The mechanism of the formation of quinonic acids in the oxidation of the bark of the Siberian larch. I

Summary 1. It has been established that in the reaction of Siberian larch bark with atmospheric oxygen in nitric acid, the hemicelluloses and the phenolic complex are oxidized most readily.2. The phenolic acids are decomposed almost completely. The considerable yield of oxalic acid shows a degradation of their aromatic nucleus during oxidation.3. In the oxidation of the bark lignin the cleavage of the lignin macromolecule to the final products does not take place directly. During oxidation an intermediate product with a quinoid system of bonds is formed in considerable amounts, and this then undergoes fragmentation as the result of the oxidative-hydrolytic action of the medium.Siberian Technological Institute, Krasnoyarsk. Translated from Khimiya Prirodnykh Soedinenii, No. 3, pp. 403–409, May–June, 1977.     

Mechanisms for the Ni-mediated oxidation of benzene to phenol by N2O

The gas-phase Ni⁺-mediated benzene oxidation by N₂O on both quartet and doublet potential energy surfaces are surveyed using density functional theory (DFT). Geometries and energies for all relevant intermediates are calculated. The initial N₂O reduction for forming NiO⁺ takes place only via reaction with the excited Ni⁺(⁴F) through the N–O insertion mechanism. The second step of the oxidation can involve two mechanisms: ‘nonradical’ through benzene H migration following the formation of the boat complex of benzene–NiO⁺ or ‘oxygen-insertion’ via rearrangement to an arenium intermediate from the chair-type benzene–NiO⁺ encounter complex. Both mechanisms are energetically available.     

Selective stoichiometric and catalytic oxidation of lower alkanes in the system Co(III)-CF3COOH

It has been found that cobalt(III) trifluoroacetate is capable of oxidizing with a high selectivity (90%) methane to methyltrifluoroacetate at 130–180°C and 10–40 atm. In the presence of oxygen the selective oxidation of methane proceeds catalytically with respect to the cobalt salt. Ethane and propane are oxidized with the formation of ethanol and isopropanol trifluoroacetates, respectively (yield 80%). The further oxidation of triethanol trifluoroacetate leads to the formation of the ethyleneglycol ester, together with products of the splitting of the C-C bond in the alkyl group of the initial ester. The data obtained lead to the assumption that the reactions of the alkanes and alkyl esters with Co(III) include an electron transfer stage from the RH molecule to the Co(III) ion, with the intermediate formation of the cation radical RH⁺.N. S. Kurnakov Institute of General and Inorganic Chemistry, Academy of Sciences of the USSR, Moscow. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 27, No. 3, pp. 301–307, May–June, 1991. Original article submitted February 19, 1991.     

Hysteresis effect and autooscillation phenomena in the oxidation of 1-hexene on palladium

The gas-phase oxidation of 1-hexene on a palladium film has been investigated in a gradient-free reactor at atmospheric pressure and temperatures of 293–693 K, at flow rates of 0.3–0.15 liter/min and an initial concentration of hexene in air of 15.5·10^–4 M. Autooscillation phenomena have been observed and a hysteresis effect in the reaction rate as function of temperature, caused by the branched-chain heterogeneous-homogeneous reaction mechanism.Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 24, No. 1, pp. 107–110, January–February, 1988.     

Electrochemical oxidation of nitroylides

Conclusions The mechanisms of the electrochemical oxidation of mono- and dinitroylides of S and Se on a Pt electrode in CH₃CN have been studied for the first time. It was shown that the cation radical formed as intermediate was able to react with the medium with removal of an H atom and with the formation of an onium cation or to undergo decomposition. Competition of these processes determines the special features of the electrooxidation of mononitroylides, while on oxidation of dinitroylides the intermediate cation radical decomposed to oxides of nitrogen.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1332–1338, June, 1981.     

The mechanism of the formation of quinonic acids in the oxidation of the bark of the Siberian larch. II

Summary 1. It has been shown by chemical and physicochemical methods of analysis that the oxidation of bark lignin by atmospheric oxygen in nitric acid is accompanied by oxidative-condensation and oxidative-hydrolytic processes.2. Oxidative condensation leads to the formation of a stable intermediate product with a quinoid system of bonds. This complex fragments under the oxidative-hydrolytic action of the medium. The oligomers obtained undergo oxidative-condensation transformations more readily. Such reactions probably take place successively as far as the formation of relatively simple compounds.Siberian Technological Institute, Kransoyarsk. Translated from Khimiya Prirodnykh Soedinenii, No. 3, pp. 409–412, May–June, 1977.     

Theory of electron transfer in systems with intermediate link

A theory is proposed for electron transfer through an intermediate link, the theory being based on solution of the time-dependent wave equation of the system with the exact Hamiltonian by assigning a wave function in form of a linear combination of wave functions of the initial state (electron on the donor), intermediate state (electron on the intermediate link), and final state (electron on the acceptor). The squares of the moduli of the time-dependent coefficients in these wave functions represent the probabilities of finding electrons in the indicated states. The coefficients have been determined by means of Laplace transforms, and an expression has been obtained for the rate of electron transfer through the intermediate link.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 3, pp. 288–293, May–June, 1985.     

The rotating ring-disk electrochemistry of the copper(II) complex of thyrotropin-releasing hormone

Thyrotropin-releasing hormone (TRH) forms an electroactive Cu(II) complex in aqueous solution. Rotating ring-disk electrochemistry reveals oxidation at the disk electrode and reduction at the ring electrode. The plot of limiting current vs. square root of rotation frequency deviates from the Levich equation, indicating both preceding and following chemical reactions. The reaction following the oxidation is a multiple-electron ECE-type of process that has been seen before in Cu(II)–peptide electrochemistry. The preceding reaction is unusual. The deviation from diffusion-controlled behavior is more pronounced at higher initial concentration of Cu(II) and peptide. We propose that a non-electroactive dimer, Cu(II)₂–TRH₂, is in a slow equilibrium with the electroactive Cu(II)–TRH. Simulation of the RRDE behavior of the postulated Cu(II)–TRH system has succeeded in matching experimental data. Capillary electrophoresis indicates that there is a negative charge on the dimer. It is suggested that a hydroxo-bridge may link the two Cu(II) centers. Calculations verify that bi-nuclear Cu(II)₂–TRH₂ complexes are possible.     

Quantum-chemical study of the mechanism of the influence of cations of d metals on the rate of the oxidation of phosphine by Cu(II) halides

A hypothesis concerning the formation of associates between intermediate complexes and cations has been introduced in order to account for the experimental dependence of the rate constant of the oxidation of phosphine by Cu(II) halides on the nature of the cation in the salt MCl₂ (M = Mn, Co, Ni). The quantum-chemical characteristics of the systems formed have been calculated by the Mulliken—Wolfsberg—Helmholz-SCCC method. It has been shown that the correct correlation between the electronic properties of the intermediates and the rate constant of the reaction upon the variation of M can be obtained only for a specific mechanism for the addition of M²⁺ to the complex.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 6, pp. 718–722, November–December, 1985.     

Possibility of using the periodate oxidation reaction in combination with PMR spectroscopy for establishing the structures of nucleosides and monosaccharides,and their analogs

A convenient method is proposed for establishing the structures of analogs and derivatives of nucleosides and monosaccharides which consists in performing the periodate oxidation reaction directly in the ampul for recording the PMR spectrum. It has been shown that the PMR spectra of the products of the periodate oxidation reaction are substantially simpler than the spectra of the initial compounds in the case of compounds with terminal diol groupings and of l-substituted pentapyranoses and tetrapyranoses.Institute of Molecular Biology of the USSR Academy of Sciences Moscow. Translated from Khimiya Prirodnykh Soedinenii, No. 1, pp. 40–43, January–February, 1987.     

Study of the mutual influence of hydrogen and carbon monoxide in their joint oxidation on palladium

The joint oxidation of hydrogen and carbon monoxide in excess oxygen on palladium was investigated in a reactor for separate calorimetry and a gradientless microreactor at atmospheric pressure. An initiating influence of hydrogen m the oxidation of carbon monoxide, as well as inhibitory effect of carbon monoxide on the heterogeneous-homogeneous oxidation of hydrogen, was detected. The conditions of transition of the reactions into the volume according to a heterogeneous-homogeneous mechanism were determined. It is suggested that the joint oxidation of CO and H₂ occurs with the participation of the same active intermediate products formed from hydrogen and oxygen.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 4, pp. 504–508, July–August, 1985.     

Inner-sphere oxidation of cis-diaquabis(oxalato)chromate(III) by periodate in aqueous weakly acidic solutions

The kinetics of oxidation of cis-[Cr^III(ox)₂(H₂O)₂]^– (ox = C₂O₄ ^2–) by IO₄ ^– showed a first-order dependence on the initial Cr^III complex concentration in the presence of a vast excess of [IO₄ ^–]. The dependence of the pseudo-first-order rate constant on [IO₄ ^–] is complex and is consistent with the formation of a precursor complex. It is proposed that this complex is formed through the coordination of the two carbonyl oxygens of the ox ligand with the IO₄ ^– ion, forming a cyclic intermediate. The kinetics are consistent with the hydroxo form of the Cr^III complex being the reactive species, whereas the aqua species forms an unreactive complex.     

One-step three-electron oxidation of tartaric and glyoxylic acids by chromium(VI) in the absence and presence of manganese(II)

Kinetics of the initial stages of oxidation of tartaric acid (TA) in the absence and presence of manganese(II) ions have been studied spectrophotometrically. The rate of the induction was slow and then gradually increased with increasing [TA] and/or [HClO₄]. The reaction followed third-order kinetics; first-order with respect to each of [TA], [HClO₄] and [Cr^VI]. The kinetic and manganese(II) effect studies are consistent with a one-step three-electron mechanism (Cr^VI Cr^III without passing through Cr^IV as an intermediate) in which a termolecular complex is formed between TA, Mn^II and HCrO₄ ^–. In order to obtain further insight, oxidation of glyoxylic acid (GA), an oxidation product of TA, was also studied under the similar conditions. Details of the process are discussed.     

Study of the electrical oxidation of 2,6-dimethyl-3,5-diethoxy-carbonyl-1,4-dihydropyridines in acetonitrile by means of a rotating disk electrode with a ring

It is shown that the electrochemical oxidation of esters of 2,6-dimethyl-1,4-dihydropyridine-3,5-dicarboxylic acids in anhydrous acetonitrile takes place as a one-electron process, while the final products of the transformations (pyridines or pyridinium salts) are formed as a result of disproportionation of the intermediate radicals. When 1–2% (by volume) water is added, the oxidation mechanism changes substantially, and the electrochemical process becomes a two-electron process.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1263–1267, September, 1980.     

Comparison of the reactivities of ketones and aldehydes of the furan series during oxidation with hydrogen peroxide

The rates of oxidation of 2-acetylfuran, 5-methyl-2-acetylfuran, furfural, and 5-methyl-furfural with hydrogen peroxide were compared. The intermediate and final oxidation products were determined. The possibility of identical reaction mechanisms for furan aldehydes and ketones is demonstrated.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 443–446, April, 1972     

Investigation of the transformations of chlorobenzene at the surface of a vanadium oxide catalyst by IR spectroscopic and thermal desorption methods

In the present work an IR spectroscopic and thermodesorption investigation of the surface compounds formed by chlorobenzene on a vanadium oxide catalyst was undertaken. It was established that in the initial period of adsorption chlorobenzene forms a weakly bonded complex with the surface of the catalyst. With subsequent increase in temperature a compound firmly bound to the surface of the catalyst appears. It is most likely that this complex is a complex formed through a bond between the reagent molecule and an electrophilic center on the surface. On the basis of the results from the IR-spectroscopic investigations and thermal desorption a mechanism involving intermediate surface compounds detected by IR spectroscopy is proposed for the catalytic oxidation of chlorobenzene.Translated from Teoreticheskaya i Éksperimental'naya, Khimiya, Vol. 26, No. 4, pp. 468–473, July–August, 1990.     

Utilization of Chromatographic and spectroscopic techniques to study the oxidation kinetics of selenomethionine

Ion-exchange LC and spectroscopic supporting techniques have been successfully used to study the kinetics and mechanism of oxidation reactions of selenomethionine (SeMet). Oxidation of selenomethionine with both cyanogen bromide (CNBr) and hydrogen peroxide (H₂O₂) proceeds through a stable intermediate which undergoes cyclization and C-Se bond cleavage to form 2-amino-4-butyrolactone. This stable intermediate was identified by IR spectroscopy as methionine dihydroxy selenide. The CH₃-Se moiety of SeMet formed methyl selenic acid upon reaction with H₂O₂ and methyl selenocyanate (CH₃SeCN), characterized by GC-MS, for the reaction with CNBr. Both reactions were of apparent first order with respect to the concentration of SeMet. A rate constant (k₁)of 4.0×10^–3 s^–1 for the reaction of SeMet with HO and 4.0×10^–3 s^–1 for the reaction with CNBr were determined at a temperature of 22°C. Oxidation of methionine (Met) gives disparate kinetics and oxidation products from SeMet. Thus the differential rate method can be utilized to quantitatively separate SeMet in biological samples in the presence of much higher concentrations of Met.     

Kinetics of the oxidation of hypophosphite,deuteriohypophosphite, phosphite and neutralized phenyl phosphonous acid by tris(pyridine-2-carboxylato)manganese(III) in picolinate-picolinic acid buffer medium

Hypophosphite, deuteriohypophosphite, phosphite and neutralized phenyl phosphonous acid are oxidized by tris(pyridine-2-carboxylato)manganese(III) in Na(pic)–picH [where Na(pic) = sodium salt of picolinic acid and picH = picolinic acid] buffer in the 4.63–5.45 pH range. All the reactions appear to follow the same mechanistic pathway in which the substrate initially forms a 1:1 intermediate complex with the oxidant. The complex then breaks down in the rate-determining step to give a free radical intermediate which subsequently reacts with another molecule of oxidant to give the products. A deuterium isotope effect of k _H/k _D = 4.26 at 313 K, was observed in the oxidation of hypophosphite indicating that proton removal from phosphorus takes place in the rate-determining step. A mechanism consistent with the experimental observations has been proposed.     

Mass spectrometric study of products and intermediate compounds in oscillating chemical reactions of oxidation of macrocyclic copper and nickel complexes by a bromate

Products and intermediate compounds formed in oscillating chemical reactions of oxidation of copper and nickel tetraazamacrocyclic complexes were studied by the method of desorption field mass spectrometry. It was shown that a cascadelike oxidation of the macrocyclic ligand takes place and its hydrolysis with rupture of the macrocyclic ring at the azomethine bonds. Products of bromination of oxygen- and nitrogen-containing compounds, participants in the reactions, were discovered.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 23, No. 2, pp. 221–225, March–April, 1987.     

Electrochemical oxidation of ferrocenylsubstituted 1,4-dihydropyridines

1-p-Ferrocenylphenyl- and 4-ferrocenylsubstituted Hantzsch esters were subjected to electrochemical oxidation using a graphite cyclic rotating disk electrode in acetonitrile medium; in each case the initial response was one-electron oxidation of the ferrocenyl substituent to give a ferrocenium cation, which was followed by a two-electron electrochemical oxidation of the dihydropyridine ring involving intermediate deprotonation of the latter and formation of a ferroceniumpyridinium dication. We have also detected for the first time at room temperature relatively stable cation radicals of 2,6-dimethyl-3,5-diethoxycarbonyl-1,4-dihydropyridine containing a cationic ferrocenium substituent in the 4-position; these proved to be more stable than cation radicals of dihydropyridines containing ferrocenyl substituents attached to the nitrogen atom.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 70–74, January, 1987.