bluuebunny/crossref_metadata_2025_split huggingface dataset:
Relative rates of bromination and chlorination of 4‐substituted cyclopentenes in methanol, ethanol and acetic acid
Authors: Zhang Ben‐Li , Qiu Jian , Gao Zhen‐Heng 
Published: January 2005

AbstractA set of 4‐monosubstituted cyclopentenes, , were synthesized and their relative rates (kX/kH) for bromination and chlorination were determined in methanol, ethanol and acetic acid at 25 °C by competitive method. log(kX/kH) for most of the substituents can be correlated by means of Taft's equation, log(kX/kH) = ρI σI + C. In methanol ρ = −2·91, ρ = −0·49, in ethanol ρ = −3·07, ρ = −0·70 and in acetic acid ρ = −1·64, ρ = −0·65. The presence of C(<0) is due to a constant steric effect. The deviation of X = H is ascribed to the absence of the steric effect and that of X = CO2Me and CO2Et is accounted for in terms of anchimeric assistance. For chlorination no anchimeric assistance was observed.
Cross interaction constants as a measure of the transition state structure (part III). Mechanism of reactions between 1‐phenylethyl benzenesulfonates with N,N‐dimethylanilines
Authors: Lee Ikchoon , Kim Hyung Yoon , Lee Hai Whang , Kim In Chul 
Published: January 2005

AbstractThe kinetics and mechanisms of the reactions between 1‐phenylethyl benzenesulfonates (1‐PEB) with N,N‐dimethylanilines are investigated in methanol at 35·0°C. Reactivity and selectivity trends were found to be similar to those for the reactions of 1‐PEB with anilines, but the magnitudes of cross interaction constants, ρXZ, between substituents X in the nucleophile and Z in the leaving group were substantially smaller indicating no hydrogen‐bond bypass bridge formation in the transition state. However, the magnitude of ρXZ suggested a direct electrostatic interaction between the reaction centers in the nucleophile and leaving group in the frontside nucleophilic attack with a loose transition state structure.
Kinetics and mechanism of the acid‐catalyzed hydration of dihydro‐1,4‐dioxin
Authors: Kresge A. J. , Yin Y. 
Published: January 2005

AbstractThe cyclic vinyl ether dihydro‐1,4‐dioxin is converted to its cyclic hemiacetal hydration product, 2‐hydroxy‐1,4‐dioxane, in aqueous solution by an acid‐catalyzed reaction for which k = 1·80 × 10−5 M−1 S−1 at 25°C. This reactivity and the solvent isotope effect k/k = 2·2 show that the reaction occurs by rate‐determining proton transfer from catalyst to substrate and not by a pre‐equilibrium mechanism as recently proposed.2
The cation radical vinylcyclobutane rearrangement
Authors: Reynolds Dan W. , Harirchian Bijan , Chiou Huh‐Sun , Marsh B. Kaye , Bauld Nathan L. 
Published: January 2005

AbstractThe cation radical vinylcyclobutane (VCB) rearrangement is found to be a reaction of substantial scope, synthetic utility, and exceptional kinetic facility. In conjuction with cation radical cyclobutanation, it constitutes an effective method for net (indirect) Diels–Alder addition to electron rich dienophiles. Reactions can be carried out with either aminium salt or photosensitized electron transfer (PET) initiation and are powerfully facilitated by ionizable substituents such as p‐anisyl, phenylthio, and phenoxy at the 2‐position of the vinylcyclobutane. The intramolecularity of the reaction is clearly established and in four discrete systems preferred sr (suprafacial/retention) stereochemistry is observed. A theoretical basis for sr stereochemistry in the cation radical VCB rearrangement is advanced. The transition state for the reaction is considered to be similar to that for the direct cation radical Diels–Alder cycloaddition, another cation radical pericyclic reaction which converges on the same product. This model of the VCB rearrangement transition state is used to rationalize the strong rate‐retarding effect of a Z‐methyl substituent attached to the vinyl group and of a methyl substituent at the 4‐position of the vinylcyclobutane ring cis to the vinyl substituent.
Selective 1,2‐addition of organocerium(III) reagents to α,β‐unsaturated carbonyl compounds
Authors: Imamoto Tsuneo , Sugiura Yasushi 
Published: January 2005

AbstractOrganocerium(III) reagents reacted with α,β‐unsaturated carbonyl compounds to give 1, 2‐addition products (allylic alcohols) in good to high yields. The reaction was studied from a mechanistic point of view by the use of (E)‐ and (Z)‐1‐(4′‐methoxyphenyl)‐3‐phenyl‐2‐propen‐1‐ones and 4,4‐ethylenedioxy‐2,6‐dimethyl‐2,5‐cyclohexadienone as the probe compounds. A polar pathway was suggested for the reaction with the former enones. On the other hand, the operability of single electron transfer processes was demonstrated in the reaction with the latter probe compound.
A preliminary quantum‐chemical study of the mechanisms for the reactions of perchlorofluoroethanes with nucleophiles initiated by chlorophilic attacks
Authors: Huang M.‐B. , Li X.‐Y. 
Published: January 2005

AbstractA preliminary theoretical study of the mechanisms for the reactions of the perchlorofluoroethanes CF2ClCCl3 (1), CF2ClCCl2F (2) and CF3CCl3 (3), with nucleophiles has been carried out by the MNDO method, following the experimentally suggested process shown in Scheme 1. The unlikely chlorophilic attack in the first step of Scheme 1 has been shown to be feasible for 1, 2 and 3 by analysis of the MO interactions. The second step has been found to be affected by the anionic hyperconjugation which stabilizes the anions CF2ClCCl (4), CF2ClCClF− (5) and CF3CCl (6) and would make reactions (2) (the second step) unfeasible in gas phase, but in solution reaction (2) may still easily occur for 4 and 5.
Multifunctional catalysis for transamination reaction of α‐amino acids with glyoxylic acid in synthetic bilayer memberane
Authors: Murakami Yukito , Kikuchi Jun‐Ichi , Shiratori Nobuyuki 
Published: January 2005

AbstractThe transamination reaction of α‐amino acids with glyoxylic acid as catalyzed by copper(II) ions was investigated kinetically in an aqueous medium at pH 5·0 and 30·0°C. L‐Phenylalanine transferred its amino group to glyoxylic acid most readily among seven different amino acids used here in the single‐walled bilayer vesicle formed with N,N‐dihexadecyl‐Nα‐[6‐(trimethylammonio)hexanoyl]‐L‐histidinamide bromide (N+C5His2C16). Such rate enhancement was found to originate from the cooperative trifunctional catalysis: a coordination effect exercised by copper(II) ions, a general acid–base catalysis by the imidazolyl group of the lipid, and a hydrophobic field effect provided by the bilayer vesicle. Lack of any of the three functions failed to give out significant rate enhancement. As regards correlation between the reactivity and the nature of α‐amino acids, the copper(II)‐catalyzed transamination was progressively enhanced as hydrophobicity of the α‐amino acid was increased in the N+C5His2C16 vesicle.
Application of Marcus theory to photochemical proton transfer reactions. II. Modifications based on intersecting state models
Authors: Yates Keith 
Published: January 2005

AbstractVarious modifications of the Marcus equation have been applied to the problem of photochemical proton transfer, using available data on general acid‐catalyzed photohydration reactions. These include incorporation of asymmetry and tightness parameters, as well as distance variation as a function of exoor endothermicity. The intersecting state model of Formosinho has also been successfully applied to these reactions. The overall conclusion from all of these approaches is that the reactions are characterized by somewhat asymmetric and ‘loose’ transition states, with a small but significant degree of charge development on the in‐flight proton at the transition state. Estimates of the intrinsic barriers and work terms place these in the 5–7 kcal and 2–3 kcal ranges respectively. A simple valence bond configuration mixing model leads to similar qualitative conclusions about the nature of the transition states in these reactions.
Lack of manifestation of an anomeric effect in 2‐diphenylphosphinoyl‐1, 3‐dioxane and 2‐diphenylphosphinoyl‐1, 3‐oxathiane1
Authors: Juaristi E. , Flores‐Vela A. , Labastida V. , Ordoñez M. 
Published: January 2005

AbstractThe spectroscopic evidence for the predominance of the equatorial conformers in the title compounds was confirmed by the study of derivatives containing counterpoise substituents, and by chemical equilibration of anancomeric models. ΔG°C [P(O)Ph2] ≳ 3·2 kcal/mol was determined in the dioxane, and ΔG°C [P(O)Ph2] = 1·42 ± 0·12 kcal/mol in the oxathiane. It follows then that the strong anomeric interaction observed previously in SCP segments does not show up in the six‐membered heterocycles 2 and 7, which contain OCP moieties. This may be due to an inherent inability of oxygen to act as an electron donor to the axial P(O)Ph2 substituent, or to a dominant repulsive steric interaction in the axial conformers.
A mechanistic study of the selenium‐catalysed carbonylation of secondary amines with carbon monoxide
Authors: Fujiwara Shin‐Ichi , Miyoshi Noritaka , Ogawa Akiya , Kambe Nobuaki , Sonoda Noboru 
Published: January 2005

AbstractThe reaction pathway of urea formation by selenium‐catalysed carbonylation of secondary amines with carbon monoxide in the presence of oxygen was studied using piperidine as a secondary amine. It was established that selenium reacts with carbon monoxide and piperidine to give carbamoselenoate as an intermediate, which affords biscarbamoyl diselenide by the oxidation with molecular oxygen. Aminolysis of biscarbamoyl diselenide gives the urea derivative, accompanying the regeneration of selenium catalyst.
Alkylating properties of phosphate esters. 4. Medium effects on methylation of pyridines by trimethyl phosphate
Authors: Modro A. M. , Modro T. A. 
Published: January 2005

AbstractThe rates of N‐methylation of pyridine and its ring‐substituted derivatives by trimethyl phosphate were measured in D2O and in CDCl3. Relative rates, together with the solvent activity coefficients of substrates for the transfer form chloroform to water, were used for determining the solvent activity coefficients of the activated complexes for this SN2 reaction. The results indicate early activated complexes for all pyridines, with the most nucleophilic substrate showing the most reactant‐like activated complex.
Lack of protonation of benzene and toluene by trifluoromethanesulfonic acid and its significance for evaluating superacid strengths
Authors: Fărcaşiu Dan , Miller Glen 
Published: January 2005

AbstractDistribution between pentane and trifluoromethanesulfonic acid (TFMSA) and carbon‐13 NMR measurements showed that benzene and toluene are not protonated to any significant extent in TFMSA. This finding contradicts previous reports, and validates the ranking of superacids based on the extent of benzene protonation.
Cross‐interaction constants as a measure of the transition state structure (part V). The transistion state structure for reactions of phenacyl benzenesulphonates with benzylamines in methanol
Authors: Lee Ikchoon , Shim Chang Sub , Lee Hai Whang 
Published: January 2005

AbstractThe Kinetics of reactions of phenacyl benzenesulphonates with benzylamines were investigated in methanol at 45·0 °C and the cross‐interaction constants λXY, λYZ and βXZ were determined in order to elucidate the transition‐state structure. The unusually small magnitude of λXY can only be accounted for by the resonance ‘shunt’ effect of the α‐CO group of the phenacyl system. Large |λYZ| values indicate a small degree of bond breaking whereas relatively large |βXZ| values compared with those for the dissociative SN2 reaction indicate a relatively tight transition state for the reactions. Further, the similar magnitudes of βXZ values compared with those of the corresponding aniline nucleophile series suggest a similar transition‐state structure for the two armatic amine nucleophile series.
Hydrogen‐bonding 8. Possible equivalence of solute and solvent scales of hydrogen‐bond basicity of non‐associated compounds
Authors: Abraham Michael H. , Buist Gabriel J. , Grellier Priscilla L. , McGill R. Andrew , Prior David V. , Oliver Susan , Turner Elizabeth , Morris Jeffrey J. , Taylor Peter J. , Nicolet Pierre , Maria Pierre‐Charles , Gal Jean‐François , Abboud José‐Luis M. , Doherty Ruth M. , Kamlet Mortimer J. , Shuely Wendel J. , Taft Robert W. 
Published: January 2005

AbstractUsing the solvatochromic indicator method, a scale of solvent hydrogen‐bond basicity, β1 (General), has been set up using a series of double regression equations,

for 11 aniline‐type indicators. A similar solvent scale, β1 (Special), has been constructed by the homomorphic comparison method using only results by Laurence et al. on the indicators 4‐nitroaniline and 4‐nitro‐N,N‐dimethylaniline. Results are available from our previous work on a general solute scale, β, and we have also obtained a special solute scale, β2 (pKHB) from available log K values for hydrogen‐bond complexation of bases with 4‐fluorophenol in CCl4. However, the two solute β2 scales are virtually identical.It is shown that there is a general connection between β1(General) and β, with r = 0·9775 and s.d. = 0·05 for 32 compounds, and between β1(Special) and β, with r = 0·9776 and s.d. = 0·06 for the same 32 compounds. The latter correlation over 60 compounds yields r = 0·9684 and s.d. = 0·07. However, there are so many compounds in these regressions for which the differences in the solvent and solute β values are larger than the total expected error of 0·07 units that the use of β1 to predict β2 or vice versa is a very hazardous procedure. About 70 new β1 values obtained by the double regression method are also reported.
Solute–solvent interactions in chemical and biological systems. IV. Correlations of ΔG, ΔH and TΔS of transfer of aliphatic and aromatic solutes from 2,2,4‐trimethylpentane to aqueous solution
Authors: Fuchs Richard , Abraham Michael H. , Kamlet Mortimer J. , Taft Robert W. 
Published: January 2005

AbstractΔG, ΔH and TΔS of transfer of 25 aliphatic and aromatic solutes from 2,2,4‐trimethylpentane to aqueous buffer (pH 7) at 298 K have been examined in terms of intrinsic volumes and the solvatochromic parameters π*, δ, β and α of the pure solutes Correlations of the form

indicate that the thermodynamic quantities of transfer are unequally affected by solute properties; most notably, for aromatic solutes the cavity term mVI/100 is a principal (unfavorable) factor affecting TΔS, but has little effect on ΔH. Transfer to water is favored by increasing solute π* (dipolarity–polarizability), β (H‐bond basicity) and α (H‐bond acidity), because water has greater dipolarity, H‐bond acidity and H‐bond basicity than trimethylpentane. Hydrogen bonding contributes exothermically to ΔH, but unfavorably to TΔS, as would be expected from a loss of transitional entropy. Correlations of ΔG, ΔH and TΔS with solute VI/100, β, α, μ2 and polarizability function [(n2 − 1)/(n2 + 2)] give closely comparable results.
Thermodynamic and spectroscopic properties of 2‐pyrrolidinones. 3. NMR spectroscopic studies on 2‐pyrrolidinone in different solvents
Authors: Ruostesuo P. , Pirilä‐Honkanen P. , Heikkinen L. 
Published: January 2005

Abstract1H, 13C, 15N and 17O NMR chemical shifts, 1JNH and 1JCH coupling constants and line widths (Δν1/2) of the 14N and 17O resonance lines were determined for 2‐pyrrolidinone neat and for several 2‐pyrrolidinone–solvent systems. The 17O NMR chemical shift of 2‐pyrrolidinone was clearly most sensitive to the solvent effects, but changes with the solvent were also observable in the 13C (CO) and 15N NMR chemical shifts, the 1JNH coupling constants and especially the line widths of the 14N and 17O resonance lines. In general, the results reflected a hydrogen bonding effect between the oxygen atom of 2‐pyrrolidinone and the proton‐donating solvents and a weak molecular interaction of the NH proton of 2‐pyrrolidinone with the proton‐accepting solvents. The results are compared with the NMR data for the corresponding binary mixtures of 1‐ethyl‐2‐pyrrolidinone.
Energy‐minimum structure of the hydrocarbon portion of [45] (1,2,3,4,5)ferrocenophane
Authors: Rudziński Jerzy M. , Ōsawa Eiji , Hisatome Masao , Watanabe Jun , Yamakawa Koji 
Published: January 2005

AbstractIn order to assist in the interpretation of the D5h molecular structure of [45] (1,2,3,4,5)ferrocenophane (1) obtained from x‐ray crystallographic analysis, a hypothetical hydrocarbon C30H40 (2) was derived from 1 by removing the iron atom and subjected to geometry optimization by molecular mechanics. A stable D5 conformer was found for 2, which has a g+ g− g+ helical bridge conformation and a ring–ring distance almost identical with that in 1 and ferrocene itself. Optimization of a perhydrogenated hydrocarbon model gave a highly strained C1 structure.
Solvation of indenyllithium and indenylpotassium studied by13C NMR spectroscopy
Authors: Eliasson Bertil , Lejon Tore , Edlund Ulf , Mueller Ronald L. , Staley Stuart W. 
Published: January 2005

AbstractPolarization of the π‐electron systems of indenyllithium (In−Li+) and indenylpotassium (In−K+), which is generally indicative of the degree and tightness of contact ion pairing, was investigated by13C NMR spectroscopy. The effect of changing solvent is about two thirds as great for In−K+as for In−Li+. The indenyl anion is more strongly polarized with Li+/dimethyl sulfoxide than with Li+/ammonia, whereas the reverse is observed for In−K+. Complexation of In−K+in tetrahydrofuran (THF) by cryptand(2,2,2) results in a π‐electron distribution identical with that of In−Li+in hexamethylphosphoramide, indicative of solvent‐ or agent‐separated ion pairs or free ions. Small but significant cation–anion interactions are observed for In−K+in hexamethylphosphoramide and for In−N+Me4in THF.
Structure and stability of 2σ/1σ* three‐electron‐bonded radical cations from 1,n‐bis(alkylthio)alkanes in aqueous solutions
Authors: Anklam Elke , Asmus Klaus‐Dieter , Mohan Hari 
Published: January 2005

AbstractA large number of 1,n‐bis(alkylthio)alkanes, R′S(CH2)nSR″, was investigated by pulse radiolysis in aqueous solutions. The sulphur‐centred radical cations, which were obtained on oxidation of the dithia compounds by ·OH radicals, are stabilized via 2σ/1σ* three‐electron interaction between two sulphur atoms. Intramolecular stabilization is the only process observed for n = 2–4, whereas for longer chain compounds with n > 4 some intermolecular sulphur–sulphur interaction is also indicated. The stability and yields of the three‐electron‐bonded species also depend on the nature of the substituents R′ and R″, and for the intermolecular systems are a function of solute concentration. The results corroborate earlier investigations and demonstrate, in particular, that the optical parameters are a very sensitive measure of changes in electron density and structure of the three‐electron‐bonded systems.
Solvolysis of highly reactive phosphorus‐sulphonic anhydrides. Stereochemical and chemical arguments for phosphathiacylium cation formation
Authors: Skrzypczyński Zbigniew 
Published: January 2005

AbstractThe stereochemistry and mechanism of solvolysis of optically active tert‐butylphenylphosphinothioic‐O‐trifluoromethanesulphonate (1) in solvents of different ionizing power were studied. It was found that in solvents of high ionizing power and low nucleophilicity 1 ionizes with the formation of a phosphathiacylium cation (2) as the reaction intermediate. Product resulting from the reaction of 2 with anisole was isolated and characterized.
Stereochemistry of silver‐ion‐assisted solvolysis of tert‐butylphenylphosphinothioic iodidate. Phosphathiacylium cation formation
Authors: Skrzypczyński Zbigniew 
Published: January 2005

AbstractStudies on the stereochemistry of silver‐ion‐assisted solvolysis of a new optically active tert‐butylphenylphosphinothioic iodidate demonstrated that the reaction proceeds with inversion of configuration at the phosphorus atom. Evidence for phosphathiacylium cation formation as a reactive intermediate is presented.
Rates of hydrogen exchange and kinetic isotope effects in the reaction of p‐nitrophenylnitromethane with amine bases in toluene solution: Absence of internal return
Authors: Kresge A. J. , Powell M. F. 
Published: January 2005

AbstractRates of loss of tritium from p‐nitrophenylnitromethane‐α‐t in toluene solution catalyzed by a series of amine bases show that hydrogen exchange in this system occurs at the same rate as ionization of the carbon acid, and that internal return is not taking place. This substantiates a previous suggestion that very large deuterium isotope effects reported for some of these reactions and attributed to extensive tunnelling were determined under conditions where the isotopic label was being lost and are therefore artifacts. This study provides isotope effects which are generally much smaller than those reported previously, but some tunnelling may still be taking place in some of these reactions.
Porphyrin–tyrosine interactions: Photo‐CIDNP and NMR studies
Authors: Le Nouen D. , Marko J. , Vermeersch G. , Febvay‐Garot N. , Combier A. Lablache , Perree‐Fauvet M. , Gaudemer A. 
Published: January 2005

AbstractStrong 1H and 13C nuclear polarizations are observed when aqueous solutions of synthetic water‐soluble porphyrins are irradiated in the presence of tyrosine and some of its derivatives. These polarizations are strongly pH dependent. Evidence is also shown for the formation, in the dark, of a complex between the reactants. The association constants are evaluated from the NMR chemical shifts on the aromatic tyrosine protons induced by the presence of the porphyrins. The nature of the intermediate radical pair generating the CIDNP effects is discussed. An electron‐transfer reaction from tyrosine to the excited triplet state of the porphyrin, or within the porphyrin–tyrosine excited complex, is expected to be the primary step in the reaction. It is followed by subsequent proton transfer within the initial ion radical pair. The spin polarizations arise principally from the back‐transfer step, as the reactants are the only products which are polarized.
Brønsted coefficients and the theory of acid–base catalysis of proton transfers from carbon acids
Authors: Arnaut Luís G. , Formosinho Sebastiāo J. 
Published: January 2005

AbstractThe intersecting‐state model previously used to interpret proton transfers in ground and excited states and enzyme catalysis was applied to the general acid–base catalysis of carbon acids. The results are consistent with the predictions published for these systems and provide a new physical meaning for the Brønsted coefficients. It is shown that in addition to the linear free energy effect, the Brønsted coefficients are also influenced by the tightness of the transition states and by electronic effects. The model suggests that the increased reactivity of carbon acids towards proton transfer in non‐hydrogen‐bonding solvents is caused by an added electronic effect on the thermodynamics of the reactions. The curvatures of the Brønsted plots are interpreted in terms of an entropic contribution to the position of the transition state.
Additivity of bond separation energies of hydrocarbons and their thermochemical data
Authors: Ibrahim Mustafa R. 
Published: January 2005

AbstractThe additivity of experimental bond separation energies is demonstrated. This thermodynamic function can be reproduced by summation of group equivalents with a correction for the ‘stabilization’ energy. For this purpose, several group equivalents have been developed for alkanes and alkenes. The correlation between the computed and the experimental bond separation energies is good. These equivalents are then used in evaluating the stabilization (destabilization) energies for aromatic hydrocarbons and polyenes. The results obtained are in good agreement with values reported in the literature.