The spin delocalization substituent parameter σ 6. The spin-delocalizing abilities of the thiomethyl,cyclopropyl and t-butyl groups. Synthesis of three substituted trifluorostyrenes

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 σ_mb 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.     

The spin delocalization substituent parameter s̀ 8. The spin-delocalizing abilities of the nitro,cyano, carbomethoxy,carboxy and carbamyl groups

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 σ_T^˙ 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.     

The spin delocalization substituent parameter σ˙. 7. Meta-substituent effects on the cyclodimerization of trifluorostyrenes. A test for the σ˙JJ approach

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 σ_mb 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 σ^X and these σ^˙_m values support our belief that the σ_j?J approach is reliable and trustworthy.     

Chemistry of trifluorostyrenes and their dimers: 2. Synthesis of substituted α, β, β-trifluorostyrenes and α, β, β-trifluoroethenylnaphthalenes. Hammett correlations of their NMR parameters and the concept of “distorted π-electron clouds”

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 Δδ_3-1 (δ (F³)—δ(f¹) or Δδ_3-2. The results are consistent with previous expectations based on the simple concept of “distorted π-electron clouds”. Facts are presented which indicate that the Δδ_3-1 (or Δδ_3-2) values may serve as empirical measures of the degree of polarization of the π bonds of these fluoroolefins.     

氟烯的结构 - 特性关系7. 取代基对α,β,β-三氟苯乙烯热环化二聚反应动力学参数的影响

在110～160℃范围内,测定了三氟苯乙烯(TFS)和六个取代三氟苯乙烯(ρ-OCH3,ρ-CH3,ρ-Cl,ρ-F,ρ-CF3,m-CH3)在正己烷溶液中热环化二聚反应的速率常数k~2,并计算其Arrhenius参数和在413K时的活化参数,讨论了取代基对反应速率常数k2的影响,发现k2与几乎所有取代基常数,如,σ,σ^+,σ^0,σ~R^0,σR^+和σR等均不能很好相关,说明除取代基的极性因素可影响k2外,不可忽略取代基自旋离域能力的作用。     

Synthesis of substituted 3‐pyrrolidinecarbonitriles

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.     

Synthesis of functionalized 1‐trimethylsiloxy‐substituted O‐trimethylsilyl alkylphosphonites and their derivatives

Nucleophilic addition of trimethylsilyl esters of tricoordinate organophosphorus acids to various functionalized aldehydes with vinyl, aryl, and heterocyclic fragments is proposed as a convenient method for the synthesis of new 1‐trimethylsiloxysubstituted alkylphosphonites and their derivatives at mild conditions. Also the new functionalized derivatives of these phosphonites, including amino groups as well as certain properties of these compounds as important precursors of new functionalized 1‐hydroxyalkylorganophosphorus acids, are presented. © 2008 Wiley Periodicals, Inc. Heteroatom Chem 19:352–359, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20430     

Chemistry of trifluorostyrenes and their dimmers: 3. Solvent effect on the kinetic parameters of the thermal cyclodimerization of α, β, β-trifluorostyrene1

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, E_a= 19.5±0.3 kcal.mol^?1; in glyme, logA = 5.31 ± 0.19, E_a= 18.0±0.3 kcal.mol^?1; in methanol, IogA=4.93±0.13, E_a=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.     

The capto-dative substituent effect on benzylic radicals ESR hyperfine splittings. An evaluation of the extra stabilization based upon spin delocalization

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.     

The competition between the intramolecular hydrogen bond and π‐electron delocalization in trifluoroacetylacetone—A theoretical study

Conformational study of trifluoroacetylacetone was carried out using the HF, B3LYP, and MP2 methods with the 6‐31G(d, p) and 6‐311++G(d, p) basis sets. All of the results show that the chelated enol structures (E11 and E31) have extra stability with respect to the other forms and one of them (E11) is global minimum. The energy gap between the chelated forms is in the range 0.7–5.9 kJ mol^?1. Theoretical calculations show that this compound has an asymmetric double minimum potential energy surface which is in contrast with the electron diffraction result. Moreover, the computational results predict that due to the withdrawing effect of CF₃ group, hydrogen bond in trifluoroacetylacetone is weaker than the acetylacetone. Because of the more stability of E11, it is expected that the hydrogen bond energy in E11 is greater than the E31, but at all of the computational levels with most extended basis set the converse results were observed. These results clearly show that the hydrogen bond is not a superior parameter in conformational preference and the contribution of resonance is probably greater than the hydrogen bond. Finally, the analysis of this system by quantum theory of atoms in molecules and natural bond orbital methods fairly support the ab initio results. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

The mass spectra of some 5′-alkul substituted 5′-thioadenosine trimethylsilyl derivatives

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.     

Delocalization of the unpaired spin density in some niobocene complexes with σ-donor, π-acceptors

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.     

Structure determination of three furan‐substituted benzimidazoles and calculation of π–π and C—H...π interaction energies

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.     

Synthesis of 8‐substituted tetrahydro‐γ‐carbolines

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.