摘要:

AbstractDecarboxylation of 6‐nitrobenzisoxazole‐3‐carboxylate ion and cyclization ofo‐3‐halopropyloxyloxyphenoxide ion (PhY7) (Y = Br, I) are accelerated by aqueous cationic and zwitterionic micelles. For cationic micelles the rate enhancements (kM′kW′) increase with increasing bulk of the surfactant head groups but are largest for decarboxylation. There are good linear free energy relationships between micellar effects on these reactions and the plots of logkM′ for cyclization against logkM′ for decarboxylation have slopes of 0.46 and 0.64 for Y = Br and I, respectively. Surfactants that have twin tail or tri‐N‐alkylbenzyloxy head groups do not fit the relationship. Despite mechanistic differences between decarboxylation and cyclization (an intramolecularSN2 reaction), similar factors control micellar effects on these reactions. In the absence of surfactant there is only a qualitative relationship between free energies of activation of cyclization and decarboxylation

ISSN:0894-3230    

DOI:10.1002/poc.610040202

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractMethodological aspects of the determination of11C/14C kinetic isotope effects (KIEs) were investigated using the reaction ofN, N‐dimethyl‐p‐toluidine with labelled methyl iodide in methanol at 30 °C as model system. The primary11C 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 the11C and14C radioactivity in these fractions. TheKIEvalue obtained agrees well with the value predicted from a previously determined12C/14C‐KIEfor the same reaction. Theoretical model calculations of both carbonKIEswere performed using theBEBOVIB IVprogram. 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

ISSN:0894-3230    

DOI:10.1002/poc.610040203

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractThe standard Gibbs energies of transfer, ΔGt° from water to water–N,N‐dimethylformamide mixtures for the dissociation process of several organic acids (formic, acetic, propionic,n‐butyric, isobutyric,n‐valeric, isovaleric, 2‐methylbutyric and trimethylacetic acids) obtained from thepKa* values are discussed in terms of the extended Scaled Particle Theory from quantum mechanical calculations in order to estimate the cavity and electrostatic contributions. The residual energy term was correlated with the hydrogen bond acceptor density and the Kamlet–Taft β parameters. The main contribution to ΔGt° was due to dipolar and speci

ISSN:0894-3230    

DOI:10.1002/poc.610040204

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractFor 34 groups, Taft's substituent constants σ* and σIare found to correlate well (r= 0.962 and 0.948) with Huheey's group electronegativity (χH) when Taft's steric parameterEsfor a group is mixed with σ* and σI:From these equations, it follows that for a particular chemical group G, σ*(G) ≈︁ 8σI(G). Since the σ* scale is essentially based on the σIscale, the small inherent steric component in σIis amplified in the case of the σ* by a factor ofca8. An analysis shows that χHrepresents the electronegativity of a group when the group is considered to be a mere collection of non‐bonded, charged and suitably hybridized atoms. Thus σ* and σIare found to represent the electronegativity of a group in the spirit of the χH. It is concluded that a chemical group can be viewed as a collection of non‐interacting atoms. Although there is evidence that a molecule can sometimes be treated similarly, for a group suc

ISSN:0894-3230    

DOI:10.1002/poc.610040205

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractKinetic isotope effects (KIE) were determined for the reactions of 2‐phenylethyl and 1‐methyl‐2‐phenylethyl benzenesulphonates with deuterated aniline nucleophiles in acetonitrile at 65.0 °C. The results are in good agreement with the transition state (TS) structures proposed based on the sign and magnitude of the cross‐interaction constants,pxz, between the substituents in the nucleophile (X) and the leaving group (Z). In the reactions of 2‐phenylethyl derivatives, all three reaction pathways,kr,ktandkΔ, were found to be contributing competitively; the inverse secondary KIE observed with a stronger nucleophile changed into the primary KIE with a weaker nucleophile owing to the predominant contribution of the four‐centre TS in thekfpath. For the reactions of the 1‐methyl‐2‐phenylethyl series, thekrpath played a major role, the contribution from the front‐side nucleophilic attack,kf, being negligible. In both reaction series, the aryl participation was important for thep‐C

ISSN:0894-3230    

DOI:10.1002/poc.610040206

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

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

ISSN:0894-3230    

DOI:10.1002/poc.610040207

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractOverall and internal rotation can be studied by means of relaxation time measurements on carbon‐13. Diffusion constants associated with overall rotation and internal rotation can be used to quantify the intermolecular and intramolecular motional constraints. During the course of the study, a very careful examination of monomethyl and dimethylnorbornanes was performed and the results were compared with data obtained by neutron inelastic scattering. This study led to a clear conclusion concerning the real usefulness of relaxation time measurements as a very good alternative method for the determination of entropy contributions, which are usually calculated on the basis of vibrational studie

ISSN:0894-3230    

DOI:10.1002/poc.610040208

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractThe crystal and molecular structure of 2,6‐diphenyl‐4‐(4‐carboxyphenyl)pyrylium perchlorate was solved by x‐ray diffraction, yielding structural parameters of high precision (estimated standard deviations for bond length ≤ 0.5 pm, for bond angles ≤ 0.2°). Comparison with other 4‐substituted derivatives of 2,6‐diphenyl‐4(4‐carboxytphenylpyrylium) perchlorate shows regularities in the variations of structural parameters following classical views

ISSN:0894-3230    

DOI:10.1002/poc.610040209

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

ISSN:0894-3230    

DOI:10.1002/poc.610040201

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY