Homogeneous catalysis of manganese(II) in the oxidation of citric acid by bromate: A novel kinetic behaviour of citric acid

407-415The kinetic and mechanistic studies on Mn(II)-catalysed oxidation of citric acid by acid bromate in presence of Hg(II) as a scavenger have been carried out under varying conditions. The reaction exhibits 1.5 order each in [bromate] and [acid], and less than unit order in [Mn(II)]. The kinetic behaviour of citric acid is novel, being 0.30 order in the lower [citric acid] range and tending to –0.25 order at higher [citric acid]. Hg(OAc)₂ and D₂O have insignificant effect on the reaction rate. Formal redox potential for the Mn(II)/Mn(III)–citric acid couple is determined as 0.32 V (vs SCE) and 0.58 V (vs NHE). These findings are accounted quantitatively by the formation of 1:1 (MnCt) and 1:2 (MnCt₂) manganese(II)-citric acid complexes of which only 1:1 (MnCt) complex is kinetically active towards the bromate oxidation. In the envisaged mechanism, acid bromate reacts with the 1:1 (MnCt) complex, whose formation is precedent to the Mn(II)-Br(V) reaction. It results in an intermediate complex, which subsequently decomposes to the products. At higher [citric acid], the ratio of 1:2 and 1:1 Mn(II)-citric acid complexes reached 1.98. The effects of [pyrophosphate], [LiF], dielectric constant of the media and ionic strength are also in accordance with the proposed mechanism. Reaction constants involved in the proposed mechanism have been evaluated and thermodynamic parameters computed

Homogeneous catalysis of manganese(II) in the acid bromate oxidation of malonic acid in the presence of bromocomplexing metal ions: Unusual kinetic behaviour of malonic acid

1737-1746Manganese(II)-catalysed pure bromate (unmixed with Br₂) oxidation of malonic acid (MA) in aqueous acetic acid media containing sulphuric acid and Hg(II)/Tl(III), bromocomplexing metal ion, is free from induction period and exhibits 1.5 order in [bromate] and 0.75 order in [Mn(II)]. The reaction rate increases with increasing [H₂SO₄] and the rate constant follows the ho function. The kinetic behaviour of MA is unusual, being 0.15 order in the lower [MA] range and tending to 0.32 order at higher [MA]. Redox potential for the Mn(II)/Mn(III)-malonic acid couple is determined as 0.89 V (vs SCE) and 1.15 V (vs NHE). These findings are accounted quantitatively and the catalytic effect of Mn(II) ion is displayed by its complex forming ability. The proposed mechanism involves oxidation of the formed Mn(II)-malonic acid binary complex to an intermediate by acid bromate which subsequently decomposes to the products. Reaction constants involved in the proposed mechanism have been evaluated and discussed

Kinetics and mechanism of acid bromate oxidation of substituted 4-oxo acids

418-424Kinetics and mechanism of oxidation of substituted 4-oxo-4-arylbutanoic acids (4-oxo acids) by acid bromate (uncontaminated with Br2) in aqueous acetic acid medium has been studied. The reaction exhibits first order each in [bromate] and [4-oxo acid] and second order in [acid]. Variation in ionic strength has no effect on the reaction rate, while the reaction rates are enhanced on lowering the dielectric constant of the reaction medium. Changing the solvent from H2O to D2O increases the rate of oxidation (kD2O/kH2O ~1.8). Electron releasing substituents in the aromatic ring accelerate the reaction rate and electron withdrawing substituents retard it. The order of reactivity among the studied 4-oxo acids is: p-methoxy>> p-methyl>-H> p-chloro> m-nitro. The Hammett’s plot of log k versus σ is a smooth curve and concaves downward. However, the plot of log k against exalted sigma (Brown’s σ+) values is found to be linear with a slope of –1.50 at 313 K. The reaction constant (ρ+) decreases with increase in temperature. The mechanism proposed involves the attack of the protonated bromate (H2Br+O3) on the enol form of the 4-oxo acid giving the carbocationic bromate ester in the slow step, which undergoes cyclization and then on carbon-carbon bond cleavage yields the products in the fast step. From the intersection of lines in the Hammett and Arrhenius plots, the isokinetic relationship is discussed

Mechanistic investigations on manganese(II) catalyzed oxidation of substituted 4-oxo-4-arylbutanoic acids by bromate in acid medium

979-990The manganese(II)-catalyzed oxidation of substituted 4-oxo-4-arylbutanoic acids (4-oxo acids) by bromate, in aqueous acetic acid medium containing sulphuric acid, has been studied. The reaction is first order each in [bromate] and [acid], less than unity order in both [substrate] and [catalyst], and also shows solvent isotope effect of 0.60 (kc′ (H2O)/kc′ (D2O)). Oxidation rate is not much affected by electron withdrawing substituents, although it is susceptible to electron demand at the seat of the reaction. The linear free-energy relationship is characterized by smooth curves in Hammett plots of log k versus σ. However, the plots are linear with excellent correlation coefficient at all the studied temperatures when Brown’s σ+ values are used. The reaction constant is negative and increases with decrease in temperature. The mechanism involving decomposition of the complex, formed by participation of the neighboring group of 4-oxo acid and intramolecular catalysis of Mn(II) is proposed, wherein, both the C–C bond-breaking and C–O bond-making in the oxidation are involved. However, these two processes are influenced in different ways by the electron withdrawing and electron releasing substituents. A π-complex formation between Mn(II) and -ene of the enol form of oxo acid is proposed

Applications of zirconium(IV) chloride in organic synthesis

Zirconium(IV) chloride is used extensively in organic synthesis as an ideal Lewis acid since it is an efficient, stable, inexpensive, environmentally friendly and convenient catalyst for the preparation of useful synthetic intermediates and for use in key steps of natural products synthesis. This review is broadly divided into four sections based on zirconium(IV) chloride mediated reactions (1) carbon-carbon bond-formation reactions, (2) protection and deprotection chemistry, (3) reduction reactions, and (4) other applications. This review is an attempt to cover the important advances in the field of synthetic organic chemistry that have been carried out to date

Synthesis, biological evaluation and docking studies of a new series of tris-heterocycles containing pyrazole, triazole and oxadiazole

687-699A new series of 3-aryl/hetaryl-6-(5-methyl-1-phenyl-1H-pyrazol-4-yl)-[1,2,4]triazolo[3,4-b][1,3,4]oxadiazoles 7a-j have been synthesized, characterized by spectral means, and evaluated for their antimicrobial and antioxidant properties. Amongst the screened compounds 7a-j, the triazole moiety bearing electron-withdrawing group on the 4th position of phenyl viz. 4-chlorophenyl 7c, 4-nitrophenyl 7f and 2,4-difluorophenyl 7h exhibit excellent antibacterial activity towards all the tested bacteria viz. Staphylococcus aureus, Bacillus licheniformis, Pseudomonas aeruginosa and Escherichia coli. On the other hand, triazole moiety bearing electron-donating group on the 4th position of phenyl viz. 4-hydroxyphenyl 7d, 4-methoxy-phenyl 7e and 4-aminophenyl 7g show prominent antifungal activity towards the tested fungi viz. Candida albicans and Fusarium oxysporum. These results are supported by molecular docking studies and through binding interactions as well. Antioxidant studies reveal that the compounds in which the triazole moiety bearing phenyl 7a, 4-methoxyphenyl 7e, 2,4-difluorophenyl 7h and 3-pyridyl 7j is present on the 4th position, display significant antioxidant properties. In brief, most of the newly synthesized compounds have emerged as potential antimicrobial and antioxidant agents for further development

Synthesis of novel bis-(1,3,4-oxadiazol-2-ylamino)-2-aryl-1,3-thiazolan-4-ones as antimicrobial, nematicidal and anticancer agents

700-714A series of novel bis-heterocycles, 3-(5-[2-methoxy-5-(4-methoxy-3-5-[(4-oxo-2-(aryl/hetaryl) -1,3-thiazolan -3-yl)amino]-1,3,4-oxadiazol-2-ylbenzyl)phenyl]-1,3,4-oxadiazol-2-ylamino)-2-(aryl/hetaryl)-1,3-thiazolan-4-ones 8a-j have been synthesized by simple and high yield routes. Structures of all the newly synthesized compounds have been established by their spectral data and elemental analysis, and they have been evaluated for their in vitro antimicrobial, nematicidal and anticancer activities. Among the tested compounds, those with 4-chlorophenyl (8c), 3-nitrophenyl (8d) and 2,4-difluorophenyl (8f) substituents at 2-position of thiazolidinone ring are found to be potent antibacterial and antifungal agents. Exploration of nematicidal activity of compounds 8a-j reveal that, compounds 8f, 8i and 8j bearing 2,4-difluorophenyl, 2-furyl and 1,3-benzodioxole substituents respectively at 2nd position of thiazolidinone ring show prominent activity comparable to the standard levamisole and have emerged as potential nematicidal agents. In vitro anticancer activity assay reveals that the compounds with electron-withdrawing groups viz., 4-chlorophenyl (8c), 3-nitrophenyl (8d) and 2,4-difluorophenyl (8f) display significant activity which is more than the standard against A549 (lung cancer) cell line, and compounds bearing electron-donating groups such as 4-methylphenyl (8b), 4-hydroxyphenyl (8e), N,N-dimethylaminophenyl (8g), and 4-methoxy-3-hydroxyphenyl (8h) show superior/comparable potency to the standard against MDAMB-231 (breast cancer) cell line. The docking scores estimated by Schrödinger software also correlate well with the experimental anticancer activity

Synthesis, biological evaluation and QSAR studies of a novel series of annelated triazolo[4,3-c]quinazolines

898-911A series of novel annelated triazolo[4,3-c]quinazolines 6a-m have been synthesized and characterized by physicochemical as well as spectral means. The bioactivity assay for the newly synthesized compounds against Gram-positive and Gram-negative bacteria, and fungi indicates that most new compounds exhibit good antibacterial and antifungal activities in comparison with standard drugs. 2-[(4-Methoxyanilino)methyl]-5-phenyl-2,3-dihydro[1,2,4] triazolo[4,3-c]quinazoline-3-thione 6f, and 2-[(4-aminoanilino)methyl]-5-phenyl-2,3-dihydro[1,2,4]triazolo[4,3-c] quinazoline-3-thione 6m have emerged as the most potent antimicrobial agents with activity (pMICam = 2.26 and 2.38 µM/mL respectively) almost equal to the reference drugs streptomycin, penicillin and amphotericin B. The results of QSAR studies demonstrate that antibacterial, antifungal and over all antimicrobial activities of synthesized quinazoline derivatives are governed by electronic parameter, hydrophobicity (log P) and chemical potential (µ). The validity of Hammett’s linear free energy relationship for the first time has been verified, from the linear plot of log activity against substituent constant (σ). The observed low residual activity and high cross validated q2 values indicate the predictive ability of the developed models, which further simplify the design and synthesis of new biologically potent molecules