The reactivity of tert-butoxyl radicals in reactions of hydrogen abstraction and β-elimination

The transition states of competing reactions of abstraction of the H atom from hydrocarbon molecules by tert-butoxyl radical and its -elimination were studied by the semiempirical quantum-chemical AM1 method. The calculated activation enthalpy and entropy of these reactions correlate with the experimental values. A correlation between the reactivity in homolytic hydrogen abstraction and the parameters of the chemical structure of the reactants was found.     

Reactivity of α-amino-peroxyl radicals and consequences for amine oxidation chemistry

A comparative theoretical study is presented on the formation and fate of α-amino-peroxyl radicals, recently proposed as important intermediates in the aerobic oxidation of amines. After radical abstraction of the weakly bonded αH-atom in the amine substrate, the α-amino-alkyl radical reacts irreversibly with O(2), forming the corresponding α-amino-peroxyl radical. HO(2)˙-elimination from various types of α-amino-peroxyl radicals (forming the corresponding imine) and the kinetically competing substrate H-abstraction (forming the α-amino-hydroperoxide) were computationally characterized. Polar solvents were found to reduce the HO(2)˙-elimination barrier, but increase the barrier for H-abstraction. Depending on the reaction conditions (gas or liquid phase, amine concentration, nature of the solvent, and temperature), either of the two mechanisms is favored. The consequences for aerobic amine oxidation chemistry are discussed.     

ESR study of the structure and reactivity of spin-adducts of element- and metal-centered radicals with perfluorinated α-triketones

The addition of element-centered radicals to perfluorinated α-triketones (CF₃)₂CFCOCOCOC₂F₅ (1) and (CF₃)₂CFCOCOCO(CF₃)₂ (2) was studied by the ESR method. In the case of Si-centered radicals, α-siloxydiketomethyl radicals are formed, which are characterized by the highest delocalization of an unpaired electron. The dimerization of these radicals is characterized by a low value of the enthalpy of the radical—dimer equilibrium (2.7 kcal mol⁻¹). The Ge(C₆F₅)₃ and ·Mn(CO)₅ radicals are added to1 at the carbonyl group directly bound to the pentafluoroethyl substituent. The hindered rotation of the (CF₃)₂CF group was observed for the spin-adduct of ·Ge(C₆F₅)₃ with2. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 474–477, March, 1999.     

Solvent effects on electronic absorption spectra of nitrochlorobenzenes,nitrophenols and nitroanilines—II. Studies in polar solvents

A general method for estimating the specific solute—solvent interaction energies (E_s) from analysis of solvent effects on electronic absorption spectra has been outlined. The E_s values for a number of mono- and disubstituted benzenes in a variety of solvents such as water, alcohols, chloroalkanes, ether and acetonitrile have been estimated and the results are discussed in relation to the interacting groups in the solute and solvent molecules. These interactions, which can be classified as H bonding or EDA type, are mainly electrostatic in nature. These studies indicate that, in the case of disubstituted benzenes, the intramolecular electronic interactions are stronger than the solute—solvent interactions.     

Effect of pH on the reactivity of dipeptides and α-amino acids in the N-acylation

The reaction kinetics of Gly, L-??-Ala, Gly-Gly, L-??-Ala-L-??-Ala and ??-Ala-??-Ala with picryl benzoate in water (40 wt %)-2-propanol was investigated. At pH = 4?C8 the rate constants of N-acylation of the anionic form of dipeptides are less than those of the corresponding amino acid anions, in agreement with their basicity, whereas the relative effective rate constants of reactions depend on pH: in acidic, neutral and slightly alkaline media the k _ef values are higher for the dipeptides, and in a strongly alkaline medium, for the amino acids. These differences are due to the changes in the concentrations of reactive forms of amino acids and dipeptides in the system at varying the medium pH.     

High-pressure rate rules for alkyl + O2 reactions. 2. The isomerization, cyclic ether formation, and β-scission reactions of hydroperoxy alkyl radicals

The unimolecular reactions of hydroperoxy alkyl radicals (QOOH) play a central role in the low-temperature oxidation of hydrocarbons as they compete with the addition of a second O(2) molecule, which is known to provide chain-branching. In this work we present high-pressure rate estimation rules for the most important unimolecular reactions of the β-, γ-, and δ-QOOH radicals: isomerization to RO(2), cyclic ether formation, and selected β-scission reactions. These rate rules are derived from high-pressure rate constants for a series of reactions of a given reaction class. The individual rate expressions are determined from CBS-QB3 electronic structure calculations combined with canonical transition state theory calculations. Next we use the rate rules, along with previously published rate estimation rules for the reactions of alkyl peroxy radicals (RO(2)), to investigate the potential impact of falloff effects in combustion/ignition kinetic modeling. Pressure effects are examined for the reaction of n-butyl radical with O(2) by comparison of concentration versus time profiles that were obtained using two mechanisms at 10 atm: one that contains pressure-dependent rate constants that are obtained from a QRRK/MSC analysis and another that only contains high-pressure rate expressions. These simulations reveal that under most conditions relevant to combustion/ignition problems, the high-pressure rate rules can be used directly to describe the reactions of RO(2) and QOOH. For the same conditions, we also address whether the various isomers equilibrate during reaction. These results indicate that equilibrium is established between the alkyl, RO(2), and γ- and δ-QOOH radicals.     

Synthesis,Stability and Reactivity of α-Fluorinated Azidoalkanes

Organic α-fluorinated azidoalkanes appeared in the literature for the first time half a century ago. However, for a long time they remained undeveloped and were regarded as chemical curiosities. Recent advances in the preparation of α-fluorinated azidoalkanes as well as studies on their stability and reactivity opened up their broader synthetic potential for the preparation of valuable fluorinated and non-fluorinated compounds.     

HPLC–MS determination of the oxidation products of the reaction between α- and β-pinene and OH radicals

Biogenic non-methane hydrocarbons such as isoprene, alpha-pinene, and beta-pinene, are emitted by forests in very large quantities. To evaluate the role of alpha- and beta-pinene and their contribution to the global production of trace gases and especially aerosol precursors, a study of the oxidation mechanism of alpha- and beta-pinene with hydroxyl radicals must be conducted.The degradation products of both monoterpenes with hydroxyl radicals were identified and quantified in a fast-flow reactor. The products were collected on a liquid-nitrogen trap coated with a 2,4-DNPH solution to which two internal standards (benzaldehyde-2,4-DNPH and tolualdehyde-2,4-DNPH) had been added. The collection method was based on the in situ conversion of aldehyde and/or ketone compounds to their 2,4-dinitrophenylhydrazone derivatives. The derivatives were analyzed by HPLC-MS using APCI(-). TIC chromatograms and mass spectral data for the various oxidation products are presented.For alpha-pinene, pinonaldehyde is the most important degradation product, with smaller amounts of acetone, formaldehyde, campholenealdehyde, and acetaldehyde. For beta-pinene, nopinone and formaldehyde are the most abundant products, of almost equal importance, whereas acetone and acetaldehyde are minor compounds.     

Kinetics of radical polymerization—XLV. Steric effects in the radical reactivity of quinones

An attempt has been made to establish the dependence on structural parameters of the reactivity of substituted 1,4-benzoquinones as inhibitors in the radical polymerization of styrene. The relative reactivity depends on the redox potential of the quinone and on the size of the ortho-substituents. The steric factor has been separated from the overall reactivity and correlated with the extent of shielding of the reaction centre in the case of substituents of spherical symmetry.     

On the reactivity of some 2-methyleneindolines with β-nitroenamines, α-nitroalkenes, and 1,2-diaza-1,3-butadienes

A study of the behaviour of some electron-rich 2-methyleneindolines (1-3) with different electron-poor reagents (formation of new carbon-carbon and nitrogen-carbon bonds) has furnished interesting results from both synthetic and the mechanistic viewpoints. Enamines 1-3 have been reacted with the β-nitroenamines 4-7 (reaction CeCl₃·7H₂O promoted), giving the polymethine dyes 14-23. The same bases 1-3 have been nitroalkylated with the nitroolefins 8-10, furnishing the indolines 24-32, and the diastereoselectivity of the reaction has been thoroughly investigated. The most unexpected results derived from the first example of reaction of Fischer's bases with 1,2-diaza-1,3-butadienes. In fact, with 11-13, the ‘unknown’ indoline spirodihydropyrroles 33-40 were formed. Their structures were unambiguously assigned, and we determined, as an example, that of 33 by X-ray analysis.     

ESR study of the free radicals formed in the photolysis of α-ketofluorosulfates

ESR spectroscopy was used to establish that the UV irradiation of -ketofluorosulfates containing an OSO ₂ F group at a tertiary carbon atom gives radicals formed upon cleavage of the C-OSO ₂ F bond. If the OSO ₂ F group is attached to a primary or secondary carbon atom, we observe ordinary photodecomposition with cleavage of the acyl—carbon bond. The ESR spectra of the new radicals were studied. Replacement of fluorine by the OSO ₂ F group in the trifluoromethyl radical leads to compression of the radical site. Branched -oxoperfluoroalkyl radicals are not heteroallylic and the unpaired electron is localized largely on the carbon atom. The [(CF ₃ ) ₂ CF] ₂ C(O)CF ₃ radical is stable in the air.A. N. Nesmeyanov Institute of Heteroorganic Compounds, Russian Academy of Sciences, 117813 Moscow. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 3, pp. 757–761, March, 1992.     

A thermodynamic study of α-, β-, and γ-cyclodextrin-complexed m-methyl red in alkaline solutions

The UV/Visible spectra of m-methyl red (m-MR) ({3-[4-(dimethyl-amino) phenylazo] benzoic acid}) were examined in basic, acidic and strongly acidic aqueous solutions. The observed spectra of m-MR were analyzed and compared with the tautomeric and resonance structures that suggested theoretically. Three isosbestic points in the spectra were observed around 508, 464 and 443 nm representing three different equilibriums between four different species of m-MR. The inclusion constant (K_f) for the inclusion of basic form of m-MR with alpha-, Beta-, and gamma-Cyclodextrin (α-, β- and γ-CD) was evaluated at different temperatures using Benesi-Hildebrand method. The values of K_f at 25 °C were found to be 8.70 × 10³, 4.93 × 10³ mol^?1 dm³ and 2.95 × 10⁷ mol^?2 dm⁶ basis on the inclusion complex ratios (m-MR:CD) of 1:1, 1:1, and 2:1 respectively. The values of the thermodynamic quantities ΔH°, ΔS°, ΔG° for the different inclusion processes were calculated by using Van’t Hoff plot. For all cases of the studied inclusion processes, these inclusions were favored through entropy and enthalpy changes.     

Solvent effects on the diastereoselection in LiAlH4 reduction of α-substituted ketones

Based on high-level DFT calculations including solvent molecules, it was found that steric effects of solvent may be responsible for the diastereoselection in LiAlH₄ reduction of acyclic ketones substituted by an oxygen-containing functional group at the α-position to the carbonyl. It was concluded that the conventional chelated transition state models lead to the predominance of the R^∗,S^∗-diastereoisomers against experimental observation.     

A piezo-resonator as a detector of α- and β-alanine in aqueous solutions

The results of analysis of aqueous solutions of α- and β-alanine using a piezo-resonator are presented. Analytical signals from analytes were found to significantly differ from each other, which is connected with the peculiarities of the piezo-resonator operation in the solution as a viscosimetric rather than a mass-sensitive device.     

Studies of the Interaction of the Salicylideneserinatecopper(II) Complex with β-Cyclodextrin: Characterization and Reactivity of the Inclusion Compound

The compound formed by the copper-Schiff base complex salicylideneserinatecopper(II), [Cu(sal-ser)(H2O)], interacting with -cyclodextrin was prepared, and characterized in the solid state by infrared, UV-visible and EPR spectroscopies, X- ray diffraction, and thermoanalytical techniques. The catalytic activity of this compound, [Cu(sal-ser)CD], in the decomposition of hydrogen peroxide, and in the dismutation of superoxide radicals was also verified, in comparison with the reactivity of the free complex, in aqueous solution. In both cases, a decreasing in the reaction rate was observed for the CD-containing compound. The results of structural characterization, in addition to the substantial differences observed in the catalytic activities of the compounds, are indicative of partial insertion of the copper complex in the cavity of the oligosaccharide.     

Oxidation of β-dicarbonyl compounds with tert-butyl hydroperoxide in the presence of vanadyl acetylacetonate

Oxidation of β-dicarbonyl compounds with tert-butyl hydroperoxide in the presence of vanadyl acetylacetonate (benzene, 20°C) involves the activated methylene group with intermediate formation of trioxo derivatives and is accompanied by decomposition of carbon skeleton. The oxidation products are carbon dioxide, carboxylic acids, and tert-butyl and peroxy esters derived from the latter.     

Free-Radical Copolymerization of Methyl Methacrylate and α-Methacrylophenone. I. Reactivity Ratios

The free-radical copolymerization of methyl methacrylate and methacrylophenone (MAP) initiated by azobisisobutyroni-trile at 60°C has been studied in ethylbenzene solution and in bulk. The process is characterized by a competitive Diels-Alder condensation of methacrylophenone and by a very low reactivity of methacrylophenone-terminated macroradicals in propagation reactions. The experimental composition data are consistent with a terminal unit model: r_A = 1.77 ± 0.02, r_B = 0.110 ± 0.006. Copolymerization with depropagation of methacrylophenone-terminated growing chains and copolymerization affected by penultimate effects have been tested as optimized possible models to take into account the inability of MAP to undergo homopolymerization.     

Chiral effect in the sechenov parameters of argon solubility in aqueous α-valine and α-phenylalanine solutions

Data on the solubility of argon in water and aqueous solutions of L and D enantiomers of α-valine and α-phenylalanine at T = 283–328 K and partial pressure of argon p ₂ = 0.1 MPa are used to calculate the standard parameters of the Sechenov salt effect. Parameters of the solute-solute pair interaction are estimated within the formalism of the McMillan-Mayer theory. Evidence is provided for the presence of the chiral effect in the Sechenov parameters.