bluuebunny/crossref_metadata_2025 huggingface dataset:
Oxygen‐atom transfer chemistry of α‐AZO hydroperoxides: Effect of competitive intramolecular hydrogen bonding and α‐methyl substitution
Authors: Baumstark Alfons L. , Vasquez Pedro C.
Published: January 2005

1‐(Phenylazo)‐1‐(2‐pyridyl)ethyl hydroperoxide, 1, 1‐(phenylazo)‐1‐(2‐furyl)ethyl hydroperoxide, 2, phenylazo(2‐furyl)methyl hydroperoxide, 3, 1‐(phenylazo)‐1‐(4‐anisyl)ethyl hydroperoxide, 4, were synthesized in moderate yield by autoxidation of the phenylhydrazones in benzene. The ionic oxidation of benzyl methyl sulfide in benzene by 1‐4 yielded the sulfoxide and the metastable α‐azo hydroxides in essentially quantitative yield. The reaction was of the first order in α‐azo hydroperoxide and sulfide, respectively. The relative reactivity series found was: 1(1.0) <4(1.4) < phenylazo(4‐anisyl)methyl hydroperoxide 5 (2·9) <2 (3·8) < 3 (9·6). α‐Methyl substitution was found to slow the rate of oxygen‐atom trasfer by a factor of 2 to 2.5. The low relative reactivity of 1 was opposite that expected based on electronic effects. Competitive intramolecular hydrogen bonding of the hydroperoxy proton to the pyridyl nitrogen in 1 accounted for the observed result.
Homocub‐1(9)‐ene trapped
Authors: Chen Ning , Jones Maitland
Published: January 2005

Decomposition of the tosylhydrazone salts formed from cubyl carboxaldehyde and homocubanone, potential precursors of cubylcarbene (5) and homocubanylidene (6), leads in each case to products derived from 6. It is suggested that homocub‐1(9)‐ene (7) is the active ingredient in the formation of 6 from both precursors. The hydrazones formed from N‐aziridylamines are useful photochemical sources of 6, and presumably other carbenes. Flash vacuum pyrolysis of the tosyl hydrazone salts and hydrazones at 600°C leads to indene.
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

A 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

The 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

The 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

The 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

Organocerium(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

A 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

The 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

Various 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

The 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

The 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

The 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

Distribution 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

The 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

Using 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

ΔG, 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

1H, 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.
Molecular mechanics (MM2) calculations on siloxanes
Authors: Frierson Manton R. , Allinger Norman L.
Published: January 2005

The MM2 force field has been extended so that calculations may be carried out on siloxanes. The parameters chosen give a good fit to available experimental data.
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

In 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

Polarization 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.
Quantification of resonance in thioamides from infrared spectral data
Authors: Randhawa H. S. , Suri S. K.
Published: January 2005

A model is proposed for the quantification of resonance in thioamides. The percentage resonance and CS and CN bond distances predicted from the model are reported and the results are compared with those for the corresponding amides.
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

A 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

The 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

Studies 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

Rates 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

Strong 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

The 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

The 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.
Deuterium fractionation factors for carbon–hydrogen bonds: Calculations using scaled quantum‐mechanical force constants
Authors: Williams Ian H.
Published: January 2005

The determination of uniform scaling factors for force constants calculated at the STO‐3G and 4‐31G levels of ab initio SCF MO theory is described; scaled 4‐31G force constants provide reasonable estimates of reduced partition function ratios for deuteriated/protiated molecules. Gas‐phase deuterium fractionation factors relative to methane calculated using scaled 4‐31G force constants are, however, consistently too high, whereas those obtained by the MP2/6‐31G* and scaled STO‐3G methods are inconsistent. Scaled 4‐31G deuterium fractionation factors relative to water for a range of structural moieties correlate linearly with experimental aqueous‐phase values, but are also consistently overestimated. Substitution at a hydrogen‐bearing carbon atom affects the deuterium fractionation factor through the combined effect of changes in reduced mass and changes in force constants other than for CH stretching.
The decay of 2‐phenyl‐1,3‐dioxolane‐2‐ylium salts in solution
Authors: Spange Stefan , Heublein Günther , Schütz Heidrun , Lukaszcyk Jan , Jedlinsky Zbigniew
Published: January 2005

The decay of 2‐phenyl‐1,3‐dioxolane‐2‐ylium salts in 1,2‐dichloroethane solution has been studied by means of 1H NMR and UV spectroscopy and conductimetry. The influence of the nature of the anion (SbCl, SbCl5Br−, AsF) and of the methyl substituents on the rate constants of decay is discussed.
Triple ion chelate structure of an equilibrium Michael adduct
Authors: Viteva L. , Stefanovsky Y. , Tsvetanov Chr. , Gorrichon L.
Published: January 2005

The equilibrium adduct from the Michael reaction between lithium dimethylphenylacetamide enolate and methyl cinnamate has been studied by means of 7Li, 13C and 1H NMR and IR spectroscopy as well as by electrical conductivity measurements in THF and diethyl ether. The 7Li NMR and the electrical conductivity measurements data are consistent with a triple ion structure of [A−M+A−]− M+ type, favoured in THF. All other spectral data support an intramolecular chelation in the triplet fragment.
Organophosphorus herbicides and plant growth regulators part 1. Synthesis and protonation behaviour of glyphosate and related compounds
Authors: Dhansay M. A. , Linder P. W. , Torrington R. G. , Modro T. A.
Published: January 2005

Ionization constants of N‐phosphonomethylglycine (glyphosate) and three structurally related plant‐growth regulators were determined by potentiometric titration. The effect of pD on the chemical shifts of the 1H and 13C nuclei of the methylene groups and of the 31P nucleus in these substrates was measured by NMR spectroscopy. The results allowed an assignment to be made of a specific site (carboxylic, phosphonic, ammonium) in the molecule to each of the determined pKa values.
Theoretical studies of hydride transfer reactions
Authors: Formosinho Sebastião J.
Published: January 2005

The intersecting‐state model (ISM) has been applied to the study of hydride transfer reactions between variously substituted and elaborated pyridinium ions in solution. Chemical bond order is conserved along the reaction coordinate, with a transition state bond order of n≠ = 0·5. This supports the view that these reactions have essentially a synchronous nature and occur by thermal activation over an energy barrier. Tunnelling of H− is negligible because the effective mass for the solvated species is high. Reaction energy and kinetic isotope effects are well accounted for by the model. For reactions in the vapour phase, ISM and the tunnel effect theory show that tunnelling becomes the dominant mechanism. The same is also valid for proton transfers in the vapour phase.
A study of a novel degenerate carbocation rearrangement of the 4,9‐dimethyl‐9‐barbaralyl cation by dynamic 13C and 1H NMR spectroscopy in superacid
Authors: Engdahl Carin , Ahlberg Per
Published: January 2005

The 4,9‐dimethyltricyclo [3.3.1.02,8]nona‐3,6‐dien‐9‐yl (4,9‐dimethyl‐9‐barbaralyl) cation (3) was generated from 4,9‐dimethyl‐9‐barbaralol (5) at −135°C in two different superacid mixtures [FSO3HSO2CIFSO2F2(1:6:1) and FSO3HSO2CIFSO2F2CHCl2 (2:10:10:1 by volume)]. Its 1H and 13CNMR spectra show a strong temperature dependence in the range −150 to −125°C. The changes in band shapes with temperature show that the following exchanges take place: 4‐methyl with 9‐methyl, C‐4 with C‐9, C‐1 with C‐3 and C‐2 with C‐8. C‐5, C‐6 and C‐7 are found not to exchange rapidly either with each other or with the other carbons in 3. The mechanism of this novel rearrangement is suggested to involve the bicyclic 2,7‐dimethylbicyclo [3.2.2] nona‐3,6,8‐trienyl cation and the secondary barbaralyl cation 4,6‐dimethyltricyclo [3.3.1.02,8] nona‐3,6‐dien‐9‐yl as intermediates rather than 7,8‐dimethyl bicyclo [3.2.2] nona‐3,6,8‐trienyl cation, which does not have a methyl group on the allyl cation moiety. Comparisons with rearrangement mechanisms for other barbaralyl cations were also made. The rate constant for the degenerate rearrangement of 3 is 160 s−1 at −140°C, which corresponds to ΔG≠s = 26 kJ mol−1 (6·3 kcal mol−1). At −125°C ion 3 rearranges non‐degenerately to the 1,4‐bishomotropylium cation 1,8‐dimethylbicyclo [4·3·0] nona, 2,4,7‐trienyl (4) with k = 3 × 10−4 s−1. A mechanism for this rearrangement and the synthesis and purification of the ion precursor 4 are also reported.
Stereochemistry and kinetics of thiophenoxide ion addition to methyl para‐substituted phenylpropiolate derivatives in methanol. I
Authors: Hamed Ezzat A. , Moussa Adel. M. , Youssef Abdel‐Hamid A.
Published: January 2005

The reactions of a series of methyl para‐substituted phenylpropiolates with thiophenoxide ion in methanol were studied and their rates measured. Methyl β‐thiophenyl esters were obtained as mixtures of Z and E isomers and their configurations were established by 1H NMR spectroscopy. Good Hammett correlations with ρ values of 1·57–1·97 were obtained, which suggest a carbanionic character of the transition state. A stepwise mechanism proceeding via an intermediate carbanion is postulated for this non‐sterospecific addition reaction.
Dephosphorylation by peroxyanions in micelles and microemulsions
Authors: Bunton Clifford A. , Mhala Marutirao M. , Moffatt John R.
Published: January 2005

Dephosphorylation of p‐nitrophenyl diphenylphosphate (pNPDPP) by OH− in aqueous cationic micelles of cetyltrimethylammonium hydroxide (CTAOH) is inhibited by tert‐butyl hydroperoxide (t‐BuO2H), which reduces the binding of OH−, and whose anion is a relatively ineffective nucleophile. Reaction is faster in microemulsions of CTACl, n‐octane and OH−, with t‐BuO2H as cosurfactant, than in otherwise similar microemulsions with t‐butyl alcohol as cosurfactant. Dephosphorylation by m‐chloroperoxybenzoate ion is slower in microemulsions of cetyltrimethylammonium mesylate or benzenesulfonate, with n‐octane and tert‐butyl alcohol, than in cationic micelles, but a wide range of concentrations can be examined. The reaction rates qualitatively follow the concentration of peroxy acid relative to surfactant, and inert anions slow the reaction. Dephosphorylation by peroxyphthalate dianion in micelles of CTACl fits a pseudophase kinetic model, except in very dilute surfactant where reaction is faster than predicted.
Homoallylic interaction of the double bond with substituents
Authors: Xue Liang , Lambert Joseph B.
Published: January 2005

Ab initio calculations were carried out on systems with a geometrically constrained relationship between a double bond and a homoallylic substituent X, as in YCCCCX. The axial–equatorial equilibria in 3‐substituted methylenecyclohexanes and 4‐substituted cyclohexenes are determined largely by the homoallylic interaction between X and the double bond. The homoallyic substituent was chosen to be OCH3, and the nature of the double bond was varied by changing Y (H, CH3, F, Cl, CN). NMR experiments previously had found very significant dependences of the axial–equatorial equilibrium on the nature of the X and Y substituents. Electrostatic calculations, based on Mulliken charges, reproduce the experimental (NMR) substituent effects on the axial–equatorial equilibrium and suffice to explain all the observations.
Evidence for formation of 2‐nitrobenzoylperoxy radical generated from 2‐nitrobenzoyl chloride and superoxide: Spin trapping of the peroxyradical intermediate and its oxidizing abilities
Authors: Kim Kyoung Soo , Lim Sang Chul , Hoshino M. , Kim Yong Hae
Published: January 2005

2‐Nitrobenzoyl chlorde was reacted with superoxide in acetonitrile to form 1 2‐nitrobenzoylperoxy radical intermediate which showed a stronger oxidizing ability than that from superoxide, as confirmed by ESR spin trapping studies.
Contribution of activity coefficients to the nucleophilicity of solvent components in solvent mixtures. Influence of added acetone on selectivity values, kE/kW, in ethanol–water mixtures
Authors: Ta‐Shma Rachel , Rappoport Zvi
Published: January 2005

Literature data on the change in the selectivity values, kE/kw, for several solvolysing systems in ethanol (E)–water (W) on addition of acetone were analysed. It is suggested that acetone acts as a basic cosolvent for both ethanol and water and that the change in the activity coefficients on changing the solvent composition is mainly responsible for the observed effect.
Cross‐interaction constants as a measure of the transition state structure. Part 8. Mechanism of the reaction of 2‐phenylethyl benzenesulphonates with benzylamine in acetonitrile
Authors: Lee Ikchoon , Lee Won Heui , Lee Hai Whang
Published: January 2005

The kinetics of reactions between 2‐phenylethyl benzenesulphonates (2‐PEB) and benzylamines in acetonitrile at 65·0°C have been studied; the mechanism was examined on the basis of the sign and magnitude of cross‐interaction constants ρij and βij. In contrast to the reactions of 2‐PEB with anilines in methanol, participation of the aryl‐assisted pathway was negligible, with a strong indication that the reaction proceeds largely by an intermolecular SNi mechanism with a four‐centre transition state (TS). The effect of substituents on the TS variation was in accord with the predictions of the quantum mechanical model.
Cross‐interaction constants as a measure of the transition state structure. Part 9. The degree of bond formation in the SN2 transition state involving anionic nucleophiles
Authors: Lee Ikchoon , Koh Han Joong , Huh Chul , Lee Hai Whang
Published: January 2005

The second‐order rate constants of the reactions between benzenesulphonyl chlorides and anionic nucleophiles, benzoates and cinnamates, in methanol at 30·0°C are reported. A marked increase in rate found with a p‐nitro substituent in the substrate indicated the development of an electron‐rich centre on the S atom in the transition state. The two types of cross‐interaction constants, ρxy and λxy, suggested that bond formation in the SN2 transition state with anionic nucleophiles is greater than that for the corresponding reaction with aniline nucleophiles.
General‐acid, specific‐base and intramolecular catalysis of the dehydration step in nitrone formation from furfurals and phenylhydroxylamine
Authors: Fett Roseane , Simionatto Edésio L. , Yunes Rosendo A.
Published: January 2005

Nitrone formation from furfural and 5‐nitrofurfural with phenylhydroxylamine was studied in water at 25°C and ionic strength 1·0 (KCl). The reaction exhibits general acid catalysis (α = 0·52), specific base catalysis and a pH‐independent process. The results are consistent with a mechanism where dehydration of an N,N′‐dihydroxy addition intermediate, formed by a rapid pre‐equilibrium, is the only rate‐determining step over the pH range studied (1–11). In contrast, nitrone formation from p‐chlorobenzaldehyde and N‐methylhydroxylamine exhibits a clear change of rate‐determining step, from dehydration of the addition intermediate to attack of the nucleophile on the aldehyde, with increase in pH. The profile of log k2 (k2 = second‐order rate constant) vs pH indicates, by comparison with the corresponding profile for similar reaction, that phenylhydroxylamine exhibits unexpected nucleophilic behaviour in this reaction. The pH‐indpendent dehydration process, which has a higher rate constant than that predicted from the Brønsted line for the water catalysis, appears to involve intramolecular catalysis rather than bifunctional catalysis by water.
Rearrangement and cyclization in the ionization of the 4‐chloro‐3‐methylbutanoyl cation
Authors: Fărcaşiu Dan , Miller Glen , Sharma Shalini
Published: January 2005

The second ionization of the 4‐chloro‐3‐methylbutanoyl ion forms a primary alkyl acyl dication, as a tight ion pair. Methyl and hydrogen shifts occur to comparable extents indicating that the relative stability of the product (sec‐ or tert‐carbocation) does not influence the energy barrier for the shift. The product of methyl shift (1,3‐sec‐alkyl acyl dication) loses the proton closest to the counterion in the tight ion pair and forms the pent‐3‐enoyl cation. Protonation–deprotonation of the latter, followed by internal acylation, gives the protonated cyclopent‐2‐enone. The dication resulting from hydrogen shift loses a proton from C‐2 and gives the 3‐methylbut‐2‐enoyl cation.
Micellar catalysis of organic reactions. 28. A comparison of the catalytic effects of micelles containing bromide and fluoride counterions on some SNAr AND BAc2 reactions
Authors: Broxton Trevor J. , Christie John R. , Wright Sallyanne
Published: January 2005

The catalytic effects of micelles of cetyltrimethylammonium bromide (CTAB) and fluoride (CTAF) on several SNAr and BAC2 reactions were studied. Rate versus detergent concentration curves predicted by the pseudophase models PPIE (ion exchange) for CTAB studies and PPMA (mass action) for the CTAF studies were fitted to the experimental results to determine the best values of the substrate binding constant (Ks), the second‐order rate constant for reaction in the micelles (K) and the exchange constant KOH/X (PPIE) or binding constant (PPMA) for the counterions. Better catalysis by micelles of CTAF is shown to be due to the combined effects of a more favourable exchange constant in CTAF than in CTAB, slightly different rates of reaction within the two micelles and different values of β, the fraction of micellar head groups neutralized in CTAF and in CTAB.

In both micelles the magnitude of catalysis depends on the charge produced in the intermediate complex and the presence of electron‐withdrawing groups to stabilize that charge. Strongest catalysis was found for a reaction leading to a monoanionic intermediate complex in which the charge was stabilized by two nitro groups, and the weakest catalysis was found for a reaction leading to a dianionic intermediate in which no electron‐withdrawing groups were present.
N‐hydroxy‐N‐arylacetamides. VI. Chemical and electrochemical oxidation of N‐hydroxy‐N‐arylacetamides
Authors: Lenk Werner , Riedl Michael
Published: January 2005

The electrochemical behaviour of seven N‐hydroxy‐N‐arylacetamides (RNOHCOCH3, R = phenyl, 4‐chlorophenyl, 3,4‐dichlorophenyl, 4‐ethoxyphenyl, 4‐biphenylyl, 2‐fluorenyl and 2‐phenanthryl) was investigated by cyclovoltammetry. Each compound showed two different oxidation potentials, attributed to a quasi‐reversible one‐electron transfer in the range between 0·55 and 0·63 V and an irreversible transfer of a second electron between 0·80 and 1·20 V. Since the one‐electron oxidation of N‐hydroxy‐N‐arylacetamides gives the corresponding nitroxides, the kinetics of the self‐reaction of these radicals and the concentration dependence of the product pattern were also studied.
Stereospecificity in anionic 1, 1‐addition to isocyanides. A re‐examination of the (H− + HNC) potential energy surface
Authors: Nguyen Minh Tho , Ha Tae‐Kyu , Hegarty Anthony F.
Published: January 2005

The potential energy surface for the addition of the hydride ion to hydrogen isocynide reported previously was re‐examined. An improved energy surface was explored at the MP2/6–31 + + G** level while relative energies were estimated at the MP4SDTQ/6–311 + + G** level and corrected for zero‐point vibrational contributions. The calculated results show the existence of transition structures for both cis and trans‐additions of H− to the carbon end of HNC even though the corresponding barrier heights are small. These transition structures did not exist at a lower level of theory (HF/4–31G) employed previously.

The cis addition is calculated to be favoured over the trans addition by 10 kcal mol−1. The overall addition is thus strongly stereoselective, if not purely stereospecific. This result confirms our previous analysis of the stereoelectronic effect in the addition–elimination process involving imidoyl anions as intermediates. Recent experimental work on the addition of OH− to ArNC is discussed.
Comparison between some semi‐empirical and ab initio Hartree–Fock models for the description of amides (formamide revisited)
Authors: Louwen J. N. , Jenneskens L. W.
Published: January 2005

A comparative study on the use of the MINDO/3, MNDO and AM1 semi‐empirical methods for the description of formamide and aliphatic amide systems is reported. For formamide, ab initio Hartree–Fock calculations are also reported using both the STO‐3G and 3–21G basis sets, as well as a basis set obtained by a minimal relaxation of the STO‐3G contraction scheme. It is shown that both the MNDO and the STO‐3G methods cannot properly reproduce the experimental results. In both cases this appears to be due to inadequacies of the basis sets used. Again in both cases only small improvements in the basis set necessary to allow for the diffuse character of the pπ orbitals lead to dramatic improvements in the calculated results. In the case of the STO‐3G basis set this is demonstrated by the fact that a small relaxation in the contraction leads to results even superior to those with the 3–21G set. The conclusions reached for amide systems can be extended to other systems where planarity around or inversion with respect to nitrogen is an issue. This is demonstrated for ammonia.
Absolute rate constants for the reactions of phenylhalocarbenes with alkynes
Authors: Moss Robert A. , Jang Eun G. , Ho Guo‐Jie
Published: January 2005

Additions of phenylchlorocarbene and phenylfluorocarbene to representative alkynes are rapid (106–107 l mol−1 s−1). The variation of the rate constants with alkyne structure parallels that observed with analogous alkene substrates. The addition of phenylchlorocarbene to 3‐hexyne is characterized by a very low activation energy (∼ 2 kcal mol−1) and a large, negative entropy of activation (∼ −20 e.u.).
Reactions of molecules with two equivalent functional groups. 2. Acetylation of the isomers of bis(hydroxymethyl)benzene
Authors: Constantinides Ioannis , Lourdes‐Guerra Maria , Macomber Roger S.
Published: January 2005

The acetylation (with acetic anhydride–pyridine in tetrahydrofuran) of the ortho, meta and para isomers of bis(hydroxymethyl)benzene (1) was studied. Third‐order rate constants k1 (formation of monoacetate 2 from 1) and k2 (formation of diacetate 3 from 2) for all three isomers were determined, as were the rate constants for acetylation of benzyl alcohol and its ortho‐ and para‐ethyl derivatives under the same conditions. The compositions of the final product mixtures (comprising 1, 2 and 3) were ascertained as a function of the initial ratio of acetic anhydride to 1. Comparisons were made with predictions based on a previously published independent functional groups model, which assumes that k2/k1 = 0·50. All three isomers gave product mixtures whose compositions were in generally good agreement with predictions based on the model. For the para and meta isomers the ratios k2/k1 were determined experimentally to be 0·548 and 0·521, respectively. The k2/k1 ratio for the ortho isomer was found to be 0·605, and its k1 and k2 values were lower than those for the para and meta isomers. Possible explanations for the anomalous behavior of the ortho isomer are discussed.
Negligible kinetic solvent interactions for the rate of photobromination of toluenes where the bromine atom is a principal chain carrier
Authors: Kim Sung Soo , Lee Chang Soo , Kim Chong Chan , Kim Hee Jin
Published: January 2005

The bromine atom is a major chain carrier in the photobromination of toluenes by N‐bromosuccinimide. Only an uncomplexed bromine atom can abstract hydrogen atoms from toluenes. The nature of the polar transition state is entirely different from that of solvolytic reactions and the solvent polarity therefore cannot infuence the rates of the bromination.
Geometry of 2‐benzopyrylium and pyrylium cations calculated by semi‐empirical methods. Crystal and molecular structure of 1,3‐dimethyl‐6,7‐dimethoxy‐2‐benzopyrylium perchlorate
Authors: Shcherbakova Irina V. , Yudilevich Iosif A. , Kuznetsov Evgenii V. , Yufit Dmitrii S. , Struchkov Yurii T.
Published: January 2005

X‐ray data on the crystal and molecular structure of 1,3‐dimethyl‐6,7‐ dimethoxy‐2‐benzopyrylium perchlorate show a good correlation with the geometry of the same cation calculated by the MINDO/3 and MNDO methods with total optimization of all geometrical parameters. The geometry of substituted pyrylium cations was determined by the MNDO and AM1 methods. On the basis of the data obtained, some effects of benzo [c] annelation are displayed for the 2‐benzopyrylium cation in comparison with the pyrylium cation.
Effect of hexadecyltrimethylammonium bromide micelles on the hydrolysis of substituted benzoate esters
Authors: Correia Valdir R. , Cuccovia Iolanda M. , Chaimovich Hernan
Published: January 2005

The alkaline hydrolysis of nine para‐substituted phenyl esters of para‐substituted benzoates was studied in the presence and absence of hexadecyltrimethylammonium bromide (CTAB) micelles. The second‐order rate constant for alkaline hydrolysis in CTAB micelles (K2m) was calculated using a pseudo‐phase ion‐exchange model. the substituent effect on the second‐order rate constant in the aqueous phase (k2w) and on k2m was analysed using Hammett's σ ρ plots. The calculated ρ values in micelles were 0.8 unit. larger than those in water. The effective low dielectric constant in the micellar surface may be responsible for the general effect of increasing the sensitivity of reactions to the electronic effect of substituents in micelles.