Gold-catalyzed amide synthesis from aldehydes and amines in aqueous medium

An efficient gold-catalyzed amide synthesis from aldehydes and amines in aqueous medium under mild reaction conditions has been developed.     

Kinetics of the autocatalytic oxidation of N-amino-3-azabicyclo[3,3,0]octane by chloramine in aqueous medium

The kinetics of the degradation of N-amino-3-azabicyclo[3,3,0]octane by chloramine has been studied by GC and HPLC in stoichiometric conditions in a solution buffered with NaOH/KH₂PO₄ and Na₂B₄O₇.10 H₂O between pH = 10.5 and 13.5. The second-order reaction exhibits specific acid catalysis which indicates competitive oxidation between the haloamine and the neutral and ionic forms of the bicyclic hydrazine. The enthalpy and entropy of activation were determined at pH = 12.89. In a nonbuffered solution, the interaction is autocatalyzed due to acidification of the mixture by the ammonium ions. In basic medium, the reaction forms an endocyclic hydrazone. A mathematical treatment based on an implicit equation allows a quantitative interpretation of all the phenomena observed over the above pH interval. This takes both the acid/base dissociation equilibria and the alkaline hydrolysis of the chloro-derivative into account. © 1995 John Wiley & Sons, Inc.     

Kinetics of oxidation of glycine by aqueous chlorine

The kinetics of oxidation of glycine with aqueous chlorine has been investigated. The reaction is first order with respect to both molecular chlorine and glycine and inverse first order with respect to H⁺. The effect of various factors on the initial rate has also been studied. The kinetic parameters have been calculated and a possible mechanism is proposed.

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. , H⁺. , . .

     

Kinetics and mechanisms of chlorine dioxide oxidation of tryptophan

The reactions of aqueous ClO2 (*) and tryptophan (Trp) are investigated by stopped-flow kinetics, and the products are identified by high-performance liquid chromatography (HPLC) coupled with electrospray ionization mass spectrometry and by ion chromatography. The rates of ClO2 (*) loss increase from pH 3 to 5, are essentially constant from pH 5 to 7, and increase from pH 7 to 10. The reactions are first-order in Trp with variable order in ClO2 (*). Below pH 5.0, the reactions are second- or mixed-order in [ClO2 (*)], depending on the chlorite concentration. Above pH 5.0, the reactions are first-order in [ClO2 (*)] in the absence of added chlorite. At pH 7.0, the Trp reaction with ClO2 (*) is first-order in each reactant with a second-order rate constant of 3.4 x 10(4) M(-1) s(-1) at 25.0 degrees C. In the proposed mechanism, the initial reaction is a one-electron oxidation to form a tryptophyl radical cation and chlorite ion. The radical cation deprotonates to form a neutral tryptophyl radical that combines rapidly with a second ClO 2 (*) to give an observable, short-lived adduct ( k obs = 48 s(-1)) with proposed C(H)-OClO bonding. This adduct decays to give HOCl in a three-electron oxidation. The overall reaction consumes two ClO2 (*) per Trp and forms ClO2- and HOCl. This corresponds to a four-electron oxidation. Decay of the tryptophyl-OClO adduct at pH 6.4 gives five initial products that are observed after 2 min and are separated by HPLC with elution times that vary from 4 to 17 min (with an eluent of 6.3% CH 3OH and 0.1% CH 3COOH). Each of these products is characterized by mass spectrometry and UV-vis spectroscopy. One initial product with a molecular weight of 236 decays within 47 min to yield the most stable product, N-formylkynurenine (NFK), which also has a molecular weight of 236. Other products also are observed and examined.     

Supramolecular binding of protonated amines to a receptor microgel in aqueous medium

Polyanionic microgels containing negatively charged tetrazole binding sites show supramolecular binding of various protonated amines (e.g. dibucaine and spermine) in a competitive aqueous medium at millimolar concentration.     

Highly efficient and enantioselective enzymatic acylation of amines in aqueous medium

A new strategy based on the unique catalytic properties, stability and enantioselectivity of the relatively unknown penicillin acylase from Alcaligenes faecalis has been developed for the effective and enantioselective acylation of amines in aqueous medium. In contrast to lipase-catalyzed acylations in organic solvents, the penicillin acylase-catalyzed acylation of amines in aqueous solution is a rapid and chemoselective process leading to a product which can subsequently be deacylated by the same enzyme, imposing secondary enantiocontrol and leading to effective resolution.     

Flow-injection spectrophotometric determination of residual free chlorine and chloramine

2,4-Dinitrophenylhydrazine has been suggested as a new reagent for the flow-injection spectrophotometric determination of residual chlorine based on its oxidation to 2,4-dinitrophenyldiazonium ion. The measurement of the decrease in colour intensity under reversed flow-injection (reagent injection) conditions has been used for the determination of 0.1–10 mg/l Cl present as free or combined chlorine, and for its speciation. The limit of detection was 0.05 mg/l Cl. Copper (II), iron (III) and many other ions have been found not to vitiate the results.     

Studies in nickel(IV) chemistry. Kinetics of electron transfer from dimethyl sulfoxide to oxohydroxonickel(IV) in aqueous acid medium

The kinetics of decomposition of “oxohydroxonickel(IV)” [Ni(IV)] with concomitant intramolecular electron transfer to produce hexaaquanickel(II) and dioxygen in aqueous acid solutions show pseudo-first-order dissappearance of the Ni(IV). The pseudo-first-order rate constants for the acid decomposition (k_ad) satisfy where K_MH and k_d refer to the equilibrium protonation constant and the decomposition constant of the protonated species of the Ni(IV) respectively. The values of K_MH and k_d in aqueous medium at 45°C and μ = 2.0M are 25.5 ± 1M^?1 and (1.7 ± 0.1) × 10^?5 s^?1, respectively. The kinetics of the intermolecular electron transfer from dimethyl sulfoxide (DMSO) to the Ni(IV), producing Ni(H₂O)₆²⁺ and dimethyl sulfone as products, have been investigated by monitoring the formation of Ni(H₂O)₆²⁺. The pseudo-first-order rate constants for the electron transfer k_obs are linearly dependent on [DMSO]₀ or [H⁺], attaining limiting values at higher relative [DMSO]₀ or [H⁺], in accordance with where K_1c and K_2c represent the formation constants of the precursors involving DMSO and the unprotonated and one-protonated Ni(IV) species, respectively, and k_1x and k_2x are the corresponding decomposition rate constants of the precursors. The values of K_2c and k_2x are (2.3 ± 0.1) × 10⁴M^?1 and 19 ± 1 s^?1, respectively, at 45°C and μ = 1.0M. Results are interpreted in terms of probable mechanisms involving (1) a rate-determining decomposition of the protonated Ni(IV) followed by rapid product formation steps, and (2) precursor complex formation between DMSO and the unprotonated or the protonated species of the Ni(IV) followed by rate-determining decomposition with electron transfer.     

Kinetics and mechanisms of chlorine dioxide and chlorite oxidations of cysteine and glutathione

Chlorine dioxide oxidation of cysteine (CSH) is investigated under pseudo-first-order conditions (with excess CSH) in buffered aqueous solutions, p[H+] 2.7-9.5 at 25.0 degrees C. The rates of chlorine dioxide decay are first order in both ClO2 and CSH concentrations and increase rapidly as the pH increases. The proposed mechanism is an electron transfer from CS- to ClO2 (1.03 x 10(8) M(-1) s(-1)) with a subsequent rapid reaction of the CS* radical and a second ClO2 to form a cysteinyl-ClO2 adduct (CSOClO). This highly reactive adduct decays via two pathways. In acidic solutions, it hydrolyzes to give CSO(2)H (sulfinic acid) and HOCl, which in turn rapidly react to form CSO3H (cysteic acid) and Cl-. As the pH increases, the (CSOClO) adduct reacts with CS- by a second pathway to form cystine (CSSC) and chlorite ion (ClO2-). The reaction stoichiometry changes from 6 ClO2:5 CSH at low pH to 2 ClO2:10 CSH at high pH. The ClO2 oxidation of glutathione anion (GS-) is also rapid with a second-order rate constant of 1.40 x 10(8) M(-1) s(-1). The reaction of ClO2 with CSSC is 7 orders of magnitude slower than the corresponding reaction with cysteinyl anion (CS-) at pH 6.7. Chlorite ion reacts with CSH; however, at p[H+] 6.7, the observed rate of this reaction is slower than the ClO2/CSH reaction by 6 orders of magnitude. Chlorite ion oxidizes CSH while being reduced to HOCl, which in turn reacts rapidly with CSH to form Cl-. The reaction products are CSSC and CSO3H with a pH-dependent distribution similar to the ClO2/CSH system.     

Kinetics and mechanisms of the reactions of hypochlorous acid,chlorine, and chlorine monoxide with bromite ion

The reaction between BrO2(-) and excess HOCl (p[H+] 6-7, 25.0 degrees C) proceeds through several pathways. The primary path is a multistep oxidation of HOCl by BrO(2)(-) to form ClO(3)(-) and HOBr (85% of the initial 0.15 mM BrO(2)(-)). Another pathway produces ClO(2) and HOBr (8%), and a third pathway produces BrO(3)(-) and Cl(-) (7%). With excess HOCl concentrations, Cl(2)O also is a reactive species. In the proposed mechanism, HOCl and Cl(2)O react with BrO(2)(-) to form steady-state species, HOClOBrO(-) and ClOClOBrO(-). Acid facilitates the conversion of HOClOBrO(-) and ClOClOBrO(-) to HOBrOClO(-). These reactions require a chainlike connectivity of the intermediates with alternating halogen-oxygen bonding (i.e. HOBrOClO(-)) as opposed to Y-shaped intermediates with a direct halogen-halogen bond (i.e. HOBrCl(O)O(-)). The HOBrOClO(-) species dissociates into HOBr and ClO(2)(-) or reacts with general acids to form BrOClO. The distribution of products suggests that BrOClO exists as a BrOClO.HOCl adduct in the presence of excess HOCl. The primary products, ClO(3)(-) and HOBr, are formed from the hydrolysis of BrOClO.HOCl. A minor hydrolysis path for BrOClO.HOCl gives BrO(3)(-) and Cl(-). An induction period in the formation of ClO(2) is observed due to the buildup of ClO(2)(-), which reacts with BrOClO.HOCl to give 2 ClO(2) and Br(-). Second-order rate constants for the reactions of HOCl and Cl(2)O with BrO(2)(-) are k(1)(HOCl) = 1.6 x 10(2) M(-1) s(-1) and k(1)(Cl)()2(O) = 1.8 x 10(5) M(-)(1) s(-)(1). When Cl(-) is added in large excess, a Cl(2) pathway exists in competition with the HOCl and Cl(2)O pathways for the loss of BrO(2)(-). The proposed Cl(2) pathway proceeds by Cl(+) transfer to form a steady-state ClOBrO species with a rate constant of k(1)(Cl2) = 8.7 x 10(5) M(-1) s(-1).     

Kinetics and mechanism of electron transfer from phenolate anion to hexacyanoferrate (III) in aqueous alkaline media

A kinetic reinvestigation of the title redox system in aqueous alkaline media at 35°C and an ionic strength of 0.5 mol dm^?3 shows that the reaction follows a pseudosecond-order Fe(CN) disappearance. While varying [phenol]₀ and [OH^?] exhibit a linear influence on the pseudo-second-order rate constant, varying[Fe(CN)]₀ and [Fe(CN)]₀, initially taken, have a complicated inhibitory effect on the same. The major phenoloxidation products isolated under a chosen condition are 2,2′- and 4,4′- dihydroxydiphenyl. Results are interpreted in terms of a probable mechanism which envisages a reversible formation, by the first one-electron transfer, of a reactive phenoxy radical (PhO^˙) which on the second one-electron transfer forms a less reactive ion-pair intermediate (stabilized by the Fe(CN) produced) to decompose rate-determiningly to phenoxonium cation (PhO⁺) and Fe(CN), the product-formation steps being very rapid and kinetically indistinguishable.     

Kinetics and mechanism of biphasic transfer of salted water from an aqueous phase to AOT reverse micelles

The transfer of salted water from an external aqueous phase to AOT reverse micelles is studied in relation with the change of properties from a percolating to a non percolating state of the micelles. A membrane-separated two-compartment cell was used to measure the transfer of salt which was followed by conductivity measurements. We expected a sharp conductivity drop which could have been used as a kind of sensor to detect a threshold of salinity. In fact, this sharp drop was not observed and this is shown to be due to the fact that no excess water is incorporated in the microemulsion phase in these conditions. For this purpose separate analysis of the water and salt incorporated in the microemulsion phase versus time has been performed. Under the conditions used the increase of the salt content is accompanied by a decrease of the water content, even though the initial system was far from being saturated with water. The results are discussed in terms of the different possible mechanisms. Molecular diffusion of water appears to be unlikely and its expulsion can be simply explained by the shrinkage of the microdroplets merging with the interfacial film, this being associated with the electrostatic shielding of the polar head repulsions.     

Hydrogen transfer from tertiary amines to trifluoroacetic anhydride

Tertiary amines are found to be oxidized by trifluoroacetic anhydride (TFAA). A discussion of the mechanism is given.     

Kinetics and adsorption behavior of carboxymethyl starch on alpha-alumina in aqueous medium

The adsorption of carboxymethyl starch (CMS) at the alpha-alumina/aqueous solution interface has been investigated through adsorption studies, electrokinetics mobility measurements, and FTIR spectroscopy. Zeta potential measurements show that the addition of CMS results in a more dramatic increase in the absolute zeta potential in the alkaline region, as well as a shift of the isoelectric point to lower values, indicating the adsorption of CMS from the aqueous solution onto the alumina surface. The positive hydrophilic surface sites of alumina are responsible for the adsorption of CMS molecules. The adsorption of CMS is possible after charge reversal by the addition of excess CMS. Nearly 30 min of contact time are found to be sufficient for the adsorption of CMS to reach equilibrium. CMS adsorption follows a Langmuir isotherm with adsorption capacities of 91.74 mg CMS per gram of alpha-alumina. For the adsorption of CMS, pseudo-second-order chemical reaction kinetics provides the best correlation with the experimental data. FTIR analysis indicated that CMS forms outer complexes with alumina surfaces depending on the shifting of the asymmetric and symmetric bands.     

Kinetics and mechanism of metal ion catalysis: Part I. Kinetics of electron transfer from cycloheptanone in aqueous alkali

The kinetics of the electron transfer from cycloheptanone to OsO₄ in alkaline medium has been studied spectrophotometrically. The oxidation of cycloheptanone by Os^VIII, continuously regenerated by Fe(CN)^3– ₆ in alkaline medium in the 0.00123–0.01 M range, is zeroth order with respect to Fe(CN)^3– ₆ and first order with respect to Os^VIII. A suitable mechanism, based on rate data analysis, is proposed.     

Oxygen uptake after electron transfer from amines, amino acids and ascorbic acid to triplet flavins in air-saturated aqueous solution

The photolysis of lumichrome, riboflavin, flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) was studied in air-saturated aqueous solution at room temperature in the presence of appropriate electron donors: ascorbic acid, aromatic amino acids or amines, e.g. ethylenediaminetetraacetate (EDTA). The overall reaction is conversion of oxygen via the hydroperoxyl/superoxide radical into hydrogen peroxide. The quantum yield of oxygen uptake increases with the donor concentration, e.g. up to 0.3 for riboflavin, FMN or FAD in the presence of EDTA or ascorbic acid (0.3-10mM). The formation of H(2)O(2) is initiated by quenching of the acceptor triplet state by the electron donor and subsequent reaction of the semiquinone radical with oxygen. Specific properties of flavins are discussed including the radicals involved and the pH and concentration dependences. The quantum yield of photodegradation is low under air, but substantial under argon, where the major product absorbing in the visible spectral range is the corresponding hydroquinone.     

Mechanism of proton transfer from 2-nitrohexafluoropropane to amines

According to the analysis of NMR kinetic data, the proton transfer from the CH-acid 2-nitrohexafluoropropane to trioctylamine occurs in a hydrogen-bonded complex. The value of the k_H/k_D ratio (6) and the linearity of the ln k vs. T^–1 dependence indicate a classical mechanism of the process.

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2- . . , . k_H/k_D 6 .