Hydrophobicity dependence of oxidation of tetrapeptides of elastin sequences with Mn(III): Synthesis,characterization, kinetics,and mechanistic study

The analogues of elastin sequences, glycyl‐glycyl‐alanyl‐proline (GGAP), glycyl‐glycyl‐phenylalanyl‐proline (GGFP), and glycyl‐glycyl‐isoleucyl‐proline (GGIP) were synthesized by classical solution phase method and characterized. The kinetics of oxidation of these tetrapeptides (TETP) by Mn(III) has been studied in the presence of sulphate ions in acidic solution at 25°C. The reaction was followed spectrophotometrically at λ_max = 500 nm. A first‐order dependence of rate on both [Mn(III)] and [TETP] was observed. The rate is independent of the concentration of the reduction product, Mn(II), and hydrogen ions. The effects of varying the dielectric constant of the medium and addition of anions such as sulphate, chloride, or perchlorate were studied. Activation parameters have been evaluated using Arrhenius and Erying plots. The oxidation products were isolated and characterized. A mechanism involving the reaction of TETP with Mn(III) in the rate‐limiting step is suggested. An apparent correlation was noted between the rate of oxidation and the hydrophobicity of these sequences, where increased hydrophobicity results in increased rate of oxidation. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 34: 39–48, 2002     

Synthesis and kinetics of oxidation of some dipeptides with anodically generated manganese(III) sulphate: Mechanistic study

Dipeptides (DP) namely phenylalanyl–proline (Phe–Pro), isoleucyl–proline (Ile–Pro), and leucyl–proline (Leu–Pro) were synthesized by classical solution method and characterized. The kinetics of oxidation of these DP by Mn(III) have been studied in the presence of sulphate ions in acidic medium at 26°C. The reaction was followed spectrophotometrically at λ_max = 500 nm. A first‐order dependence of rate on both [Mn(III)]₀ and [DP]₀ was observed. The rate is independent of the concentration of reduction product, Mn(II), and hydrogen ions. The effects of varying dielectric constant of the medium and addition of anions such as sulphate, chloride, and perchlorate were studied. The activation parameters have been evaluated using Arrhenius and Eyring plots. The oxidation products were isolated and characterized. A mechanism involving the reaction of DP with Mn(III) in the rate‐limiting step is suggested. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 438–444, 2002     

Kinetics and mechanism of ruthenium(III) catalysis in the oxidation of erythritol and dulcitol byN-bromoacetamide

Summary The kinetics of Ru^III catalysed oxidation of erythritol (1,2,3,4-tetrahydroxybutane) and dulcitol (1,2,3,4,5,6-hexahydroxyhexane) byN-bromoacetamide (NBA) in HClO₄ in the presence of Hg(OAc)₂ as a scavenger for Br^– have been investigated. The reactions are zeroth order with respect to both alcohols, and first order at low concentration of NBA tending to zero order at high NBA concentrations. The oxidation rate is directly proportional to [Ru^III] and a positive effect on the rate is observed for [H⁺] and [Cl^–] whereas a negative effect is observed for acetamide and ionic strength. D₂O and Hg(OAc)₂ do not influence the oxidation rate; (H₂OBr)⁺ is postulated as the oxidising species. A suitable mechanism consistent with the observed kinetic data is proposed.     

Polymeric ionic liquid material‐anchored Mn–porphyrin anion: Heterogeneous catalyst for aerobic oxidation of olefins

P(APTMACl)‐[Mn(TPPS)(OAc)] heterogeneous catalyst system comprised of anionic [Mn(tetrakis(4‐sulfonatophenyl)porphyrin)(OAc)] ([Mn(TPPS)(OAc)]) embedded within cationic cross‐linked polymeric ionic liquid (poly[(3‐acrylamidopropyl)trimethylammonium chloride], p(APTMACl)) hydrogel matrices was used for the selective aerobic oxidation of olefins. P(APTMACl)‐[Mn(TPPS)(OAc)] hydrogel was synthesized by radical polymerization in a solution of cationic APTMACl as an ionic liquid monomer, N,N′‐methylenebisacrylamide as cross‐linking agent, ammonium persulfate as initiator and N,N,N′,N′‐tetramethylmethylenediamine as accelerator in the presence of anionic [Mn(TPPS)(OAc)]. P(APTMACl)‐[Mn(TPPS)(OAc)] was characterized using Fourier transform infrared, diffuse reflectance UV–visible and atomic absorption spectroscopies and scanning electron microscopy. Differential scanning calorimetry was used for measuring the glass transition temperature. Catalytic activity of p(APTMACl)‐[Mn(TPPS)(OAc)] was investigated in the aerobic oxidation of olefins with emphasis on the effect of various parameters such as temperature, catalyst amount, substituent effect, etc. The catalyst was easily recovered from the reaction medium and could be re‐used for another seven runs without significant loss of activity.     

Autocatalytic Intermolecular versus Intramolecular Deprotonation in C?H Bond Activation of Functionalized Arenes by Ruthenium(II) or Palladium(II) Complexes

The activation of the C? H bond of 1‐phenylpyrazole ( 2 ) and 2‐phenyl‐2‐oxazoline ( 3 ) by [Ru(OAc)₂(p‐cymene)] is an autocatalytic process catalyzed by the co‐product HOAc. The reactions are indeed faster in the presence of acetic acid and water but slower in the presence of a base K₂CO₃. A reactivity order is established in the absence of additives: 2‐phenylpyridine>2‐phenyl‐2‐oxazoline>1‐phenylpyrazole (at RT). The accelerating effect of added acetate ions reveals an intermolecular deprotonation after C? H bond activation by a cationic Ru^II center (S_E3 mechanism). The reactions of 1‐phenylpyrazole and 2‐phenyl‐2‐oxazoline first lead to the neutral cyclometalated complexes A₂ and A₃ ligated by one acetate. The latter dissociate to the cationic complexes B₂ ⁺ and B₃ ⁺ , respectively, and acetate. A slow incorporation of one or two D atoms into 2 , 3 , and 2‐phenylpyridine ( 1 ) was observed in the presence of deuterated acetic acid. The “reversibility” of the C? H bond activation/deprotonation takes place from the cationic complexes B _n ⁺ (n=1–3). They are also involved in oxidative additions to PhI, which are rate‐determining and lead to the mono‐ and bis‐phenylated products at high temperatures. A general mechanism is proposed for the arylation of arenes 1–3 catalyzed by [Ru(OAc)₂(p‐cymene)]. In contrast, the reaction of Pd(OAc)₂ with 2‐phenylpyridine ( 1 ), is much faster: Pd(OAc)₂>[Ru(OAc)₂(p‐cymene)]. Since the kinetics is not affected by added acetates, the reaction proceeds through a CMD mechanism assisted by a ligated acetate (intramolecular process) and is irreversible. A bis‐cyclometalated Pd^II^Pd^II dimer D′₁ is formed whose bielectronic electrochemical oxidation leads to a [Pd^III^Pd^III]²⁺ dimer, in agreement with the result of a reported chemical oxidation used in arene functionalizations catalyzed by Pd(OAc)₂.     

Palladium(II) complexes of aliphatic amines and their oxidation by chloramine‐T in perchloric acid medium

The formation of palladium(II) complexes with aliphatic amines and their oxidation by chloramine‐T in perchloric acid medium has been studied. The spectrophotometric studies showed the formation of 1:1 and 1:2 complexes between palladium(II) and amine in absence of HClO₄. An increase in [HClO₄] in reaction mixture suppresses the complex formation and in presence of [HClO₄] ～10^?3 mol dm^?3 only a 1:1 complex between palladium(II) and amine has been observed. The effect of Cl^? on the complex formation has also been studied. Palladium(II)‐catalyzed oxidation of these amines by chloramine‐T showed a first‐order dependence of rate with respect to each—oxidant, substrate, catalyst, and H⁺. The mechanism consistent with kinetic data for the oxidation process has been proposed in absence as well as in presence of initial [Cl^?]. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 603–612, 2002     

Mechanism of iridium(III) catalysis in the oxidation of diols by N-bromoacetamide

Summary The kinetics of iridium(III)-catalysed oxidation of 1,2-ethanediol and 1,4-butanediol by N-bromoacetamide (NBA) in HClO₄ in the presence of [Hg(OAc)₂] as a scavenger for Br^– have been investigated. The reactions are zero-order with respect to both diols, and first-order in NBA at low NBA concentrations, tending to zero order at high concentrations. The order in Ir^III decreases from unity to zero at high iridium(III) concentrations. A positive effect on the oxidation rate is observed for [H⁺] and [Hg^II] whereas a negative effect is observed for acetamide and [Cl^–]. Ionic strength does not influence the oxidation rate. (H₂OBr)⁺ is postulated as the oxidizing species. A mechanism consistent with the observed kinetic data is proposed.     

氢过碘酸）合银（III）配离子氧化L-脯氨酸的反应动力学及机理

L-脯氨酸独有的亚胺基使其在生物医药领域具有许多独特的功能,并广泛用作不对称有机化合物合成的有效催化剂。本文在碱性介质中研究了二（氢过碘酸）合银（III）配离子氧化 L-脯氨酸的反应。经质谱鉴定,脯氨酸氧化后的产物为脯氨酸脱羧生成的 γ-氨基丁酸盐;氧化反应对脯氨酸及Ag(III) 均为一级;二级速率常数 k′ 随 [IO₄^-] 浓度增加而减小,而与 [OHˉ] 的浓度几乎无关;推测反应机理应包括 [Ag(HIO₆)₂]^5-与 [Ag(HIO₆)(H₂O)(OH)]^2-之间的前期平衡,两种Ag（III）配离子均作为反应的活性组分,在速控步被完全去质子化的脯氨酸平行地还原,两速控步对应的活化参数为： k₁ (25 ^oC)＝1.87±0.04（mol·L^-1)^-1s^-1,∆ H₁^≠＝45±4 kJ · mol^-1, ∆ S₁^≠＝-90±13 J· K^-1·mol^-1 and k₂ (25 ^oC) ＝3.2±0.5（mol·L^-1)^-1s^-1, ∆ H₂^≠＝34±2 kJ · mol^-1, ∆ S₂^≠＝-122 ±10 J· K^-1·mol^-1。本文第一次发现 [Ag(HIO₆)₂]^5-配离子也具有氧化反应活性。     

Manganese(II) catalyzed periodate oxidation of a ternary dipicolinatochromium(III) complex with iminodiacetate as co-ligand: mechanistic and kinetic study

The kinetics and mechanism of the oxidation of [Cr^III(DPA)(IDA)(H₂O)]^? (DPA = dipicolinate and IDA = iminodiacetate) by periodate in the presence of Mn(II) as a catalyst have been investigated. The rate of the reaction increases with increasing pH, due to the deprotonation equilibria of the complex. Addition of Mn(II) in the concentration range of (2.5–10) × 10^?6 mol dm^?3 enhanced the reaction rate; the reaction is first order with respect to both [IO₄ ^?] and the Cr complex, and obeys the following rate law: \( {\text{Rate}} = [ {\text{Cr}}^{\text{III}} ({\text{DPA}})({\text{IDA}})({\text{H}}_{2} {\text{O}})^{ - } ][{\text{Mn}}^{\text{III}} ]\{ (k_{7} + K_{1} k_{8} /[{\text{H}}^{ + } ]) + [{\text{I}}^{\text{VII}} ]((k_{9} k_{11} /k_{ - 9} + k_{11} ) + (K_{1} k_{10} k_{12} )/(k_{ - 10} + k_{12} )[{\text{H}}^{ + } ])\} . \) Catalysis by Mn(II) is believed to be due to initial oxidation of Mn(II) to Mn(III), which acts as the oxidizing agent. It is proposed that electron transfer proceeds through an inner-sphere mechanism via coordination of IO₄ ^? to Cr(III). Thermodynamic activation parameters were calculated using the transition state theory equation.     

Kinetics of oxidation of pyridoxine by anodically generated manganese(III) in aqueous acetic acid medium

Summary Manganese(III) acetate was prepared by the electrolytic oxidation of Mn(OAc)₂ in aqueous AcOH. The electro-generated manganese(III) species was characterised by spectroscopic and redox potential studies. The kinetics of oxidation of pyridoxine (PRX) by manganese(III) in aqueous AcOH were investigated and is first order with respect to [Mn^III]. The effects of varying [Mn^III], [PRX], added manganese(II), pH and added anions such as AcO⁻, F⁻, Cl⁻ and ClO _inf4 ^sup− and SO _inf4 ^sup2− were studied. The rate decreased slowly with increasing [H⁺] up to 0.2 mol dm⁻³ and increased steeply thereafter. The orders in [PRX] and [Mn^II] were unity and inverse fractional, respectively, in both low and high [H⁺] ranges. The dependence of reaction rate on temperature was studied and activation parameters were computed from Arrhenius and Eyring plots. A mechanism consistent with the observed results is proposed and discussed.     

TI(III) oxidation of alkenes and alkynes and Ru(III) catalysis in the oxidation of alkenes: A kinetic and mechanistic study

The oxidation of trans-stilbene, phenylacetylene, and diphenylacetylene by Tl(OAc)₃ in aqueous acetic acid medium in the presence of HClO₄ follows the rate law in [H⁺] of 0.1–1.0M, the [H⁺] dependence below 0.1M being marginal. The reactions are strongly dielectric dependent. The order of reactivity among the substrates is styrene > phenylacetylene and trans-stilbene > diphenylacetylene. A mechanism involving the oxythallation adduct by the Tl⁺(OAc)₂ species has been discussed. The use of Ru(III) as a homogeneous catalyst brings a change in the kinetic orders for trans-stilbene, the rate law being The formation constants K for the Ru(III)–alkene π complex at 40, 50, and 60°C are 90.14M^?1, 105.2M^?1, and 127.7M^?1, respectively. Interestingly the oxidation of phenylacetylene and diphenylacetylene does not undergo catalysis by Ru(III). The mechanism involving the metal–arene π complex is discussed.     

Kinetics of the Oxidation of D‐Glucose and Cellobiose by Acidic Solution of N‐Bromoacetamide Using Transition Metal Complex Species, [RuCl3(H2O)2OH] −, as Catalyst

The kinetics of Ru(III)‐catalyzed and Hg(II)‐co‐catalyzed oxidation of D‐glucose (Glc) and cellobiose (Cel) by N‐bromoacetamide (NBA) in the presence of perchloric acid at 40 °C have been investigated. The reactions exhibit the first order kinetics with respect to NBA, but tend towards the zeroth order to higher NBA. The reactions are the first order with respect to Ru(III) and are fractional positive order with respect to [reducing sugar]. Positive effect of Cl^? and Hg(OAc)₂ on the rate of reaction is also evident in the oxidation of both reducing sugars. A negative effect of variation of H⁺ and acetamide was observed whereas the ionic strength (µ) of the medium had no influence on the oxidation rate. The rate of reaction decreased with the increase in dielectric constant and this enabled the computation of d_AB, the size of the activated complex. Various activation parameters have been evaluated and suitable explanation for the formation of the most reactive activated complex has been given. The main products of the oxidation are the corresponding arabinonic acid and formic acid. HOBr and [RuCl₃(H₂O)₂OH]^? were postulated as the reactive species of oxidant and catalyst respectively. A common mechanism, consistent with the kinetic data and supported by the observed effect of ionic strength, dielectric constant and multiple regression analysis, has been proposed. Formation of complex species such as [RuCl₃·S·(H₂O)OH]^? and RuCl₃·S·OHgBr·OH during the course of reaction was fully supported by kinetic and spectral evidences.     

Kinetic Study of Oxidation of DL-Serine by N-Bromophthalimide in the Presence of Sodium Dodecyl Sulfate

The kinetics of oxidation of DL-serine (Ser) by N-bromophthalimide (NBP) was studied in the presence of an anionic surfactant, sodium dodecyl sulfate, in acidic medium at 308 K. The rate of reaction was found to have first-order dependence on [NBP], fractional order dependence on [Ser] and inverse fractional order dependence on [H⁺]. The addition of reduced product of the oxidant [Phthalimide] and [Hg(OAc)₂] has no effect on the rate of reaction. The change in ionic strength of the medium had no effect on oxidation velocity. The rate of reaction increased with increasing [Br^?] and decreased with increasing [Cl^?]. The rate of reaction decreased with decrease in dielectric constant of the medium. OHCH₂CN was identified as the main oxidation product of the reactions. The various activation parameters have been computed. A suitable mechanism consistent with the experimental findings has been proposed. The micelle-binding constant has been calculated.     

Manganese(III) oxidation of L‐serine in aqueous sulfuric acid medium: Kinetics and mechanism

Kinetics and mechanism of oxidation of L‐serine by manganese(III) ions have been studied in aqueous sulfuric acid medium at 323 K. Manganese(III) sulfate was prepared by an electrolytic oxidation of manganous sulfate in aqueous sulfuric acid. The dependencies of the reaction rate are: an unusual one and a half‐order on [Mn(III)], first‐order on [ser], an inverse first‐order on [H⁺], and an inverse fractional‐order on [Mn(II)]. Effects of complexing agents and varying solvent composition were studied. Solvent isotope studies in D₂O medium were made. The dependence of the reaction rate on temperature was studied and activation parameters were computed from Arrhenius‐Eyring plots. A mechanism consistent with the observed kinetic data has been proposed and discussed. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 525–530, 1999     

Production and isolation of ligated metal(IV)‐oxo ions by tandem mass spectrometry

High valent metal(IV)‐oxo species, [M(?O)(MeIm)_n(OAc)]⁺ (M = Mn–Ni, MeIm = 1‐methylimidazole, n = 1–2), which are relevant to biology and oxidative catalysis, were produced and isolated in gas‐phase reactions of the metal(II) precursor ions [M(MeIm)_n(OAc)]⁺ (M = Mn–Zn, n = 1–3) with ozone. The precursor ions [M(MeIm)(OAc)]⁺ and [M(MeIm)₂(OAc)]⁺ were generated via collision‐induced dissociation of the corresponding [M(MeIm)₃(OAc)]⁺ ion. The dependence of ozone reactivity on metal and coordination number is discussed. Copyright © 2010 John Wiley & Sons, Ltd.     

Kinetics and mechanism of oxidation of the drug mephenesin by bis(hydrogenperiodato)argentate(III) complex anion

Mephenesin is being used as a central‐acting skeletal muscle relaxant. Oxidation of mephenesin by bis(hydrogenperiodato)argentate(III) complex anion, [Ag(HIO₆)₂]^5?, has been studied in aqueous alkaline medium. The major oxidation product of mephenesin has been identified as 3‐(2‐methylphenoxy)‐2‐ketone‐1‐propanol by mass spectrometry. An overall second‐order kinetics has been observed with first order in [Ag(III)] and [mephenesin]. The effects of [OH^?] and periodate concentration on the observed second‐order rate constants k′ have been analyzed, and accordingly an empirical expression has been deduced: k′ = (k_a + k_b[OH^?])K₁/{f([OH^?])[IO^?₄]_tot + K₁}, where [IO^?₄]_tot denotes the total concentration of periodate, k_a = (1.35 ± 0.14) × 10^?2M^?1s^?1 and k_b = 1.06 ± 0.01 M^?2s^?1 at 25.0°C, and ionic strength 0.30 M. Activation parameters associated with k_a and k_b have been calculated. A mechanism has been proposed to involve two pre‐equilibria, leading to formation of a periodato‐Ag(III)‐mephenesin complex. In the subsequent rate‐determining steps, this complex undergoes inner‐sphere electron transfer from the coordinated drug to the metal center by two paths: one path is independent of OH^? whereas the other is facilitated by a hydroxide ion. In the appendix, detailed discussion on the structure of the Ag(III) complex, reactive species, as well as pre‐equilibrium regarding the oxidant is provided. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 440–446, 2007     

Robust and efficient amide-based nonheme manganese(III) hydrocarbon oxidation catalysts: substrate and solvent effects on involvement and partition of multiple active oxidants

Two new mononuclear nonheme manganese(III) complexes of tetradentate ligands containing two deprotonated amide moieties, [Mn(bpc)Cl(H₂O)] ( 1 ) and [Mn(Me₂bpb)Cl(H₂O)] ? CH₃OH ( 2 ), were prepared and characterized. Complex 2 has also been characterized by X‐ray crystallography. Magnetic measurements revealed that the complexes are high spin (S=5/2) Mn^III species with typical magnetic moments of 4.76 and 4.95 μ_B, respectively. These nonheme Mn^III complexes efficiently catalyzed olefin epoxidation and alcohol oxidation upon treatment with MCPBA under mild experimental conditions. Olefin epoxidation by these catalysts is proposed to involve the multiple active oxidants Mn^V?O, Mn^IV?O, and Mn^III? OO(O)CR. Evidence for this approach was derived from reactivity and Hammett studies, KIE (k_H/k_D) values, H₂¹⁸O‐exchange experiments, and the use of peroxyphenylacetic acid as a mechanistic probe. In addition, it has been proposed that the participation of Mn^V?O, Mn^IV?O, and Mn^III? OOR could be controlled by changing the substrate concentration, and that partitioning between heterolysis and homolysis of the O? O bond of a Mn‐acylperoxo intermediate (Mn? OOC(O)R) might be significantly affected by the nature of solvent, and that the O? O bond of the Mn? OOC(O)R might proceed predominantly by heterolytic cleavage in protic solvent. Therefore, a discrete Mn^V?O intermediate appeared to be the dominant reactive species in protic solvents. Furthermore, we have observed close similarities between these nonheme Mn^III complex systems and Mn(saloph) catalysts previously reported, suggesting that this simultaneous operation of the three active oxidants might prevail in all the manganese‐catalyzed olefin epoxidations, including Mn(salen), Mn(nonheme), and even Mn(porphyrin) complexes. This mechanism provides the greatest congruity with related oxidation reactions by using certain Mn complexes as catalysts.     

Ruthenium(III) catalysed oxidation of glycerol by acidified KBrO3: a kinetic study

Kinetic investigations on the Ru^III-catalysed oxidation of glycerol by an acidified solution of KBrO₃ in the presence of Hg(OAc)₂ as a scavenger have been carried out in the 30–50 °C range. First order kinetics in the lower KBrO₃ concentration range tended to zero order at higher concentrations. The reaction follows zero order kinetics in glycerol and [H⁺]; the order is one with respect to [Ru^III]. An increase in [Cl^–] showed a positive effect, while addition of NaClO₄ has a negligible effect on the reaction rate. Hg(OAc)₂ and D₂O have an insignificant effect on the reaction rate. A suitable mechanism in conformity with the kinetic observations has been proposed and thermodynamic parameters computed.     

Biomimetic Decarboxylation of Carboxylic Acids with PhI（OAc）2 Catalyzed by Manganese Porphyrin [Mn（TPP）OAc]

Manganese（Ⅲ） meso-tetraphenylporphyrin acetate [Mn（TPP）OAc] served as an effective catalyst for the oxidative decarboxylation of carboxylic acids with （diacetoxyiodo）benzene [PhI（OAc）2] in CH2CI2-H2O（95：5, volume ratio）. The aryl substituted acetic acids are more reactive than the less electron rich linear carboxylic acids in the presence of catalyst Mn（TPP）OAc. In the former case, the formation of carbonyl products was complete within just a few minutes with 〉97% selectivities, and no further oxidation of the produced aldehydes was achieved under these catalytic conditions. This method provides a benign procedure owing to the utilization of low toxic（diacetoxyiodo） benzene, biologically relevant manganese porphyrins, and carboxylic acids.     

Kinetics and mechanism of the ruthenium(III)-catalyzed oxidation of hydroxy-acids byN-bromosuccinimide

Summary The kinetics of the Ru^III-catalyzed oxidation of the hydroxy acids; lactic, tartaric, malic and citric acids byN-bromosuccinimide in HClO₄ and in the presence of Hg(OAc)₂ have been studied. The reactions exhibit a first order rate dependence with respect to the oxidant and zeroth order rate dependence with respect to substrate. The rate is retarded by [H⁺], accelerated at law Ru^III concentrations but independent of [Ru^III] at higher Ru^III concentrations. A mechanism consistent with the observed kinetic data is proposed.