Asymmetric epoxidation of styrenes catalyzed by molybdenum complexes with amino alcohol ligands

Two common amino alcohols, prolinol and isolucinol, and their derivatives have been screened to coordinate with MoO₂(acac)₂ to form in situ catalysts for asymmetric epoxidation of styrenes with the highest enantioselectivity of 84% for 4-fluoro-styrene under the optimized reaction conditions.     

Kinetics of propylene epoxidation with hydrogen peroxide catalyzed by extruded titanium silicalite in methanol

The kinetics of propylene oxidation into propylene oxide in the presence of extruded titanium silicalite was studied. Based on the experimental data, a kinetic model of the process was designed and the activation energies of the target and side reactions, the rate constants, and the adsorption equilibrium constants were determined. The adequacy of the proposed kinetic model was verified on a continuously-operated test bench laboratory unit.     

Catalytic epoxidation of propylene with tert-butyl peroxide in presence of molybdenum complexes on polymer carriers

A polymer catalyst has been synthesized on the basis of Amberlite IRA-45 anion-exchange resin modified with molybdenum hexacarbonyl. Its catalytic activity in the epoxidation of propylene with tert-butyl hydroperoxide is similar to the activity of Mo(CO)₆ used under homogeneous conditions.

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, - Amberlite IRA-45, . - Mo(CO)₆, .

     

Catalytic epoxidation of propylene over a Schiff-base molybdenum complex supported on a silanized mesostructured cellular foam

Research on Chemical Intermediates - A Schiff-base molybdenum complex (MoO2–salen) supported on mesostructured cellular foam (MCF) was initially prepared by an in situ synthesis method under...     

Olefin epoxidation by molybdenum peroxo compound: molecular mechanism characterized by the electron localization function and catastrophe theory

The oxygen atom transfer reaction from the Mimoun-type complex MoO(η(2)-O(2))(2)OPH(3) to ethylene C(2)H(4) affording oxirane C(2)H(4)O has been investigated within the framework of the Bonding Evolution Theory in which the corresponding molecular mechanism is characterized by the topological analysis of the electron localization function (ELF) and Thom's catastrophe theory (CT). Topological analysis of ELF and electron density analysis reveals that all Mo-O bonds in MoO(η(2)-O(2))(2)OPH(3) and MoO(2)(η(2)-O(2))OPH(3) belong to closed-shell type interactions though negative values of total energy densities E(e)(r(BCP)) imply some covalent contribution. The peroxo O(i)-O(j) bonds are characterized as charge-shift or protocovalent species in which pairs of monosynaptic basins V(3)(O(i)), V(3)(O(j)) with a small electron population of ~0.25e each, are localized between core basins C(O(i)), C(O(j)). The oxygen transfer reaction from molybdenum diperoxo complex MoO(η(2)-O(2))(2)OPH(3) to C(2)H(4) system can be described by the following consecutive chemical events: (a) protocovalent peroxo O(2)-O(1) bond breaking, (b) reduction of the double C(1)=C(2) bond to single C(1)-C(2) bond in ethylene, (c) displacement of oxygen O(1) with two nonbonding basins, V(i=1,2)(O(1)), (d) increase of a number of the nonbonding basins to three (V(i=1,2,4)(O(1))); (e) reorganization and reduction in the number of nonbonding basis to two basins (V(i=1,4)(O(1))) resembling the ELF-topology of the nonbonding electron density in oxirane, (e) formation of the first O(1)-C(2) bond in oxirane, (f) C(2)-O(1)-C(2) ring closure, (g) formation of singular nonbonding basin V(O(2)) in new Mo=O(2) bond. The oxygen atom is transferred as an anionic moiety carrying a rather small electronic charge ranging from 0.5 to 0.7e.     

On the mechanism of allylic alkylations catalyzed by palladium

Reults are reported indicating the intermediacy of a symmetrical π-allyl species in the alkylation of an allylic lactone. A previous report indicating that another mechanism was operative is disputed.     

A study of the mechanism and kinetics of cyclooctene epoxidation catalyzed by iron(III) tetrakispentafluorophenyl porphyrin

A study has been conducted of the mechanism and kinetics of cyclooctene epoxidation by hydrogen peroxide catalyzed by iron(III) tetrakispentafluorophenyl [F(20)TPPFe(III)] porphyrin. The formation of cyclooctene oxide, the only product, was determined by gas chromatography, and the consumption of hydrogen peroxide was determined by (1)H NMR. UV-visible spectroscopy was used to identify the state of the porphyrin as a function of solvent composition and reaction conditions and to follow the kinetics of porphyrin degradation. F(20)TPPFe(III) was found to be inactive in the chloride-ligated form, but became active when the chloride ligand was replaced by a methoxide ligand. The methoxide-ligated form of F(20)TPPFe(III) reacts with hydrogen peroxide to form an iron(III) hydroperoxide species, which then undergoes both heterolytic and homolytic cleavage to form iron(IV) pi-radical cations and iron(IV) oxo species, respectively. The iron(IV) pi-radical cations are responsible for the epoxidation of cyclooctene, whereas the iron(IV) oxo species are responsible for hydrogen peroxide decomposition. The kinetics of cyclooctene epoxidation and hydrogen peroxide decomposition developed from the proposed mechanism describe the experimentally observed kinetics accurately. The rate parameters derived from a fit of the model to the experimental data are consistent with previous estimates of the magnitude of these parameters.     

Kinetics of propylene epoxidation with hydrogen peroxide

The kinetics of propylene oxidation to propylene oxide in the presence of a titanium-containing zeolite in an isopropyl alcohol medium is reported. The influence of the concentrations of the starting substances and reaction products and temperature on the rate of the process is considered. A mathematical model has been derived from the experimental data obtained. The rate constants, adsorption equilibrium constants, and activation energies of the reactions have been calculated.     

Highly diastereoselective epoxidation of allyl-substituted cycloalkenes catalyzed by metalloporphyrins

Highly diastereoselective epoxidations of allyl-substituted cycloalkenes including allylic alcohols, esters, and amines using sterically bulky metalloporphyrins [Mn(TDCPP)Cl] (1) and [Ru(TDCPP)CO] (2) as catalysts have been achieved. The "1 + H(2)O(2)" and "2 + 2,6-Cl(2)pyNO" protocols afforded trans-epoxides selectively in good yields (up to 99%) with up to >99:1 trans-selectivity.     

Mechanisms of the stabilization of a molybdenum epoxidation catalyst by nitrogen-containing compounds

Nitrogen-containing compounds have been studied as stabilizing additives for a complex molybdenum catalyst for propylene epoxidation with ethylbenzene hydroperoxide. Introducing the additives increases the half-life period by a factor of 10–14. The effective constants and rates of the decomposition of the catalytic complex in the presence of stabilizing agents have been calculated. Two possible mechanisms of the action of the nitrogen-containing additives have been suggested.     

Extraction of molybdenum(VI) by propylene carbonate

A condensed molybdate complex was extracted from acidic solutionsof pH 0·5−5 by propylene carbonate (PrCO₃). The extracted complex exhibited a yellow color and had three characteristic absorption maxima at 225 mμ, 260 mμ, and 325 mμ. The extraction data indicated that the extract was highly condensed and adducted three propylene carbonate molecules. Propylene carbonate behaves not only as an extracting reagent but also as the reagent which reacts with molybdate in adequate acidic solution to produce the extractable species.     

Solvent-free ketone hydrogenations catalyzed by molybdenum complexes

Et2C=O is hydrogenated under solvent-free conditions using a catalyst prepared by hydride abstraction from HMo(CO)2[eta5:eta1-C5H4(CH2)(2)PCy2]; the catalyst functions at low catalyst loadings (< 0.4 mol%).     

Biomimetic epoxidation in aqueous media catalyzed by cyclic dipeptides

We have developed a practical epoxidation of electron-deficient enones in aqueous media using cyclic dipeptides as bioinspired green catalyst. Optimizing the reaction conditions in a triphasic system led to efficient conditions providing epoxides with good enantioselectivities. Depending on the catalyst substituent chirality, both enantiomers are obtained. The cyclic rigidity impacts significantly the enantioselectivity.     

DFT study on mechanism of carbonyl hydrosilylation catalyzed by high-valent molybdenum (IV) hydrides

Density functional theory (DFT) calculations have been employed to investigate hydrosilylation of carbonyl compounds catalyzed by three high-valent molybdenum (VI) hydrides: Mo(NAr)H(Cp)(PMe₃) (1A), Mo(NAr)H(PMe₃)₃ (1B), and Mo(NAr)H (Tp)(PMe₃) (Tp?=?tris(pyrazolyl) borate) (1C). Three independent mechanisms have been explored. The first mechanism is “carbonyl insertion pathway”, in which the carbonyls insert into Mo?H bond to give a metal alkoxide complex. The second mechanism is the “ionic hydrosilylation pathway”, in which the carbonyls nucleophilically attacks η¹-silane molybdenum adduct. The third mechanism is [2 + 2] addition mechanism which was proposed to be favorable for the high-valent di-oxo molybdenum complex MoO₂Cl₂ catalyzing the hydrosilylation. Our studies have identified the “carbonyl insertion pathway” to be the preferable pathway for three molybdenum hydrides catalyzing hydrosilylation of carbonyls. For Mo(NAr)H (Tp)(PMe₃) (Tp?=?tris(pyrazolyl) borate), the proposed nonhydride mechanism experimentally is calculated to be more than 32.6?kcal/mol higher than the “carbonyl insertion pathway”. Our calculation results have derived meaningful mechanistic insights for the high-valent transition metal complexes catalyzing the reduction reaction.     

Iron-catalysed propylene epoxidation by nitrous oxide: dramatic shift of allylic oxidation to epoxidation by the modification with alkali metal salts

A dramatic shift of allylic oxidation to epoxidation has been observed during the oxidation of propylene by N(2)O when the FeO(x)/SBA-15 catalyst is modified with alkali metal salts, and the roles of alkali metal salts are to suppress the reactivity of lattice oxygen and to induce an iron coordination structure effective for epoxidation with N(2)O.