Kinetics of the interaction of ketyl and semiquinone radicals with dioxygen. Concerted electron and proton transfer involving ketyl and semiquinone radicals

Kinetics of the interaction of ketyl and neutral semiquinone radicals with dioxygen was studied by the flash photolysis technique. The reactivity of neutral semiquinone radicals in the transfer of a hydrogen atom to O₂ is lower than that of ketyl radicals and increases as the reduction ability of the radicals increases, which give evidence for the charge transfer from the radicals to O₂ in the transition state of the reaction. The deuterium kinetic isotope effect of the reaction (up to 2.6) suggests considerable weakening of the O−H bond of the seminquinone radical in the transition state. A cyclic structure of the transition state similar to that in the reactions of ketyl radicals with hydrogen atom acceptors is proposed. In aprotic volvents, solvation has essentially no effect on the reactivity of neutral anthrasemiquinone radicals up to solvent nucleophilicityB≈240. In solvents with higher nucleophilicity and in protic solvents, their reactivity drops sharply. Hydrogen atom transfer reactions involving ketyl and neutral semiquinone radicals are shown to involve concerted electron and proton transfers, and to have transition states in which the partial transfer of an electron and a proton from the ketyl or semiquinone radical to an acceptor occurs. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1131–1137, June, 1997.     

A tr esr study of the reactions of excited organic triplet molecules with inorganic sulfur oxoanions

The reactions of excited triplet ketones including benzophenone, 4,4′-dihydroxybenzophenone, and acetone with inorganic sulfur oxoanions: sulfite (SO₃ ⁼), metabisulfite (S₂O₅ ⁼), and dithionite (S₂O₄ ⁼) in solution were studied by TRESR. Observation of radical anion intermediates polarized by the Triplet Mechanism demonstrated the reactions of electron transfer from the sulfur dianions to the organic triplet states. Thus, electron transfer from SO₃ ⁼ to the triplet organic carbonyl compounds gave rise to the SO₃ ^? radical; reactions with metabisulfite and dithionite were more complex and may be interpreted as occurring via electron transfer to the carbonyl system followed by dissociation of the S-S bond in the resulting sulfur oxoanion radical. The initial radical ion pairs thus formed consist of pairs of anion radicals, one organic and one inorganic.     

Generation of ketyl radical anions by photoinduced electron transfer (PET) between ketones and amines. Synthetic applications

Photoreduction of ketones in the presence of amines led to ketyl radicals through photoinduced electron transfer (PET). Tertiary amines, such as triethylamine (Et₃N) have frequently been used in these reactions. Different reactions can occur from ketyl radicals such as photoreduction, coupling reactions, additions on activated double bonds, cyclizations, bond cleavage of strained rings, tandem reactions such as cyclization-ring opening or ring opening-cyclization.     

The electron transfer reactions of cyano substituted pyridines and quinolines with thermally generated diphenyl ketyl

Electron transfer reactions of cyano substituted pyridines and quinolines with thermally generated diphenyl ketyl are reported. When E¹/₂(-) for the heterocycle is less negative than -2V, electron transfer occurs from the ketyl to the heterocyclic base. In some cases the products obtained from the thermal reaction are the same as those obtained from the photochemical reaction with benzophenone and alcohols. In other cases different products are formed. Two bases, 2-pyridinecarbonitrile and 2,4-pyridinedicarbonitrile, undergo regiospecific reactions in which the course of the reaction is determined by the acidity of the medium. A mechanism in which the heterocyclic nitrogen is involved in the substitution process at the 2 position is proposed.     

Zur synthese von siloxanen: II. Sterische substituentenkonstanten für siloxyreste

The acetolysis reactions of compounds of the type XSiMe₂Cl (I) with acetic acid in the presence of acetic anhydride were studied kinetically by means of ¹H NMR spectroscopy. We found these reactions exclusively influenced by steric effects (? = 0)The steric susceptibility constant (δ) of the acetolysis reaction, using alkylchlorodimethylsilanes was found to be 1.3. In this investigation the Taft E_s values showed a better correlation than the E_s(Si) values of Cartledge.From the rate constants of the acetolysis reactions with I (X = Cl, acetoxy or siloxy) we could then obtain for the first time E_s values for Cl, acetoxy and a series of siloxy groups at silicon.     

Low-field CIDNP study of photoinduced electron transfer reactions

Chemically induced dynamic nuclear polarization in low magnetic field (low-field CIDNP) has been detected and studied in photoinduced electron transfer reactions in the polar solvent acetonitrile. For the radical-ion reactions two different approaches to interpret the low-field CIDNP are demonstrated: interpretation of the low-field CIDNP sign on the basis of quality relationships, and numerical calculations of the CIDNP field dependence. Analysis shows that low-field CIDNP in these reactions is sensitive to the value of the electron exchange interactions in radical-ion pairs.     

Thermal decomposition of diisopropyl peroxydicarbonate. CIDNP and product studies

The thermal decomposition of diisopropyl peroxydicarbonate (IPP) in two solvents (chlorobenzene and tetrachloroethylene) was studied by [¹H]NMR. In both solvents the major decomposition products, acetone and isopropanol, showed interesting CIDNP effects during the first few minutes of the decomposition at 60 and 75 respectively. For decomposition in the presence of excess di-t-butyl-p-cresol, no acetone was formed while the other CIDNP effects were completely suppressed. From these results and further analysis of the by-products, it is concluded that the observed CIDNP effects are closely connected with induced decomposition of IPP, starting by abstraction of the secondary H-atom of the isopropyl group.     

Reversal of low-field CIDNP due to deuterium substitution

A perturbation method is applied to the problem of low-field CIDNP. It is shown that in the intermediate magnetic field region the most important deviation from the high-field formulae is due to a perturbation of the S—T₀ mixing by interaction with the T_± levels of the radical pair. In the case of a symmetrical radical pair this contribution decreases to zero and higher order contributions become dominant. An experimental example is the reversal of low-field CIDNP of ethane formed during the thermal decomposition of acetyl peroxide compared with that of ethane-d₃ during the decomposition of acetylperoxide-d₃. A qualitative rule is given for net CIDNP effects in the intermediate field region.     

Darstellung und EPR-spektroskopie m-phenylen-verknuepfter biradikalischer bistetracyclon-metallketyle

The synthesis of several substituted bistetracyclones bridged by phenyl rings at meta positions is described. From the corresponding metal ketyls—obtained by alkali metal reduction—EPR triplet powder spectra have been recorded. The spectra can be interpreted in terms of electron-electron dipolar coupling between the unpaired spins of randomly oriented molecules. The influences of substitution, concentration of the paramagnetic species and of the nature of the alkali metal counter ion on the magnitude of the zero field splitting parameters are investigated. Assuming two different conformations of the bis-tetracyclone ketyl biradicals (cis cis and cis trans—referred to the position of the oxygen atoms) the EPR data can be explained satisfactorily.     

Study on chain transfer to acetic anhydride of THF polymerization

In the presence of acetic anhydride, the catalyst containing protons makes the polymerization complex.^1,2 Thus, whether the living center, during the polymerization process of THF, transfers to the acetic anhydride is a difficult problem to answer. In this article, CH₃COSbCl₆ is used as the catalyst to avoid the interference of the protons. It is found that acetic anhydride is an effective chain transfer agent, by the experiment at different temperatures for two systems in the presence or absence of acetic anhydride, and by the comparison of kinetic behavior. In the system without acetic anhydride at the temperature of 7, 15, and 22°C, the propagation rate constants of THF, k_p are 7.90, 14.23, and 23.35 10^?3L/Mol.S, respectively. In the presence of acetic anhydride, k_p are 1.51, 2.85, and 4.98 10^?3L/Mol.S; and k_tr are 2.04, 3.59, and 6.49 10^?4L/Mol.S, respectively. © 1994 John Wiley & Sons, Inc.     

Antianexiety activity of pyridine derivatives synthesized from 2-chloro-6-hydrazino-isonicotinic acid hydrazide

A series of oxadiazole pyridine derivatives were synthesized by using 2-chloro-6-hydrazinoisonicotinic acid hydrazide as starting material. Treatment of the hydrazide with carbon disulfide to afford the oxadiazole derivative, which was treated with 5-methyl-2-furancarbaldehyde, formic acid, acetic acid/acetic anhydride, or phthalic anhydride to yield the corresponding pyridinodiazoles and on imide. Condensation of the hydrazide with p-fluorobenzaldehyde in ethanol or acetic acid in the presence of sodium acetate afforded hydrazone and oxadiazole derivatives, which were acetylated and cyclized with acetic anhydride to N-acetyloxadiazole derivatives. The hydrazone was treated with acetic acid in the presence of sodium acetate, or bromine water/sodium acetate to give on oxadiazole, while it was cyclized with chloroacetyl chloride in the presence of TEA to oxoazetidinaminoisonicotinamide. Finally, condensation of the hydrazide with acid anhydrides in refluxing glacial acetic acid afforded the corresponding bisimide derivatives. The pharmacological screening showed that many of these obtained compounds have good antianexiety activity comparable to diazepam^® as positive control.     

The structure ofO-arylmercury derivatives of dihydroxy-9,10-anthraquinones and their reactions with anions

The structure of mono- and di-O-arylmercury-derivatives of quinizarin (1,4-dihydroxy-9, 10-anthraquinone) and anthrarufin (1,5-dihydroxy-9, 10-anthraquinone) and their reactions with Br^–, Cl^–, OH^–, and^tBuO^– anions in the solid state and in aprotic solvents were examined by vibrational and electron spectroscopy. These reactions result in cleavage of the O-Hg bond. The formation of ions or contact ion pairs depends on the size and nature of the counterion; quinizarin dianions give very strong ion pairs with K⁺ cations, which do not cleave in DMSO. The electronic structure of mono- and dianions of the compounds studied is discussed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2933–2940, December, 1996.     

Photochemical radical cyclization of γ,δ-unsaturated ketone oximes to 3,4-dihydro-2H-pyrroles

3,4-Dihydro-2H-pyrroles are synthesized from γ, δ-unsaturated oximes by photochemical radical cyclization with 1,5-dimethoxynaphthalene (DMN) as the sensitizer. The cyclization of alkyl ketone O-acetyloximes proceeds via photosensitized electron transfer in the presence of acetic acid, while conjugated oximes of aryl and α,β-unsaturated ketones are cyclized via energy transfer.     

Gas chromatographic determination of isocitric and malic acid in the presence of a large excess of citric acid

Derivatization yields of different esters (methyl, ethyl, n-propyl, isopropyl, n-butyl and isobutyl) of citric, malic and isocitric acids with various acylating agents, such as acetic anhydride, propionic anhydride, trifluoroacetic anhydride and heptafluorobutyric anhydride, were investigated. The formation of acylated malic and isocitric acid esters is rapid and quantitative whereas the acylation of citric acid esters was slow and partial in most instances. It was found that selective analytical conditions can be achieved with the O-heptafluorobutyryl n-butyl esters. The analytical applicability of the separation and determination of the malic, isocitric and citric acid contents of model solutions and of pressed lemon juice is discussed. Concentrations of 1.2 × 10^?3 ?9.0 × 10^?3g per 100 g of isocitric acid and 4.2 × 10^?3 ?3.5 × 10^?2 g per 100 g of malic acid in the presence of 9.3 × 10^?1 g per 100 g of citric acid were measured with relative standard deviations of 7.5 and 4.2%, respectively.     

CIDNP effects in various magnetic fields during the reaction of substituted benzyl chlorides with n-butyllithium

We detected and studied CIDNP effects in the reaction of substituted benzyl chlorides with n-butyllithium in hexane (Scheme 1) in various magnetic fields (from 0·5 Oe to about 25,000 Oe). The polarisation of ¹⁹F nuclei in the reaction of p-F-benzyl chloride in strong fields has been observed by Rakshys.¹ We succeeded, however, in finding the CIDNP effects for both ¹⁹F and ¹H in all the reactions investigated. The CIDNP effects observed offer a number of remarkable features; therefore, a detailed analysis of these effects can be useful for a better understanding of the nature of CIDNP.     

The field dependence of CIDNP in gas-phase reactions of biradicals

The field dependence of CIDNP effects in gas-phase biradical reactions is for the first time detected during the photolysis of the homological series of cyclic aliphatic ketones from cyclodecanone to cyclopentadecanone. The influence of magnetic field on nuclear polarization is studied up to 0.47 T. The experimental data are interpreted on the basis of nuclear-spin-selective ST₋ transitions in intermediate biradicals. In contrast to liquid-phase reactions, mainly protons in the γ-position are polarized. The relatively weak α-CH₂ and β-CH₂ protons in the gas are explained by compensation of various conversion channels in biradicals. The strong spin—rotation interaction in gases excludes polarization transfer observed in solutions. The field dependence of CIDNP effects differs noticeably in gases and liquids. It is associated with the different dynamics of conformational motion in liquids and gases.     

Kinetics and mechanism of the reaction of propylene oxide with chlorine atoms and hydroxy radicals

The kinetics and mechanism of gas‐phase propylene oxide (PPO) reactions were studied in a 142‐L reaction chamber by long‐path Fourier transform infrared spectroscopy at atmospheric pressure and 298 K. Rate coefficients for the reaction of PPO with ozone (O₃), chlorine atoms (Cl), and hydroxyl radicals (OH) were measured using the relative rate technique. Product yields of acetic acid, acetic formic anhydride, formic acid, and carbon monoxide were determined for the following reactions: PPO with Cl both in the presence and absence of NO, PPO with OH and NO, methyl acetate with Cl both in the presence and absence of NO, and ethyl formate with Cl both in the presence and absence of NO. The measured rate coefficients for PPO with O₃, Cl, and OH are <3.5 × 10^?21 cm³ molecule^?1 s^?1, (3.0 ± 0.7) × 10^?11 cm³ molecule^?1 s^?1, and (3.0 ± 1.0) × 10^?13 cm³ molecule^?1 s^?1, respectively. The carbon balance for the products measured ranged from 10% (for OH + PPO) to 100% (for Cl + methyl acetate in the absence of NO). The mechanistic and atmospheric implications of these measurements are discussed. © 2011 Wiley Periodicals, Inc. Int J Chem Kinet 43: 507–521, 2011     

Photoinitiated oxidation of NADH catalyzed by horseradish peroxidase studied by chemically induced dynamic nuclear polarization

The photoinitiated oxidation of β-NADH catalyzed by horseradish peroxidase (Per³⁺) was studied by time-resolved photoinitiated chemically induced dynamic nuclear polarization (CIDNP). The polarization observed on protons at the C(4) atom of the β-NADH molecule is evidence for the reversible one-electron transfer between the radical cation NADH^+· and the ferroperoxidase intermediate (Per²⁺). A new approach based on electron transitions in the (NADH^+· Per²⁺) pair was proposed to describe the formation of CIDNP effects in systems including quartet (Q)—doublet (D) electron transitions. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1090–1094, July, 2006.     

Fe2(SO4)3·xH2O-catalyzed per-O-acetylation of sugars compatible with acid-labile protecting groups adopted in carbohydrate chemistry

Fully acetylated saccharides are inexpensive and very useful starting materials for the synthesis of many naturally occurring glycosides, oligosaccharides, and glycoconjugates. Ferric sulfate hydrate (Fe₂(SO₄)₃·xH₂O) was found to be a valuable Lewis acid promoter in the per-O-acetylation reaction of saccharides with acetic anhydride in 100% of conversion rate and 88-99% yields. Interestingly, the procedure is perfectly compatible with the presence of a variety of acid-labile protecting groups, such as isopropylidene, benzylidene, trityl, and TBDMS groups. The reactions were simply performed by stirring the mixture of a sugar with a slight excessive acetic anhydride in the presence of 2.0 mol % of Fe₂(SO₄)₃·xH₂O at rt and the pure products were obtained by a simple dilution of the reaction mixture with dichloromethane and washings with aqueous Na₂CO₃.     

13C CIDNP effects in the decomposition products of acetylbenzoyl peroxide

¹³C CIDNP effects in the decomposition of acetylbenzoyl peroxide in cyclohexanone are described. A comparison between our data, and those on the ¹H CIDNP effects and the ¹³C CIDNP observed in the decomposition of ABP in C₂Cl₄¹ leads to definite conclusions on the signs of the HFI constants of the benzoyl radical and to an identification of the separate products of the radical-induced reactions with the solvent.