Three substituted α, β, β-trifluorostyrenes (TFS's), i. e., p-thiomethyltrifluorostyrene (4), p-cyclopropyltrifluorostyrene (5) and p-t-butyltrifluorostyrene (6), have been synthesized. The rate constants (k2) of the thermal cyclodimerization of these compounds have been measured in the temperature range 120–160°C. The polar parameters σmb of these TFS's calculated from 19F NMR chemical shifts are: for p-thiomethyl, -0.18; p-cyclopropyl, -0.31 and p-t-butyl, -0.22. The spin delocalization substituent parameters σ′T (140°) of p-thiomethyl, cyclopropyl and t-butyl groups are 0.59, 0.27 and 0.30 respectively. Thus all of these groups act as electron-donating groups which can also effectively stabilize a spin. 相似文献
The rate constants (k) of the homo-cycloaddition reactions of five substituted α,β,β-trifluorostyrenes (TFS's), i.e. p-nitrotrifluorostyrene (4), p-cyanotrifluorostyrene (5), p-carbomethoxytri-fluorostyrene (6), p-carboxytrifluorostyrene (7) and p-carbamyltrifluorostyrene (8), have been measured in the temperature range of 110–160°C. The s?mb polar substituent parameters of these TFS's calculated from 10F NMR chemical shifts are: NO2, 0.86; CN, 0.86; CO3CH3, 0.40; CO2H, 0.31; CONH2, 0.10. The spin delocalization substituent parameters σT˙ of NO2, CN, CO2CH3, CO2H and CONH2 are 0.32, 0.38, 0.31, 0.37 and 0.37 respectively. Thus all these electron-pair attracting groups are also very effective spin-stabilizers. 相似文献
Rate constants (k's) of the thermal cyclodimerization of ten meta-substituted trifluorostyrenes (m-Y-TFS's) have been measured in the temperature range 110–160°C. On the basis of these k's and the σmb values derived from 19F NMR data the spin-delocalization substituent constant σ˙m values of these ten meta-substituents have been calculated. The averaged σ˙m values are: m-Et, 0.09; m-t-Bu, 0.11; m-F, 0.03; m-Cl,?0.05; m-Br, 0.12: m-CF3, ?0.07; m-OMe, 0.10; m-NO2, ?0.002; m-CN, 0.11 and m-CO2Me, 0.08. Both the smallness of these values and the nonexistence of correlation between Hammett-Type polar σX and these σ˙m values support our belief that the σj?J approach is reliable and trustworthy. 相似文献
Six α, β, β-trifluorostyrenes with the following substituents, viz., p-MeO, p-Me, m-Me, p-Cl, m-Cl, and m-CF3, were synthesized by the reaction of the corresponding Grignard reagents with tetrafluoroethylene in tetrahydrofuran. Similarly, α-and β-trifluoroethenylnaphthalenes were prepared. The substituent electronic effects on the 19F-NMR parameters were investigated for the trifluorostyrenes (I). Linear correlations between the Hammett σ constants and the following 19F-NMR parameters were established, namely, chemical shifts δ. (F1) and δ (F2), coupling constants J12, differences of chemical shifts Δδ3-1 (δ (F3)—δ(f1) or Δδ3-2. The results are consistent with previous expectations based on the simple concept of “distorted π-electron clouds”. Facts are presented which indicate that the Δδ3-1 (or Δδ3-2) values may serve as empirical measures of the degree of polarization of the π bonds of these fluoroolefins. 相似文献
Cyclopropanes substituted at the same ring carbon by two electron‐withdrawing groups such as alkoxycarbonyl or cyano group react with the primary arylamines via a ring‐opening reaction and ensuing intramolecular cyclization to form substituted pyrrolidines. 相似文献
Rate constants for the thermal cyclodimerization of α, β, β-trifluorostyrene (TFS) were determined in six solvents at 393°K. The products of this reaction were mixtures of roughly equal amounts of cis-trans isomers. The rate constants in 3 solvents, were calculated according to Arrhenius equation. In n-hexane, log A = 6.02±0.18, Ea= 19.5±0.3 kcal.mol?1; in glyme, logA = 5.31 ± 0.19, Ea= 18.0±0.3 kcal.mol?1; in methanol, IogA=4.93±0.13, Ea=17.1±0.3 kcal mol?1. All data are consistent with a stepwise radical mechanism, and our reaction in this solvent series obeys an isokinetic relationship, with β = 478°K. 相似文献
Appropriate hyperfine splittings in ESR spectra of disubstituted (cyano/methoxy) benzylic radicals were compared with the corresponding values of monosubstituted ones. On the basis of the dependence of radical stabilization on spin delocalization, it is concluded that captodative substitution implies a synergetic substituent effect on radical stabilization. On the contrary, an antagonistic effect is observed in the case of two identical substituents. 相似文献
Trimethylsilyl derivatives of 5′-S-alkyl substituted 5′-thioadenosines give mass spectra that are characterized by a number of highly intense ions. These ions are formed by the fragmentation of the alkyl substituted sugar moicty and are specific for the alkyl group in the substitution. 相似文献
Extended Hückel calculations and qualitative MO have been used to analyze the EPR data relevant to the localization of the unpaired spin density in several paramagnetic niobocene complexes with σ-donor π-acceptors, acetylene, aldehyde, ketene, ketenimine, and carbon disulfide. 相似文献
The synthesis and structural characterization of 2‐(furan‐2‐yl)‐1‐(furan‐2‐ylmethyl)‐1H‐benzimidazole [C16H12N2O2, (I)], 2‐(furan‐2‐yl)‐1‐(furan‐2‐ylmethyl)‐1H‐benzimidazol‐3‐ium chloride monohydrate [C16H13N2O2+·Cl−·H2O, (II)] and the hydrobromide salt 5,6‐dimethyl‐2‐(furan‐2‐yl)‐1‐(furan‐2‐ylmethyl)‐1H‐benzimidazol‐3‐ium bromide [C18H17N2O2+·Br−, (III)] are described. Benzimidazole (I) displays two sets of aromatic interactions, each of which involves pairs of molecules in a head‐to‐tail arrangement. The first, denoted set (Ia), exhibits both intermolecular C—H...π interactions between the 2‐(furan‐2‐yl) (abbreviated as Fn) and 1‐(furan‐2‐ylmethyl) (abbreviated as MeFn) substituents, and π–π interactions involving the Fn substituents between inversion‐center‐related molecules. The second, denoted set (Ib), involves π–π interactions involving both the benzene ring (Bz) and the imidazole ring (Im) of benzimidazole. Hydrated salt (II) exhibits N—H...OH2...Cl hydrogen bonding that results in chains of molecules parallel to the a axis. There is also a head‐to‐head aromatic stacking of the protonated benzimidazole cations in which the Bz and Im rings of one molecule interact with the Im and Fn rings of adjacent molecules in the chain. Salt (III) displays N—H...Br hydrogen bonding and π–π interactions involving inversion‐center‐related benzimidazole rings in a head‐to‐tail arrangement. In all of the π–π interactions observed, the interacting moieties are shifted with respect to each other along the major molecular axis. Basis set superposition energy‐corrected (counterpoise method) interaction energies were calculated for each interaction [DFT, M06‐2X/6‐31+G(d)] employing atomic coordinates obtained in the crystallographic analyses for heavy atoms and optimized H‐atom coordinates. The calculated interaction energies are −43.0, −39.8, −48.5, and −55.0 kJ mol−1 for (Ia), (Ib), (II), and (III), respectively. For (Ia), the analysis was used to partition the interaction energies into the C—H...π and π–π components, which are 9.4 and 24.1 kJ mol−1, respectively. Energy‐minimized structures were used to determine the optimal interplanar spacing, the slip distance along the major molecular axis, and the slip distance along the minor molecular axis for 2‐(furan‐2‐yl)‐1H‐benzimidazole. 相似文献
The Fischer reaction is applied to the synthesis of 8‐substituted tetrahydro‐γ‐carbolines with electron‐donating or electron‐withdrawing groups, using catalytic or thermal methods. The reaction conditions must be varied according to the nature of the N 1 substituent of the piperidone. The best results are observed when a releasing group is present on the arylhydrazine and a benzyl substituent on the nitrogen of piperidone. Formation of carbolines with a withdrawing substituent is observed in soft acidic conditions; in others, reaction ended at the hydrazone level or did not evolve. 相似文献