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Switching on the electrocatalytic ethene epoxidation on nanocrystalline RuO2
Jirkovsky, Jakub S.,Busch, Michael,Ahlberg, Elisabet,Panas, Itai,Krtil, Petr
, p. 5882 - 5892 (2011)
Ruthenium-based oxides with rutile structure were examined regarding their properties in electrocatalytic ethene oxidation in acid media. A possible promoting effect of chloride ions toward oxirane formation was explored. Online differential electrochemical mass spectrometry combined with electrochemical polarization techniques were used to monitor the potential dependence of organic products resulting from ethene oxidation as well as the reaction solution decomposition products. Quantum chemical modeling by means of density functional theory was employed to study key reaction steps. The ethene oxidation in acid media led to CO2, whereas oxirane was formed in the presence of 0.3 M Cl-. In the Cl- promoted oxidation on RuO2, oxirane and a small amount of CO2 were the only detected electro-oxidation products at potentials below the onset of Cl2 and O2 evolution, resulting from Cl- and water oxidation. It is demonstrated here that the epoxidation is a surface-related electrocatalytic process that depends on the surface properties. Cl acts as the epoxidation promoter that switches off the combustion pathway toward CO2 and enables the epoxidation reaction channel by surface reactive sites blocking. The proposed epoxidation mechanism implies binuclear (recombination) mechanism for O2 evolution reaction on considered surfaces.
An Ultrasensitive Fluorescence Assay for the Detection of Halides and Enzymatic Dehalogenation
Aslan-üzel, A?k?n S.,Beier, Andy,Ková?, David,Cziegler, Clemens,Padhi, Santosh K.,Schuiten, Eva D.,D?rr, Mark,B?ttcher, Dominique,Hollmann, Frank,Rudroff, Florian,Mihovilovic, Marko D.,Bury?ka, Tomá?,Damborsky, Ji?í,Prokop, Zbyněk,Badenhorst, Christoffel P. S.,Bornscheuer, Uwe T.
, p. 2032 - 2039 (2020)
Halide assays are important for the study of enzymatic dehalogenation, a topic of great industrial and scientific importance. Here we describe the development of a very sensitive halide assay that can detect less than a picomole of bromide ions, making it very useful for quantifying enzymatic dehalogenation products. Halides are oxidised under mild conditions using the vanadium-dependent chloroperoxidase from Curvularia inaequalis, forming hypohalous acids that are detected using aminophenyl fluorescein. The assay is up to three orders of magnitude more sensitive than currently available alternatives, with detection limits of 20 nM for bromide and 1 μM for chloride and iodide. We demonstrate that the assay can be used to determine specific activities of dehalogenases and validate this by comparison to a well-established GC-MS method. This new assay will facilitate the identification and characterisation of novel dehalogenases and may also be of interest to those studying other halide-producing enzymes.
Reaction Mechanism from Structure-Energy Relations. 2. Acid-Catalyzed Addition of Alcohols to Formaldhyde
Grunwald, Ernest
, p. 4715 - 4720 (1985)
Previous theory of structure-energy relations is extended to mechanisms with three concerted reaction events.Data for 25 reactions (five alcohols, five acid catalysts) are examined on the basis of four different mechanisms.Only one of the mechanisms fits well.It involves concerted C...O bond formation, proton donation by the acid catalyst, and proton acceptance by a water molecule, according to H2O + HOCH2R + H2C=O + HOOCCH2R' -> H2OH(+) + RCH2OCH2OH + (-)OOCCH2R'.The fit for this mechanism is decisively good in three practically independent tests.The contrast between the present mechanism and that which fits the base-catalyzed reaction (which does not involve HOH as a reagent) can be explained as due to peculiarities of relative acid-base properties of RCH2OCH2OH and H2O.No inconsistency appears with other data.Transition-state coordinates for the acid-catalyzed reaction are tabulated.Progress of C...O bond formation, though variable within the reaction series, is well ahead of that of the proton transfers.Further analysis of the theoretical free-energy surfaces indicates that disparity of progress of the concerted reaction events reaches a maximum at the transition state.
Structure-Reactivity Effects in the Breakdown of Hemiacetals of α-Bromoacetophenone. Change in Rate-Limiting Step for Base Catalysis
Soerensen, Poul E.,Pedersen, K. J.,Pedersen, P. R.,Kanagasabapathy, V. M.,McClelland, Robert A.
, p. 5118 - 5123 (1988)
Kinetics of breakdown are reported for the ethyl, methyl, 2-chloroethyl, and 2,2,2-trifluoroethyl hemiacetals of α-bromoacetophenone, the hemiacetals being generated by the aqueous bromination of the appropriate α-alkoxystyrene.Acid catalysis is characterized by a Broensted α value of 0.6 and a β1g (for H+) of 0.3.With these values the transition state has been located on a reaction coordinate diagram for the class e mechanism of acid catalysis and a comparison made with positions of transition states for acetaldehyde and formaldehyde hemiacetals.With changing electrophilicity of the carbonyl there is a consistent shift in the position of the transition state, but the effect is relatively small.The base-catalyzed breakdowns of the ethyl and 2-chloroethyl hemiacetals have β vlues near 0.7, significantly higher than the values associated with analogous formaldehyde and acetaldehyde hemiacetals.The difference can be explained in terms of the reaction coordinate diagram of the class n base mechanism, recognizing the decreased susceptibility of the ketone to nucleophilic addition.With the trifluoroethyl hemiacetal, buffer dilution plots are curved and there is a downward break in the rate-pH profile.These are explained by a mechanism-hemiacetal hemiacetal anion -> product-where at low buffer concentrations or low H+ concentration the deprotonation step is rate-limiting.As shown through a kinetic analysis, rate-limiting proton transfer is a result of a large rate constant for anion breakdown (ca. 8E8 s-1).Thus, at low buffer concentration breakdown occurs more quickly than reprotonation.The curvature arises because the buffer acid provides efficient protonation.At high buffer concentration the deprotonation step is (approaching) a rapid equilibrium preceding rate-limiting breakdown.Comparison with the other two hemiacetals shows that the behavior of the trifluoroethyl compound is predictable on the basis of structure-reactivity correlations.A reaction with rate-limiting proton transfer is enforced by the very short lifetime of the hemiacetal anion.
Chemistry of Nitrosoureas. Intermediacy of 4,5-Dihydro-1,2,3-oxadiazole in 1,3-Bis(2-chloroethyl)-1-nitrosourea Decomposition
Brundrett, Robert B.
, p. 1245 - 1247 (1980)
A new product, ethylene glycol, was identified from BCNU decomposition.The variation of ethylene glycol yield with pH indicates that there are two competing mechanisms of decomposition.At pH 7.4 the decomposition is predominantly through 2-chloroethyldiazohydroxide, and chloroethanol and acetaldehyde are the major products.At pH 5, the decomposition is predominantly through 4,5-dihydro-1,2,3-oxadiazole and 2-hydroxyethyldiazohydroxide, and ethylene glycol and acetaldehyde are the major products.Deuterium labeling shows that atboth pH's the acetaldehyde arises through a mechanism involving a hydride shift.At pH 5 in the presence of bromide, 2-bromoethanol is a major product and deuterium labeling shows that the hydroxyl is predominantly on the carbon which bore the chlorine in BCNU.
Characterization of transition states by isotopic mapping and structure-reactivity coefficients: Solvent and secondary deuterium isotope effects for the base-catalyzed breakdown of acetaldehyde hemiacetals
Coleman, Charolotte A.,Murray, Christopher J.
, p. 1677 - 1684 (1991)
Rate constants and structure-reactivity coefficients for the breakdown of acetaldehyde and acetaldehyde-d4 hemiacetals were determined in water and deuterium oxide by trapping the acetaldehyde formed with α-effect nucleophiles. General-base catalysis by substituted acetate and cacodylate ion catalysts represents equilibrium ionization of the hemiacetal CL3CL(OL)OR (L = H or D) to form the hemiacetal anion, CL3CL(O-)OR followed by rate-determining general-acid catalysis of the cleavage of the hemiacetal anion to form acetaldehyde and ROL. Solvent isotope effects for the catalytically active proton kpBH/kpBD = 0.9-2.5 do not change significantly with changes in the pK of the catalyst or the leaving group alcohol. The increase in the secondary αβ-deuterium isotope effects kαβH/kαβD = 1.21-1.30 with decreases in the pK of the leaving group alcohol can be described by the interaction coefficient pyy′ = ?ρn/-?pK1g = -0.069. The increase in Br?nsted β = 0.48-0.72 with decreases in the pK of the leaving group alcohol in water can be described by the interaction coefficient pxy′, = ?β/-?pK1g = 0.090 and in D2O by pxy′ = 0.078. The interaction coefficients and the observation of both solvent and secondary deuterium isotope effects are consistent with a coupling between proton transfer to the leaving group oxygen and changes in hybridization about the central carbon in the transition state for cleavage of the hemiacetal anion. The results are discussed in the context of proposals for stable hydrogen-bonded protons in concerted acid- and base-catalyzed reactions in water.
Reactions of gold(III) complexes with alkenes in aqueous media: Generation of bis-(β-hydroxyalkyl)gold(III) complexes
Rezsnyak, Chad E.,Autschbach, Jochen,Atwood, Jim D.,Moncho, Salvador
, p. 1153 - 1165 (2013)
Reactions of gold(III) complexes with ethylene and propylene in water provide the first such examples not accompanied by reduction of the gold. HAuC14 and AuCL3(TPPTS) produce organic products (alcohol, aldehyde/ketone, etc.) with gold reduction to the metal. However, [Au(bipy)CL2] Cl in water produces the gold(III) β-hydroxy complexes, [Au(bipy)(CH2CH2OH)2]X (X =Cl, PF6) and [Au(bipy)(CH2CH(OH)CH3]Cl, which are stable in solution. These complexes could not be isolated, but were characterized by NMR and high-resolution mass spectra. [Au(terpy)Cl]CL2 fails to react with ethylene in water, even at elevated temperatures. DFT computations were performed to investigate the reaction mechanism.
Synthesis of Oxochromium(VI) Alkoxides via Epoxide Cleavage. Structure, Reactivity, and Mechanism
Limberg, Christian,Wistuba, Tobias
, p. 6169 - 6176 (1999)
In an NMR spectroscopic study the cleavage of epoxides [ethylene, propylene, and cis/trans-butylene oxide] by chromyl chloride giving access to oxochromium(VI) alkoxides was shown to proceed via a bimolecular rate-determining step where two molecules of a complex CrO2Cl2...epoxide collide. Subsequently one Cl ligand at the first Cr center attacks the backside of an epoxide molecule complexed at the Cr center of a second CrO2Cl2...epoxide molecule and vice versa. The trans-opening of the epoxides was proved by determining the configuration of the chlorohydrins resulting from hydrolysis of the corresponding alkoxide products in the cases of cis- and trans-butylene oxide. The NMR data provide evidence that each oxochromium(VI) alkoxide adopts one preferred conformation in solution although DFT calculations did not indicate any special stabilizing effects. The product formation was rationalized by DFT calculations concerning the thermodynamics of the reactions.
Decomposition of ethylene, a flower-senescence hormone, with electrolyzed anode water.
Harada, Kazuo,Yasui, Keiko
, p. 790 - 796 (2003)
Electrolyzed anode water (EAW) markedly extended the vase life of cut carnation flowers. Therefore, a flower-senescence hormone involving ethylene decomposition by EAW with potassium chloride as an electrolyte was investigated. Ethylene was added externally to EAW, and the reaction between ethylen and the available chlorine in EAW was examined. EAW had a low pH value (2.5), a high concentration of dissolved oxygen, and extremely high redox potential (19.2 mg/l and 1323 mV, respectively) when available chlorine was at a concentration of about 620 microns. The addition of ethylene to EAW led to ethylene decomposition, and an equimolar amount of ethylene chlorohydrine with available chlorine was produced. The ethylene chlorohydrine production was greatly affected by the pH value (pH 2.5, 5.0 and 10.0 were tested), and was faster in an acidic solution. Ethylene chlorohydrine was not produced after ethylene had been added to EAW at pH 2.6 when available chlorine was absent, but was produced after potassium hypochlorite had been added to such EAW. The effect of the pH value of EAW on the vase life of cut carnations was compatible with the decomposition rate of ethylene in EAW of the same pH value. These results suggest that the effect of EAW on the vase life of cut carnations was due to the decomposition of ethylene to ethylene chlorohydrine by chlorine from chlorine compounds.
Effects of Volume and Surface Property in Hydrolysis by Acetylcholinesterase. The Trimethyl Site
Cohen, Saul G.,Elkind, Jerome L.,Chishti, S. Bano,Giner, Jose-L. P.,Reese, Heide,Cohen, Jonathan B.
, p. 1643 - 1647 (1984)
β-Substituted ethyl acetates, XCH2CH2OCOCH3, have been prepared, and their hydrolysis by acetylcholinesterase has been studied.Log of enzymic reactivity, normalized for intrinsic reactivity in hydrolysis by hydroxide, log(kcat/Km)n, rises linearly with increasing refraction volume, MR (or RD25), for substrates with β-X = H, Cl, Br, CH3CH2, (CH3)2CH, (CH3)2S+, (CH3)3N+, and (CH3)3C.Larger substituents may by accommodated, (CH3)3Si and (CH3CH2)3N+, with no further increase in rate.Substrates with β-substituents CH3S, CH3S(O), (CH3)3N+(OH), and CH3S(O2) are less reactive than consistent with the relation with MR by factors of 5-40, indicating that hydrophobic surface and desolvation of the substrate-enzyme interface may be necessary for maximum reactivity correlated with MR.Values of log (kcat/Km)n for substrates with β-substituents X = CH3S, Cl, Br, CH3CH2, (CH3)2CH, (CH3)3C, and (CH3)3Si rise linearly with increasing hydrophobicity, ?, but reactivity of substrates with X = (CH3)3N+ and (CH3)2S+ are more reactive than consistent with a relation to ? by factors of 300 and 40 and with X = CH3S(O2), CH3S(O), and (CH3)2N+(OH), by factors of 7-100.Reactivity appears related to (i) volume of the β-substituent and its fit in its subsite, which is trimethyl rather than anionic, and (ii) the hydrophobicity of its surface.