Electrophilicity parameters for benzylidenemalononitriles

Kinetics of the reactions of stabilized carbanions (derived from nitroethane, diethyl malonate, ethyl cyanoacetate, ethyl acetoacetate, acetyl acetone) with benzylidenemalononitriles have been determined in dimethyl sulfoxide solution at 20 degrees C. The second-order rate constants are employed to determine the electrophilicity parameters E of the benzylidenemalononitriles according to the correlation equation log k (20 degrees C) = s(E + N). Comparison with literature data shows that this equation allows the semiquantitative prediction of the reactivities of benzylidenemalononitriles toward a wide variety of nucleophiles, including carbanions, enamines, amines, water, and hydroxide.     

A new scale of electronegativity based on electrophilicity index

By calculating the energies of neutral and different ionic forms (M2+, M+, M, M-, and M2-) of 32 elements (using B3LYP/6-311++G** level of theory) and taking energy (E) to be a Morse-like function of the number of electrons (N), the electrophilicity values (omega) are calculated for these atoms. The obtained electrophilicities show a good linearity with some commonly used electronegativity scales such as Pauling and Allred-Rochow. Using these electrophilicities, the ionicities of some diatomic molecules are calculated, which are in good agreement with the experimental data. Therefore, these electrophilicities are introduced as a new scale for atomic electronegativity, chi(omega)0. The same procedure is also performed for some simple polyatomic molecules. It is shown that the new scale successfully obeys Sanderson's electronegativity equalization principle and for those molecules which have the same number of atoms, the ratio of the change in electronegativity during the formation of a molecule from its elements to the molecular electronegativity (Delta chi/chi omega) is the same.     

Electrophilicity of 5-benzylidene-1,3-dimethylbarbituric and -thiobarbituric acids

The kinetics of reactions of acceptor-stabilized carbanions 2a-m with benzylidenebarbituric and -thiobarbituric acids 1a-e has been determined in a dimethyl sulfoxide solution at 20 degrees C. Second-order rate constants were employed to determine the electrophilicity parameters E of the benzylidenebarbituric and -thiobarbituric acids 1a-e according to the correlation equation log k(20 degrees C) = s(N + E). With E parameters in the range of -10.4 to -13.9, the electrophilicities of 1a-e are comparable to those of analogously substituted benzylidenemalononitriles.     

Ranking the reactivity of superelectrophilic heteroaromatics on the electrophilicity scale

The kinetics of the reactions of a series of reference carbon nucleophiles, consisting of N-methylpyrrole A, indole B, N-methylindole C, and enamines D-G, with 10 electron-deficient aromatic and heteroaromatic substrates (1-10), resulting in the formation of stable anionic sigma-adducts, have been investigated in acetonitrile at 20 degrees C. It is shown that the second-order rate constants k(1) pertaining to the carbon-carbon coupling step of these processes fit nicely the three-parameter equation log k (20 degrees C) = s(N + E), allowing the determination of the electrophilicity parameters E of 1-10 and therefore the ranking of these neutral electron-deficient compounds on the comprehensive electrophilicity scale defined for cationic electrophiles by Mayr et al. (Mayr, H.; Kempf, B,; Ofial, A. R. Acc. Chem. Res. 2003, 36, 66). The E values of 1-10 are found to cover a range from -13 to -5, going from 1,3,5-trinitrobenzene 1, the least reactive molecule, to 4,6-dinitrotetrazolo[1,5-a]pyridine 8, 4-nitro-6-trifluoromethanesulfonylbenzofuroxan 3, and 4,6-dinitrobenzofuroxan 2, the three most reactive heterocycles. Of major interest is that the E value of 2 is essentially the same as that for 4-nitrobenzenediazonium cation (E = -5.1), approaching that of the tropylium cation family (E approximately -3 to -6) as well as a number of metal-coordinated carbenium ions. Such a ranking holds promise for expanding the range of coupling reactions which can be envisioned with such strongly electron-deficient neutral heteroaromatics as nitrobenzofuroxans and related compounds. Arguments are also given which exclude the possibility for the reactions studied to proceed via an electron-transfer mechanism.     

Electrophilicity parameters of 5-benzylidene-2,2-dimethyl[1,3]dioxane-4,6-diones (benzylidene Meldrum's acids)

Kinetics of the reactions of four benzylidene Meldrum's acids 1 with acceptor-substituted carbanions 2 were studied photometrically in DMSO at 20 degrees C. The reactions follow second-order kinetics, and the second-order rate constants were found to follow the correlation log k2 (20 degrees C) = s(N + E) (eq 1), which was used to calculate the electrophilicity parameters E for compounds 1. Hammett correlations are given, which allow one to assign electrophilicity parameters for various beta,beta-acceptor substituted styrenes and thus to predict a large number of absolute rate constants for a manifold of Michael additions. The reactions of primary and secondary amines are approximately 2 orders of magnitude faster than predicted by the correlation (1), supporting transition states which are stabilized by hydrogen bridges from NH to the carbonyl groups of the benzylidene Meldrum's acids.     

Electrophilicity parameters for sigma-complexation by uncharged electron-deficient aromatic and heteroaromatic structures

Using appropriate sets of reference nucleophiles, the reactivity of neutral electrophiles of widely different reactivity and structure has been ranked on the comprehensive electrophilicity scale of Mayr (Acc. Chem. Res., 2003, 36, 66), holding promise of a general rationalization of sigma-complexation processes and related SNAr substitutions.     

Quantitative characterization of the global electrophilicity pattern of some reagents involved in 1,3-dipolar cycloaddition reactions

The global electrophilicity power, ω, of a series of dipoles and dipolarophiles commonly used in 1,3-dipolar cycloadditions may be conveniently classified within a unique relative scale. The effects of chemical substitution on the electrophilicity of molecules have been evaluated using a representative set of electron-withdrawing and electron-releasing groups for a series of dipoles including nitrone, nitrile oxide and azide derivatives. The absolute scale of electrophilicity is used to rationalize the chemical reactivity of these species as compared to the static reactivity pattern of the reagents involved in the Diels-Alder reactions.     

Fulgides based on 1,3-dimethyl-2-formylindole and 1,3-dimethyl-2-acetylindole

Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 422–423, March, 1992.     

Determination of the electrophilicity parameters of diethyl benzylidenemalonates in dimethyl sulfoxide: reference electrophiles for characterizing strong nucleophiles

The second-order rate constants of the reactions of nine substituted diethyl benzylidenemalonates 1 a-i with the carbanions 2 a-e have been determined spectrophotometrically in dimethyl sulfoxide (DMSO). Product studies show that the nucleophiles attack regioselectively at the electrophilic C==C double bond of the Michael acceptors to form the carbanionic adducts 4. The correlation log k(20 degrees C)=s(N+E) allows the determination of the electrophilicity parameters E for the electrophiles 1 a-i from the rate constants determined in this work and the previously published N and s parameters for the nucleophiles 2 a-e. The electrophilicities E for compounds 1 a-i cover a range of six units (-17.7>E>-23.8) and correlate excellently with Hammett's substituent constants sigma(p). The title compounds are roughly ten orders of magnitude less reactive than analogously substituted benzylidene Meldrum's acids, their cyclic analogues. Due to their low reactivities, compounds 1 a-i are suitable reference electrophiles for determining the reactivities of highly reactive nucleophiles, such as carbanions with 16相似文献   

Simple Method for the Preparation of 1,3-Dithiol-2-one and 1,3-Dithiol-2-thione

Abstract

The reaction of diisopropyl xanthogen and thio-diisopropyl xanthogen disulfide with alkynes furnished 1,3-dithiol-2-one and 1,3-dithiol-2-thione.     

1,3-Dichloro-2-butene: a useful precursor for the 2-butene-1,3-dianion and its corresponding 1,3-dipolar synthon

The reaction of 1,3-dicloro-2-butene (1; 5:1 Z:E-mixture) with lithium powder and a catalytic amount of 4,4′-di-tert-butylbiphenyl (DTBB, 1% molar) in the presence of different electrophiles [EtCHO, PrⁱCHO, Bu^tCHO, c-C₆H₁₁CHO, Me₂CO, Et₂CO, (CH₂)₄CO, (CH₂)₅CO, (c-C₃H₅)₂CO, Me₃SiCl] in THF at temperatures ranging between −78 and −50°C gives, after hydrolysis with water, the corresponding products 2 in different Z:E-ratios depending on the electrophile used. Treatment of some diols 2 with hydrochloric acid gives dienic alcohols 3 or substituted dihydropyrans 4, depending on the structure of the starting diol. Finally, the same dichlorinated starting material is transformed into the corresponding allylic amines derived from morpholine and benzyl methyl amine and submitted to the same DTBB-catalysed lithiation as above, so after reaction with different electrophiles [Bu^tCHO, c-C₆H₁₁CHO, Me₂CO, Et₂CO, (CH₂)₄CO, (CH₂)₅CO, Me₃SiCl] and final hydrolysis with water, compounds 7 are isolated having a Z-configuration. A mechanistic explanation for this behaviour is given.     

1,3-Dimethyl-2-phenyl-1,3-diazaphospholidine-2-oxide as ligand for the preparation of luminescent lanthanide complexes

Abstract

Europium(III) coordination compounds having general formula [Eu(β-dike)₃L₂] (β-dike?=?dibenzoylmethanate, tenoyltrifluoroacetonate; L?=?1,3-dimethyl-2-phenyl-1,3-diazaphospholidine-2-oxide) were isolated and characterized. The complexes exhibited bright red emission associated to the ⁵D₀→⁷F_J transitions of the metal center upon excitation with near-UV light, with intrinsic quantum yields around 51% and 65%, respectively, for the dibenzoylmethanate and tenoyltrifluoroacetonate derivatives. More information about the behavior of 1,3-dimethyl-2-phenyl-1,3-diazaphospholidine-2-oxide as an antenna-ligand towards trivalent lanthanide ions was obtained by its coordination to [Ln(NO₃)₃] (Ln?=?Eu, Gd, Tb) metal fragments.     

Investigation of the growth parameters of hydrothermal ZnO nanowires for scale up applications

Zinc oxide nano-wires (ZnO NWs) are synthesized reproducibly with high yield via a low temperature hydrothermal technique. The influence of the growth duration time, growth temperature, zinc precursor and base concentration of Na₂CO₃ on the morphology of NWs is investigated. The growth products are characterised using scanning electron microscopy (SEM), Transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL). SEM analysis shows that the optimum growth temperature is 140 °C and finds that length and diameter of ZnO NWs have a relationship with growth duration time and base concentrations of Na₂CO₃. In addition, it is reported that a high (～90%) yield of ZnO NWs can be synthesised via using any of three different precursors: zinc chloride, zinc acetate and zinc nitrate. TEM and XRD results indicate the high purity and the single crystalline nature of the ZnO NWs. XPS confirms the absence of sodium contaminants on the surface and indicates a near flat band surface condition. PL shows a large visible band in the yellow part of the spectrum, and a small exciton emission peak, indicating a large defect concentration, which is reduced after annealing in air.     

Rates and equilibria of the reactions of tertiary phosphanes and phosphites with benzhydrylium ions

The kinetics of the reactions of benzhydrylium ions and quinone methides with eight tertiary phosphanes and two phosphites were investigated photometrically. The nucleophilicity parameters N and slope parameters s of these nucleophiles were derived according to the equation log k(20 degrees C) = s(N + E). Correlations of the nucleophilicity parameters N with pK(Ha) and sigma(p) values as well as with the rate constants of reactions with other electrophiles are discussed. In some cases, equilibrium constants for the formation of phosphonium ions were measured, which allow one to determine the Marcus intrinsic barriers of DeltaG(0) (not equal) = 58 kJ mol(-1) for the reactions of triarylphosphanes with benzhydrylium ions. The N parameters [5.51 for P(OPh)3, 10.36 for P(OBu)3, 14.33 for PPh3, 15.49 for PBu3, 18.39 for P(4-Me2NC6H4)3] are compared with the reactivities of other classes of nucleophiles (see, www.cup. uni-muenchen.de/oc/mayr).     

Kinetic parameters of cationic polymerization of 1,3-dienes

Russian Chemical Bulletin - A new technique for the determination of the concentration of active centers of polymerization and propagation rate constants for processes of cationic polymerization of...     

Relationship between local electrophilicity and rate coefficients for the hydrolysis of carbenium ions

The local electrophilicity of a series of 28 carbenium ions has been ranked within a theoretical absolute scale. New substituent constants are introduced to account for the responses of the electrophilicity pattern induced by multiple substitutions at the carbocation site. The model is used to predict rate coefficients ordering in terms of the experimental hierarchy of electrophilicity established for these systems [Minegishi, S.; Mayr, H. J. Am. Chem. Soc.2003, 125, 286].     

Methodologies for 2-alkylidene-1,3-cyclopentanediones

Methods for the synthesis of 2-alkylidene-1,3-cyclopentanediones are given along with some preliminary information on the reactivity of these ene-diones.     

Synthesis of benzimidazolone derivatives based on 2-acylcyclohexane-1,3-diones alkoximes

2-(1-Alkoxyiminoalkyl)cyclohexane-1,3-diones undergo at heating Beckmann rearrangement to give 6,7-dihydro-1,3-benzoxazol-4(5H)-one derivatives that under treatment with amines in acid medium are converted into 1,5,6,7-tetrahydro-4H-benzimidazol-4-ones. In reaction of 6,7-dihydro-1,3-benzoxazol-4(5H)-ones with O-ethylhydroxylamine 4-ethoxyimino derivatives were obtained that treated with hydrochloric acid formed the corresponding N-ethoxybenzimidazolones.     

A conceptual framework for predicting the toxicity of reactive chemicals: modeling soft electrophilicity

Although the literature is replete with QSAR models developed for many toxic effects caused by reversible chemical interactions, the development of QSARs for the toxic effects of reactive chemicals lacks a consistent approach. While limitations exit, an appropriate starting-point for modeling reactive toxicity is the applicability of the general rules of organic chemical reactions and the association of these reactions to cellular targets of importance in toxicology. The identification of plausible "molecular initiating events" based on covalent reactions with nucleophiles in proteins and DNA provides the unifying concept for a framework for reactive toxicity. This paper outlines the proposed framework for reactive toxicity. Empirical measures of the chemical reactivity of xenobiotics with a model nucleophile (thiol) are used to simulate the relative rates at which a reactive chemical is likely to bind irreversibly to cellular targets. These measures of intrinsic reactivity serve as correlates to a variety of toxic effects; what's more they appear to be more appropriate endpoints for QSAR modeling than the toxicity endpoints themselves.