bluuebunny/crossref_metadata_2025_split huggingface dataset:
Mechanism of formation of phosphonate carbanions with the use of a lithium salt and 1,8‐diazabicyclo[5.4.0] undec‐7‐ene
Authors: Froment F. , Corset J. 
Published: January 2005

AbstractThe deprotonation reaction mechanism of methyl diethylphosphonoacetate in the presence of lithium salts and 1,8‐diazabicyclo[5.4.0]undec‐7‐ene (DBU) was examined by infrared spectroscopy. It was found that the intermediate species formed by chelation of the bidentate phosphonate with a lithium cation or an ion pair of the salt deaggregated by DBU takes part in the reaction path. The role of the anion X− is to induce the elimination of a protonated DBUH+X− ion pair and that of the solvent is to compete with the bidentate phosphonate in cation solvation. A similar mechanism via a related intermediate is proposed to interpret the results obtained by other workers on the catalysis by Ba(OH)2 of the Wittig–Horner reaction.
Absolute hardness and aromaticity: MNDO study of benzenoid hydrocarbons
Authors: Zhou Zhongxiang , Navangul Himanshoo V. 
Published: January 2005

AbstractAbsolute hardness calculated by the MNDO method has been shown to be a good measure of aromaticity for 14 benzenoid hydrocarbons. Comparisons among hardnesses at different levels of approximation are given. The parallelism of the aromaticity and hardness concepts in benzenoids is elaborated.
Radical mechanism of nucleophilic substitution on halocyclohexane systems
Authors: Palacios Sara M. , Rossi Roberto A. 
Published: January 2005

AbstractChlorocyclohexane does not react in the dark with diphenylphosphide ions in liquid ammonia, but it reacts under irradiation to give the substitution product. Bromocyclohexane reacts slowly in the dark, but this reaction is strongly accelerated by light, and inhibited by p‐dinitrobenzene. 3‐Bromo‐2‐tetrahydropyranyl allyl ether (used as a radical probe) reacts with diphenylphosphide ions under irradiation to give both the straightforward and the cyclized substitution products. This photostimulated reaction is also inhibited by p‐dinitrobenzene. It is suggested that the chlro‐bromocyclohexanes and related compounds react under irradiation by the SRN1 mechanism of nucleophilic substitution.
Micellar catalysis of the intramolecular aminolysis of the β‐lactam antibiotic cephaclor
Authors: Oliveira Anselmo G. , Nothenberg Michael S. , Cuccovia Iolanda M. , Chaimovich Hernan 
Published: January 2005

AbstractThe rate of intramolecular degradation of cephaclor, involving the attack of the amino group at C‐17 (N‐18) on the β‐lactam carbonyl, was increased up to 50‐fold by neutral (polyoxyethylene‐23 lauryl ether; Brij) and zwitterionic [3‐(N‐dodecyl‐N, N‐dimethylammonium) propane 1‐sulphonate; (SDP)] micelles. The rate of OH− attack on cephaclor was increased 2–3‐fold by Brij and SDP micelles. In the absence of micelles the rate of intramolecular degradation of cephaclor increased by up to 2‐fold by addition of organic solvents. Distance calculations, based on the crystal structure of the antibiotic, showed that the intramolecular degration can only proceed in a conformation involving a cis‐amide bond (N‐14—C‐15). Micellar catalysis of the intramolecular degradation process was proposed to be due to the stabilization of the reactive conformation.
Aggregation of amphiphilic molecules in water. I. α‐phenylethylamine: 1H and 13C NMR study
Authors: Kozerski Lech , Hansen Poul Erik 
Published: January 2005

AbstractThe concentration dependence of the 1H and 13C NMR chemical shifts in D2O and in CDCl3 solutions were determined for (±)‐α‐phenylethylamine (I). Aqueous solutions of (−)‐I and (+)‐I, 50% enantiomerically enriched in (−)‐ and (±)‐2, 2, 2,‐trifluoro‐1‐phenylethanol and the (+)‐ and (−)‐N‐formyl derivatives of I, were also studied. 1H nuclear Overhauser enhancements were used to check the conformations of the solutes at various concentrations and 1H T1 values were used to monitor the changes in molecular tumbling in solutions. The results are interpreted in terms of a spontaneous aggregation of solute molecules in water, with the possible determination of the critical micelle concentration. The time‐dependent splittings in the 1H NMR spectra suggest further, more detailed, studies of the structures of the aggregates and the possibility of chiral recognition in water.
The 11C/14C kinetic isotope effect method. The 11C/14C kinetic isotope effect in the SN2 reaction of N,N‐dimethyl‐p‐toluidine with labelled methyl iodide
Authors: Svante Axelsson B. , Matsson Olle , Långström Bengt 
Published: January 2005

AbstractMethodological aspects of the determination of 11C/14C kinetic isotope effects (KIEs) were investigated using the reaction of N, N‐dimethyl‐p‐toluidine with labelled methyl iodide in methanol at 30 °C as model system. The primary 11C KIE was determined to be 1.202 ± 0.008 for this reaction. The isotope effect was determined by a one‐pot technique based on liquid chromatographic fractionation of the reactant and product followed by liquid scintillation counting of the 11C and 14C radioactivity in these fractions. The KIE value obtained agrees well with the value predicted from a previously determined 12C/14C‐KIE for the same reaction. Theoretical model calculations of both carbon KIEs were performed using the BEBOVIB IV program. Transition‐state models ranging from reactant‐ to product‐like were employed in the calculations, and three different reaction coordinate models with more or less pronounced Walden inversion of the methyl hydrogens were investigated.
Addition of hydroxylamine to cyclohexanone and bicyclic ketones. Steric, electronic and hydrogen bonding effects on the general mechanism of addition of amines to carbonyl compounds
Authors: Brighente I. M. C. , Vottero L. R. , Terezani A. J. , Yunes R. A. 
Published: January 2005

AbstractThree different types of pH‐rate profiles were observed for the addition of amines to carbonyl compounds, depending on the relative predominance, at weakly acidic pH, of either a concerted or a stepwise mechanism. The predominant mechanism depends on the basicity of the amine and the mutual equilibrium affinity for adduct formation between the carbonyl compound and the amine. The reaction of hydroxylamine with cyclohexanone and bicyclic ketones was studied in order to examine the roles of steric and electronic effects in this mechanism. Cyclohexanone and 3‐chlorobicyclo [2.2.1] heptan‐2‐one exhibit the profile expected for a single change in the rate‐determining step with increasing pH from uncatalysed attack of the amine on the carbonyl group to hydronium ion‐catalysed dehydration of the carbinolamine. In contrast, both bicyclo [2.2.1] heptan‐2‐one and bicyclo [2.2.2] octan‐2‐one show an unexpected profile with two negative breaks and five kinetically significant regions. This profile is explained by stabilization of the zwitterionic intermediate T± by intramolecular hydrogen bonding, leading to the formation of the carbinolamine by a stepwise mechanism at weakly acidic pH.
Formation of adsorption complexes in phase‐transfer nucleophilic substitution. Kinetic analysis of solid‐phase system
Authors: Esikova I. A. , Yufit S. S. 
Published: January 2005

AbstractThe kinetics of the nucleophilic substitution between n‐hexyl bromide and solid potassium chloride (KCl) in toluene were studied in the presence of catalytic amounts of tetra‐n‐butylammonium bromide at 84°C. This reaction is characterized by an induction period that disappears on activation of the solid salt. The substitution with the activated salt constitutes a reversible pseudo‐first‐order reaction with variable orders in the substrate and the catalyst equal to 0 ≤ n ≤ 1. A quantitative increase in the solid KCl at first accelerates the substitution reaction, but when the level reaches KCl/RBr ≥ 7 the rate constant is no longer influenced by the amount of salt. Comparing data obtained with kinetic equations for several possible substitution schemes, the mechanism of the reaction can be derived, including the formation of a ternary adsorption complex (TC) from the substrate, catalyst and solid salt. The data obtained allow the evaluation of the equilibrium constant of TC formation (KΣ= 20.48 l2 mol−2) and the rate constant of its disintegration into substitution products (k+ = 0.75 × 10−2 s−1). Through kinetic analysis the adsorption sequence on the surface of the solid salt was determined, including primary formation of the binary complex KCl.QX and subsequent TC formation. The concentration of active KCl molecules, estimated on the basis of the primary kinetic data is 10−2 M and is commensurate with the catalyst concentration. This concentration of active KCl molecules is achieved owing to the formation of subcolloidal‐sized particles in the process of the solid salt activation.
Micellar effects on the alkaline hydrolysis of N‐alkyl‐4‐cyanopyridinium ions. An example of micelle‐induced regiochemical selectivity
Authors: Politi Mario J. , Chaimovich Hernan 
Published: January 2005

AbstractThe rates of the reaction of N‐alkyl‐4‐cyanopyridinium (RCP) ions [alkyl = CH3 (MCP), n‐C4H9 (BCP), n‐C8H17 (OCP), n‐C12H25 (DCP)] with OH− ion in water are independent of chain length and RCP concentration up to 0.001 M. The ratios of the concentrations of the reaction products, N‐alkyl‐4‐pyridone (P) and N‐alkyl‐4‐carboxamidopyridinium (A), P/A, are similar for all substrates. In water the P/A ratios increase with pH, reaching a plateau value of ca 2 at pH ≥ 13. Added salts slightly decrease the reaction rate and do not affect the P/A ratios. In aqueous solutions, addition of dioxane increases both rates and P/A ratios. Micelles of hexadecyltrimethylammonium bromide (cetyltrimethylammonium bromide; CTAB) and 3‐(N,N‐dimethyl dodecyl ammonium)propane‐1‐sulphonate (SDP) catalyse the alkaline hydrolysis of OCP and DCP and increase the yield of pyridone with BCP, OCP and DCP. In micellar CTAB and SDP the attack of OH− on RCP occurs almost exclusively at the 4‐position of the pyridinium ring. Sodium dodecyl sulphate micelles inhibit the reaction and produce small changes in the P/A ratio. The regiochemical selectivity produced by micelles, leading to a preference for the attack of OH− at the pyridinium ring, was rationalized in terms of the low effective dielectric constant at the micelle–water interface.
Kinetics of oxidation of hydrocarbons by quinolinium dichromate
Authors: Sarma G. C. , Mahanti M. K. 
Published: January 2005

AbstractQuinolinium dichromate (QDC) oxidizes hydrocarbons (toluenes and phenanthrenes) smoothly in dimethylformamide in the presence of acid. The rate of the reaction was first order in each substrate, oxidant and acid. The effects of variations in solvent composition and temperature were studied. The rate data obeyed Hammett's relationship and the values of ρ were −0.20 for toluenes and −1.79 for phenanthrenes. Induced polymerization of acrylonitrile and the reduction of mercury(II) chloride were not observed. For the oxidation of toluenes, a kinetic isotope effect, kH/kD = 5.13, was observed. The initial reaction for the oxidation of toluenes involved hydrogen abstraction, forming an intermediate which was rapidly converted to the product, the corresponding aldehyde. For the oxidation of phenanthrene, the experimental data have been rationalized in terms of a hydride ion transfer in the ratedetermining step.
Long‐range deuterium isotope effects in tautomeric β‐thioxoketones. A 1H and 13C NMR study
Authors: Hansen Poul Erik , Skibsted Ulrik , Duus Fritz 
Published: January 2005

AbstractEquilibrium displacements within the enol–enthiolic tautomeric systems of β‐thioxoketones caused by long‐range isotope effects were monitored by 1H and 13C NMR spectroscopy. Thioacetylacetone and 1,3‐diphenyl–3‐thioxopropanone deuteriated at various positions were investigated. Both compounds are in fast (Z)‐enol‐(Z)‐enethiol equilibrium on the NMR time scale. The investigations showed that CD3 and C6D5 groups changes the equilibrium so that the tautomer having a C  X bond next to the CD3 or C6D5 groups decreases. Deuteriation at the methine position pushes the equilibrium towards the (Z)‐enol form. The study further showed that deuteriation at the phenyl ring next to the C  O group causes the largest effect if the deuterium is at the ortho position and the smallest if it is at the para position. Long‐range effects on equilibrium are discussed in general in order to establish a common pattern. Deuterium scrambling is observed both during the coupling reaction of ethyl thionoacetate with hexadeuterioacetone and during preparation of specifically labelled acetophenone. The latter compounds are synthesized by acetylation of specifically labelled benzenes. The scrambling reaction is suggested to be a transdeuteriation of the deuteriated benzenes rather than involving the acetophenones.
Kinetic and equilibrium study of the interaction of N‐(1‐Ethylpropyl)‐3, 4‐Dimethyl‐2, 6‐Dinitroaniline with hydroxide ion
Authors: Nassetta Mirtha , De Vargas Elba B. , De Rossi Rita H. 
Published: January 2005

AbstractThe reversible interaction of N‐(1‐ethylpropyl)‐3, 4‐dimethyl‐2, 6‐dinitroaniline (1) with HO− is characterized by several kinetic processes. The relaxation times of two of them were measured. It is suggested that the faster one is related to the protonation‐deprotonation of the 3‐methyl group and the value of the protonation rate is 0.19 s−1. The slowest process leads to the dianionic species with maximum absorption at 340 nm where one HO− group adds to the unsubstituted ring position of an anion derived from 1 by deprotonation of either the NH or the 3‐methyl group. The rate and equilibrium constants for the formation of this species are reported.
Depurination of (dien)Pt(II) complexes of purine deoxyribonucleosides. Comparison with the effects of (dien)Pd(II) ion complexing
Authors: Arpalahti J. , Jokilammi A. , Hakala H. , Lönnberg H. 
Published: January 2005

AbstractFirst‐order rate constants for the hydrolytic depurination of 2′‐deoxyinosine and its various (dien)Pt(II) ion complexes were measured over the acidic pH range. The rate profiles obtained indicate that the uncomplexed nucleoside and its N1‐(dien)Pt(II) complex are depurinated via mono‐ and di‐protonated species, whereas the N7−(dien)Pt(II) and N1, N7−di(dien)Pt(II) complexes exhibit significant spontaneous hydrolysis, which competes with a markedly retarded acid‐catalysed reaction. Rate constants for the various partial reactions were calculated and the results were employed to explain the effects that (dien)Pd(II) ion exerted on depurination rates of the same compounds. Similar measurements were carried out with the N1, N7−di(dien)Pt(II) complex of 2′‐deoxyadenosine in order to further the understanding of the previously reported rate‐enhancing effect of (dien)Pd(II) ion on the depurination of 2′‐deoxyadenosine.
New ideas on phase transfer catalysts: SN2 reaction with a solid ionophoric salt
Authors: Yufit S. S. , Esikova I. A. 
Published: January 2005

AbstractA new mechanism of substitution between n‐hexyl bromide (RBr) and solid potassium chloride in toluene under phasetransfer catalysis conditions is suggested, involving the formation of intermediates, adsorbed on the solid phase, viz., two binary (KCl·QX and KX·QBr) and two ternary (QX·KCl·RBr and QBr·KX·RCl) complexes. According to this mechanism, the catalytic activity of onium salts (QX) in the substitution changes in the order QI > QBr > QCl. The nature of the catalyst cation has little or no effect on the reaction rate. A general rule is proposed according to which in this reaction the substrate is attacked by a stronger nucleophile whereas a weaker nucleophile accepts the leaving group of the substrate. These and earlier data are interpreted in term of the SN2 process occurring on the solid‐phase surface. The structure of the ternary complexes and the role of the phase‐transfer catalyst are discussed.
Activation and thermodynamic parameters of nucleophilic substitution. Reaction of alkyl halides with solid salts under phase‐transfer catalysis conditions
Authors: Esikova I. A. , Yufit S. S. 
Published: January 2005

AbstractThe paper is concerned with the effect of temperature and the nature of a solid ionophore (MCl, where M = Li, Na, K, Rb or Cs) on the rate and equilibrium constants in the process of substitution of chlorine for bromine in hexyl bromide. The non‐linearity of the log k vs 1/T, dependence is shown to be linked with a change in the physical meaning of the constants with temperature variations. This is attributed to the occurrence of a strong exothermic process consisting in the formation of a kinetically independent intermediate, that precedes the limiting stage. A mechanism including the formation of two stable ternary complexes coordinated on the solid‐phase surface, one of which is structurally close to the starting reagents and the other to the final reaction products, is proposed. The observed activation energies, enthalpies of formation of the ternary complex and activation energies of the limiting stages for the forward and reverse substitution processes were calculated. Thermal effects of the substitution in the presence of various solid MCl species were determined from the thermodynamic data and observed activation energies for both reaction types. The substitution is shown to involve a limiting stage represented by a transition between the ternary complexes. A linear relationship between the activation energy for the limiting stage and the solid ionophore crystalline lattice was established, suggesting incorporation of the solid salt molecule into the transition reaction state. Potential energy changes in the course of substitution in the presence of the various MCl solid salts are discussed. The Bell—Evans—Polyani principle is shown to hold for these studies.
Orientation of benzoic acid and terephthalic acid on an alumina surface and their reactivities observed by infrared spectroscopy
Authors: Ogawa Haruo 
Published: January 2005

AbstractThe orientation of benzoic acid and terephthalic acid on alumina was ascertained by IR measurements. Benzoic acid was chemisorbed as a carboxylate anion with a vertical conformation of the benzene ring (0 < θ < 1.0) and as a mixture of the anion and the acid (1.0 < θ). Terephthalic acid was chemisorbed as a mixture of monocarboxylate anion (hydrogenterephthalate) and dicarboxylate anion (0 < θ < 0.3), as the monocarboxylate anion with a vertical conformation of benzene ring (0.3 < θ < 1.0) and a mixture of the monocarboxylate anion and terephthalic acid (1.0 < θ). The carboxyl group of expected hydrogenterephthalate adsorbed through the carboxylate anion on alumina (θ = 0.8 : 0.37 mmol g−1 alumina) readily reacted with dimethyl sulphate to form monomethyl terephthalate selectively.
Correlation of alkyl and polar substituents in the elimination kinetics of 2‐substituted ethyl methanesulphonates in the gas phase
Authors: Chuchani Gabriel , Alvarez G Jaime , Martín Ignacio 
Published: January 2005

AbstractThe kinetics of the elimination of several polar 2‐substituted ethyl methanesulphonates in the gas phase were determined in a static system, seasoned with allylbromide, and in the presence of at least equal amount of the freeradical suppressor propene and/toluene. The working temperature and pressure ranges were 290–360°C and 31°199 Torr (1 Torr = 133.3 Pa), respectively. The reactions are homogeneous, unimolecular, show a first‐order rate law and take place according to the following equations: for 3‐chloropropyl methanesulphonate, log k1 (s−1) = (12.01 ± 0.18) ‐ (171.7 ± 2.1) kJ mol−1 (2.303RT)−1; for 4‐chlorobutyl methanesulphonate, log k1 (S−1) = (11.78 ± 0.31) ‐ (166.1 ± 3.5) kJ mol−1 (2.303RT)−1; for 3‐methoxypropyl methanesulphonate, log k1 (s−1) = (11.50 ± 0.36) ‐ (163.3 ± 4.0) kJ Mol−1 (2.303RT)−1; and for 2‐ethoxyethyl methanesulphonate, log k1 (s−1) = (11.52 ± 0.37) ‐ (167.3 ± 4.1) kJ mol−1 (2.303RT)−1. The present data together with those reported in the literature show that alkyl 2‐substituted ethyl methanesulphonates give an approximate straight line when log k/ko vs σ* values (ρ* = −0.82 ± 0.088, intercept = 0.0084 ± 0.0209 and correlation coefficient r = 0.967 at 320°C) are plotted. However, polar substituents at the 2‐position of ethyl methanesulphonates give rise to an inflection point at σ*(CH3) = 0.00 into another very good straight line (σ* = −0.29 ± 0.013, intercept = −0.0065 ± 0.0213 and r = 0.994 at 320°C). The mechanism of these reactions is considered to be heterolytic in nature, proceeding in terms of an intimate ion‐pair intermediate.
AM1 studies on the intramolecular cyclization of ω‐haloenolate anions
Authors: Lee Ikchoon , Kim Chang Kon , Kong Byung Hoo , Lee Byung Choon 
Published: January 2005

AbstractThe intramolecular cyclizations of ω‐haloenolate anions,−CαH2C(O)(CH2)n−3CωH2X with X = F, Cl and Br andn= 3–7, were investigated by the AM1 method. In most cases, cycloketone formation proceeds more favourably than cycloether formation, as predicted by the HSAB principle. The reactivity increases in the order X = F < Br < Cl for both processes, and for cycloether formation it is in the ordern= 3 < 4 < 7 < 6 < 5 for all X but the relative order forn= 5 and 6 reverses in favour ofn= 6 for cycloketone formation with X = Cl and Br. The softness of the acid centre, Cω, decreases for a harder X( = F) whereas it increases for a softer X( = Br). Thus the reactivity order with respect to X suggests that the softness of the base centres O and Cαbelongs to the borderline class. The transition state (TS) structures indicate that the BEP principle is obeyed in all cases, and a less reactive process has a more product‐like TS.
Treatment of electrostatic effects within the molecular mechanics method. 3. Double bonds and conjugated systems
Authors: Došen‐Mićović Ljiljana , Li Shusen , Allinger Norman L. 
Published: January 2005

AbstractThe previously developed general method (IDME), which includes calculation of electrostatic and solvation effects on conformational equilibria, has been extended to molecules containing double bonds and conjugated systems. It was applied in connection with MM2 studies to the calculation of the charge distributions and dipole moments, and to the conformational energies of some β‐substituted cyclohexenes, exo‐methylenecyclohexanes and benzocycloheptenes. Conformational equilibria and dipole moments were well calculated. It was concluded that electrostatic interactions do not play a major role in determining conformational preferences in these compounds. On the other hand, the energies of the preferred conformations of the ten‐membered ring in (E)‐3β‐acetoxy‐5,10‐seco‐1(10)‐cholesten‐5‐one are poorly calculated without the IDME procedure, and fairly well with it.
Preparation, characterization and rearrangement pathways of 10‐alkyl‐9‐decalyl carbocations
Authors: Kirkbride Paul K. , Sorensen Ted S. 
Published: January 2005

AbstractThe preparation and characterization of 10‐methyl‐ and 10‐ethyl‐9‐decalyl cations is described. Both cations undergo a rapid, degenerate 10, 9‐alkyl shift, but the ethyl group migration is about 700 times slower at −120°C than that of the methyl group. This preference is opposite to that usually found for alkyl migration rates and, on the basis of MO calculations, it is suggested that a conformational interchange may be the rate‐determining step. Both cations irreversibly rearrange at about −100°C, giving in the methyl case the rearrangement cascade 10‐methyl‐9‐decalyl → cis‐1‐methyldecalyl → trans‐1‐methyldecalyl → trans‐2‐methyldecalyl. Each of these rearrangement ions was independently prepared and characterized by NMR spectroscopy. One can rationalize the initial formation of the less stable cis ring junction on the basis of suprafacial hydride and methyl shifts. Attempts to add molecular hydrogen to the 10‐methyl‐9‐decalyl cation were unsuccessful.