Showing papers on "Epoxide published in 2009"

Self-supported catalysts.

Abstract: 2.2.6. Poly-Carbene Coordination Polymers 335 2.3. Oxidation 335 2.3.1. Poly-N-Ligand Coordination Polymers 335 2.3.2. Poly-Carboxylate Polymers 336 2.3.3. Lanthanide Disulfonate Polymers 338 2.4. Polymerization 338 2.5. Ring Opening of Epoxides 340 2.6. Miscellaneous 340 3. Self-Supported Chiral Catalysts 342 3.1. Background 342 3.2. Enantioselective Hydrogenation 344 3.3. Enantioselective Oxidations 347 3.3.1. Enantioselective Epoxidation 347 3.3.2. Enantioselective Sulfoxidation 349 3.4. Asymmetric C-C Bond-Formation Reactions 350 3.4.1. Michael Addition 350 3.4.2. Carbonyl-Ene Reaction 350 3.4.3. Addition of Diethylzinc to Aldehydes 351 3.5. Asymmetric Ring Opening of Epoxide 353 3.6. Miscellaneous 353 4. General Considerations in Self-Supported Catalyst Design 354

Mechanistic Aspects of the Copolymerization of CO2 with Epoxides Using a Thermally Stable Single-Site Cobalt(III) Catalyst

Abstract: The mechanism of the copolymerization of CO(2) and epoxides to afford the corresponding polycarbonates catalyzed by a highly active and thermally stable cobalt(III) complex with 1,5,7-triabicyclo[4,4,0] dec-5-ene (designated as TBD, a sterically hindered organic base) anchored on the ligand framework has been studied by means of electrospray ionization mass spectrometry (ESI-MS) and Fourier transform infrared spectroscopy (FTIR). The single-site, cobalt-based catalyst exhibited excellent activity and selectivity for polymer formation during CO(2)/propylene oxide (PO) copolymerization even at temperatures up to 100 degrees C and high [epoxide]/[catalyst] ratios, and/or low CO(2) pressures. The anchored TBD on the ligand framework plays an important role in maintaining thermal stability and high activity of the catalyst. ESI-MS and FTIR studies, in combination with some control experiments, confirmed the formation of the carboxylate intermediate with regard to the anchored TBD on the catalyst ligand framework. This analysis demonstrated that the formed carboxylate intermediate helped to stabilize the active Co(III) species against decomposition to inactive Co(II) by reversibly intramolecular Co-O bond formation and dissociation. Previous studies of binary catalyst systems based on Co(III)-Salen complexes did not address the role of these nucleophilic cocatalysts in stabilizing active Co(III) species during the copolymerization. The present study provides a new mechanistic understanding of these binary catalyst systems in which alternating chain-growth and dissociation of propagating carboxylate species derived from the nucleophilic axial anion and the nucleophilic cocatalyst take turns at both sides of the Co(III)-Salen center. This significantly increases the reaction rate and also helps to stabilize the active SalenCo(III) against decomposition to inactive SalenCo(II) even at low CO(2) pressures and/or relatively high temperatures.

A thermally remendable epoxy resin

Abstract: To provide epoxy resin with crack healing capability, an epoxy containing both furan and epoxide groups, N,N-diglycidyl-furfurylamine (DGFA), was synthesized through a two-step approach. When it reacted with N,N′-(4,4′-diphenylmethane) bismaleimide (DPMBMI) and methylhexahydrophthalic anhydride (MHHPA), respectively, a crosslinked polymer with two types of intermonomer linkage was yielded. That is, thermally reversible Diels–Alder (DA) bonds from the reaction between furan and maleimide groups, and thermally stable bonds from the reaction between epoxide and anhydride groups. In this way, cured DGFA possessed not only similar mechanical properties as commercial epoxy, but also thermal remendability that enabled elimination of cracks. The latter function took effect as a result of successive retro-DA and DA reactions, which led to crack healing in a controlled manner through chain reconnection.

Epoxide reduction with hydrazine on graphene: A first principles study

Abstract: Mechanisms for epoxide reduction with hydrazine on a single-layer graphene sheet are examined using quantum mechanical calculations within the framework of gradient-corrected spin-polarized density-functional theory. We find that the reduction reaction is mainly governed by epoxide ring opening which is initiated by H transfer from hydrazine or its derivatives. In addition, our calculations suggest that the epoxide reduction by hydrazine may predominantly follow a direct Eley–Rideal mechanism rather than a Langmuir–Hinshelwood mechanism. We also discuss the generation of various hydrazine derivatives during the reduction of graphene oxide with hydrazine and their potential contribution to lowering the barrier height of epoxide ring opening.

H2-Driven Deoxygenation of Epoxides and Diols to Alkenes Catalyzed by Methyltrioxorhenium

Abstract: Catalytic deoxygenation of epoxides and diols is underdeveloped. This reaction is appealing in the context of making value-added organics from biomass. Methyltrioxorhenium (MTO) catalyzes the conversion of epoxides and vicinal diols to olefins with dihydrogen (H2) as the reductant under reasonably mild conditions (150 °C and 80−300 psi). The only reaction byproduct is water. The reaction is selective for cis cyclic diols, signaling a mechanism of alkene extrusion from a coordinated epoxide via a metallaoxetane intermediate.

Assessing the Potential for Diol and Hydroxy Sulfate Ester Formation from the Reaction of Epoxides in Tropospheric Aerosols

Abstract: Polyols and sulfate esters have recently been identified in the secondary organic aerosol (SOA) formed in the photooxidation of biogenic hydrocarbons both in the laboratory and under ambient atmospheric conditions. In the present study, the potential role of the reactions of epoxides in SOA to form diols and hydroxy sulfate esters is explored. Nuclear magnetic resonance methods were used to monitor the bulk reaction kinetics of the epoxide hydrolysis reactions for a number of simple epoxides. The experiments were carried out at various acid concentrations in order to confirm the acid-catalysis rate order and to determine the second-order rate constants for such reactions in aerosols under the previously studied laboratory conditions and under ambient atmospheric conditions. The measured rate constants depended systematically on the carbon substitution nature of the epoxide ring, with the tertiary epoxides characterized by the largest rate constants. The hydroxy sulfate yield was observed to depend linearly on the total sulfate concentration, with yields as high as 30% observed at high sulfate concentrations. Due to the large values of the observed rate constants, these reactions are expected to be efficient even for mostly neutralized tropospheric SOA, let alone the much more acidic SOA particles previously studied in laboratory experiments. Therefore, the epoxide hydrolysis mechanism appears to be a kinetically feasible route to the formation of the diols and hydroxy sulfate esters observed in the SOA resulting from the photooxidation of biogenic hydrocarbons.

Enantioselective Ring-Opening Reactions of Racemic Ethynyl Epoxides via Copper−Allenylidene Intermediates: Efficient Approach to Chiral β-Amino Alcohols

Abstract: Enantioselective copper-catalyzed ring-opening reactions of racemic ethynyl epoxides with amines using (R)-DTBM-MeO-BIPHEP as a chiral ligand have been found to give the corresponding amino alcohols in high yields with up to 94% ee. The reaction is considered to proceed via copper-allenylidene complexes as key intermediates. This methodology may provide a novel synthetic approach to optically active amino alcohols, the structures of which are widely found in many natural products, biologically active compounds, and chiral ligands.

Catalytic reactivity of a meso-N-substituted corrole and evidence for a high-valent iron-oxo species.

Abstract: It is shown that an iron(III) meso-N-substituted corrole (TBP8Cz)FeIII (1) (TBP8Cz = octakis(4-tert-butylphenyl)corrolazinato), is a potent catalyst for the oxidation of alkenes in the presence of pentaflouroiodosylbenzene (C6F5IO) as oxidant. In the case of cyclohexene, complex 1 performs on a par with one of the best porphyrin catalysts ((TPPF20)FeCl), exhibiting rapid turnover and a high selectivity for epoxide (CzFeIII/C6F5IO/cyclohexene (1:100:1000) in CH2Cl2/CH3OH (3:1 v:v) gives 33 turnovers of epoxide in <2 min). Reaction rates for 1 are greatly enhanced compared to other Fe or Mn corroles under similar catalytic conditions, consistent with an increase in the electrophilicity of a high-valent iron−oxo intermediate induced by meso-N substitution. Reaction of dark-green 1 (λmax = 440, 611, 747 nm) under single-turnover-like conditions at −78 °C leads to the formation of a new dark-brown species (2) (λmax = 396, 732, 843 nm). The FeIII complex 1 is restored upon the addition of 2 equiv of ferrocene t...

Total Synthesis of ent-Dioxepandehydrothyrsiferol via a Bromonium-Initiated Epoxide-Opening Cascade

Abstract: In the first total synthesis of ent-dioxepandehydrothyrsiferol, the signature trans-anti-trans 7,7,6-fused tricyclic polyether framework was constructed in a single bromonium-initiated epoxide-opening cascade that incorporates both endo- and exo-selective epoxide openings, each directed by the substitution pattern of the epoxide (methyl groups). This study thus demonstrates the feasibility of a possible biogenesis.

Novel coordination complexes and process of producing polycarbonate by copolymerization of carbon dioxide and epoxide using the same as catalyst

Abstract: Provided are a complex prepared from ammonium salt-containing ligands and having such an equilibrium structural formula that the metal center takes a negative charge of 2 or higher, and a method for preparing polycarbonate via copolymerization of an epoxide compound and carbon dioxide using the complex as a catalyst. When the complex is used as a catalyst for copolymerizing an epoxide compound and carbon dioxide, it shows high activity and high selectivity and provides high-molecular weight polycarbonate, and thus easily applicable to commercial processes. In addition, after forming polycarbonate via carbon dioxide/epoxide copolymerization using the complex as a catalyst, the catalyst may be separately recovered from the copolymer.

A diacetate ketone-catalyzed asymmetric epoxidation of olefins

Abstract: A fructose-derived diacetate ketone has been shown to be an effective catalyst for asymmetric epoxidation. High ee values have been obtained for a variety of trans and trisubstituted olefins including electron-deficient α,β-unsaturated esters as well as certain cis olefins.

Synthesis of epoxides by palladium-catalyzed reactions of tertiary allyl alcohols with aryl or alkenyl halides.

Abstract: A novel synthetic method for the preparation of tri- or tetrasubstituted epoxides is reported. Treatment of readily available tertiary allyl alcohol with aryl or alkenyl halide under palladium catalysis resulted in arylative cyclization to form the epoxide. The reaction includes intramolecular C-O bond and intermolecular C-C bond construction.

Total synthesis of (+)-schweinfurthins B and E.

Abstract: The first total synthesis of (+)-schweinfurthin B, a potent and differentially active cytotoxic agent, has been accomplished. Completion of the synthesis required just 16 steps in the longest linear sequence from commercially available vanillin. Key synthetic transformations included a Shi epoxidation and an efficient cascade cyclization initiated by treatment of the resulting epoxide with BF3·OEt2. Furthermore, use of a methyl ether as a stable protecting group for benzylic alcohols dramatically increased the efficiency of the overall sequence. The benzylic ether can be removed from this electron-rich aromatic system through oxidation with DDQ that provided the desired aldehyde intermediate in quantitative yield and shortened the synthetic sequence. Introduction of the A-ring diol in the required cis stereochemistry then became viable through a short sequence highlighted by an aldol condensation with benzaldehyde to introduce an olefin as a latent carbonyl group at the C-3 position. This synthesis establ...

Conversion of carbon dioxide to cyclic carbonates using diimine Ru(II) complexes as catalysts

Abstract: Cationic diimine Ru(II) complexes were synthesized and tested as catalysts for the formation of cyclic organic carbonates from CO2 and liquid epoxides (propylene oxide, epichlorohydrine, 1,2-epoxybutane and styrene oxide) which served as both reactant and solvent. The reaction rates not only depended on the type of ligand, but also on reaction conditions such as temperature, pressure, base, the epoxide substrates and the use of an additional solvent. Reaction rates in terms of turnover frequencies up to 4050 molproduct molcat.−1 h−1 at 99% selectivity were achieved by optimizing the diimine ligand as well as the reaction temperature and CO2 pressure. Consistent with CV measurements, the electron donating group on the p-position of the aryl ring accelerated the reaction rate. Copyright © 2008 John Wiley & Sons, Ltd.

Epoxidation of olefins catalysed by vanadium–salan complexes: a theoretical mechanistic study

Abstract: Plausible mechanisms of olefin epoxidation catalysed by a V–salan model complex [VIV(O)(L)(H2O)] (1, L = (CH2NHCH2CHCHO−)2) in the presence of H2O2 are investigated and compared by theoretical methods using density functional theory. Three main routes, i.e. the Mimoun, Sharpless and biradical mechanisms, were examined in detail, and the Sharpless pathway was found to be the most favourable one. The reaction starts from the formation of an active catalytic species [VV(O)(OO)(LH)] (3c) upon interaction of 1 with H2O2, then concerted, highly synchronous attack of the olefin to 3c occurs yielding the epoxide and catalyst [VV(O)2(LH)], the latter being oxidized by H2O2 to 3c. The activation barrier strongly depends on the proton location in the catalyst molecule and is the lowest when one of the oxygen atoms of the salan ligand is protonated and the vanadium atom is penta-coordinated with one vacant coordination position (complex 3c). The olefin in this reaction acts as an electron donor (nucleophile) rather than as an electron acceptor (electrophile).