bluuebunny/crossref_metadata_2025 huggingface dataset:
A valence and charge criterion for reactivity of π electron systems
Authors: Jug Karl , Köster Andreas M.
Published: January 2005

A new criterion for the reactivity of π electron systems is proposed. The method incorporates and modifies ideas of Coulson and Fukui et al. about free valence. The proposed reactivity index includes covalent and lonic components and is suitable for elctrophilic, radical and nucleophillic attack. The method is tested for a few simple π systems and differences from Coulson and Fukui et al. are discussed. Finally, it is applied to the electrophilic attack of a series of substituted benzenes and to nucleophilic attack of a series of subsituted benzoic acid esters.
Electronic absorption spectra of aliphatic diamine radical cations. Conformation‐dependent charge delocalization
Authors: Gȩbicki Jerzy , Marcinek Andrzej , Stradowski Czesław
Published: January 2005

Electronic absorption spectra of radical cations generated by steady‐state and pulse radiolysis from a series of α,ω‐diaminoalkanes (R2N(CH2)nNR2; n = 1–6, R = CH3 or C2H5), 1,4‐dimethylpiperazine and 1,4‐diazabicyclo [2.2.2] octane (DABCO) were measured. In some cases spectral evidence was obtained for an interaction between the nitrogen atoms leading to charge delocalization in the radical cation. A delayed formation of radical cations was associated with a conformational change necessary for achieving the most stable structure having a favourable conformation for charge delocalization.
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

The 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

Absolute 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

Chlorocyclohexane 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

The 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

The 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

Methodological 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

Three 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

The 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

The 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

Quinolinium 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

Equilibrium 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

The 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

First‐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

A 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

The 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

The 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

The 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

The 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

The 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

The 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.
Short communication. Structural studies of bromine adducts of 1,2‐ and 1,4‐bis(phenylseleno)benzenes. Evidence for the (TB, MC) structure of 1,2‐(PhSeBr2)2C6H4
Authors: Nakanishi W. , Okumura Y. , Hayashi S.
Published: January 2005

1,2‐Bis(phenylseleno)benzene reacted with equimolar amount of bromine to yield a trigonal bipyramidal (TB) adduct, in contrast to the molecular complex (MC) structure of selenanthrene with bromine. Whereas the structure of 1,4‐(PhSeBr2)2C6H4 was (TB, TB) at the two selenium atoms, that of the 1,2‐derivative was demonstrated to be (TB, MC), the first example of an MC structure of PhSeC6H4Y with Br2 [Y = 2‐PhBr2Se(TB)]. Both electronic and steric effects of the group must play an important role in MC formation. Rapid structural exchange in this tetrabromide, (TB, MC) ⇄ (MC, TB), was also observed.
Review commentary. Solvation, the electrophilic driving force of ionic bromination of ethylenic compounds. The addition‐solvolysis analogy
Authors: Ruasse Marie‐Françloise , Motallebi Shahrokh
Published: January 2005

Solvation is the main driving force of electrophilic bromination since it is impossible to obtain a bromonium ion from bromine and alkene in the gas phase, whereas it is a very fast reaction in solution. The role of a protic solvent in this addition was investigated experimentally by extended Winstein–Grunwald relationships, kinetic solvent isotope effects and R, the rate ratios in two solvents of similar ionizing powers but different nucleophilicities. It is shown that electrostatic medium effects and electrophilic assistance to bromide ion departure are the main rate‐determining factors of the reaction. These two contributions are roughly independent of the double bond substituents. Nucleophilic solvent assistance to positive charge development is also found; however, it provides only a small acceleration, the magnitude of which depends on alkene structure. This nucleophilic solvent involvement is annulled when crowded substituents inhibit approach of the solvent to the cationic part of the transition state or when positive charge is delocalized by conjugated electron‐donating groups. These several solvent roles are identical in nature and in magnitude with those observed in heterolytic solvolysis. In halogenated solvents, the driving force of bromination arises from catalysis by a second bromine molecule which assists heterolysis of the bromine‐bromine bond, leading to the bromonium‐tribromide ion pair. Similar halogen catalysis occurs also in some solvolyses. Finally, return is also found in both reactions; reversible formation of bromonium ions is observed when their nucleophilic attack, the productforming last step, is made energetically difficult either by steric inhibition or by poor nucleophilicity of the trapping nucleophiles. Similarities and differences between electrophilic bromine addition and limiting solvolysis are discussed in terms of respective intermediate stabilities and heats of formation.
Steric effects of alkyl groups: A ‘cone angle’ approach
Authors: Datta Dipankar , Majumdar Devashis
Published: January 2005

A ‘cone angle’, θR, is defined for an alkyl group (R), which is proposed as a measure of the steric effect exerted by the group. The θR values for some 57 groups have been calculated mathematically by constructing the corresponding Corey—Pauling—Koltun molecular models. The variation in θR with the bulk of a group is satisfactory. However not all types of alkyl groups can be accommodated in this cone angle approach. In the case of the groups for which the cone angle can be measured, correspondences between θR and the two existing steric parameters (Taft's Es scale and Dubois' E′s scale) have been established. For some 23 alkyl groups θR has been used to explain the variation in the rate constant of the base‐catalysed hydrolysis of RCOOEt with the nature of R. Our studies independently substantiate the basic assumptions of the Taft—Ingold hypothesis that, while the base‐catalysed ester hydrolysis is mostly controlled by the steric and electronic effects of the R group, the acid‐catalysed one is controlled almost solely be the steric effect.
Relationships between electronic substituent parameters
Authors: Sæthre Leif J. , Thomas T. Darrah
Published: January 2005

Core‐electron energies for sulphur in a series of 2‐substituted thiophenes have been used to investigate the relationship between electronic substituent parameters as developed by Taft and Topsom and by Charton. Initial‐ and final‐state energies are analysed in terms of familiar substituent effects, such as field, delocalization and polarizability. The results show that the data correlate equally well with both sets of parameters and that there are many common features between the two methods. Both agree that resonance delocalization contributes significantly to the initial state, but has virtually no effect on the final state. In connection with this investigation we have also correlated the parameters for 31 substituents that are parameterized in the two approaches. The results of this more general study indicate some real differences between these and suggest that neither of the parameterizations incorporates all the electronic effects.
Effects of structures and concentrations of mono and BIS‐aryliminodimagnesium on the reactions with nitrobenzenes. Formation of azoxy and azo products assisted by aggregated excess of magnesium reagent
Authors: Ōkubo Masao , Tanaka Mikio , Shiku Haruko , Yamauchi Akira , Matsuo Koji
Published: January 2005

The relative yields of azoxy‐ and azo‐type products in the reactions of aryliminodimagnesium [ArN(MgBr)2 (IDMg)] with nitrobenzene were greatly affected by changes in the molar concentration in tetrahydrofuran, addition of hexamethylphosphoramide to the reagent solution and the structures of mono‐ and bis‐IDMg reagents. All the modes of variation of the product distribution, including extreme cases, were roughly but systematically related to a series of qualitative degrees of aggregation of IDMg. The role of aggregation is ascribed to the assistance of excess reagent molecules in the product formation process in the reaction. From the present and previous results and discussion on the reactions of IDMg and RMgBr, the IDMg aggregation is concluded to be an essential modification factor for useful reactions of the NMg (and also CMg) reagents having a suitable efficiency of single electron transfer.
Towards a general approach to the deprotonation of carbon acids, including nitroalkanes
Authors: Arnaut Luís G.
Published: January 2005

The deprotonation of nitroalkanes and ketones was studied through the intersecting state model of Formosinho. The anomalous Brønsted coefficients observed when substituted nitroalkanes react with a common base are explained in terms of variable transition‐state electronic structures. These are very sensitive to substitution in the nitroalkane, owing to the high electron affinity of the nitro group. In ketones, such electronic effects are much attenuated. The transition states are characterized by the bond order, n,*, which is quantitatively correlated to the field/inductive and resonance effects of the substituents. The deprotonation of fluorence and other carbon acids was also studied. The theoretical model applied shows that all these reactions follow a mechanistic continuum, dictated by the extent into which the group directly bonded to the acidic carbon is mixed in the reaction coordinate.
Kinetic model for acid‐catalysed hydrolysis of benzohydroxamic acid
Authors: Ghosh Kallol K. , Rajput Surendra K. , Krishnani Kishore K.
Published: January 2005

The kinetics of the hydrolysis of benzohydroxamic acid have been investigated in hydrochloric, sulphuric and perchloric acids in 10% (v/v) dimethyl sulphoxide—water at 55 °C. Activation parameters were also determined. Rate correlation by the Cox—Yates excess acidity method shows an A‐2 mechanism involving rapid pre‐equilibrium protonation of the substrate followed by rate‐limiting attack of water at the carbonyl carbon atom to form a tetrahedral intermediate which collapses, in a fast step, to the products.
Transimination reaction between pyridoxal‐5′‐phosphate schiff bases with dodecylamine and amino acids
Authors: Vázquez M. A. , Muñtoz F. , Donoso J. , García Blanco F.
Published: January 2005

Some transimination processes were studied using an intermolecular model formed by pyridoxal‐5′ ‐phosphate (PLP) an amino acid and dodecylamine (DOD) in an aqueous medium. All the kinetic constants for the reversible reaction were determined. The results show that in these cases transimination proceeds through an addition‐elimination by forming a diamine geminal intermediate. Equilibria are always shifted to dodecylamine‐PLP Schiff base formation. Differences between the stability of this Schiff base and the ϵ‐aminocaproic Schiff base cannot be explained only on the basis of the different nucleophicities of amine groups and therefore differences in the imine double bond environment must be taken into account to explain this behaviour.
Source of the intraannular hydrogens in the dehydroxylation of calix [4] arene diethyl phosphate ester derivatives
Authors: Grynszpan Flavio , Bialio Silvio E.
Published: January 2005

The different possible sources of the intraannular hydrogens in the dehydroxylated calixarenes obtained by reductive cleavage of the calix [4] arene diethyl phosphate esters 2 and 6 are analysed. Two calixarene diethyl phosphate esters (4 and 5) full deuterated in the ethyl groups were synthesized. Reductive cleavage of 4 and 5 (potassium‐ammonia) resulted in the formation of the OH‐depleted calixarenes 3 and 7, respectively, which did not incorporate any deuterium at the intraannular positions, as judged by integration of the NMR signals. Quenching with D2O of the reaction mixture of either 2 or 6 and potassium‐ammonia did not result in any deuterium incorporation in the products. The labelling experiments rule out the possibility that the source of the intraannular hydrogens is the diethyl phosphate ester groups or the quencher. It is concluded that the most probable sources of the hydrogen atoms in the OH‐depleted calixarenes is the ammonia molecule.
Micellar effects upon oxidation of organic sulfides by anionic oxidants
Authors: Bacaloglu Radu , Blaskó Andrei , Bunton Clifford A. , Foroudian Houshang
Published: January 2005

Peroxymonosulfate ion (HSO5−) rapidly oxidizes 2‐chloroethyl phnyl sulfide (1). Dilute micelle‐forming cationic surfactants slightly speed the reaction but the reaction rate decreases sharply with increasing surfactant concentration. Rate‐surfactant concentration profiles in solutions of cetyltrimethylammonium hydrogensulfate can be treated quantitatively by estimating concentrations of 1 and HSO5− at the micellar surface with allowance for competition between HSO5− and HSO4−. The second‐Order rate constant in the micellar pseudo‐phase is lower that in water by a factor of ca 400. Similar rate–surfactant concentration profiles were observed for the oxidation of di‐n‐propyl sulfide by periodate ion in solutions of cetyltrimethylammonium bromide, chloride and mesylate, and second‐order rate constants in the micellar pseudo‐phases are lower than in water by factors of ca 400. These oxidations are slowed by a decrease in the water content of aqueous acetonitrile and solvent, and micellar rate effects are consistent with transition states in which positive charge builds up on sulfur.
Multi‐step processes in gas‐phase reactions of halomethyl anions XCH2− (X = Cl,Br) with CH3X and NH3
Authors: Matthias Bickelhaupt F. , De Koning Leo J. , Nibbering Nico M. M. , Jan Baerends Evert
Published: January 2005

A mechanistic investigation of the gas‐phase reactivity of the halomethyl anions XCH2− (X = Cl, Br) towards a mixture of the corresponding halomethane and ammonia was performed using Fourier transform ion cyclotron resonance mass spectrometry. The interpretation of the experimental data is supported by high‐level density functional theoretical (DFT) calculations for the chlorine‐containing systems (X = Cl). When the specific isotopomer AXCH2− (AX = 35 Cl, 79Br) is allowed to react in an atmosphere of CH3X and NH3, the exclusive formation of the isotope cluster of the halide anions AX− and BX− (BX = 37Cl, 81Br) is observed. However, the intensity ratio I(AX−)/I(BX−) exceeds significantly the value expected from the natural relative isotope abundances and depends linearly on the pressure ratio p(NH3)/P(CH3X). The experimental results are interpreted in terms of three competing reaction mechanisms: (i) The by far dominating process is the more than 70 kcal mol−1 exothermic one‐step SN2 substitution of AXCH2− on CH3X, generating haloethane AXCH2CH3 and X− isotopomers, the latter in the proportion of their natural abundances (direct SN2). The experimentally observed excess of AX− stems from two minor reaction pathways: (ii) in a secondary reaction, the halide X− in the primary product anion‐molecule complex [AXCH2CH3 … X−] * of the SN2 substitution induces a 1,2‐elimination, leading to the formation of the AX− isotopomer (two‐step SN2/E2). (iii) Finally, ACH2− can react with ammonia by consecutive endothermic proton transfer (PT) from NH3 to AXCH2− and a very exothermic SN2 substitution of the resulting amide on AXCH3 leading to CH3NH2 and an excess of AX− which depends linearly on p(NH3)/p(CH3X) (PT/SN2). Theoretical calculations show that in the case of [ClCH2−…NH3] *, the PT/SN2 reaction has no stable intermediate. Therefore, it is concluded that this reaction is not a two‐step but a one‐step process.
ESR study of solvent effects on the electronic structure of 2‐(4′ ‐dialkylaminophenyl)indan‐1,3‐dionyl radicals
Authors: Sueishi Yoshimi , Isozaki Takanori , Yamamoto Shunzo , Nishimura Norio
Published: January 2005

The ESR spectra of 2‐arylindan‐1,3‐dionyl radicals in various solvents at 343 K were obtained and assignments of the hyperfine structures were made. The hyperfine coupling constant (hfcc) of the dimethylamino nitrogen increases whereas the 2',6'‐hydrogen hfcc decreases as the ET value, a solvent polarity parameter, increases. MO calculations were made according to the semi‐empirical procedure of Rieger and Fraenkel; the nitrogen and oxygen Coulomb integrals were adjusted so that the calculated spin density on the nitrogen atom agrees with that estimated from its hfcc. It is found that the π‐moment of the radicals increases from 9·13 D in toluene to 9·85 D in acetic acid. The solvent dependence of the equilibrium between the radicals and its dimer has been explained in terms of the solventinduced π‐moment in polar solvents.
Solvolysis of 1,1‐diphenylethyl derivatives in highly aqueous media. Reaction via ion pairs
Authors: Thibblin Alf
Published: January 2005

Solvolysis of 1,1‐diphenyl‐1‐X‐ethane (1‐X) [X = 4‐nitrobenzoate (PNB), 3,5‐dinitrobenzoate (DNB), AcO, MeOH+, EtOH+ or H2O+] was studied in 20 vol% dimethyl sulphoxide in water, in which specific salt effects are very small, and in 25 vol% acetonitrile in water. Substrates with negatively charged leaving groups yield more elimination product 1,1‐diphenylethene (3) than those with uncharged groups, indicating the intermedicacy of ion pairs. Thus, three times more alkene is formed in the reactions of the substrates 1‐OAc and 1‐PNB than in the acid‐catalysed hydrolysis of 1‐OMe and 1‐OAc. The substitution products may be formed via the solvent‐equilibrated carbocation since the measured nucleophilic selectivities are very similar with different leaving groups.
Complexes of TRI‐ and tetraprotonated forms of 1,4,7,10,13‐pentaazacyclopentadecane with various mono‐ and divalent anions in aqueous media
Authors: Gelb Robert I. , Alper Joseph S. , Schwartz Marietta H.
Published: January 2005

Acid dissociation and anion complexation properties of 3+ and 4+ forms of 1,4,7,10,13‐pentaazacyclopentadecane were investigated by pH potentiometry, conductometric titration and13C NMR experiments. The 3+ and 4+ species form complexes with a variety of mono‐ and divalent anions, and their complexation constants can be determined by pH potentiometric methods. However, no iodide complexes are detected and neither is a pentaprotonated species. Enthalpy and entropy changes for the acid dissociation of the 3+ ligand and for its complexation with chloride are estimated. The complexation and acid dissociation properties of the ligand are discussed in terms of internal hydrogen bonding interactions and solvation effects.
Kinetic evidence for interdigitation in model lipid bilayers
Authors: Moss Robert A. , Bhattacharya Soumendu
Published: January 2005

Pseudoglyceryl ammonium ion coliposomes constructed from an arachidoyl probe lipid and either stearoyl or palmitoyl host lipids display unusually slow transverse bilayer migration (‘flip‐flop’) of the arachidoyl probe molecule compared with the behavior of shorter probe lipids in similar colipsomes. This phenomenon is probably associated with interdigitation of the arachidoyl lipid across the bilayer's midplane.
Metal ion complexation by tetraester derivatives of bridged calix [4] arenes
Authors: Arnaud‐neu Francoise , Böhmer Volker , Guerra Lourdes , McKervey M. Anthony , Paulus Erich F. , Rodriguez Amado , Schwing‐weill Marie‐José , Tabatabai Moniralsadat , Vogt Walter
Published: January 2005

A series of bridged calix [4] arenes (5–10 methylene groups in the bridge) were synthesized and transformed into their tetraester derivatives by reaction with ethyl bromoacetate. The stability constants for complexes of the tetraester derivatives with sodium, potassium and silver cations, determined in methanol by spectroscopic or potentiometric techniques, show a drastic decrease (more than 105 for Na +) for the shorter methylene chains. 1H NMR studies demonstrate a conformational rearrangement of the calixarene part to fourfold symmetry to be necessary for the complexation of a cation, which is prevented by the shorter chains. This is further confirmed by the X‐ray structure of a tetraester derivative with a CH2CH2COCH2CH2 bridge; crystals were monoclinic, space group P21/c,a = 11.847(2), b = 39.773(5), c = 12 · 127(2) Å, β = 109.24(1), V = 5395.1 Å3, Z = 4, Dx = 1.186 Mg m−3.
Host–guest chemistry. Methanol, ethanol and propan‐1‐ol inclusion compounds of 2‐[o‐(triphenylphosphoranylidenamino) benzyliden] amino‐1H‐2, 3‐dihydroindazol‐3‐one. X‐ray structural characterization of the free host and its ethanol inclusion compound
Authors: Molina Pedro , Arques Antonio , Obón Rosario , Llamas‐saiz Antonio L. , Foces‐Foces Concepcion , Claramunt Rosa Maria , López Concepcion , Elguero Jose
Published: January 2005

The title compound (3) which is a 2‐aminoindazol‐3‐one derivative linked through a methinephenyl bridge to an iminophosphorane, and unrelated to any previous host, is described. The bulky triphenylphosphino group provides the cavity and the indazol‐3‐one the hydrogen bonds (both donor NH and acceptor CO). X‐ray structural analyses of the free host 3 and that of its ethanol inclusion compound 3 · EtOH are reported. Hydrogen bonds resulting in infinite chains dominate the molecular packing. The use of thermal analysis and 13C CP/MAS NMR shed light in the structures of two other inclusion compounds, 3 · MeOH and 3 PrOH. The conclusion is that all three compounds are similar. An analysis of the cavities, using a model of interpenetrating spheres of van der Waals radii, demonstrates that in the case of the ethanol derivative, the cavity has the space and form requirements necessary to include the propanol guest.
Supramolecular metallocatalysts for the cleavage of amino acid esters
Authors: Scrimin Paolo , Tecilla Paolo , Tonellato Umberto
Published: January 2005

Transition metal ions are effective catalysts of the hydrolytic cleavage of amino acid esters and their effects can be enhanced and properly directed when they are chelated to functionalized ligands. The resulting metallocatalysts are attracting increasing attention and the systems so far investigated are briefly reviewed. Particular emphasis is given to supramolecular systems which may add to the metallocatalysts the benefits of the cooperativity, set upon convergent non‐covalent interactions of their components, needed for substrate recognition. The results obtained with metallomicellar aggregates and molecular metalloreceptors, with particular reference to those studied in the authors' laboratory, are reported in more detail. In the case of loosely structured metallomicelles, remarkable accelerations and, generally, modest selectivities have been observed; less spectacular kinetic effects, but promising substrate selectivity, have been obtained with structurally well defined metalloreceptors.
Molecular recognition by macrocyclic receptors having multiple hydrophobic branches in a synthetic bilayer membrane
Authors: Kikuchi Jun‐Ichi , Matsushima Chiemi , Tanaka Yumi , Hie Ken‐Ichi , Suehiro Kazuaki , Hayashida Osamu , Murakami Yukito
Published: January 2005

Hybrid molecular assemblies were prepared in combinations of a synthetic peptide lipid, involving an L‐alanine residue interposed between an anionic head group and a hydrophobic double‐chain segment, with cationic macrocyclic hosts, a steroid cyclophane bearing four rigid steroid moieties and octopus cyclophanes having eight flexible hydrocarbon branches. On addition of the cyclophanes to multi‐walled bilayer membranes composed of the anionic lipid, thermodynamic parameters (ΔHand ΔS) associated with the phase transition between the gel and liquidcrystalline states were subjected to changes that are consistent with the formation of the hybrid assemblies. Anionic fluorescent guests, 8‐anilinonaphthalene‐1‐sulphonate and 6‐p‐toluidinonaphthalene‐2‐sulphonate, were effectively incorporated into the hydrophobic cavities provided by the cationic cyclophanes embedded in the bilayer membrane through hydrophobic and electrostatic interactions. The guest‐binding modes of the hybrid assemblies are classified into two types; a guest is included in the proximity of the hydrogen‐belt domain of the bilayer membrane in one mode, and a guest is incorporated into the hydrophobic bilayer domain composed of double hydrocarbon chains of the lipid in another.
The photochemistry of pyrene–cytosine conjugates: Modelling the carcinogenic action of aromatic hydrocarbons
Authors: Sessler Jonathan L. , Kubo Yuji , Harriman Anthony
Published: January 2005

Photoreduction, but not the corresponding photooxidation, of cytosine can be sensitized by a covalently appended pyrene molecule in a process that may have some importance for understanding the known carcinogenic activity of polynuclear aromatic hydrocarbons.