摘要:

AbstractThe origin of the π‐facial selectivity in nucleophilic additions to cyclohexanone was analysed by means ofab initioMO calculations (MP2/6–31G*//HF/6–31G*). The calculations showed that BH3attacks at the axial side whereas MeLi attacks at the equatorial side. Calculated kinetic and equilibrium deuterium isotope effects and the structures of the transition states (TSs) demonstrated that four factors, stabilization of the axial TS by antiperiplanar allylic bonds, destabilization of the equatorial TS by torsional strain, destabilization of the axial TS by steric hindrance between a nucleophile and the 3,5‐axial hydrogens and destabilization due to the conformational deformation of the ring in both the equatorial and axial TSs, operate in the same direction and can bring about the apparent selectivity changeover. The magnitudes ofanti‐D KIEs for the MeLi and the EtLi additions to acetone were consistent with the Anh‐Eisenstein model rather than the C

ISSN:0894-3230    

DOI:10.1002/poc.610080702

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

Abstract1,3‐Dipolar cycloadditions ofN‐(phenylmethylene)benzenesulphonamide with mesionicN‐methyl‐1,3‐oxazolium‐5‐olates give 2,5‐disubstituted imidazole derivatives with high regioselectivity. The intermolecular interactions underlying this regioselectivity were investigated. The conformational and electronic properties of the reagents were characterized separately. The approach in the early stages of the reactions was then modelled by considering the steric and electrostatic molecular interactions. The interaction energies related to different reaction paths were calculated by perturbation molecular orbital (PMO) treatme

ISSN:0894-3230    

DOI:10.1002/poc.610080703

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

Abstract1H and13C NMR data for 2‐diethylaminomethyl‐3,4,6‐tri‐ and ‐tetrachlorophenols in CH2Cl2solutions at a variety of temperatures are reported. The results are consistent with intramolecular hydrogen bonding and proton transfer as shown in the structure 3. Concentration dependence of the NMR data for the perchloro compound shows an additional hydrogen bonding process attributed to the solute self‐association. The1H,13C and15N NMR data support the proposed proton transfe

ISSN:0894-3230    

DOI:10.1002/poc.610080704

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractThe normal electron‐demand Diels–Alder cycloaddition between cyclopentadiene and methyl vinyl ketone is inhibited by the cycloadduct, when catalyzed by kaolinite‐supported zinc chloride. This inhibition stems from occupation of the catalytic sites by both the cycloadduct and methyl vinyl ketone. Catalysis occurs in this case exclusively by a decrease in the enthalpy of activ

ISSN:0894-3230    

DOI:10.1002/poc.610080705

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractAb initiostudies on theSN2 identity exchange reactions RCH2X + X−→ X−+ RCH2X for R = CH2CH with X−= H, NH2, OH, F, PH2, SH and Cl, and for R = CH3and CHC with X−= Cl were carried out at the HF and MP2 levels using the 6–31 ++ G** basis sets. The activation barriers, ΔE≠, and major structural changes, Δd≠(C–X), in the activation process are closely related to the electronegativity of the R and X groups. The effect of electronegativity is twofold: a stronger electronegativity of R and/or X leads to a lesser electronic as well as structural reorganization required in the activation and to a greater correlation energy in the transition state. The former effect lowers the energy barriers at both the HF and MP2 levels whereas the latter lowers only the correlated (MP2) activation energies. Results with R = CH2CH, as a model for R = C6H5, indicate that ‘benzylic effect’ arises mainly from the relatively stronger electron acceptor ab

ISSN:0894-3230    

DOI:10.1002/poc.610080706

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractThe fluorescence of some aliphatic diamines was studied in the gas phase and in some solvents at several temperatures. It was found thatN,N,N′,N′‐tetramethylpropane‐1,3‐diamine (TMPD) andN,N,N′,N′‐tetramethylbutane‐1,4‐diamine (TMBD) have two emission bands in the gas phase. TMBD also shows two bands in solution. These bands were assigned previously to an excited monomer and an intramolecular excimer. From the temperature dependence of the intensity ratio of the two bands, the enthalpy changes for the excimer formation were evaluated. The stability of the cyclic excimer is discussed in terms of the ring size effect and the solvent effect on

ISSN:0894-3230    

DOI:10.1002/poc.610080707

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractThe pK BH +values of the conjugate acids ofN‐phenylhydroxylamines change drastically when the pK BH +values are determined in dimethyl sulfoxide (DMSO) rather than methanol. The Hammett ρ values change from −5·69 to −1·20 on going from methanol to DMSO for protonatedN‐phenylhydroxylamines, in contrast to a shift of −4·70 to −4·83 for protonatedN‐methylanilines in the same two solvents. This large change in susceptibility indicates that the species from which the proton departs is not the same for protonatedN‐phenylhydroxylamines in the two solvents. Experimental and computational evidence supports ionization of the H+from the O atom for the protonatedN

ISSN:0894-3230    

DOI:10.1002/poc.610080708

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractMolecular reactivity parameters based on the concept of an effective electronic potential and defined as a simple function of the molecular charge distribution were applied in a study of various aspects of the chemical reactivity of azabenzenes and azanaphthalenes. Excellent linear correlations were obtained for proton affinities, pKavalues, H–D exchange rate exponents and13C shifts induced by the paramagnetic shift reagent TEMPO. In the last case, the predictions for quinazoline prompted a reinvestigation of the13C NMR spectrum of this compound, resulting in a reassignment of the spectrum relative to the assignment assumed by Grant and co‐work

ISSN:0894-3230    

DOI:10.1002/poc.610080709

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

ISSN:0894-3230    

DOI:10.1002/poc.610080701

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY