2NCHNH2+ , HNCHNH 3+, NH3, CH3NH2, CH2NH, OCNH, OCHNH2,and HNCHNH2. Gas-phase acidities were calculated at the G2(MP2) level where experimental values are not available in order to have benchmark values for all acidities. Barriers to proton transfer relative to the separated reactants, ΔHTS, show a straight-line relation to acidity for all of the neutral acids and for all but four of the cationic acids. Three show ΔHTS values well above the line: HNCHNH3+, OCNH2+, and OCHNH3 +, in increasing order of positive deviation. One shows a small negative deviation: H2NNH3+ . The first three acids have localized pi systems but can yield delocalized transition structures and conjugate bases. The barriers result from a lag in delocalization relative to proton transfer in the transition structures. All of the other acids give transition structures that can only be localized, or if they can be delocalized they prefer to adopt conformations in which the unshared pair on nitrogen delocalizes rather than the electrons of the N---H bond. The negative deviation for H2NNH3 + is attributed to polarizability of the NH2 group.Key words: ab initio, nitrogen acids, proton transfer, acidity.
The rhodium(II)-catalyzed aziridination of olefins with {[(4-nitrophenyl)sulfonyl]imino}phenyl-lambda3-iodane
Authors: Müller Paul , Baud Corine , Jacquier Yvan
Published: April 2011

The aziridination of olefins with {(4-nitrophenylsulfonyl)imino}phenyl-lambda3-iodane, NsN==IPh (1c), in the presence of [Rh2(OAc)4] proceeds in yields of up to 85% when the olefin is used in large excess. Under optimized conditions, styrene (4a) is aziridinated with 1 equiv. of NsN==IPh (1c) in 64% yield with 2 mol% of catalyst. The aziridines derived from electron-rich olefins undergo ring-opening under the conditions of the aziridination and afford rearrangement products or pyrrolidines. The aziridination is sterospecific with 1,2-dialkyl- and 1,2-arylalkyl-disubstituted olefins, but nonstereospecific with stilbene.The rho -value for aziridination of substituted styrenes is -0.61. No ring-opened products are observed upon aziridination of vinylcyclopropanes. In the presence of chiral RhII catalysts, the aziridination is enantioselective, affording an ee of 73% with cis- β -methylstyrene (4k) and Pirrungs [Rh2{(R)-(-)-bnp}4] catalyst. The experimental results are consistent with a one-step mechanism for transfer of the nitrenoid moiety from the catalyst to the olefin.Key words: aziridination, nitrene transfer, rhodium catalysis.
The solid-state structure of bis(pentafluorophenyl)zinc
Authors: Sun Yimin , Piers Warren E , Parvez Masood
Published: April 2011

Using the published method of Wiedenbruch, bis(pentafluorophenyl)zinc, 1, was prepared from anhydrous ZnCl2 and 2 equiv. of LiC6F5 in diethyl ether. Base-free 1 was obtained in 60-65% yield by repeated distillation of the initially formed bis(diethyl) ether adduct of 1. The X-ray quality crystals of 1 were obtained from benzene solution. The molecular structure of 1 revealed a near linear geometry for the two-coordinate zinc center (C(1)-Zn-C(7) = 172.6(2)°), typical of monomeric ZnR2 derivatives. In the crystal structure, stacking interactions between C6F5 rings on adjacent molecules is a dominant motif, with ring centroid to ring centroid distances of 3.503 and 3.563 Å observed. A weak intermolecular C-F ...Zn interaction between F(2) and an adjacent zinc center, as judged by the close contact of 2.849(2) Å, also appears to be an important aspect of the crystal structure. Compound 1 is an effective but nonselective C6F5 transfer agent to BCl3; 1: monoclinic, space group P21/n, a = 11.902(2) Å, b = 7.732(2) Å, c = 13.735(2) Å, β = 110.58(1)°, V = 1183.4(4) Å3, Z = 4, R = 0.048, Rw = 0.069.Key words: organozinc complex, pentafluorophenyl transfer agent.
The mechanism of solvolysis of 2-adamantyl azoxytosylate: isotopic labelling, medium effect, and attempted deoxygenation studies
Authors: Conner John K , Haider Johanna , Hill MN Stuart , Maskill Howard , Pestman Monique
Published: April 2011

Rates of solvolysis of 2-adamantyl azoxytosylate (1) have been measured over a range of temperatures in ethanoic acid, methanoic acid, 50:50 (v/v) trifluoroethanol:water, 80:20 (v/v) trifluoroethanol:water, 97:3 (w/w) trifluoroethanol:water, and 70:30 (v/v) ethanol:water. For comparison, rates of solvolysis of 2-adamantyl tosylate (2) have also been measured in 50:50, 80:20, and 90:10 (v/v) trifluoroethanol:water, and for both compounds, activation parameters have been determined. These and results published earlier allow a correlation of the two reactions and indicate that the m value for 2-adamantyl azoxytosylate solvolysis is only 0.46. This is one of the lowest m values for a reaction that is unambiguously an SN1 solvolysis. We have also recorded the 17O NMR spectrum of the 2-adamantyl tosylate formed from 17O-labelled 2-adamantyl azoxytosylate in deuteriochloroform, and the millimeter-wave spectrum of the nitrous oxide evolved in the hydrolysis of 18O-labelled 2-adamantyl azoxytosylate. These labelling studies have provided more detailed knowledge of the SN1 fragmentation mechanism of 1 and exclude a mechanism of reaction via rearrangement to N-nitroso,N-(2-adamantyl),O-(p-toluenesulfonyl)hydroxylamine (5). Attempted deoxygenation of 1 to give 2-adamantyl diazotosylate (8) and a subsequent fragmentation proved unsuccessful.Key words: nitrous oxide, carbenium ion, isotopic labelling, solvolysis, m value.
Article
Authors: Segstro Edward Paul , Davie Kerry , Huang Xiaoling , Janzen Alexander Frank
Published: April 2011

The equilibrium in the Ph2P(O)OC6H4OH-base system may be shifted by varying the base concentration, where base = imidazole, triethylamine, dimethyl sulfoxide, pyridine, and 1,4-dioxane. This equilibrium was studied by 31P and 1H NMR, and information about the symmetry of phosphorus-containing intermediates is provided by the 1H NMR spectrum of the catecholyl ring, with its ABCD spin system. The equilibrium is also affected by trifluoroacetic acid. A mechanism is proposed that involves protonation-deprotonation, cyclic-acyclic equilibria, and selective P-O bond cleavage, with all steps occurring rapidly on the NMR time scale.Key words: Ph2P(O)OC6H4OH-base system, cyclic-acyclic equilibrium, and selective P-O bond cleavage.
Regioselectivity and stereoelectronic effects in the reactions of the dinitroaniline herbicides trifluralin and benefin with nucleophiles
Authors: Annandale Michael T , vanLoon Gary W , Buncel Erwin
Published: April 2011

The reactions of two members of the dinitroaniline class of herbicides, N,N-di-n-propyl-2,6-dinitro-4- (trifluoromethyl)aniline (trifluralin; 1) and N-ethyl-N-n-butyl-2,6-dinitro-4-(trifluoromethyl)aniline (benefin; 2), along with their analogue, N-phenyl-2,6-dinitro-4-(trifluoromethyl)aniline (3), with the nucleophiles, OD- and SO32- , have been investigated using 400 MHz 1H NMR spectroscopy. The reactions of both 1 and 2 with OD- result in formation of Meisenheimer anionic σ -complexes according to a K3T1 (kinetic preference for formation of the C-3 adduct with thermodynamic preference for formation of the C-1 adduct) reaction sequence while the reaction of 3 with OD- and that of 1 with SO32- follow a K3T3 (kinetic and thermodynamic preference for formation of the C-3 adduct) sequence. There was no observation of the C-1 adducts of 1 and 2 with OD-, but the products of SNAr displacement at C-1 were observed as the final thermodynamic products. Geometry optimization calculations support our hypothesis of n –> σ * stabilization of the C-1 adduct leading to SNAr displacement. In the reaction of 3 with OD-, initial N-deprotonation to form the anion, 3a, is followed by σ -complex formation. The final thermodynamic product observed in this system is 3,5-dinitro-4-(N-phenylamino)benzoic acid formed through hydrolysis of the trifluoromethyl group on the anion, 3a. Aryl H-D exchange has been found for the systems of 1 and 2 with OD-, but not for the SO32- system and neither for the reaction of 3 with OD-. Since dimethylpicramide showed significantly slower H-D exchange under identical conditions, it is argued that this discrepancy has as origin the ability of the amino N lone electron pair to interact with the π -system of the ring. With both 1 and 2 the larger size of the amino alkyl chains prevent the amino N lone pair from aligning with the π -system of the ring, thus hindering electron density donation to the electron-deficient ring carbons.Key words: trifluralin, benefin, herbicides, Meisenheimer complexes, stereoelectronic effects.
Phase behaviour of model stratum corneum lipid mixtures: an infrared spectroscopy investigation
Authors: Lafleur Michel
Published: April 2011

The stratum corneum, the top layer of the epidermis, is the material that constitutes the membrane enveloping our body. The lipids that are present are responsible for the permeability properties of the skin and, as a consequence, are essential to maintain the hydration of the internal components and to protect our body from external agents. In the present work, the mixing and the structural properties of model mixtures formed by the main lipids of the stratum corneum have been examined by infrared spectroscopy. The model is formed by an equimolar mixture of ceramides (type III), cholesterol, and perdeuterated palmitic acid. Binary mixtures as well as mixtures for which the ceramides were substituted by sphingomyelin, a ceramide precursor, have also been studied. The results indicate that the stratum corneum model mixture exhibits a rich polymorphism, ranging from crystalline domains with heterogeneous lipid composition and orthorhombic chain packing, to a fluid and homogeneous phase. To obtain this particular behaviour, the three components are essential and the specific role of each species is discussed. In addition, the results reveal that the homogeneous lipid distribution observed for temperatures higher than 70°C can be maintained at low temperatures, leading to the formation of a metastable phase. Several weeks are needed to obtain the thermodynamically stable phase if the sample is incubated at 5°C. However, it is rapidly induced by annealing the sample at 40°C.Key words: stratum corneum, lipid, infrared spectroscopy, ceramide.
Uniaxial deformation of nylon-6 and nylon-11: changes in orientation and crystal phase
Authors: Moffatt Sonia , Ajji Abdellah , Lotz Bernard , Brisson Josée
Published: April 2011

The effect of uniaxial drawing on orientation of the crystalline fraction of two polymers forming hydrogen bonds, nylon-6 and nylon-11, has been investigated using X-ray diffraction. These two polymers have similar crystal phases, although their hydrogen bond density differs. For both polymers, the deformation occurs in two steps, the first being a plastic deformation of the α-phase spherulites. This leads to two popul ations of crystals, one with the chain axis oriented parallel to the draw direction, the second with the a axis (hydrogen bond direction) aligned along the draw direction. In the second step, the a-axis aligned population gradually tilts, leading to a uniaxial orientation of the samples with chain axis aligned along the draw direction. For nylon-11, the onset of this step corresponds to the emergence of crystals of the γ phase, which rapidly becomes the major phase. It reaches a higher orientation than the α phase, and stems from crystallization upon tension of the polymer. For nylon-6, although the γ phase also appears during drawing, at the maximum draw ratio only a small fraction is present. This difference is attributed to the relative stability of the two phases, which is different for nylon-6 than for nylon-11.Key words: orientation, X-ray diffraction, nylon, hydrogen bonds.
Article
Authors: Carlow G R , Zinke-Allmang M
Published: April 2011

We present results on the formation of buried silicide layers at ion implantation doses in the range of 1-60% of the critical dose for formation of a uniform layer. We emphasize observations for the low-dose range of 1-5% where the precipitate density is quite dilute. The Co redistribution during post-implant annealing is measured using Rutherford backscattering techniques and secondary ion mass spectrometry. Experimental observations during post-implantation annealing at 1000°C involves (i) a contraction of the Co depth profile for all doses, (ii) shifting of the peak of the profile towards the bulk, and (iii) formation of a secondary Co peak near the surface. The secondary peak is only present in samples implanted to greater than 3% of the critical dose. The interpretation of the shift of the main peak and the occurrence of the secondary peak requires a model exceeding the standard ripening model used previously to describe mesotaxy. We suggest that more recent ripening-based concepts allow for a full description of these observations with a minimum of parameters, particularly not requiring interaction with the complex defect profiles formed initially during implantation. Essential for this model is a proper inclusion of precipitate-precipitate interactions and the role of diffusion screening.Key words: silicide, Co implantation, mesotaxy, precipitate, ripening, screening.
The supramolecular architecture of 4-aminoacetophenone (1-(4-fluorophenyl)ethylidene)hydrazone hydrate. Double T-contacts and extremely low-density water layers in a mixed azine
Authors: Lewis Michael , Barnes Charles L , Glaser Rainer
Published: April 2011

The crystal structure of unsymmetrical azine 4-aminoacetophenone (1-(4-fluorophenyl)ethylidene)hydrazone hydrate, 1·H2O, was determined. Comparison to the X-ray structures of the symmetrical 4-fluoro- and 4-aminoacetophenone azines, 2 and 3, respectively, reinforces the view of the azine bridge as a conjugation stopper. The focus of the analysis lies with the supramolecular architecture. The structure demonstrates azines as double T-contact synthons and suggests that novel L-type arene-arene contacts also might play a significant role in the supramolecular architecture. The absence of inter-water hydrogen bonding in an extremely low-density water layer is one of the surprising and unique features of the solid-state structure of the title compound. The water layers connect azine layers through a hydrogen bonded network which is described by graph set analysis.Key words: arene-arene interaction, hydrogen bonding, graph set analysis, azine, supramolecular synthon, low density water monolayer.
Chemical kinetics and thermodynamics of tin ionization in H2-O2-N2 flames and the proton affinity of SnO
Authors: Goodings John M , Chen QingFeng
Published: April 2011

A small amount (<10-6 mol fraction) of tin was introduced into five, fuel-rich, H2-O2-N2 flames in the temperature range 1820-2400 K at atmospheric pressure. Ions in a flame were observed by sampling the flame along its axis through a nozzle into a mass spectrometer. The major neutral tin species in these flames were SnO (>97%) and Sn (<3%). The principal tin ions observed were SnOH+ and Sn+. Thermodynamic functions for SnOH+, Sn+, SnO, and Sn were calculated by statistical mechanics using published data from ab initio calculations and spectroscopy. The SnOH+ ion was formed initially by proton transfer to SnO by H3O+, a natural flame ion, with which it is in equilibrium. It was also produced by chemi-ionization of SnO reacting with H; SnOH+ rapidly equilibrates with Sn+. Ion ratio measurements of SnOH+/H3O+ led to the proton affinity PA°298 (SnO) = 911 ± 21 kJ mol -1 (218 ± 5 kcal mol-1). A calculated equilibrium constant provided the SnOH+/Sn+ ion ratio. When electron-ion recombination of SnOH+ with free electrons was made dominant by the addition of CH4 and K, the measured recombination coefficient of SnOH+ was (0.116 ± 0.065)T-(1.66±0.16) cm3 molecule-1 s-1; the temperature dependence is in good agreement with the T-1.5 dependence predicted by simple theory. The rate constant for chemi-ionization could not be measured due to impurity ions from potassium and sodium, but the equilibrium constant for chemi-ionization/recombination was calculated to be 0.004 863 exp (-52 070/T). Assuming detailed balance and the experimental recombination coefficient, the relatively small rate constant for chemi-ionization was given by 3.27 × 10-10 exp (-48 630/T) cm3 molecule-1 s-1. Finally, calculated values were obtained for the bond energy D°0(HO-Sn+) = 408 ± 21 kJ mol-1, and the standard zero-point enthalpy of formation deltafH°0(SnOH+) = 637 ± 21 kJ mol-1.Key words: flame ionization, chemi-ionization, proton affinity, mass spectrometry, tin.
Spectroscopic and differential scanning calorimetric studies of the phase transitions in oxanorbornane
Authors: Paroli Ralph M , Gilson Denis FR , Butler Ian S
Published: April 2011

Oxanorbornane (7-oxabicyclo[2.2.1]heptane, C6H10O) resembles other bicycloheptane-like molecules by exhibiting order-disorder transitions in the solid state. Differential scanning calorimetric studies show that five different phases may occur, with hysteresis in the transition temperatures. The vibrational spectra have been examined, and a factor group analysis of the vibrational splittings indicates that the crystal structure of the ordered phase has D2d symmetry. Activation energies for molecular motions were derived from proton spin-lattice relaxation time measurements.Key words: oxanorbornane, order-disorder, phase transitions.
General base-catalyzed hydrolysis and carbonyl-18O exchange of N-(4-nitrobenzoyl)pyrrole
Authors: Beach Laurence J , Batchelor Raymond J , Einstein Frederick WB , Bennet Andrew J
Published: April 2011

Base-promoted hydrolysis kinetics for N-(4-nitrobenzoyl)pyrrole (1) have been measured as a function of buffer concentration at several pH values at 25°C. In addition carbonyl-18O exchange kinetics have been determined at a single pH value (9.48) as a function of 1,4-diazobicyclo[2.2.2]octane (DABCO) concentration. At zero buffer concentration the measured ratio of 18O exchange to hydrolysis (kex/khyd) is approximately 0.04, and this value increases and finally levels off at about 0.23 as the DABCO concentration is increased. These observations are consistent with the buffer acting as a general-base to catalyze both the attack of water to generate an anionic tetrahedral intermediate (To-) and the breakdown of To- to give hydrolysis products.Key words: amide, hydrolysis, catalysis, general-base, tetrahedral intermediate.
Molecular modelling studies of a nerve growth factor receptor
Authors: Shamovsky Igor L , Ross Gregory M , Riopelle Richard J , Weaver Donald F
Published: April 2011

Using computer simulations, a geometry for the receptor binding site for nerve growth factor (NGF) has been proposed. Variable basis Monte Carlo simulated annealing calculations have been used to ascertain the structures of the complexes of four fully active NGF analogues with the second leucine-rich motif (LRM-2) of trkA, a putative binding site for NGF. The previously suggested bioactive conformation of the amino and carboxyl termini of NGF docks favourably with the receptor defined by the LRM-2 of trkA: only minor conformational changes take place in the NGF analogues upon docking. Extensive intermolecular van der Waals contacts arise from the geometric fit of the NGF binding domain to the LRM-2. Within this receptor environment, five distinct binding areas reveal a highly selective multiple-point NGF-trkA recognition based on hydrophobic, ionic, hydrogen bonding, and van der Waals interactions. Binding specificity is determined primarily by residues Lys100, Asp109, and Phe113 of trkA which bind to conserved NGF residues Asp16, Arg114, Lys115, and Phe7. An explicit atom-level model of the high-affinity NGF receptor is thus developed.Key words: NGF, trkA, leucine-rich motif, protein docking, Alzheimer's disease.
Application of [Ru((R)-BINAP)(MeCN)(1-3:5,6-η-C8H11)](BF4) as a catalyst precursor for enantioselective hydrogenations
Authors: Daley Christopher JA , Wiles Jason A , Bergens Steven H
Published: April 2011

A catalyst system employing [Ru((R)-BINAP)(MeCN)(1-3:5,6-η-C8H11)](BF4) as catalyst precursor was evaluated using the enantioselective hydrogenations of tiglic acid, α-acetamidocinnamic acid, itaconic acid, methyl tiglate, dimethyl itaconate, geraniol, ethyl acetoacetate, and dimethyl oxaloacetate as a series of typical substrates. Acetone and MeOH were used as model aprotic and protic solvents, respectively. The hydrogenation of substrates containing an α, β-unsaturated carboxylic acid functionality required stoichiometric quantities of NEt3 to occur at reasonable rates in acetone solution, while in MeOH solution it did not. The enantioselectivities were typically higher in acetone than in MeOH. This catalyst system is among the more enantioselective ruthenium-BINAP type systems reported for the catalytic hydrogenation of substrates containing an α, β-unsaturated acid or ester functionality. The enantioselectivities for the hydrogenation of ketones ranged from poor (15%) to moderate (74%). 1,4-Dicarboxylate substrates with the prochiral olefin or ketone at the 2-position were all hydrogenated in good to high ee with the same enantioface selectivity both with our system and other catalysts reported in the literature. This raised the possibility that these substrates were hydrogenated through intermediates with similar structural features. Key words: ruthenium, BINAP, enantioselective, hydrogenation, catalysis.
Oxidation of 2-methylpyrroles with perchlorinated iron(III) metalloporphyrin catalysts: a versatile synthesis of symmetric and asymmetric dipyrromethanes
Authors: Karunaratne Veranja , Dolphin David
Published: April 2011

A variety of substituted 2-methylpyrroles (3-8) were oxidized using the metalloporphyrin catalysts iron(III) meso-tetra(2,6-dichloro-3-sulphonatophenyl)-β-octachloroporphyrin chloride 1 and iron(III) meso-tetra(2,6-dichlorophenyl)-β-octachloroporphyrin chloride 2 under very mild conditions. Treatment of the resulting allylic alcohols 3a-8a with α-free pyrroles 9 and 10 resulted in a very efficient synthesis of the corresponding dipyrromethanes 3b-8b and 3c-8c. Furthermore, the above allylic alcohols when treated with furfurylamine produced the novel (2-furylmethyl)-2-pyrrolylmethylamines 3d-8d.Key words: catalytic oxidation, metalloporphyrins, pyrroles, dipyrromethanes, polyhalogenated porphyrins.
Diastereoselective alkoxymethylation of aromatic aldehydes with chiral lithiomethyl ethers. Synthesis of optically active monoprotected glycols
Authors: Ponzo Viviana L , Kaufman Teodoro S
Published: April 2011

The synthesis of optically active α-alkoxymethyl benzyl alcohols by reaction of polysubstituted benzaldehydes with chiral α-alkoxymethyllithiums, easily obtained by transmetallation of the corresponding α-alkoxystannanes with n-butyllithium, is reported. Chemical yields of the transformation are good while low asymmetric induction (<1.6:1) is obtained. Key words: diastereoselective synthesis, monoprotected glycols, chiral alkoxymethyllithiums, addition to aldehydes, optically active alcohols.
Strain in bicyclobutane: on the interpretation of quantitative data
Authors: Rozental Esther , Azran Carmela , Basch Harold , Hoz Shmaryahu
Published: April 2011

In small rings, strain energy is believed to be dominated by bond angles (Baeyer strain). However, it was found (Dill et al.) that, in bicyclobutane, bridgehead substitution effects on strain energy range from +10 to -30 kcal. Ab initio computations at the 6-31G and 6-31+G* levels have shown that the deformation caused by the bridgehead substitution is minimal and so are its energetic consequences.Key words: bicyclobutane, strain energy, geometry distortion.
Article
Authors: Al-Awadi Nouria A , Kaul Kamini , El-Dusouqui Osman ME
Published: April 2011

The formate ester of benzoin (1) and the methyl ethers of benzoin and anisoin (2, 3) have been converted separately under thermal gas-phase conditions cleanly into 1,2-diarylethanone and have given, respectively, the following Arrhenius log A (s-1) and Ea (kJ mol-1): 10.11 and 132.0, 11.29 and 173.0, and 11.85 and 151.1. A plausible mechanism is suggested to account for these transformations based on the kinetics and products of reaction.Key words: gas-phase, pyrolysis, kinetics, mechanism, synthesis.
Distortions in square-planar palladium(II) halides: a nonempirical molecular orbital model study
Authors: Harvey Pierre D , Reber Christian
Published: April 2011

Density functional theory is used to characterize the lowest energy excited spin triplet orbital configurations of square-planar halide complexes of palladium(II). Calculations for the eg –> b1g orbital excitation (D4h labels) predict a non-totally symmetric distortion along the b1g normal coordinate, leading to unequal bond lengths for perpendicular metal-ligand bonds (D2h symmetry). Calculated bond length changes are 0.15 Å (0.11 Å) and 0.05 Å (0.00 Å) for PdCl42- (PdBr42-). These values compare favorably to the emitting-state distortions of 0.12 Å and 0.09 Å for K2PdCl4 (0.12 Å, and 0.07 Å for K2PdBr4) determined from resolved single-crystal luminescence spectra. The calculations indicate that the non-totally symmetric distortion is an intrinsic molecular property of these complexes.Key words: palladium(II) complexes, excited state distortions, density functional theory.
13C NMR and crystallographic evidence of push-pull effects in furoindolic β-enamino esters
Authors: Morales-Ríos Martha S , Suárez-Castillo Oscar R , Alvarez-Cisneros Celina , Joseph-Nathan Pedro
Published: April 2011

A series of cyclic enamine derivatives, stabilized through conjugation with an electron-attracting CO2Me group at the β-carbon of the enamine system, has been prepared. The influence of substituents at the vinylogous donor NH2 group on 13C chemical shifts has been investigated, and the results analyzed on the basis of resonance and steric effects on the dipolar charge distribution in the molecule. Intramolecular H-bonding was established for the N-methylated enamines 4a and 4b based on the high-frequency shift of the NH proton and its solvent immutability. Several deuterium-induced isotope effects on 13C chemical shift [nΔC(ND)] due to deuteration at the enamine nitrogen have been measured. The crystal structures of the enamine 1b and the N,N-diacetylated enamine 3a, determined by X-ray analyses, show that these compounds, in the solid state, adopt an s-trans disposition around the C-C=O bond, with the carbonyl ester group and the C=C double bond in an almost coplanar conformation. Comparison of bond lengths of 1b and 3a in the crystalline state provides information on changes in the extent of n-π delocalization arising from the nature of the N-substituent.Key words: β-enamino esters, furo[2,3-b]indoles, push-pull effects, 13C NMR, X-ray.
The molecular complexes of deoxycholic acid and cholesterol with CH3COOD studied by infrared spectroscopy
Authors: Diallo Alpha O , Meyer Claude , Van-Thanh Nguyen
Published: April 2011

The frequencies of the O-D stretching vibrations of CH3COOD have been measured in the liquid and vapour phases and in the form of molecular associations with deoxycholic acid and cholesterol. The spectra confirm that the vapour phase is a mixture of monomeric and dimeric forms at room temperature. The vibrational assignment made for the gaseous phase is used in conjunction with prior data available on the monomeric O-D stretching frequencies in solutions to explain the spectroscopic behaviour of the molecular complexes. The pattern of frequency shifts suggests that deuterium bonding links the cholesterol and CH3COOD in the hydrophilic regions and that the self association of the heavy acid is limited to the dimeric species in the complex with deoxycholic acid. Extra absorption bands observed in the spectral region characteristic of the deuterium bond vibrations are tentatively considered to arise from O-D groups in different local environments with different vibrational energies compared with the D-bonded species. Further evidence for steroid-CH3COOD interactions is obtained from a study of the O-H and CbetaO stretching vibrations of the compounds.Key words: acetic acid-d, complexation with deoxycholic acid and cholesterol, infrared spectroscopy.
Chemoenzymatic enantioselective synthesis of (-)-enterolactone
Authors: Chênevert Robert , Mohammadi-Ziarani Ghodsi , Caron Dave , Dasser Mohammed
Published: April 2011

Enterolactone, a lignan isolated from biological fluids of animals and humans, was synthesized via enzymatic desymmetrization of 2-(3-methoxybenzyl)-1,3-propanediol.Key words: enterolactone, synthesis, lipase, desymmetrization, lignan.
The 4,4'-dimethoxytrityl carbenium ion by ionization of 4,4'-dimethoxytrityl alcohol in acetonitrile - aqueous perchloric and nitric acids containing electrolytes: kinetics, equilibria, and ion-pair formation
Authors: Crugeiras Juan , Maskill Howard
Published: April 2011

We have studied the equilibration shown in eq. [3] of 4,4prime-dimethoxytrityl alcohol in aqueous perchloric and nitric acids containing low proportions of acetonitrile using stopped-flow kinetics techniques. The rate constants for the overall progress to equilibrium, kobs, have been resolved into forward and reverse components using the equilibrium UV absorbance and a value for the molar absorptivity of the 4,4prime-dimethoxytrityl carbenium ion determined in concentrated aqueous perchloric acid. The forward reaction (rate constant kf) is first order in both the alcohol and the acid concentrations; the reverse reaction (rate constant kr) is pseudo first order with respect to the carbocation. At constant hydronium ion concentration, the forward rate constant increases linearly with the concentration of electrolyte, whereas the reverse rate constant decreases. These effects depend upon the nature of the anion, but not the cation, and are not ionic strength effects. At constant anion concentrations, kf in both acids, and kr in perchloric acid, are independent of hydronium ion concentration; however, kr decreases with increasing hydronium ion concentration at constant nitrate concentration. At nonconstant ionic strength, changes in kf and kr observed in increasing concentrations of perchloric acid are attributable wholly to changes in perchlorate concentration. A mechanism is proposed which involves pre-equilibrium protonation of the alcohol, heterolysis of the protonated alcohol to give a 4,4prime-dimethoxytrityl carbenium ion - water ion-molecule pair, then conversion of this into a dissociated carbenium ion in equilibrium with ion pairs. To account for the strong effects of perchlorate and nitrate upon the forward rate constants, it is proposed that these anions provide additional reaction channels from the ion-molecule pair. However, we find no evidence of acid catalysis in the reaction of the ion-molecule pair (in contrast to our finding for the reaction of the corresponding ion-molecule pair formed from dimethoxytritylamine in acidic media). Some of the elementary rate and equilibrium constants of the proposed mechanism have been evaluated.Key words: trityl, carbenium ion, stopped-flow, ion pair, ion-molecule pair.
Redox switching of carbohydrate binding to ferrocene boronic acid
Authors: Moore Andrew NJ , Wayner Danial DM
Published: April 2011

The redox properties of ferrocene boronic acid in aqueous buffers from pH 4 to pH 12 have been investigated using cyclic voltammetry. It is shown that the proton transfer equilibrium is under thermodynamic control over the range of scan rates employed (20-5000 mV/s), leading to pKa values of 10.8 and 5.8 for the ferrocene and ferrocenium forms, respectively. In the presence of sorbitol, fructose, or glucose, the voltammetric behaviour of ferrocene boronic acid at pH 7 is consistent with reversible formation of a carbohydrate - boronic acid complex. However, in all cases, the complexation-dissociation is under kinetic control. Analysis of the voltammetric response as a function of scan rate and carbohydrate concentration, together with an independent spectroscopic determination of the binding constant to the ferrocene form, allows all of the kinetic and thermodynamic constants for each system to be determined. In general, it is found that the binding constants ferrocenium form are about two orders of magnitude greater than those for the ferrocene form. It is possible that this redox dependent switching of the carbohydrate binding can be exploited in amperometric or potentiometric sensing devices.Key words: electrochemistry, boronic acid, binding constants, pKa.
Excess molar volumes of liquid 1-bromoalkane + alkane mixtures. Nitta–Chao characterization of the bromine–bromine and bromine–methylene interactions in binary 1-bromoalkane + alkane mixtures
Authors: García J , Comuñas M J , Lugo L , López E R , Fernández J
Published: April 2011

Excess molar volumes at 298.15 K for the binary mixtures of a 1-bromoalkane with a n-alkane are reported. The VE values were obtained from measurements of the density with an Anton Paar densimeter. Literature data of excess Gibbs energies, excess enthalpies, excess volumes, and activity coefficients at infinite dilution for this kind of mixtures together with molar volumes and vaporization enthalpies of pure bromoalkanes were used to calculate the characteristic parameters of the Nitta-Chao group contribution model. With the obtained parameters, this model consistently describes the experimental data of the 1-bromoalkane + n-alkane mixtures. Furthermore, we have determined the energetic parameters for the Larsen version of the UNIFAC model.Key words: liquids, mixtures, thermodynamics, group contributions.
Computational study on small water clusters using a semiempirical valence bond approach
Authors: Wang Youliang , Gunn John R
Published: April 2011

Small clusters of water (H2O)n, n = 3-8, are studied using a semiempirical valence bond approach to investigate the bonding energy contribution and hydrogen-bonding structure in the most stable conformation. The energy contribution was decomposed into electron pair-pair interactions and valence-bond energy for each water monomer. Our study shows that there is significant bonding difference between small clusters (n [Formula: see text] 5) of water and larger clusters (n > 5). In the larger clusters, there are structures containing tetravalent oxygen centers, which is impossible in the small clusters. The contribution to the binding energy from each H-bond varies from -4.7 kcal/mol to -7.3 kcal/mol in the water clusters considered here. The contribution of -5.9 kcal/mol per H-bond in the cubic octamer is comparable to the experimental value (-6.7 kcal/mol) of the binding energy in ice.Keywords: semi-empirical, valence bond, hydrogen bonding, water clusters.
A theoretical study on the reactivity of vinyl iodonium ions
Authors: Okuyama Tadashi , Yamataka Hiroshi
Published: April 2011

Reactions of methyl(vinyl)iodonium ion and the β-substituted derivatives as well as divinyliodonium ion with chloride ion were examined theoretically by ab initio MO (MP2) calculations at the double-zeta (DZ) + d level. Interaction of the iodonium ion 1 with Cl- leads to chloro-λ3-iodane 2. Transition states for the SN2, ligand-coupling substitution (LC), and β-elimination (βE) are found for reactions at the vinyl group. The barrier to LC is usually the lowest in the gas phase, but relative barriers to SN2 and to βE change with the substituents. Effects of solvent were evaluated by a dielectric continuum model and found to be large on SN2 but small on LC. The experimental observations in solution that the SN2 is the most facile for the β-methyl-substituted 1 while it is retarded by the β-tert-butyl and β-chloro substitutions are reproduced by the calculations that take medium effects into account.Key words: vinyliodonium salt, polyvalent iodine, vinylic SN2 reaction, ligand coupling, ab initio MO calculation.
Aggregation and ion pair acidity in THF: lithium and cesium enolates of α-phenylcyclohexanone
Authors: Wang Daniel Ze-Rong , Streitwieser Andrew
Published: April 2011

The lithium (1) and cesium (2) enolates of α-phenylcyclohexanone, PCH, have been studied in THF at 25°C. Both form monomer-dimer equilibria with K1,2 = 2.8 × 103 M-1 for 1 and 1.8 × 103 M-1 for 2. The ultraviolet-visible (UV-vis) spectra of the monomers and dimers were derived from the spectra of the enolates at a series of concentrations by singular value decomposition. Measurements of the ion pair pK values gave pK = 12.7 for 1 and 19.8 for 2. The monomers of 1 and 2 are alkylated by benzyl halides faster than are the dimers.Key words: carbon-acid ionization, enolate ions, ion pairs, ion pair acidity, aggregation, dimerization constants, alkylation.
Tetraorganodichalcogenoimidodiphosphinato derivatives of dimethyltellurium(IV) compounds. Crystal structures of Me2Te[(SPPh2)2N]2, Me2TeCl[(SPPh2)2N], Me2TeI[(SPPh2)2N], and Me2TeCl[(OPPh2)(SPPh2)N]
Authors: Drake John E , Drake Robert J , Silvestru Anca , Yang Jincai
Published: April 2011

Isolation of Me2Te[(SPPh2)2N]2, Me2Te[(OPPh2)(SPPh2)N]2, and the halo derivatives Me2TeX[(SPPh2)2N] and Me2TeX[(OPPh2)(SPPh2)N], where X = Cl, Br, I, was attempted. Four of these were characterized by single crystal X-ray diffraction, while the formation of the others was confirmed by multinuclear NMR spectroscopy. Me2Te[(SPPh2)2N]2, 1; P21/n (No. 14) with cell parameters a = 14.908(3) Å, b = 11.122(5) Å, c = 30.384(6) Å, β = 92.67(2)°, V = 5032(2) Å3, Z = 4, R = 0.0661, Rw = 0.0696. Me2TeCl[(SPPh2)2N], 2; P[Formula: see text] (No. 2) with cell parameters a = 10.553(2) Å, b = 15.512(2) Å, c = 8.769(3) Å, α = 103.19(2)°, β = 94.27(2)°, γ = 85.76(2)°, V = 1391.3(7) Å3, Z = 2, R = 0.0377, Rw = 0.0328. Me2TeI[(SPPh2)2N], 4; P[Formula: see text] (No. 2) with cell parameters a = 10.954(3) Å, b = 15.580(3) Å, c = 8.751(3) Å, α = 103.49(2)°, β = 93.11(3)°, γ = 87.33(2)°, V = 1449.3(7) Å3, Z = 2, R = 0.0586, Rw = 0.0613. Me2TeCl[(OPPh2)(SPPh2)N], 6; P21/c (No. 14) with cell parameters a = 10.441(9) Å, b = 13.672(10) Å, c = 19.521(9) Å, β = 97.16(6)°, V = 2764(3) Å3, Z = 4, R = 0.0546, Rw = 0.0572. The immediate environment about Te is that of a sawhorse structure in which the lone pair is apparently stereochemically active and occupying an equatorial position in a distorted trigonal bipyramid. In 1, 2, and 4, one sulfur atom of (SPPh2)2N occupies an axial position. If the second sulfur atom is included in the coordination sphere, then the environment about Te becomes distorted octahedral, for 1, and square pyramidal, for 2 and 4. By contrast, in Me2TeCl[(OPPh2)(SPPh2)N], 6, the oxygen atom, rather than sulfur, is bound to tellurium in the axial position, with the S atom, at best, having a very weak interaction with Te.Key words: structure, tellurium, methyl, tetraphenyldichalogenoimidodiphosphinates.
The electrochemical reduction of α-nitrocumene in a protic and basic medium on large surface area (porous) electrodes: electronation-protonation or electrocatalytic hydrogenation?
Authors: Chan-Shing Elisa Soazara , Boucher Denys , Lessard Jean
Published: April 2011

The electrochemical reduction of α-nitrocumene (1) has been investigated under controlled potential, at a mercury pool cathode and at Raney metal (Raney nickel, Raney cobalt and Devarda copper) and fractal nickel electrodes in basic aqueous ethanol. A comparison of the product distribution from the reduction at Hg (electronation-protonation (EP) mechanism) and that from the reduction at Raney metals and fractal nickel has shown that both the electrocatalytic hydrogenation (ECH) and EP mechanisms can be involved at large surface area (porous) transition metal electrodes in a basic protic medium. Cyclic voltammetry was used to determine the reduction potential of 1 at Hg and at bright polycrystalline nickel, cobalt, and copper cathodes in the same medium.Key words: electrocatalytic hydrogenation, electroreduction, 2-nitro-2-phenylpropane, Raney metal cathodes, mechanism.