Voltammetric Determination of Thiourea at Conductive Diamond Electrodes

The oxidation of thiourea (TU) at boron‐doped diamond (BDD) electrodes was investigated by the use of anodic voltammetry. The results indicated that the overall TU oxidation reaction is rather complicated and takes place via two steps: a slow electron‐transfer yielding the corresponding free radical, followed by a fast oxidation of this radical, prior to its dimerization. It was found that in acidic media the voltammetric response is suitable for analytical applications, and unlike glassy carbon (GC), BDD electrodes exhibit very low susceptibility to adsorption. The same conclusion was supported by the results of AC voltammetric measurements. Based upon the voltammetric peak for the first step of TU oxidation, a method is proposed for the determination of this compound in the micromolar concentration range. The analytical performance characteristics of the method are comparable to those reported for TU determination by the use of platinum electrodes or enzyme‐modified platinum electrodes.     

Imidazolinone herbicides in strongly acidic media: Speciation and electroreduction

From UV-visible measurements and potentiometric titrations it follows that the lowest pK values (pK₁) of imidazolinone herbicides correspond to the simultaneous protonation/dissociation equilibria of both the pyridinic (or quinolinic) nitrogen and the carboxyl group, the following pK (pK₂) to the imminium nitrogen and the basic pK (pK₃) to the dissociation of the imido nitrogen. Below pH 6 and down to pH c.a. 2.5, the dominant form of the herbicide is a double ion having both positive and negative charges, this being important in discussing the effect of pH in the natural dynamics of imidazolinone herbicides, especially in their soil sorption. Electrochemical studies of the reduction of the herbicides were made on mercury and carbon electrodes in strongly acidic media (0.1 to 2.7 M H₂SO₄) as well up to pH 7. The reduction signals were all attributed to the reduction of the imidazolinone ring except the second peak/wave that was found to have originated by the reduction of the pyridine/quinoline ring. A signal observed in strongly acidic media and at highly negative overpotentials was attributed to the reduction of the imidazolinone ring of the product of the previous reduction in a process consisting of two reversible electron transfers followed by a protonation reaction.     

Reactions between technetium(VII) and some hydrazones and the spectrofluorimetric determination of technetium(VII) with 2,2′-dipyridylketone hydrazone

The reactions between pertechnetate and five hydrazones are described. Of these, the technetium(VII)/2,2′-dipyridylketone hydrazone system was found to be most sensitive, and is studied in detail. Spectrofluorimetric procedures for the determination of technetium(VII) over the range 0.01–12 mg l^?1 are reported. The reaction proceeds most favourably under acidic conditions (1.4 M hydrochloric acid). For 1 mg l^?1 technetium(VII), 100 mg l^?1 levels of U(VI), Re(VII), Mo(VI) or W(VI) do not interfere when the reaction proceeds at room temperature. Sensitivity improves at higher temperatures.     

A structural study of 4‐aminoantipyrine and six of its Schiff base derivatives

Six derivatives of 4‐amino‐1,5‐dimethyl‐2‐phenyl‐2,3‐dihydro‐1H‐pyrazol‐3‐one (4‐aminoantipyrine), C₁₁H₁₃N₃O, (I), have been synthesized and structurally characterized to investigate the changes in the observed hydrogen‐bonding motifs compared to the original 4‐aminoantipyrine. The derivatives were synthesized from the reactions of 4‐aminoantipyrine with various aldehyde‐, ketone‐ and ester‐containing molecules, producing (Z)‐methyl 3‐[(1,5‐dimethyl‐3‐oxo‐2‐phenyl‐2,3‐dihydro‐1H‐pyrazol‐4‐yl)amino]but‐2‐enoate, C₁₆H₁₉N₃O₃, (II), (Z)‐ethyl 3‐[(1,5‐dimethyl‐3‐oxo‐2‐phenyl‐2,3‐dihydro‐1H‐pyrazol‐4‐yl)amino]but‐2‐enoate, C₁₇H₂₁N₃O₃, (III), ethyl 2‐[(1,5‐dimethyl‐3‐oxo‐2‐phenyl‐2,3‐dihydro‐1H‐pyrazol‐4‐yl)amino]cyclohex‐1‐enecarboxylate, C₂₀H₂₅N₃O₃, (IV), (Z)‐ethyl 3‐[(1,5‐dimethyl‐3‐oxo‐2‐phenyl‐2,3‐dihydro‐1H‐pyrazol‐4‐yl)amino]‐3‐phenylacrylate, C₂₂H₂₃N₃O₃, (V), 2‐cyano‐N‐(1,5‐dimethyl‐3‐oxo‐2‐phenyl‐2,3‐dihydro‐1H‐pyrazol‐4‐yl)acetamide, C₁₄H₁₄N₄O₂, (VI), and (E)‐methyl 4‐{[(1,5‐dimethyl‐3‐oxo‐2‐phenyl‐2,3‐dihydro‐1H‐pyrazol‐4‐yl)amino]methyl}benzoate, C₂₀H₁₉N₃O₃, (VII). The asymmetric units of all these compounds have one molecule on a general position. The hydrogen bonding in (I) forms chains of molecules via intermolecular N—H...O hydrogen bonds around a crystallographic sixfold screw axis. In contrast, the formation of enamines for all derived compounds except (VII) favours the formation of a six‐membered intramolecular N—H...O hydrogen‐bonded ring in (II)–(V) and an intermolecular N—H...O hydrogen bond in (VI), whereas there is an intramolecular C—H...O hydrogen bond in the structure of imine (VII). All the reported compounds, except for (II), feature π–π interactions, while C—H...π interactions are observed in (II), C—H...O interactions are observed in (I), (III), (V) and (VI), and a C—O...π interaction is observed in (II).     

Synthesis and properties of derivatives of 2-mercaptobenzothiazole

2,3-Dihydro-4H-[1,3]-thlazino[2,3-b]benzothiazolium chlorides have been synthesized and their reduction with sodium borohydride has given previously-unknown derivatives of (perhydro-1,3-thiazino) [2,3-b]benzothiazoline. The stability of the compounds in acid and alkaline media has been studied.For part VII, see [1].     

Heterocyclic Thiones and Their Analogs in Reactions of 1,3-Dipolar Cycloaddition: III. Reaction of Benzothiazole-2-thione with a Double Excess of Nitrilimine

Main products of reaction between benzothiazole-2-thione and a double excess of nitrilimine are substituted 1-aryl-1-(1,3-benzothiazol-2-yl)-2-(2,3-dihydro-1,3,4-thiadiazol-2-yl)hydrazines. In the first step presumably forms a spiro compound via 1,3-dipolar cycloaddition of nitrilimine molecule to the C=S bond of the benzothiazolethione. The second nitrilimine molecule adds to the new C=S bond of the intermediate product arising by the rupture of CS bond in the thiadiazole ring of the spiro compound.     

Electrochemical studies of potassium ferrate(VI)

The electrochemical reduction of potassium ferrate(VI) in 6N-KOH occurs in three steps, at approximately +0.3, ?0.7, and ?1.4 V vs. Hg/HgO, 6N-KOH. The first reduction appears as a voltammetric wave on a platinum electrode and the second two are observed as polarographic waves with a dropping mercury electrode. Controlled potential coulometric, voltammetric, and chronopotentiometric studies indicate the reduction proceeds stepwise first to iron(III), then to iron(II) and finally to iron(0).     

Synthesis and NMR characterization of 6‐Phenyl‐6‐deoxy‐2,3‐di‐O‐methylcellulose

Cellulose ( 1 ) was converted for the first time to 6‐phenyl‐6‐deoxy‐2,3‐di‐O‐methylcellulose ( 6 ) in 33% overall yield. Intermediates in the five‐step conversion of 1 to­ 6 were: 6‐O‐tritylcellulose ( 2 ), 6‐O‐trityl‐2,3‐di‐O‐methylcellulose ( 3 ), 2,3‐di‐O‐methylcellulose ( 4 ); and 6‐bromo‐6‐deoxy‐2,3‐di‐O‐methylcellulose ( 5 ). Elemental and quantitative carbon‐13 analyses were concurrently used to verify and confirm the degrees of substitution in each new polymer. Gel permeation chromotography (GPC) data were generated to monitor the changes in molecular weight (DP_w) as the synthesis progressed, and the compound average decrease in cellulose DP_w was ～ 27%. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were used to characterize the decomposition of all polymers. The degradation temperatures ( °C) and percent char at 500 °C of cellulose derivatives 2 to 6 were 308.6 and 6.3%, 227.6 °C and 9.7%, 273.9 °C and 30.2%, 200.4 °C and 25.6%, and 207.2 °C and 27.0%, respectively. The glass transition temperature (T_g) of­6‐O‐tritylcellulose by dynamic mechanical analysis (DMA) occurred at 126.7 °C and the modulus (E′, Pa) dropped 8.9 fold in the transition from ?150 °C to + 180 °C (6.6 × 10⁹ to 7.4 × 10⁸ Pa). Modulus at 20 °C was 3.26 × 10⁹ Pa. Complete proton and carbon‐13 chemical shift assignments of the repeating unit of the title polymer were made by a combination of the HMQC and COSY NMR methods. Ultimate non‐destructive proof of carbon–carbon bond formation at C6 of the anhydroglucose moiety was established by generating correlations between resonances of CH₂6 (anhydroglucose) and C1′, H2′, and H6′ of the attached aryl ring using the heteronuclear multiple‐bond correlation (HMBC) method. In this study, we achieved three major objectives: (a) new methodologies for the chemical modification of cellulose were developed; (b) new cellulose derivatives were designed, prepared and characterized; (c) unequivocal structural proof for carbon–carbon bond formation with cellulose was derived non‐destructively by use of one‐ and two‐dimensional NMR methods. Copyright © 2002 John Wiley & Sons, Ltd.     

Synthesis and Spectroscopic Characterisation of 2,3‐Epoxy/ 3‐Chloro‐2‐hydroxy Propyl Derivatives of Quaternary Ammonium Salts:Useful Cationic Fixing Agents

Eight mono-reactive and four bis-reactive 2,3-epoxy/3-chloro-2-hydroxy propyl derivatives of quaternary ammonium salts have been synthesised by reacting epichlorohydrin with suitable tertiary amines and diamines. Mono-reactive derivatives also contain cyano and carboxylic groups in addition to 2,3-epoxy/3-chloro-2-hydroxy propyl groups. The structures of these quaternary ammonium salts have been characterised using IR and ¹H NMR spectroscopic techniques. These quaternary ammonium chlorides have been applied to the cotton fabrics as pretreatments and aftertreatments and promising results have been obtained.     

Heterolytic cleavage of peroxide by a diferrous compound generates metal-based intermediates identical to those observed with reactions utilizing oxygen-atom-donor molecules

Under cryogenic stopped-flow conditions, addition of 2-methyl-1-phenylprop-2-yl hydroperoxide (MPPH) to the diiron(II) compound, [Fe(2)(H(2)Hbamb)(2)(NMeIm)(2)] (1; NMeIm=N-methylimidazole; H(4)HBamb: 2,3-bis(2-hydroxybenzamido)dimethylbutane) results in heterolytic peroxide O-O bond cleavage, forming a high-valent species, 2. The UV/Vis spectrum of 2 and its kinetic behavior suggest parallel reactivity to that seen in the reaction of 1 with oxygen-atom-donor (OAD) molecules, which has been reported previously. Like the interaction with OAD molecules, the reaction of 1 with MPPH proceeds through a three step process, assigned to oxygen-atom transfer to the iron center to form a high-valent intermediate (2), ligand rearrangement of the metal complex, and, finally, decay to a diferric mu-oxo compound. Careful examination of the order of the reaction with MPPH reveals saturation behavior. This, coupled with the anomalous non-Arrhenius behavior of the first step of the reaction, indicates that there is a preequilibrium peroxide binding step prior to O-O bond cleavage. At higher temperatures, the addition of the base, proton sponge, results in a marked decrease in the rate of O-O bond cleavage to form 2; this is assigned as a peroxide deprotonation effect, indicating that the presence of protons is an important factor in the heterolytic cleavage of peroxide. This phenomenon has been observed in other iron-containing enzymes, the catalytic cycles of which include peroxide O-O bond cleavage.     

Derivation of Oridonin with Bioreduction‐Responsive Disulfide Bond

A novel oridonin derivative substituted with 3‐((2,3‐dihydroxypropyl)dithio)‐propionyl group at 14‐O position was synthesized through a two‐step procedure: Esterification of oridonin with 3‐(2‐pyridyldithio)propanoic acid and the subsequent thiol‐disul?de exchange reaction with 3‐mercapto‐1,2‐propanediol. The bioreduction‐responsive disulfide bond in the compound leads to high leaving ability of the 14‐O acyl moiety when treated with glutathione as monitored by reverse phase high‐performance liquid chromatography.     

Electrochemical Study of 4‐Substituted Analogues of Megazol

The electrochemical behavior of three different megazol analogues substituted at position 4 and their comparison with the parent compound megazol in protic and aprotic media by cyclic voltammetry, Tast and differential pulse polarography was studied. All the compounds were electrochemically reducible in both media with the reduction of the nitroimidazole group the main voltammetric signal. The one‐electron reduction couple due to the nitro radical anion formation was visualized only in aprotic media for all these compounds. By applying cyclic voltammetric methodology we have calculated the dimerization reaction decay constants (k₂) of the corresponding nitro radical anions in aprotic media. The nitro radical anion obtained from the synthesized nitroimidazole compound having a bromine substituent in 4‐position (GC‐141) was significantly more stable than the corresponding radical formed from the compound lacking of the substituent in 4‐position, megazol.     

Determination of Ag(I) with chemically modified carbon paste electrode based on 2,3-dicyano 1,4-naphthoquinone

The utility of carbon paste modified with 2,3-dicyano 1,4-naphthoquinone (CYNQ) for the voltammetric determination of Ag(I) is demonstrated. The method is based on the formation of Ag(I) complex with CYNQ by accumulation from the aqueous solution to the electrode surface without an applied potential. Using the medium exchange technique, the electrode was transferred to another media followed by linear-scan voltammetric measurements. The reduction peak of the Ag(I)-CYNQ complex was observed at +0.22 V (vs. SCE). The silver response is studied with respect to paste composition, pH of the measurement solution, reproducibility, interference and other variables. A detection limit of 5 × 10⁻⁸M was obtained and the calibration curve was linear over the range 1 × 10⁻⁶−8 × 10⁻⁵M.     

Voltammetric study of the redox behaviour of the Hg(II)/Hg(I)/Hg system at a rotating metal-ring/glassy-carbon disc electrode

The reduction of Hg(II) at a glassy-carbon electrode in various electrolytes has been studied by rotating ring-disc voltammetry. Reduction proceeds directly to metallic mercury in a single 2-electron step. However, at the foot of the wave, and only during the first reduction sweep after pretreatment of the electrode surface, a small amount of Hg(I) species is detected at the ring. The appearance of an Hg(I) intermediate is most pronounced in sulphuric acid solution. The reduction of Hg(II) is found to proceed irreversibly and to be of first order. At sufficiently negative potentials the reduction is convective-diffusion controlled. Stripping voltammetric experiments indicate that the dissolution of mercury gives Hg(II) in complexing electrolytes. In non-complexing electrolytes the initially formed Hg(II) reacts with mercury atoms on the electrode surface to give Hg(I). During electrodissolution, two stripping peaks may be observed as a result of underpotential adsorption of mercury on glassy carbon. The difference in peak potential between the adsorption (mono) layer peak and the bulk mercury peak has been related to the difference in work functions of the deposit (mercury) and substrate (carbon). A rotating glassy-carbon electrode has been used for the anodic stripping determination of mercury. When an appropriate amount of a cation such as cadmium(II) or copper(II) is added to the test solution, mercury down to 2 x 10(-9)M (0.4 ng ml ) can be determined in acidified thiocyanate electrolyte with a relative standard deviation of about 22%.     

Microwave‐Assisted Aqueous Synthesis: A Rapid Approach to Poly‐functionalized Indoline‐Spiro Benzofurodiazepines

New cycloaddition has been established for the synthesis of indoline‐spiro benzofurodiazepine derivatives. Those reactions were conducted by reacting readily available and inexpensive starting materials, such as benzene‐1,2‐diamines, tetronic acid and indoline‐2,3‐diones, in aqueous solution under microwave irradiation. When mono‐substituted benzene‐1,2‐diamines as an amine component was employed, the reaction regioselectively resulted in the poly‐functionalized indoline‐spiro benzofurodiazepine with good yields. The present green synthesis shows attractive characteristics such as the use of water as reaction media, concise one‐pot conditions, short reaction periods, easy work‐up/purification and reduced waste production without the use of any strong acids or metal promoters.     

Free‐radical polymerization of dioxolane and dioxane derivatives: Effect of fluorine substituents on the ring opening polymerization

Partially fluorinated and perfluorinated dioxolane and dioxane derivatives have been prepared to investigate the effect of fluorine substituents on their free‐radical polymerization products. The partially fluorinated monomer 2‐difluoromethylene‐1,3‐dioxolane (I) was readily polymerized with free‐radical initiators azobisisobutyronitrile or tri(n‐butyl)borane–air and yielded a vinyl addition product. However, the hydrocarbon analogue, 2‐methylene‐1,3‐dioxolane (II), produced as much as 50% ring opening product at 60 °C by free‐radical polymerization. 2‐Difluoromethylene‐4‐methyl‐1,3‐dioxolane (III) was synthesized and its free‐radical polymerization yielded ring opening products: 28% at 60 °C, decreasing to 7 and 4% at 0 °C and −78 °C, respectively. All the fluorine‐substituted, perfluoro‐2‐methylene‐4‐methyl‐1,3‐dioxolane (IV) produced only a vinyl addition product with perfluorobenzoylperoxide as an initiator. The six‐membered ring monomer, 2‐methylene‐1,3‐dioxane (V), caused more than 50% ring opening during free‐radical polymerization. However, the partially fluorinated analogue, 2‐difluoromethylene‐1,3‐dioxane (VI), produced only 22% ring opening product with free‐radical polymerization and the perfluorinated compound, perfluoro‐2‐methylene‐1,3‐dioxane (VII), yielded only the vinyl addition polymer. The ring opening reaction and the vinyl addition steps during the free‐radical polymerization of these monomers are competitive reactions. We discuss the reaction mechanism of the ring opening and vinyl addition polymerizations of these partially fluorinated and perfluorinated dioxolane and dioxane derivatives. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5180–5188, 2004     

Application of ortho-chloro-β-aroylthioamides in synthesis(II): an efficient one-pot, three-component synthesis of tricyclic thiochromeno[2,3-b]pyridine derivatives

Tricyclic thiochromeno[2,3-b]pyridine derivatives have been successfully synthesized in an unusual one-pot multicomponent cascade reaction from ortho-halo-β-aroylthioamides, Meldrum's acid, and aromatic aldehydes. The reaction presumably proceeds via Knoevenagel condensation-Michael addition-cyclocondensation-decarboxylation-rearrangement-intramolecular S_NAr reaction sequence. High bond forming efficiency of this reaction makes it attractive for the synthesis of thiochromeno[2,3-b]pyridine derivatives in a single step operation.     

Polarographic and Voltammetric Assays of Sulfonamides as α‐Oxo‐γ‐Butyrolactone Arylhydrazones

Abstract

2‐Acetylbutyrolactone was characterized as a novel reagent of analytical potential in polarographic and voltammetric analyses. It forms α‐oxo‐γ‐butyrolactone arylhydrazones through Japp‐Klingemann coupling reaction with primary arylamines. α‐Oxo‐γ‐butyrolactone arylhydrazones possess an electro‐active site (azomethine center) that displays a cathodic activity at the mercury electrode. The protonated azomethine center of α‐oxo‐γ‐butyrolactone arylhydrazones is reduced by 2e/2H⁺ reaction to the hydrazo form. The differential pulse polarographic behavior of α‐oxo‐γ‐butyrolactone arylhydrazones was investigated in aqueous media ranging from pH 2 to 10.5. In aqueous acidic solution, α‐oxo‐γ‐butyrolactone arylhydrazones were shown to adsorb on a hanging mercury drop electrode and to be amenable to determination by adsorptive stripping voltammetry. Procedures for applying the polarographic and voltammetric methods to determination of sulfadiazine and sulfamethoxazole in pharmaceutical preparations have been developed. An analogous study on sulfas‐azo derivatives of ethyl acetoacetate was also considered. Furthermore, the differential pulse voltammetric method was adopted for determination of sulfamethoxazole in spiked plasma and urine samples. The recoveries turned out to be satisfactory, showing relative standard deviations from 2.4 to 4.6%.     

The Reduction of 2,6‐Dimethoxy‐4‐chloro‐1,3,5‐triazine on Mercury Electrodes in Aqueous Solutions in Relation with the Reduction of s‐Triazine Herbicides

This paper presents polarographic and voltammetric studies on the electroreduction of 2,6‐dimethoxy‐4‐chloro‐1,3,5‐triazine on mercury electrodes in aqueous solutions and in the acidity range 2 M H₂SO₄ to pH 7. Above pH 6.5 no signals were obtained. In both DC and DP polarography and voltammetry, one or two partially overlapped reduction waves were observed, depending on the pH of the medium. The overall process corresponded to a four‐electron irreversible reduction. In strongly acidic media (pH<4.5) the protonated reactant was reduced through a two‐electron process to give a dechlorinated molecule that is subsequentely reduced at the potentials of the second wave. It was concluded that the reduction of the chlorinated s‐triazine rings seems to occur through the reductive cleavage of the chlorine irrespective of the nature of the groups bonded to the ring in positions 2, 6.     

Rhodium‐Catalyzed C?H Annulation of Nitrones with Alkynes: A Regiospecific Route to Unsymmetrical 2,3‐Diaryl‐Substituted Indoles

The direct C H annulation of anilines or related compounds with internal alkynes provides straightforward access to 2,3‐disubstituted indole products. However, this transformation proceeds with poor regioselectivity in the synthesis of unsymmetrically 2,3‐diaryl substituted indoles. Herein, we report the rhodium(III)‐catalyzed C H annulation of nitrones with symmetrical diaryl alkynes as an alternative method to prepare 2,3‐diaryl‐substituted N‐unprotected indoles with two different aryl groups. One of the aryl substituents is derived from NC‐aryl ring of the nitrone and the other from the alkyne substrate, thus providing the indole products with exclusive regioselectivity.