Kinetics and mechanism of hydration of o-thioquinone methide in aqueous solution. Rate-determining protonation of sulfur

o-Thioquinone methide, 2, was generated in aqueous solution by flash photolysis of benzothiete, 1, and rates of hydration of this quinone methide to o-mercaptobenzyl alcohol, 3, were measured in perchloric acid solutions, using H2O and D2O as the solvent, and also in acetic acid and tris(hydroxymethyl)methylammonium ion buffers, using H2O as the solvent. The rate profiles constructed from these data show hydronium-ion-catalyzed and uncatalyzed hydration reaction regions, just like the rate profiles based on literature data for hydration of the oxygen analogue, o-quinone methide, of the presently examined substrate. Solvent isotope effects on hydronium-ion catalysis of hydration for the two substrates, however, are quite different: k(H)/k(D) = 0.42 for the oxygen quinone methide, whereas k(H)/k(D) = 1.66 for the sulfur substrate. The inverse nature (k(H)/k(D) < 1) of the isotope effect in the oxygen system indicates that this reaction occurs by a preequilibrium proton-transfer reaction mechanism, with protonation of the substrate on its oxygen atom being fast and reversible and capture of the benzyl-type carbocationic intermediate so formed being rate-determining. The normal direction (k(H)/k(D) > 1) of the isotope effect in the sulfur system, on the other hand, suggests that protonation of the substrate on its sulfur atom is in this case rate-determining, with carbocation capture a fast following step. A semiquantitative argument supporting this hypothesis is presented.     

Ab initio molecular dynamics simulations of an excited state of X(-)(H(2)O)(3) (X = Cl, I) complex

Upon excitation of Cl(-)(H(2)O)(3) and I(-)(H(2)O)(3) clusters, the electron transfers from the anionic precursor to the solvent, and then the excess electron is stabilized by polar solvent molecules. This process has been investigated using ab initio molecular dynamics (AIMD) simulations of excited states of Cl(-)(H(2)O)(3) and I(-)(H(2)O)(3) clusters. The AIMD simulation results of Cl(-)(H(2)O)(3) and I(-)(H(2)O)(3) are compared, and they are found to be similar. Because the role of the halogen atom in the photoexcitation mechanism is controversial, we also carried out AIMD simulations for the ground-state bare excess electron -- water trimer [e(-)(H(2)O)(3)] at 300 K, the results of which are similar to those for the excited state of X(-)(H(2)O)(3) with zero kinetic energy at the initial excitation. This indicates that the rearrangement of the complex is closely related to that of e(-)(H(2)O)(3), whereas the role of the halide anion is not as important.     

Decay of the peroxide intermediate in laccase: reductive cleavage of the O-O bond.

Laccase is a multicopper oxidase that contains four Cu ions, one type 1, one type 2, and a coupled binuclear type 3 Cu pair. The type 2 and type 3 centers form a trinuclear Cu cluster that is the active site for O(2) reduction to H(2)O. To examine the reaction between the type 2/type 3 trinuclear cluster and dioxygen, the type 1 Cu was removed and replaced with Hg(2+), producing the T1Hg derivative. When reduced T1Hg laccase is reacted with dioxygen, a peroxide intermediate (P) is formed. The present study examines the kinetics and mechanism of formation and decay of P in T1HgLc. The formation of P was found to be independent of pH and did not involve a kinetic solvent isotope effect, indicating that no proton is involved in the rate-determining step of formation of P. Alternatively, pH and isotope studies on the decay of P revealed that a proton enhances the rate of decay by 10-fold at low pH. This process shows an inverse k(H)/k(D) kinetic solvent isotope effect and involves protonation of a nearby residue that assists in catalysis, rather than direct protonation of the peroxide. Decay of P also involves a significant oxygen isotope effect (k(16)O(2)/k(18)O(2)) of 1.11 +/- 0.05, indicating that reductive cleavage of the O-O bond is the rate-determining step in the decay of P. The activation energy for this process was found to be approximately 9.0 kcal/mol. The exceptionally slow rate of decay of P is explained by the fact that this process involves a 1e(-) reductive cleavage of the O-O bond and there is a large Franck-Condon barrier associated with this process. Alternatively, the 2e(-) reductive cleavage of the O-O bond has a much larger driving force which minimizes this barrier and accelerates the rate of this reaction by approximately 10(7) in the native enzyme. This large difference in rate for the 2e(-) versus 1e(-) process supports a molecular mechanism for multicopper oxidases in which O(2) is reduced to H(2)O in two 2e(-) steps.     

Ru/ZrO2·xH2O催化剂催化肉桂醛选择性加氢制肉桂醇

采用共沉淀法制备了7.5%Ru/ZrO2·xH2O催化剂,运用N2物理吸附-脱附法、X射线衍射、X射线光电子能谱和高分辨透射电子显微镜等技术对催化剂进行了表征,并用于催化肉桂醛选择加氢制肉桂醇反应中,考察了温度、H2压力和溶剂对肉桂醛转化率和肉桂醇选择性的影响.结果表明,肉桂醛转化率随着温度或H2压力的升高而升高,而肉...     

Non-oxidative decarboxylation of glycine derivatives by a peroxidase

Under anaerobic, peroxide-free conditions (pH 5.5, 25 degrees C), horseradish peroxidase (HRP) catalyzes the rapid, non-oxidatve decarboxylation of N-alkyl-N-phenylglycine derivatives to the corresponding N-alkyl-N-methylanilines in 100% yield. When the reaction is conducted in D2O buffer, the product contains a single deuterium in the methyl group. The reactions are very fast compared to the oxidative decarboxylation of the same substrates under standard peroxidatic conditions (i.e., hydrogen peroxide added, air present) and in fact are inhibited by peroxide and oxygen. To account for these unprecedented observations, we propose a cyclic mechanism in which ferric HRP abstracts an electron from the substrate, giving an aminium ion intermediate that decarboxylates; protonation of the resulting alpha-aminoradical on carbon gives an aminium ion that is reduced by ferrous HRP to complete the cycle.     

Polyfluorinated quaternary ammonium salts of polyoxometalate anions: fluorous biphasic oxidation catalysis with and without fluorous solvents

[reaction: see text] Polyfluorinated quaternary ammonium cations, [CF(3)(CF(2))(7)(CH(2))(3)](3)CH(3)N(+) (R(F)N(+)), were synthesized and used as countercations for the [WZnM(2)(H(2)O)(2)(ZnW(9)O(34))(2)](12)(-) (M = Mn(II), Zn(II)) polyoxometalate. The (R(F)N(+))(12)[WZnM(2)(H(2)O)(2)(ZnW(9)O(34))(2)] compounds were fluorous biphasic catalysts for alcohol and alkenol oxidation, and alkene epoxidation with aqueous hydrogen peroxide. Reaction protocols with or without a fluorous solvent were tested. The catalytic activity and selectivity was affected by both the hydrophobicity of the solvent and the substrate.     

钨过氧酸盐离子液体催化过氧化氢氧化苯甲醇制苯甲酸

用甲基三辛基氯化铵和钨酸钠一步法合成甲基三辛基季铵钨酸盐离子液体[(CH3)N(n-C8H17)3]2W2O11,以该离子液体为催化剂,在无反应溶剂条件下催化过氧化氢氧化苯甲醇生成苯甲酸。 考察了反应温度、催化剂用量以及氧化剂过氧化氢用量对苯甲酸产率的影响。 确定优化条件：反应温度70 ℃,苯甲醇用量5 mmol,催化剂用量是底物的0.4%(摩尔分数),30%过氧化氢用量2 mL,苯甲醇的转化率可达99%,苯甲酸选择性为98%。 该方法具有反应条件温和、产率高和选择性好的优点。     

Kinetics of two pathways in peroxyoxalate chemiluminescence

It has been shown that 1,1'-oxalyldiimidazole (ODI) is formed as an intermediate in the imidazole-catalyzed reaction of oxalate esters with hydrogen peroxide. Therefore, the kinetics of the chemiluminescence reaction of 1,1'-oxalyldiimidazole (ODI) with hydrogen peroxide in the presence of a fluorophore was investigated in order to further elucidate the mechanism of the peroxyoxalate chemiluminescence reaction. The effects of concentrations of ODI, hydrogen peroxide, imidazole (ImH), the general-base catalysts lutidine and collidine, and temperature on the chemiluminescence profile and relative quantum efficiency in the solvent acetonitrile were determined using the stopped-flow technique. Pseudo-first-order rate constant measurements were made for concentrations of either H2O2 or ODI in large excess. All of the reaction kinetics are consistent with a mechanism in which the reaction is initiated by a base-catalyzed substitution of hydrogen peroxide for imidazole in ODI to form an imidazoyl peracid (Im(CO)2OOH). In the presence of a large excess of H2O2, this intermediate rapidly decays with both a zero- and first-order dependence on the H2O2 concentration. It is proposed that the zero-order process reflects a cyclization of this intermediate to form a species capable of exciting a fluorophore via the "chemically initiated electron exchange mechanism" (CIEEL), while the first-order process results from the substitution of an additional molecule of hydrogen peroxide to the imidazoyl peracid to form dihydroperoxyoxalate, reducing the observed quantum yield. Under conditions of a large excess of ODI, the reaction is more than 1 order of magnitude more efficient at producing light, and the quantum yield increases linearly with increasing ODI concentration. Again, it is proposed that the slow initiating step of the reaction involves the substitution of H2O2 for imidazole to form the imidazoyl peracid. This intermediate may decay by either cyclization or by reaction with another ODI molecule to form a cyclic peroxide that is much more efficient at energy transfer with the fluorophore. The reaction kinetics clearly distinguishes two separate pathways for the chemiluminescent reaction.     

Mechanism of the reaction of the [W3S4H3(dmpe)3]+ cluster with acids: evidence for the acid-promoted substitution of coordinated hydrides and the effect of the attacking species on the kinetics of protonation of the metal-hydride bonds

The cluster [W(3)S(4)H(3)(dmpe)(3)](+) (1) (dmpe=1,2-bis(dimethylphosphino)ethane) reacts with HX (X=Cl, Br) to form the corresponding [W(3)S(4)X(3)(dmpe)(3)](+) (2) complexes, but no reaction is observed when 1 is treated with an excess of halide salts. Kinetic studies indicate that the hydride 1 reacts with HX in MeCN and MeCN-H(2)O mixtures to form 2 in three kinetically distinguishable steps. In the initial step, the W-H bonds are attacked by the acid to form an unstable dihydrogen species that releases H(2) and yields a coordinatively unsaturated intermediate. This intermediate adds a solvent molecule (second step) and then replaces the coordinated solvent with X(-) (third step). The kinetic results show that the first step is faster with HCl than with solvated H(+). This indicates that the rate of protonation of this metal hydride is determined not only by reorganization of the electron density at the M-H bonds but also by breakage of the H-X or H(+)-solvent bonds. It also indicates that the latter process can be more important in determining the rate of protonation.     

Epoxidation of olefins with hydrogen peroxide catalyzed by a reusable lacunary-type phosphotungstate catalyst

Olefins and allylic alcohols have been epoxidized with commercially available hydrogen peroxide (30% H2O2) using a phase transfer catalyst,composed of cetyltrimethylammonium cations and a lacunary-type phosphotungstate anion [PW11O39]7-or the complete Keggin-type heteropolyanion [PW12O40]3-,under two-phase conditions using ethyl acetate as the solvent. It was found that the lacunary-type catalyst showed higher activity and better recyclability than the complete Keggin-type catalyst under the same reaction c...     

氯苄双羰化合成苯丙酮酸新型催化剂吡啶-2-羧酸钴 研究

实验发现吡啶-2-羧酸钴是氯苄双羰化合成苯丙酮的新颖催化剂。在水和1,4-二氧六环混合溶剂中,当T=353K,p=2.4mPa,V(H2O):V(dioxane)=1:1.1,氧化钙与氯苄克分子比为1.00,氯苄与吡啶-2-羰酸钴的摩尔比为1:0.05时,氯苄转化率为74.5%,选择性达99%,苯丙酮酸产率为73.8%。研究了反应条件对苯丙酮酸产率和选择性的影响,并使用IR,UV,GC-MS等对产物进行了测定。     

Structure and function of vanadium haloperoxidases

A quantum mechanics/molecular mechanics study of the resting state of the vanadium dependent chloroperoxidase from fungi Curvularia inaequalis and of the early intermediates of the halide oxidation is reported. The investigation of different protonation states indicates that the enzyme likely consists of an anionic H2VO4- vanadate moiety where one hydroxo group is in axial position. The calculations suggest that the hydrogen peroxide binding may not involve an initial protonation of the vanadate cofactor. A low free energy reactive path is found where the hydrogen peroxide directly attacks the axial hydroxo group, resulting in the formation of an hydrogen peroxide intermediate. This intermediate is promptly protonated to yield a peroxo species. The free energy barrier for the formation of the peroxo species does not depend significantly upon the protonation state of the cofactor. The most likely protonation states of the peroxo cofactor are neutral forms HVO2(O2) with a hydroxo group either H-bonded to Ser402 or coordinated to Arg360. The peroxo cofactor is also coordinated to an axial water molecule, which could be important for the stability of the peroxovanadate/His496 adduct. Our calculations strongly suggest that the halide oxidation may take place with the preliminary formation of a peroxovanadate/halogen adduct. Subsequently, the halogen reacts with the peroxo moiety yielding a hypohalogen vanadate. The most reactive protonation state of peroxovanadate is the neutral HVO2(O2) with the hydroxo group H-bonded to Ser402. The important role of Lys353 in determining the catalytic activity is also confirmed.     

Nucleophilic substitution reaction of benzyl benzenesulfonates with anilines in MeOH-MeCN mixtures-I : Effects of substituent and solvent on the transition-state structure

Kinetic studies on nucleophilic substitution reaction of benzyl tosylates with anilines are reported. The reaction was found to proceed via a dissociative S_N2 mechanism with less than 50 % bond formation and extensive bond breaking at the transition state. It was found that positive charge development at the benzylic carbon is substantial and para-substituent effect on the substrate is predominantly of resonance type. Bond formation is shown to be favored by a better nucleophile, by an electron withdrawing group on the substrate and by the more polar(higher MeCN content) solvent. The substrate, nucleophile and solvent were found to follow the RSP.     

Chemiluminescence of diphenoyl peroxide electron transfer revealed by laser spectrophotometric analysis

The reaction of diphenoyl peroxide with a series of electronically excited electron donors was investigated by nanosecond laser spectroscopy. The results indicate that electron transfer from the excited state to the peroxide is the predominant reaction. The radical ions formed in this process may diffuse from the solvent cage or annihilate to regenerate the excited state. Kinetic data is presented that show an analogous process occurs for ground state electron donors resulting in chemilumine escence by the chemically initiated electron exchange luminescence (CIEEL) mechanism.     

Reaction of Aplysia limacina metmyoglobin with hydrogen peroxide

Myoglobin (Mb) from gastropod mollusc Aplysia limacina shows only 20% sequence homology to the 'prototype' sperm whale Mb but exhibits a typical Mb fold and can reversibly bind oxygen. An intriguing feature of aplysia Mb is that it lacks the distal histidine and displays a ligand stabilisation based on an arginine. Here we report the reaction of aplysia metMb with hydrogen peroxide studied by optical and electron paramagnetic resonance (EPR) spectroscopies. Two electron oxidation of the protein by H2O2 results in formation of two intermediates typical for this class of reactions, the oxoferryl haem state and a globin-bound free radical. An unusual characteristic of the aplysia Mb reaction is formation, prior to haem oxidation, of an optically distinct compound with an EPR spectrum typical of the low spin Fe3+ haem state. This compound is interpreted as the complex between H2O2 and the ferric haem state (Compound), formed prior to cleavage of the dioxygen bond. We conclude that H2O2 is singly deprotonated in Compound which can thus be notated as [Fe3+--OOH]. A new low spin ferric haem state has been observed over the period of Compound decay, and hypotheses have been formulated as to its identity and role. The location of the protein bound radical observed in aplysia Mb is discussed in light of the fact that the protein does not have any tyrosine residues, the most common site of free radical formation in the haem protein/peroxide systems. All intermediates of the reaction are kinetically characterised.     

Control of the evolution of iron peroxide intermediate in superoxide reductase from Desulfoarculus baarsii. Involvement of lysine 48 in protonation

Superoxide reductase is a nonheme iron metalloenzyme that detoxifies superoxide anion radicals O(2)(?-) in some microorganisms. Its catalytic mechanism was previously proposed to involve a single ferric iron (hydro)peroxo intermediate, which is protonated to form the reaction product H(2)O(2). Here, we show by pulse radiolysis that the mutation of the well-conserved lysine 48 into isoleucine in the SOR from Desulfoarculus baarsii dramatically affects its reaction with O(2)(?-). Although the first reaction intermediate and its decay are not affected by the mutation, H(2)O(2) is no longer the reaction product. In addition, in contrast to the wild-type SOR, the lysine mutant catalyzes a two-electron oxidation of an olefin into epoxide in the presence of H(2)O(2), suggesting the formation of iron-oxo intermediate species in this mutant. In agreement with the recent X-ray structures of the peroxide intermediates trapped in a SOR crystal, these data support the involvement of lysine 48 in the specific protonation of the proximal oxygen of the peroxide intermediate to generate H(2)O(2), thus avoiding formation of iron-oxo species, as is observed in cytochrome P450. In addition, we proposed that the first reaction intermediate observed by pulse radiolysis is a ferrous-iron superoxo species, in agreement with TD-DFT calculations of the absorption spectrum of this intermediate. A new reaction scheme for the catalytical mechanism of SOR with O(2)(?-) is presented in which ferrous iron-superoxo and ferric hydroperoxide species are reaction intermediates, and the lysine 48 plays a key role in the control of the evolution of iron peroxide intermediate to form H(2)O(2).     

Hydrogen isotope effects and mechanism of aqueous ozone and peroxone decompositions

Hydrogen peroxide exalts the reactivity of aqueous ozone by reasons that remain obscure. Should H2O2 enhance free radical production, as it is generally believed, a chain mechanism propagated by (.OH/.O2-) species would account for O3 decomposition rates in neat H2O, HR-O3, and in peroxone (O3 + H2O2) solutions, HPR-O3. We found, however, that: (1) the radical mechanism correctly predicts HR-O3 but vastly overestimates HPR-O3, (2) solvent deuteration experiments preclude radical products from the (O3 + HO2-) reaction. The modest kinetic isotope effect (KIE) we measure in H2O/D2O: HR-O3/DR-O3 = 1.5 +/- 0.3, is compatible with a chain process driven by electron- and/or O-atom transfer processes. But the large KIE found in peroxone: HPR-O3/DPR-O3 = 19.6 +/- 4.0, is due to an elementary (O3 + HO2-) reaction involving H-O2- bond cleavage. Since the KIE for the hypothetical H-atom transfer: O3 + HO2- HO3. +.O2-, would emerge as a KIE1/2 factor in the rates of the ensuing radical chain, the magnitude of the observed KIE must be associated with the hydride transfer reaction that yields a diamagnetic species: O3 + HO2- HO3- + O2. HO3-/H2O3 may be the bactericidal trioxide recently identified in the antibody-catalyzed addition of O2(1Deltag) to H2O.     

Characterization and Reactivity of a Tetrahedral Copper(II) Alkylperoxido Complex

A tetrahedral Cu^II alkylperoxido complex [Cu^II(TMG₃tach)(OOCm)]⁺ ( 1^OOCm ) (TMG₃tach={2,2′,2′′-[(1s,3s,5s)-cyclohexane-1,3,5-triyl]tris-(1,1,3,3-tetramethyl guanidine)}, OOCm=cumyl peroxide) is prepared and characterized by UV/Vis, cold-spray ionization mass spectroscopy (CSI-MS), resonance Raman, and EPR spectroscopic methods. Product analysis of the self-decomposition reaction of 1^OOCm in acetonitrile (MeCN) indicates that the reaction involves O−O bond homolytic cleavage of the peroxide moiety with concomitant C−H bond activation of the solvent molecule. When an external substrate such as 1,4-cyclohexadiene (CHD) is added, the O−O bond homolysis leads to C−H activation of the substrate. Furthermore, the reaction of 1^OOCm with 2,6-di-tert-butylphenol derivatives produces the corresponding phenoxyl radical species (ArO^.) together with a Cu^I complex through a concerted proton-electron transfer (CPET) mechanism. Details of the reaction mechanisms are explored by DFT calculations.     

Photochemical synthesis of H2O2 from the H2O...O(3P) van der Waals complex: experimental observations in solid krypton and theoretical modeling

Productive photochemical synthesis of hydrogen peroxide, H(2)O(2), from the H(2)O...O((3)P) van der Waals complex is studied in solid krypton. Experimentally, we achieve the three-step formation of H(2)O(2) from H(2)O and N(2)O precursors frozen in solid krypton. First, 193 nm photolysis of N(2)O yields oxygen atoms in solid krypton. Upon annealing at approximately 25 K, mobile oxygen atoms react with water forming the H(2)O...O complex, where the oxygen atom is in the triplet ground state. Finally, the H(2)O...O complex is converted to H(2)O(2) by irradiation at 300 nm. According to the complete active space self-consistent field modeling, hydrogen peroxide can be formed through the photoexcited H(2)O+-O- charge-transfer state of the H(2)O...O complex, which agrees with the experimental evidence.     

Dramatic acceleration of olefin epoxidation in fluorinated alcohols: activation of hydrogen peroxide by multiple h-bond networks

In 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) as solvent, the epoxidation of olefins by hydrogen peroxide is accelerated up to ca. 100 000-fold (relative to that in 1,4-dioxane as solvent). The mechanistic basis of this effect was investigated kinetically and theoretically. The kinetics of the epoxidation of Z-cyclooctene provided evidence that higher-order solvent aggregates (rate order in HFIP ca. 3) are responsible for the rate acceleration. Activation parameters (DeltaS++ = -39 cal/mol.K) indicated a highly ordered transition state in the rate-determining step. In line with these findings, DFT simulations revealed a pronounced decrease of the activation barrier for oxygen transfer from H(2)O(2) to ethene with increasing number of (specifically) coordinated HFIP molecules. The oxygen transfer was unambiguously identified as a polar concerted process. Simulations (combined DFT and MP2) of the epoxidation of Z-butene were in excellent agreement with the experimental data obtained in the epoxidation of Z-cyclooctene (activation enthalpy, entropy, and kinetic rate order in HFIP of 3), supporting the validity of our mechanistic model.