Phase-transfer catalysis in reactions of electrophilic aromatic substitution

Electrophilic substitution in aromatic compounds under conditions of phase-transfer catalysis is considered. Catalysts of phase transfer of electrophilic reagents are used; their efficiency and the mechanism of their action in organic solvent-water systems are discussed.This review is based on materials of the report delivered at the Conference Phase-Transfer Catalysis. New Ideas and Methods (March 1994).Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1895–1900, October, 1995.     

Steric factor in electrophilic substitution reactions of aromatic hydrocarbons

Summary In supplementation of previous work on halomethylation reactions, the nitration, chlorination, bromination, and iodination of toluene, ethylbenzene, and eumene were investigated. By the method of oxidizing the derivatives formed, the relative amounts of o- and p-isomers were determined quantitatively, and the results reveal the steric effects of substituents in the aromatic nucleus on orientation in all of the above-mentioned electrophilic substitution reactions.     

Intervention of phenonium ion in Ritter reactions

The transformation of phenylethyl chloride to 3,4-dihydroisoquinolines is shown to proceed via phenonium ion. The evidence comes from a study of dideuterated analogue 4, and the monomethylated and dimethylated compounds 2 and 3.     

Theoretical analysis of reactions of electrophilic iodination and chlorination of benzene and polycyclic arenes in density functional theory approximation

Quantum-chemical method DFT B3LYP/6-311G* was applied to stage by stage thermodynamic calculation of reactions of electrophilic iodination of benzene, naphthalene, phenanthrene, and anthracene with iodine and iodine monochloride, and comparison with chlorination reactions was performed. The main distinction of iodination process from chlorination was an enhanced reversibility owing to protodeiodination. The reversibility of iodination grows with the electron-donor properties of aromatic substrates. The calculations permit an assumption that the chlorination of anthracene and phenanthrene with iodine monochloride occurs most probably through stages of electrophilic iodination-dehydroiodination.     

Gradient paths of the reactions of electrophilic addition of HF to ethylene and 1,2 H-shift in H2PN

It is shown that the gradient path of the electrophilic addition of HF to ethylene includes two different gradient lines. The first gradient line goes through two sequential saddle points (transition structures) and bypasses the minimum of reaction products. The other gradient line, which corresponds to internal rotation in ethane, joins two minima of reaction products and goes only through the second saddle point. The gradient path of the reaction of 1,2 H-shift in H₂PN consists of two equivalent gradient lines that simultaneously join the minima of reactants and products. This work is devoted to V. I. Minkin, Member of Russian Academy of Sciences, on the occasion of his sixtieth anniversary. Research Institute for Physical and Organic Chemistry, Rostov State University. Chemical Laboratories, Cambridge University. Translated fromZhurnal Struktumoi Khimii, Vol. 36, No. 4, pp. 579–592, July–August, 1995. Translated by I. Izvekova     

Kinetics and mechanism of aromatic electrophilic substitution reactions: VII. Temperature dependence of substrate selectivity of methylbenzene hydroxylation in the system H2O2-H2SO4

The relative rates of the hydroxylation of toluene,ortho-, meta-, andpara-xylenes, mesitylene, and pseudocumene are measured by the kinetic distribution method in the H₂O₂-H₂SO₄ (70.0 wt %) system at 15 to 55°C. The activation entropies correlate with the basicities of arenes and the activation enthalpies correlate with both the basicities and ionization potentials of ArH. Substrate selectivity is described with a high accuracy by the general equation with three variables (basicity, ionization potentials, and temperature) and implies the compensation effect between changes in standard entropies and enthalpies for the basicity constants of ArH. The structure of the transition state of the slow step of hydroxylation is intermediate between the structure of a charge-transfer complex and that of a σ-complex.     

A comparison of the relative energies of isomeric intermediates formed in electrophilic,radical, and nucleophilic aromatic substitution reactions

The relative energetics of isomeric σ-complex intermediates formed in electrophilic, free radical, and nucleophilic attack of Cl, H, and Me on PhCl and PhMe are compared for all positions: o, m, p, and ipso. The results are presented as ΔH, computed by the MINDO/3 method, for the appropriate isodesmic reaction with benzene. For the chlorobenzene intermediates, there is a marked increase in the relative preference for the ipso-position as the reactions change from electrophilic to free radical to nucleophilic. For toluene intermediates, the order of stability-p > o > m > ipso-is seen in all reactions except for one case, free radical methylation. In general, the free radical intermediates show the smallest range of energy differences. Comparison of predictions from these calculations with experimental results (largely partial rate factors and product ratios) shows some qualitative agreement.     

Formation of stable Meisenheimer adduct ion pairs in apolar solvents: implications for stereoselective reactions

[reaction: see text] A detailed study concerning the formation of Meisenheimer adducts in biphasic solvent systems is described. The process relies on utilizing a significantly lipophilic quaternary ammonium salt to transfer a nucleophile (e.g., hydroxide ion) between an aqueous and organic layer containing the electron-deficient aromatic substrate. Provided that the organic layer is sufficiently apolar, the resultant Meisenheimer adduct is considerably stable, likely the result of a strong ion-pairing interaction between the large polarizable anionic complex and the diffusively charged tetraalkylammonium cation. Using the diethylamide of 3,5-dinitrobenzoic acid as a model compound, the influence of ion-pairing reagents and solvents on adduct formation was investigated. Dramatically increased equilibrium formation of the Meisenheimer adduct is observed in the biphasic medium (e.g., benzene/2 M NaOH) relative to the same adduct generated in single-phase systems. Spectroscopic studies on this adduct are consistent with those conducted in single-phase polar or dipolar aprotic solvents. The methodology is extended to performing highly enantioselective biphasic kinetic resolutions of several racemic electron-deficient amides.     

Benzindoles. 18. Some electrophilic substitution reactions involving the hydrogen atoms in 4,5,6,7-dibenzindole

The principal electrophilic substitution reactions involving the hydrogen atom of 4,5,6,7-dibenzindole — the Vilsmeier reaction, the Mannich reaction, diazo coupling, and acylation — were studied, and a qualitative comparison of its reactivities with indole and 4,5- and 6,7-benzindoles was made. It was established that 4,5,6,7-dibenzindole has the most clearly expressed electron-donor properties. The structures of the synthesized compounds were proved by data from the IR, UV, PMR, and mass spectra.See [1] for communication 17.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 366–370, March, 1979.     

Radical nucleophilic substitution mechanism in the reactions of arenediazonium cations with nitrite ion

Reactions of arenediazonium cations with nitrite ion generally believed to involve nucleophilic displacement have been found to occur via single electron transfer where free radicals intervene.