Mechanism of positive iodine reactions: Kinetics of oxidation of semicarbazide by iodamine-T, iodine monochloride and iodine

Kinetics of oxidation of semicarbazide (SC) by iodamine-T (IAT), iodine monochloride and aqueous iodine has been studied in aqueous perchloric acid medium. The rate laws followed by the oxidation of SC were determined. The rates decreased slightly with increase in ionic strength of the medium in IAT and ICI oxidations, while the reverse trend was observed with I₂. Decrease in dielectric constant of the medium increased the rates with IAT and ICI, while it decreased the rate in I₂ oxidations. Addition of the reduced product,p-toluene-sulphonamide had no effect on the rate with IAT. Addition of I⁻ had slight negative and positive effects on the rates of oxidations with IAT and ICI, respectively, but the negative effect was considerable in I₂ oxidations. Mechanisms consistent with the observed rate laws have been proposed and discussed. Rate determining steps have been identified and their coefficients calculated. These constants were used to predict the rate constants from the deduced rate laws as [SC], [H⁺] and [I⁻] varied. Reasonable agreement between the calculated constants and experimental values provide support for the suggested mechanisms.     

Substituent effects in addition of iodine thiocyanate to alkenes

The plots of logarithms of relative rates of ISCN addition to alkenes versus alkene IPs and versus alkene HOMO energies reveal that the alkene relative reactivity depends upon both electronic and steric effects of the substituents. Steric effects are related not only to the degree of substitution on the CC bond but also to the relative position, size, and branching of alkyl substituents.     

A density functional study on the formation of charge transfer complexes between alkaloids and iodine monochloride

The formation of charge-transfer (CT) complexes between N-methyl-6,7-methylenedioxy-tetrahydro-4-isoquinolone (MMDTIQ, the model molecule of alkaloids) and iodine monochloride has been studied with the density functional B3LYP method. The competition for ICl between n-donor(s) and π-donor in the same molecule has been compared for the first time. The results indicate that the electron-donating ability of various donor sites decreases in the following order: amino nitrogen>carbonyl oxygen>etheric oxygen≈aromatic ring. Complexes with a stoichiometry of 1:1 to 1:5 (MMDTIQ–ICl) might form. Among complexes with a given stoichiometry, the one in which the phenyl ring is involved in the binding is always the least stable. According to these results, in order to ensure an alkaloid iodinated, the amount of ICl should be far beyond that of alkaloids and a low polar solvent should be used, which is quite in agreement with our recent experimental results.     

Stereoelectronic and solvent effects on the allylic oxyfunctionalization of alkenes with singlet oxygen

The factors that control the stereochemistry of sensitized photooxygenation of alkenes via singlet oxygen (ene reaction) are selectively reported. We also introduce our most recent stereoelectronic effects on the singlet oxygen-ene reaction. The origin of site selectivity and solvent-dependent stereoselectivity in this classical ene reaction with simple as well as functionalized alkenes is highlighted. These studies and other similar studies have enhanced substantially the utility of singlet oxygen in the synthesis of natural and non-natural products.     

Direct iodination of 3- and 4-nitropyrazoles with a reagent based on iodine monochloride and silver sulfate

The reagent obtained from iodine monochloride and silver sulfate in sulfuric acid easily iodinates I-methyl-3-nitropyrazole under mild conditions to give 4-iodo or 4,5-diiodo derivatives, 1-Methyl-4-nitropyrazole was also directly iodinated with this reagent for the first time. Published inIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1482–1484, August, 2000.     

Kinetics and mechanism of base catalysed ethyl cyanoformate addition to aldehydes

The mechanism by which tertiary amines catalyse the formation of cyanohydrin carbonates from aldehydes and alkyl cyanoformates is investigated by means of a kinetic study. The reaction rate shows a second order dependence on amine concentration unless the amine is sterically hindered, when the rate has a first order dependence on amine concentration. The catalytic activity of the amine correlated with its pK_aH. On the basis of these results, a mechanism is proposed in which the amine acts as a base to activate a water molecule, which reacts with the ethyl cyanoformate generating cyanide in situ.     

Kinetics of the periodate oxidation of octacyanomolybdate(IV) in ethanol-water solvent mixtures

The kinetics of the oxidation of Mo(CN) ₈ ^4– by IO ₄ ^– has been studied in ethanol—water solvent mixtures over a temperature range of 15–35 °C. The effect of solvent composition on the reaction rate and the mechanism has been investigated. Activation parameters are given. An inner-sphere mechanism, consistent with the kinetic results, is proposed.

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Kinetik der Periodat-Oxidation von Octacyanomolybdat(IV) in Ethanol-Wasser

Zusammenfassung Die Kinetik der Oxidation von Mo(CN) ₈ ^4– mit IO_4/– wurde in Ethanol—Wasser über einen Temperaturbereich von 15–35 °C untersucht. Der Effekt der Lösungsmittelzusammensetzung auf die Reaktionsgeschwindigkeit und der Mechanismus der Reaktion werden diskutiert. Die Aktivierungsparameter sind angeführt. Es wird ein Inner-Sphere-Mechanismus vorgeschlagen, der mit den kinetischen Ergebnissen konsistent ist.

     

INDO-SCPT calculations of the solvent dependence of some carbon-proton spin-spin coupling constants

Summary A theoretical study of the effects of non-specific solute-solvent interactions on the directly bonded carbon-proton couplings¹ J(C–H), in acetaldehyde, acetone and acrylonitrile is presented. The solvent induced changes in these couplings calculated by an INDO-SCPT procedure incorporating a modified solvaton model are in satisfactory agreement with experiment and show significant improvement over the INDO-FPT calculations using the Klopman-Germer solvaton model.

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INDO-SCPT-Rechnungen der Lösungsmittelabhängigkeit einiger C-H-Kopplungskonstanten

Zusammenfassung Es wird eine theoretische Studie zu den nichtspezifischen Lösungsmittelwechselwirkungen auf die Spin-Spin-Kopplungen direkt aneinander gebundener C- und H-Atome von Acetaldehyd, Aceton und Acrylnitril vorgestellt. Die vom Solvens induzierten Änderungen der Kopplungskonstanten werden von der INDO-SCPT-Methode unter Verwendung eines modifizierten Solvationsmodells zufriedenstellend wiedergegeben. Die Übereinstimmung mit den experimentellen Daten ist besser als INDO-FPT-Rechnungen mit dem Klopmann-Germer-Solvaton-Modell.

     

反相HPLC中溶剂极性对容量因子的影响

考查了反相HPLC中,溶剂极性改变对容量因子的影响及其相关规律。结果发现:其影响是复杂的,不同溶质对不同溶剂极性改变的敏感程度差异较大。反映在容量因子的变化上就是:有时极性改变较大,但某些溶质的容量因子变化却较小;反之有时极性改变较小,但某些溶质的容量因子变化却较大。随着溶剂极性的改变,原来容量因子小的溶质变化幅度小,原来容量因子大的溶质变化幅度大。最后探讨了产生这种复杂变化的机理。     

On the theory of solvent effects

A simple electrostatic analysis is given of the virtual charge (solvaton) model to represent the environment effect on the electronic wave function of a solute immersed in a polarizable surrounding. New features of this model are found. The classical aspects are discussed and secondly the quantal implications are considered. A correct Hartree-Fock-like operator is derived which represents an electron in a molecular orbital subjected to the average effect of the other electrons and to the reaction field produced by the virtual charges on the atomic centers.A general formalism based on the preceding model is presented in appendix. The final equations have a form similar to Newton's equation to represent a solvated electron. Unlike some other theories in this field, there is no cut-off involved in the evaluation of the molecular integrals.     

Interaction of benzene-1,3-disulfonylamid-kriptofix[22] with iodine in chloroform and dichloromethane solutions

The interaction of iodine as an cr-acceptor with a synthetic ligand,benzene-1,3-disulfonylamid-kriptofix[22]（BDSAK） as a π-donor has been investigated spectrophotometrically in chloroform（CHCl3） and dichloromethane（DCM） solutions.The results of mole ratio plots and Job＇s method show the stoichiometry of complexation I2/BDSAK is 1：1.Stability constants have been calculated in various temperatures and thermodynamic parameters have also been determined from the temperatures dependence of the stability constants by using van＇t Hoff equation.The results indicate the iodine complex with BDSAK is enthalpy stabilized but entropy destabilized.     

Comparison of the electrophilic and free-radical addition of halogens with hexafluoro-1,3-butadiene and 1,3-butadiene

Ionic and photochemical reaction of chlorine (Cl₂), bromine (Br₂) and iodine monochloride (ICl) to hexafluoro-1,3-butadiene (1) and 1,3-butadiene (2) were carried out under conditions that would provide product distributions under controlled ionic or free-radical conditions. Product distributions for ionic reaction of Cl₂ and Br₂ with 1 are similar and suggest a weakly-bridged halonium ion species. Theoretical calculations support weakly-bridged chloronium and bromonium ions for both dienes 1 and 2. There are more of the 1,4-dihalo-2-butene products from ionic halogenation of 1 than 2 which correlates with the greater charge density on carbon-4 of halonium ions from 1. Ionic and free-radical reactions of ICl with 1 give 8 and 2% of 3-chloro-4-iodohexafluoro-1-butene and 4-chloro-3-iodohexafluoro-1-butene, respectively. The minor cis-1,4-dihalo-2-butene products from 1 and 2 are reported when formed.     

Solvent polarity studies. Part I. NewZ values and relationships with other solvent polarity scales

Z values are reported for 41 solvents, thereby almost doubling present knowledge. Linear correlations amongZ, Y, logk _ion, _B, , andF solvent polarity scales have been examined statistically. Correlation coefficients are greater than 0.95, and it is shown that solvent polarity values may be predicted to within a few percent. TheZ value for water is reexamined and a new lower value of 91.8 kcal-mole^–1 is advocated.     

Effect of solvent polarity on N-H insertion versus rearrangement of alkylidene carbenes

Alkylidene carbenes, when generated from o-aminobenzophenones and lithiated trimethylsilyldiazomethane, give a mixture of alkyne (by rearrangement) and indole (by N-H insertion). It has been found that this ratio of indole to acetylene shows a linear dependence on the polarity of the ethereal solvent.     

Oxidation of substituted pyridines by dimethyldioxirane: kinetics and solvent effects

The oxidation of a series of substituted pyridines by dimethyldioxirane (1) produced the expected N-oxides in quantitative yields. The second order rate constants (k₂) for the oxidation of a series of substituted pyridines (2a-g) by dimethyldioxirane were determined in dried acetone at 23 °C. An excellent correlation with Hammett sigma values was found (ρ = −2.91, r = 0.995). Kinetic studies for the oxidation of 4-trifluoromethylpyridine by 1 were carried out in the following dried solvent systems: acetone (k₂ = 0.017 M⁻¹ s⁻¹), carbon tetrachloride/acetone (7:3; k₂ = 0.014 M⁻¹ s⁻¹), acetonitrile/acetone (7:3; k₂ = 0.047 M⁻¹ s⁻¹), and methanol/acetone (7:3; k₂ = 0.68 M⁻¹ s⁻¹). Kinetic studies of the oxidation of pyridine by 1 versus mole fraction of water in acetone [k₂ = 0.78 M⁻¹ s⁻¹ (χ = 0) to k₂ = 11.1 M⁻¹ s⁻¹ (χ = 0.52)] were carried out. The results showed the reaction to be very sensitive to protic, polar solvents.     

Mechanistic studies with positive iodine: Kinetics of oxidation of thiocyanate ion by iodine monochloride and aqueous iodine in perchloric acid medium

Kinetics of oxidation of thiocyanate ion (NCS^?) by iodine monochloride and iodine has been studied in aqueous perchloric acid medium. The rates of oxidations followed the rate laws: Variations in ionic strength and dielectric constant of the medium had little effects on the rates of reactions with both the oxidants. Mechanisms consistent with the observed rate laws have been suggested. Rate limiting steps have been identified and the constants of some of these steps have been evaluated by varying [NCS^?] at each temperature. Activation parameters were computed from the Arrhenius plots. The rate constants predicted from the rate law as [NCS^?], and [H⁺], varied in iodine monochloride oxidation, are in good agreement with the experimental values providing support to the proposed mechanism.     

Kinetics of furfuryl alcohol polymerisation by iodine in methylene dichloride

The kinetic studies of furfuryl alcohol polymerisation by iodine in methylene dichloride showed a pseudo-first-order behaviour in monomer. The pseudo-first-order constant obtained varies linearly with the concentration of iodine. Our previous work on the system concluded for a pseudo-cationic polymerisation mechanism based on the admission of non-ionic propagating species. The good fitting among the experimental monomer concentration data and the one obtained from computer simulation justifies the mechanism proposed for the system. The set of estimated values of the rate parameters is also reported.     

Kinetics and mechanism of Ru(III) catalyzed iodination of anisole and substituted anisoles by iodine monochloride in aqueous acetic acid

The order with respect to ICl and Ru(III) is unity and fractional with respect to [substrate]. The effect of varying [acid] is negligible at lower [HClO₄] but at higher [acid] (>0.01 M) there is first order dependence on [H⁺]. The Hammett plot gives a value of –1.6. A plausible mechanism has been postulated.

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ICl Ru(III) . [HClO₄], (>0,01 M) [H⁺]. -1,6. .

     

Metallic samarium and iodine promoted facile and efficient syntheses of trisubstituted alkenes from the acetates of Baylis-Hillman adducts

Promoted by samarium metal in the presence of a catalytic amount of iodine, the Baylis-Hillman adducts underwent reductive elimination to form (E)-methylcinnamic ester derivatives. When the iodine was used in 1:1 ratio with metallic samarium, stereospecific syntheses of allylic iodide derivatives, (2Z)-2-(iodomethyl)alk-2-enoates, were achieved. Thus, this gives a new approach to the selective construction of stereo-defined trisubstituted alkenes with the simple Sm/I₂ system.