Kinetics and Mechanism of the Epoxidation of Alkyl-Substituted Alkenes by Hydrogen Peroxide, Catalyzed by Methylrhenium Trioxide

Epoxidations of alkyl-substituted alkenes, with hydrogen peroxide as the oxygen source, are catalyzed by CH(3)ReO(3) (MTO). The kinetics of 28 such reactions were studied in 1:1 CH(3)CN-H(2)O at pH 1 and in methanol. To accommodate the different requirements of these reactions, (1)H-NMR, spectrophotometric, and thermometric techniques were used to acquire kinetic data. High concentrations of hydrogen peroxide were used, so that diperoxorhenium complex CH(3)Re(O)(eta(2)-O(2))(2)(H(2)O), B, was the only predominant and reactive form of the catalyst. The reactions between B and the alkenes are about 1 order of magnitude more rapid in the semiaqueous solvent than in methanol. The various trends in reactivity are medium-independent. The rate constants for B with the aliphatic alkenes correlate closely with the number of alkyl groups on the olefinic carbons. The reactions become markedly slower when electron-attracting groups, such as halo, hydroxy, cyano, and carbonyl, are present. The rate constants for catalytic epoxidations with B and those reported for the stoichiometric reactions of dimethyldioxirane show very similar trends in reactivity. These findings suggest a concerted mechanism in which the electron-rich double bond of the alkene attacks a peroxidic oxygen of B. These data, combined with those reported for the epoxidation of styrene (a term intended to include related molecules with ring and/or aliphatic substituents) by B and by the monoperoxo derivative of MTO, suggest that all of the rhenium-catalyzed epoxidations occur by a common mechanism. The geometry of the system at the transition state can be inferred from these data, which suggest a spiro arrangement.     

Electron transfer. 137. Formal potentials involving indium(II)

Aqueous solutions of indium(III) undergo 1e⁻ reductions by Sm(II) (E⁰-1.55 V) and by Yb(II) (−1.05 V), but not by U(III) (−0.52 V). The facile and irreversible reduction, by In⁺ _aq, of Ru(NH₃)₆ ³⁺ (E⁰+0.067 V) at reagent concentrations near 10⁻³ M implies a potential more negative than −0.23 V for In(II,I) and, in conjunction with the known value of −0.44 V for In(III,I), a complementary potential less negative than −0.65 V for In(III,II). These observations, taken together, support le⁻ indium potentials E_II,I ^o−0.2 V and E_III,II ⁰−0.6 V.     

Indigo dyes as indicators for oxidation reactions: Competition kinetic studies

The use of indigo dyes as competiters to study the kinetics of oxidation reactions is reported. The kinetic model was theoretically investigated using the kinetic simulation program KinSim under conditions involving very limiting concentrations of the indigo dye relative to both the oxidant and the reductant. Complete kinetic results on the reaction of an oxidant with a reductant can be obtained indirectly from observing the absorbance changes due to the loss in the indigo dye at 600 nm (ε₆₀₀∼2 × 10⁴ M⁻¹cm⁻¹). Experimental data from the oxidation of an olefin (cyclohexene) with m-chloroperoxybenzoic acid in the presence of an indigo dye were used to check the validity of the kinetic model. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 532–537, 2005     

A Collocation-Based Algorithm for Computing the Pull-In Range of a Class of Phase-Locked Loops

The pull-in range (ω_p) of a phase-locked loop (PLL) is defined as the maximum value of loop detuning ω_0s for which pull-in occurs from anywhere on the PLL's phase plane. That is, pull-in is guaranteed from anywhere on the phase plane if ω_0s < ω_p. Simple approximation is available for computing ω_p for the high gain PLL where saddle-node bifurcation occurs at ω_0s = ω_p. Unlike the high gain case, a simple approximation for ω_p is not available for the low gain case where bifurcation from a separatrix cycle occurs at ω_0s = ω_p. The vector field model for a class of second-order PLLs is shown to have rotational properties, which imply the existence of a separatrix cycle. The external stability of this separatrix cycle is an indicator of the type of bifurcation (saddle-node or separatrix cycle) which terminates the limit cycle associated with the PLL's stable false lock state and the PLL pulls-in (i.e. achieve phase lock). A formula is given for determining the separatrix cycle's stability, which indicates that these paratrix cycle is externally stable for small values of closed loop gain. A collocation-based algorithm is presented for computing the PLL's separatrix cycle and the value of pull-in range frequency ω_0s = ω_p at which a stable separatrix cycle exists.     

Kinetic and mechanistic studies on the reactions of peroxynitrite with estrone and phenols

Reaction of peroxynitrite with estrone, a female sex hormone, was carried out in tetrahydrofuran (THF)/H₂O (8: 2) basic solutions. The major products are the corresponding o-quinone, nitroestrone and 2,2′-biphenol. The reaction of phenols with peroxynitrite under the same conditions leads also to the formation of quinones, nitrophenols and biphenols. The major mechanistic pathways take place via a one-electron oxidation of the phenolic group leading to the formation of a phenoxyl radical intermediate which is further oxidized by peroxynitrite (or by intermediates generated from peroxynitrite) to give the final products. A Hammett correlation of the rate constants for the oxidation of meta substituted phenols support a radical mechanism. The kinetic isotope factors rule out the involvement of a C-H bond cleavage in the rate-determining step. A multistep mechanism showing major intermediates involved in the reaction and the final products has been proposed. Published in Russian in Kinetika i Kataliz, 2009, Vol. 50, No. 1, pp. 96–105. The article is published in the original.     

Synthesis and catalytic application of octahedral lewis base adducts of dichloro and dialkyl dioxotungsten(VI)

Complexes of the composition W(O)(2)(Cl)(2)L(2) and W(O)(2)(R)(2)L(2) (R = Me, Et; L(2) = bidentate Lewis base ligand) have been prepared and are fully characterized (including an exemplary X-ray crystal structure of W(O)(2)(Cl)(2)(4,4'-di-tert-butyl-2,2'-bipyridine)). This latter compound crystallizes in the orthorhombic space group P2(1)2(1)2(1) with a = 8.3198(1) A, b = 13.3224(2) A, c = 18.0415(2) A, and Z = 4. The title complexes are applied as catalysts in olefin epoxidation catalysis with tert-butyl hydroperoxide (TBHP) as the oxidizing agent. The W(VI) complexes display only moderate turnover frequencies but can be reused several times without loss of catalytic activity. The highest activity can be achieved at reaction temperatures of ca. 90 degrees C. Chloro derivatives are somewhat more active than alkyl complexes, and sterically less crowded complexes show also higher activities than their congeners with bulky ligands L(2). Kinetic examinations show that the catalyst formation is the rate determining step and it is observed that tert-butyl alcohol, the byproduct of the epoxidation reaction, acts as a competitor for TBHP, thus lowering the reaction velocity during the course of the reaction but not irreversibly destroying the catalyst.     

Activation of H2O2 by methyltrioxorhenium(VII) inside β-cyclodextrin

The effect of β-cyclodextrin on the catalytic stability and reactivity of methylrhenium trioxide (MTO), CH₃ReO₃, which has been used for activation of hydrogen peroxide toward oxidation and epoxidation reactions, was studied using UV–Vis spectrophotometery. The stability and reactivity of the new catalytic system (MTO/β-CD) to activate H₂O₂ toward oxidation of indigo blue dye were investigated in basic media. Furthermore, effects of inclusion stoichiometry, temperature and concentrations of hydrogen peroxide on the stability and reactivity of the MTO/β-CD system were investigated. The formation of the inclusion complex between MTO and β-CD was confirmed experimentally using the changes in the UV–Vis absorption spectra. The results of this study demonstrate that the complexation process was better guaranteed when the amount of β-CD is higher than that of MTO, using a 1:2 molar ratio of MTO:β-CD enhances both the activity and stability of the catalyst. The results showed that the stability of the catalytic system was enhanced in presence of β-CD with maintaining good reactivity of the MTO even in the presence of high concentration of NaOH. The changes of thermodynamic activation parameters (ΔH^≠ and ΔS^≠) for the oxidation reaction of indigo with H₂O₂ catalyzed by MTO/β-CD system were determined on the basis of the experimental data.