Olefin Epoxidation in Aqueous Phase Using Ionic-Liquid Catalysts

Hydrophobic imidazolium-based ionic liquids (IL) act as catalysts for the epoxidation of unfunctionalized olefins in water using hydrogen peroxide as oxidant. Although the catalysts are insoluble in both the substrate and in water, surprisingly, they are very well soluble in aqueous H2 O2 solution, owing to perrhenate-H2 O2 interactions. Even more remarkably, the presence of the catalyst also boosts the solubility of substrate in water. This effect is crucially dependent on the cation design. Hence, the imidazolium perrhenates enable both the transfer of hydrophobic substrate into the aqueous phase, and serve as actual catalysts, which is unprecedented. At the end of the reaction and in absence of H2 O2 the IL catalyst forms a third phase next to the lipophilic product and water and can easily be recycled.

Oxygen activation and catalytic aerobic oxidation by Mo(iv)/(vi) complexes with functionalized iminophenolate ligands

Synthesis of molybdenum(vi) dioxido complexes 1-3, coordinated by one or two functionalized iminophenolate ligands HL1 or HL2, bearing a donor atom side chain or a phenyl substituent, respectively, allowed for systematic investigation of the oxygen atom transfer (OAT) reactivity of such complexes towards phosphanes. Depending on stoichiometry and employed phosphane (PMe3 or PPh3), different molybdenum(iv) and molybdenum(v) complexes 4-7 were obtained. Whereas molybdenum(iv) complexes 4 and 5, bearing a terminal PMe3 ligand, readily reacted with molecular O2 to form oxido peroxido complexes 8 and 9, phosphane free μ-oxido bridged dinuclear molybdenum(v) complexes 6 and 7 proved to be stable towards oxidation with molecular O2 under ambient conditions. Single-crystal X-ray diffraction analyses revealed different isomeric structures in the solid state for dioxido complexes 1 and 2 in comparison with oxido phosphane complex 5, dinuclear oxido μ-oxido complex 6 and oxido peroxido complexes 8 and 9, pointing towards an isomeric rearrangement during OAT. Compounds 1 and 2 were furthermore tested for their ability to catalyze the aerobic oxidation of PMe3 and PPh3. A significant difference in catalytic activity has been observed in the oxidation of PMe3, where complex 1 bearing donor atom functionalized ligands led to higher conversion and selectivity than complex 2 coordinated by phenyl iminophenolate ligands. In the oxidation of PPh3, complex 2 leads to higher conversion compared to 1. In a control experiment, phenyl-based dinuclear μ-oxido complex 7, derived from complex 2, was found to be catalytically active, which suggests a lower energy barrier for disproportionation into [MoO(L)2] and [MoO2(L)2] in comparison with methoxypropylene based compound 6, a prerequisite for subsequent reactivity toward molecular O2.

Epoxidation using molecular oxygen in flow: facts and questions on the mechanism of the Mukaiyama epoxidation

Mukaiyama reaction was performed G/L continuous-flow microreactor. In less than 5 minutes at room temperature, cyclooctene was efficiently transformed to the corresponding epoxide using O₂ as oxidant and aldehyde as co-reductant.

Aerobic epoxidation catalysed by transition metal substituted polyfluorooxometalates

Polyfluorooxometalates with a nitro ligands, Q₉[NaH₂Cu^II(NO₂)W₁₇F₆O₅₅], are oxygen donors for epoxidation of alkenes and catalytic with O₂ as terminal oxidant.

Kinetic studies of fluorinated aryl molybdenum(ii) tricarbonyl precursors in epoxidation catalysis

Benzyl substituted molybdenum tricarbonyl complexes displaying CF₃ groups are synthesized and applied as catalyst precursors in olefin epoxidation reactions.

Fighting Fenton Chemistry: A Highly Active Iron(III) Tetracarbene Complex in Epoxidation Catalysis

Organometallic Fe complexes with exceptionally high activities in homogeneous epoxidation catalysis are reported. The compounds display Fe(II) and Fe(III) oxidation states and bear a tetracarbene ligand. The more active catalyst exhibits activities up to 183 000 turnovers per hour at room temperature and turnover numbers of up to 4300 at -30 °C. For the Fe(III) complex, a decreased Fenton-type reactivity is observed compared with Fe(II) catalysts reported previously as indicated by a substantially lower H2 O2 decomposition and higher (initial) turnover frequencies. The dependence of the catalyst performance on the catalyst loading, substrate, water addition, and the oxidant is investigated. Under all applied conditions, the advantageous nature of the use of the Fe(III) complex is evident.

Immobilisation of a molecular epoxidation catalyst on UiO-66 and -67: the effect of pore size on catalyst activity and recycling

Amino-functionalised metal-organic frameworks UiO-66 and -67 were post-synthetically modified with salicylaldehyde. A molybdenum complex was immobilised on the resulting materials. They were characterised by (13)C-MAS-NMR, XPS and PXRD to confirm immobilisation and stability. The immobilised complex is an active and reusable catalyst for olefin epoxidation with tert-butyl hydroperoxide (TBHP) as an oxidant. It is shown that the effective pore size, probed with Brunauer-Emmett-Teller (BET) surface area analysis and the number of amino groups affect the diffusion of reactants and products, as well as catalyst recycling.

Activation of molecular oxygen by a molybdenum complex for catalytic oxidation

A sterically demanding molybdenum(VI) dioxo complex was found to catalytically activate molecular oxygen and to transfer its oxygen atoms to phosphines. Intermediate peroxo as well as reduced mono-oxo complexes were isolated and fully characterized. Monomeric Mo(IV) monooxo species proved to be of an unusual nature with the coordinated phosphine trans to the oxo group. The reduced molybdenum centers can activate O2 to form a stable Mo(VI) oxo-peroxo complex unambiguously characterized by single crystal X-ray diffraction analysis. NMR experiments demonstrate that both oxygen atoms of the peroxo unit are transferred to an accepting substrate, generating the Mo(IV) intermediate and restarting the catalytic cycle.

Influence of structural and electronic properties of organomolybdenum(ii) complexes of the type [CpMo(CO)3R] and [CpMo(O2)(O)R] (R = Cl, CH3, CF3) on the catalytic olefin epoxidation

Six compounds of the type [CpMo(CO)₃R] (R = Cl (1), CH₃ (2), CF₃ (3)) and [CpMo(O₂)(O)R] (R = Cl (4), CH₃ (5), CF₃ (6) (Cp = η⁵-cyclopentadienyl)) have been synthesised and characterised.

An environmentally friendly route for grafting of molybdenum carbonyl onto a diaminosilane-modified SBA-15 molecular sieve and its catalytic behaviour in olefin epoxidation

Molybdenum carbonyl (Mo(CO)₆) grafted onto a diaminosiloxane functionalized SBA-15 surface and showed promising activity for epoxidation of olefins.

Preparation of SnO2/graphene nanocomposite and its application to the catalytic epoxidation of alkenes with H2O2

The SnO₂/graphene nanocomposites have been achieved via hydrothermal method and used as an efficient catalyst for the epoxidation of alkenes with H₂O₂ in nitrile based systems for the first time.

Catalytic epoxidation by perrhenate through the formation of organic-phase supramolecular ion pairs

Organic-phase supramolecular ion pair (SIP) host–guest assemblies of perrhenate anions (ReO₄⁻) with ammonium amide receptor cations are reported.

Assignment of O–O and Mo=O Stretching Frequencies of Molybdenum/Tungsten Complexes Revisited

The assignment of peroxo stretching frequencies for Molybdenum and Tungsten complexes is studied by DFT and MP2 calculations. We found that M06 functional is unsuitable for assignment of Mo=O and O–O stretches in CpMo(η2-O2)OCH3and we found that MP2 and even the def2-TZVP do not give accurate order of asymmetric and symmetric Mo=O stretching. We recommend the M06L, which is a good compromise between speed and accuracy, for works involving these complexes. For a series of ten Molybdenum and Tungsten complexes studied we found that, for Mo/W=O stretching frequencies at M06L/def2-TZVP, after scaling, a small RMSD of 15 cm−1could be obtained. However peroxo stretching frequencies RMSD remains high at 40 cm−1after scaling. This could potentially point out the need for reassignment of experimental peroxo frequencies in some of the works cited in this report.

Artificial sunlight and ultraviolet light induced photo-epoxidation of propylene over V-Ti/MCM-41 photocatalyst

The light irradiation parameters, including the wavelength spectrum and intensity of light source, can significantly influence a photocatalytic reaction. This study examines the propylene photo-epoxidation over V-Ti/MCM-41 photocatalyst by using artificial sunlight (Xe lamp with/without an Air Mass 1.5 Global Filter at 1.6/18.5 mW·cm(-2)) and ultraviolet light (Mercury Arc lamp with different filters in the range of 0.1-0.8 mW·cm(-2)). This is the first report of using artificial sunlight to drive the photo-epoxidation of propylene. Over V-Ti/MCM-41 photocatalyst, the propylene oxide (PO) formation rate is 193.0 and 112.1 µmol·gcat (-1)·h(-1) with a PO selectivity of 35.0 and 53.7% under UV light and artificial sunlight, respectively. A normalized light utilization (NLU) index is defined and found to correlate well with the rate of both PO formation and C3H6 consumption in log-log scale. The light utilization with a mercury arc lamp is better than with a xenon lamp. The selectivity to PO remains practically unchanged with respect to NLU, suggesting that the photo-epoxidation occurs through the same mechanism under the conditions tested in this study.

Dinuclear Ru-aqua complexes for selective epoxidation catalysis based on supramolecular substrate orientation effects

Ru-aqua complex {[Ru(II)(trpy)(H2O)]2(μ-pyr-dc)}(+) is a powerful epoxidation catalyst for a wide range of linear and cyclic alkenes. High turnover numbers (TNs), up to 17000, and turnover frequencies (TOF), up to 24120 h(-1) (6.7 s(-1)), have been obtained using PhIO as oxidant. This species presents an outstanding stereospecificity for both cis and trans olefins towards the formation of their corresponding cis and trans epoxides. In addition, it shows different reactivity to cis and trans olefins due to a substrate orientation supramolecular effect transmitted by its ligand scaffold. This effect together with the impressive reaction rates are rationalized using electrochemical techniques and DFT calculations.

Cleavage of C-O bonds in lignin model compounds catalyzed by methyldioxorhenium in homogeneous phase

Methyldioxorhenium (MDO)-catalyzed C-O bond cleavage of a variety of lignin β-O-4-model compounds yields phenolic and aldehydic compounds in homogeneous phase under mild reaction conditions. MDO is in situ generated by reduction of methyltrioxorhenium (MTO) and is remarkably stable under the applied reaction conditions allowing its reuse for least five times without significant activity loss. Based on the observed and isolated intermediates, 17 O- and 2 H-isotope labeling experiments, DFT calculations, and several spectroscopic studies, a reaction mechanism is proposed.

Epoxidation of styrene over Fe(Cr)-MIL-101 metal–organic frameworks

Fe(Cr)-MIL-101 metal–organic frameworks as efficient heterogeneous catalysts for the selective oxidation of styrene.

Efficient epoxidation of propene using molecular catalysts

The epoxidation of propene to propene oxide at mild conditions using molecular rhenium and molybdenum catalysts is presented.