Mechanism of oxidation of organic sulphides by oxo(salen)manganese(v) complexes

Bis (Salicylaldehyde-1, 2-Phenylene Diimine)Mn(III) Chloride (Mn(III)-Salophen) Catalysed Oxidation of Thiols to Symmetrical Disulfides Using Urea Hydrogen Peroxide (UHP) as Mild and Efficient Oxidant

A variety of thiols were oxidised efficiently by a catalytic amount of Mn(III)-salophen 1 in the presence of urea hydrogen peroxide adduct 2 as a convenient and mild oxidant to afford the corresponding disulfides in high yields in quite short reaction times.

Kinetics and Mechanism of the Oxidation of Substituted Benzyl Alcohols by oxo(salen)manganese(V) complexes

The oxidation of benzyl alcohol by oxo(salen)manganese (V) complexes proceeds via either a hydride-ion or a hydrogen-atom transfer from the alcohol to the manganese (V) complexes.

Kinetics and mechanism of the oxidation of Substituted Benzylamines by Oxo(Salen)Manganese (V) Complexes

The oxidation of substituted benzylamines by oxo(salen) Mn(V) complexes, to the corresponding aldimine, proceeds through a hydride ion transfer from the amine to the oxidant.

The reactivity–selectivity principle in the oxidation of aryl methyl sulfides with sodium hypochlorite catalysed by (salen)MnIII complexes

The kinetics of oxygen atom transfer from four oxo(salen)manganese(V) complexes to various para-substituted phenyl methyl sulfides have been studied spectrophotometrically in 90% acetonitrile-10% water(v/v) at 20oC. Electron-releasing substituents in sulfides and electron-withdrawing substituents in oxo(salen)manganese(V) complexes enhance the rate of oxidation. Correlation analyses establish that there is an inverse relationship between reactivity and selectivity in both the sulfide and the complex series. Mathematical treatment of the results shows the operation of a valid reactivity-selectivity principle in this redox system.

Electrophilic and nucleophilic pathways in ligand oxide mediated reactions of phenylsulfinylacetic acids with oxo(salen)chromium(V) complexes

The mechanism of oxidative decarboxylation of phenylsulfinylacetic acids (PSAA) by oxo(salen)Cr(V)+ ion in the presence of ligand oxides has been studied spectrophotometrically in acetonitrile medium. Addition of ligand oxides (LO) causes a red shift in the λ max values of oxo(salen) complexes and an increase in absorbance with the concentration of LO along with a clear isobestic point. The reaction shows first-order dependence on oxo(salen)-chromium(V)+ ion and fractional-order dependence on PSAA and ligand oxide. Michaelis-Menten kinetics without kinetic saturation was observed for the reaction. The order of reactivity among the ligand oxides is picoline N-oxide > pyridine N-oxide > triphenylphosphine oxide. The low catalytic activity of TPPO was rationalized. Both electron-withdrawing and electron-donating substituents in the phenyl ring of PSAA facilitate the reaction rate. The Hammett plots are non-linear upward type with negative ρ value for electron-donating substituents, (ρ - = -0.740 to -4.10) and positive ρ value for electron-withdrawing substituents (ρ + = +0.057 to +0.886). Non-linear Hammett plot is explained by two possible mechanistic scenarios, electrophilic and nucleophilic attack of oxo(salen)chromium(V)+-LO adduct on PSAA as the substituent in PSAA is changed from electron-donating to electron-withdrawing. The linearity in the log k vs. E ox plot confirms single-electron transfer (SET) mechanism for PSAAs with electron-donating substituents.

Tailoring the structure of two-dimensional self-assembled nanoarchitectures based on ni(ii) -salen building blocks

The synthesis of a series of Ni(II) -salen-based complexes with the general formula of [Ni(H2 L)] (H4 L=R(2) -N,N'-bis[R(1) -5-(4'-benzoic acid)salicylidene]; H4 L1: R(2) =2,3-diamino-2,3-dimethylbutane and R(1) =H; H4 L2: R(2) =1,2-diaminoethane and R(1) =tert-butyl and H4 L3: R(2) =1,2-diaminobenzene and R(1) =tert-butyl) is presented. Their electronic structure and self-assembly was studied. The organic ligands of the salen complexes are functionalized with peripheral carboxylic groups for driving molecular self-assembly through hydrogen bonding. In addition, other substituents, that is, tert-butyl and diamine bridges (2,3-diamino-2,3-dimethylbutane, 1,2-diaminobenzene or 1,2-diaminoethane), were used to tune the two-dimensional (2D) packing of these building blocks. Density functional theory (DFT) calculations reveal that the spatial distribution of the LUMOs is affected by these substituents, in contrast with the HOMOs, which remain unchanged. Scanning tunneling microscopy (STM) shows that the three complexes self-assemble into three different 2D nanoarchitectures at the solid-liquid interface on graphite. Two structures are porous and one is close-packed. These structures are stabilized by hydrogen bonds in one dimension, while the 2D interaction is governed by van der Waals forces and is tuned by the nature of the substituents, as confirmed by theoretical calculations. As expected, the total dipolar moment is minimized.

Visible-light activation of the bimetallic chromophore-catalyst dyad: analysis of transient intermediates and reactivity toward organic sulfides

In order to develop a new photocatalytic system, we designed a new redox-active module (5) to hold both a photosensitizer part, [Ru(II)(terpy)(bpy)X](n+) (where terpy = 2,2':6',2''-terpyridine and bpy = 2,2'-bipyridine), and a popular Jacobsen catalytic part, salen-Mn(III), covalently linked through a pyridine-based electron-relay moiety. On the basis of nanosecond laser flash photolysis studies, an intramolecular electron transfer mechanism from salen-Mn(III) to photooxidized Ru(III) chromophore yielding the catalytically active high-valent salen-Mn(IV) species was proposed. To examine the reactivity of such photogenerated salen-Mn(IV), we employed organic sulfide as substrate. Detection of the formation of a Mn(III)-phenoxyl radical and a sulfur radical cation during the course of reaction using time-resolved transient absorption spectroscopy confirms the electron transfer nature of the reaction. This is the first report for the electron transfer reaction of organic sulfide with the photochemically generated salen-Mn(IV) catalytic center.

Iron and Manganese (III) – Porphyrins as New Applicable Catalysts for Selective Oxidation of Imines with Urea–hydrogen Peroxide

A variety of imines were oxidised with urea–hydrogen peroxide using iron(III) and manganese (III)- tetraphenylporphyrins [Fe(TPP)Cl], [Mn(TPP)Cl] and manganese (III)-octabromotetraphenyl porphyrin [Mn(TPPBr8)Cl] as catalysts. Experimental results showed the released urea from UHP acts as an axial ligand. These catalysts showed high selectivity in oxidation of imines to corresponding nitrones and oxaziridines at 0°C to room temperature.

Rapid and efficient oxidation of Hantzsch 1,4-dihydropyridines with sodium periodate catalyzed by manganese (III) Schiff base complexes

Rapid and efficient oxidation of Hantzsch 1,4-dihydropyridine with sodium periodate is reported. The Mn(III)-salophen/NaIO4 catalytic system converts 1,4-dihydropyridines to their corresponding pyridine derivatives at room temperature in a 1:1, CH3CN/H2O mixture. The ability of various Schiff base complexes in the oxidation of 1,4-dihydropyridine was also investigated.

Cytochrome P-450 dependent monooxygenases model system: rapid and efficient oxidation of primary aromatic amines to azo derivatives with sodium periodate catalyzed by manganese(III) Schiff base complexes

Rapid and efficient oxidation of primary aromatic amines was investigated. Mn(III)-salophen catalyst can catalyze the oxidation of primary aromatic amines to azo derivatives with sodium periodate. The ability of various Schiff base complexes in this oxidation system was also investigated.

Electron Transfer Reaction of Oxo(salen)chromium(V) Ion with Anilines

The kinetics of oxidation of 16 meta-, ortho-, and para-substituted anilines with nine oxo(salen)chromium(V) ions have been studied by spectrophotometric, ESIMS, and EPR techniques. During the course of the reaction, two new peaks with lambda(max) at 470 and 730 nm appear in the absorption spectrum, and these peaks are due to the formation of emeraldine forms of oligomers of aniline supported by the ESIMS peaks with m/z values 274 and 365 (for the trimer and tetramer of aniline). The rate of the reaction is highly sensitive to the change of substituents in the aryl moiety of aniline and in the salen ligand of chromium(V) complexes. Application of the Hammett equation to analyze kinetic data yields a rho value of -3.8 for the substituent variation in aniline and +2.2 for the substituent variation in the salen ligand of the metal complex. On the basis of the spectral, kinetic, and product analysis studies, a mechanism involving an electron transfer from the nitrogen of aniline to the metal complex in the rate controlling step has been proposed. The Marcus equation has been successfully applied to this system, and the calculated values are compliant with the measured values.

Rapid and efficient oxidative decarboxylation of carboxylic acids with sodium periodate catalyzed by manganese (III) Schiff base complexes

Rapid and efficient oxidative decarboxylatoin of alpha-aryl carboxylic acids was observed. In the chemical system containing Mn(III)-salophen complex as catalyst, carboxylic acids are converted efficiently to the corresponding carbonyl derivatives with sodium periodate. The ability of various Schiff base complexes in the oxidative decarboxylation of carboxylic acids was also investigated.

Catalytic Asymmetric Sulfoxidation by Chiral Manganese Complexes: Acetylacetonate Anions as Chirality Switches

Three manganese(II) complexes, namely [Mn(1)(ClO(4))(2)] (3), [Mn(1)(acac)(2)] (4), and [Mn(2)(1)(acac)(4)] (5), were isolated from solutions of Mn(ClO(4))(2) or Mn(acac)(2), and an easily accessible diimine ligand (1S,2S)-N,N'-bis-pyridin-2-ylmethylene-cyclohexane-1,2-diamine (1). Their structure was determined by X-ray crystallography, and these complexes proved to be catalysts for asymmetric sulfide oxidation by H(2)O(2). Enantiomeric excesses ranging from 5% to 62% were obtained with a variety of aryl alkyl sulfides. We also observed an interesting "chirality switch" effect by the achiral acac anion reversing the enantioselectivity of the complex [Mn(1)(ClO(4))(2)] from the S to the R sulfoxide enantiomer.

Electronic and steric effects on the oxygenation of organic sulfides and sulfoxides with oxo(salen)chromium(V) complexes

The kinetics of oxygenation of several para-substituted phenyl methyl sulfides and sulfoxides with a series of 5-substituted and sterically hindered oxo(salen)chromium(V) complexes have been studied by a spectrophotometric technique. Though the reaction of sulfides follows simple second-order kinetics, sulfoxides bind strongly with the metal center of the oxidant and the oxygen atom is transferred from the oxidant-sulfoxide adduct to the substrate. The reduction potentials, E(red), of eight Cr(V) complexes correlate well with the Hammett sigma constants, and the reactivity of the metal complexes is in accordance with the E(red) values. The metal complexes carrying bulky tert-butyl groups entail steric effects. Organic sulfides follow a simple electrophilic oxidation mechanism, and the nonligated sulfoxides undergo electrophilic oxidation to sulfones using the oxidant-sulfoxide adduct as the oxidant. Sulfoxides catalyze the Cr(V)-salen complexes' oxygenation of organic sulfides, and the catalytic activity of sulfoxides is comparable to pyridine N-oxide and triphenylphosphine oxide. The rate constants obtained for the oxidation of sulfides and sulfoxides clearly indicate the operation of a pronounced electronic and steric effect in the oxygenation reaction with oxo(salen)chromium(V) complexes.

Iron(III) [bond] Salen complexes as enzyme models: mechanistic study of oxo(salen)iron complexes oxygenation of organic sulfides

The oxidation of a series of para-substituted phenyl methyl sulfides was carried out with several oxo(salen)iron (salen = N,N'-bis(salicylidine)ethylenediaminato) complexes in acetonitrile. The oxo complex [O=Fe(IV)(salen)](*+), generated from an iron(III) [bond] salen complex and iodosylbenzene, effectively oxidizes the organic sulfides to the corresponding sulfoxides. The formation of [O [double bond] Fe(IV)(salen)](*+) as the active oxidant is supported by resonance Raman studies. The kinetic data indicate that the reaction is first-order in the oxidant and fractional-order with respect to sulfide. The observed saturation kinetics of the reaction and spectral data indicate that the substrate binds to the oxidant before the rate-controlling step. The rate constant (k) values for the product formation step determined using Michaelis-Menten kinetics correlate well with Hammett sigma constants, giving reaction constant (rho) values in the range of -0.65 to -1.54 for different oxo(salen)iron complexes. The log k values observed in the oxidation of each aryl methyl sulfide by substituted oxo(salen)iron complexes also correlate with Hammett sigma constants, giving positive rho values. The substituent effect, UV-vis absorption, and EPR spectral studies indicate oxygen atom transfer from the oxidant to the substrate in the rate-determining step.

Mechanism of selective oxidation of organic sulfides with Oxo(salen)chromium(V) complexes

The selective oxidation of organic sulfides to sulfoxides by oxo(salen)chromium(V) complexes in acetonitrile is overall second-order, first-order each in the oxidant and the substrate. The rate constant, k(2), values of several para-substituted phenyl methyl sulfides correlate linearly with Hammett sigma constants and the rho values are in the range of -1.3 to -2.7 with different substituted oxo(salen)chromium(V) complexes. The reactivity of different alkyl sulfides is in accordance with Taft's steric substituent constant, E(S). A mechanism involving direct oxygen atom transfer from the oxidant to the substrate rather than electron transfer is envisaged. Correlation analyses show the presence of an inverse relationship between reactivity and selectivity in the reaction of various sulfides with a given oxo(salen)chromium(V) complex and vice versa. Mathematical treatment of the results shows that this redox system falls under strong reactivity-selectivity principle (RSP).

Reduction of permanganate by thioanisole: lewis acid catalysis

The oxidation of sulfides and sulfoxides by permanganate in anhydrous acetone solutions is catalyzed by Lewis acids such as iron(III) chloride, zinc chloride, and mercury(II) chloride. The reaction kinetics unequivocally confirm that the function of these catalysts is to activate the oxidant by forming permanganate/Lewis acid complexes analogous to the protonation of Mn by Bronsted acids. A Hammett analysis of the rate constants for the oxidation of a series of substituted thioanisoles gives a negative rho value (-1. 11) indicative of an electron deficient transition state. No secondary kinetic isotope effect is observed when the hydrogens of the methyl group are replaced by deuterium. Despite previous observations that sulfoxides are preferentially oxidized in competitive experiments, sulfides are oxidized more rapidly when individual rates are measured. All of these observations are most consistent with a mechanism in which the reductant reacts with the oxidant via initial ligation.

Kinetics and Mechanism of the Oxidation of Substituted Benzaldehydes by Oxo(salen)manganese(v) Complexes

The oxidation of benzaldehyde by oxo(salen)manganese(v) complexes proceeds via either a hydride-ion transfer or a hydrogen-atom transfer from the aldehyde to the manganese(v) complex.