Showing papers on "Epoxide published in 2000"

Catalytic Enantioselective meso-Epoxide Ring Opening Reaction with Phenolic Oxygen Nucleophile Promoted by Gallium Heterobimetallic Multifunctional Complexes

Abstract: The catalytic enantioselective meso-epoxide ring opening reaction with phenolic oxygen nucleophile (4-methoxyphenol) is described for the first time herein This reaction was first found to be promoted by (R)-GaLB (Ga = gallium, L = lithium, B = (R)-BINOL), giving a variety of epoxide opening products in good to high ee (67−93% ee) However, chemical yield was only modest (yield 31−75%), despite the use of more than 20 mol % GaLB This was due to the undesired ligand exchange between BINOL and 4-methoxyphenol, which resulted in the decomplexation of GaLB Application of various known chiral ligands such as 6,6‘-bis((triethylsilyl)ethynyl)-BINOL and H8-BINOL were examined, but satisfactory results were not obtained To overcome this problem a novel linked-BINOL containing coordinative oxygen atom in the linker has been developed By linking two BINOL units in GaLB, the stability of the Ga-complex was greatly improved Using 3−10 mol % (R,R)-Ga-Li-linked-BINOL complex, a variety of epoxide opening reactions

Evidence for the Participation of Two Distinct Reactive Intermediates in Iron(III) Porphyrin Complex-Catalyzed Epoxidation Reactions

Abstract: We have studied the competitive epoxidations of olefins with cis- and trans-stilbenes and with cyclooctene and trans-stilbene in iron porphyrin complex-catalyzed epoxidation reactions by H2O2, tert-butyl hydroperoxide (t-BuOOH), and m-chloroperoxybenzoic acid (m-CPBA) in protic solvent (i.e., a solvent mixture of CH3OH and CH2Cl2) and aprotic solvent (i.e., a solvent mixture of CH3CN and CH2Cl2) at room temperature under catalytic reaction conditions. The competitive epoxidations were also carried out with in situ generated high-valent iron(IV) oxo porphyrin cation radical complexes in aprotic solvent under stoichiometric reaction conditions. By determining the ratios of epoxide products formed in the competitive epoxidations, we were able to conclude unambiguously that the reactive species generated in protic solvent are high-valent iron(IV) oxo porphyrin cation radical complexes 3 and the intermediates formed in aprotic solvent are oxidant−iron porphyrin intermediates 2. A protic solvent such as methano...

Chiral Ketone-Catalyzed Asymmetric Epoxidation of Olefins

Abstract: Dioxiranes are remarkably versatile oxidizing agents that show encouraging potential for asymmetric synthesis, particularly asymmetric epoxidation of unfunctionalized olefins. This review outlines the recent development in this area. Discussions are focused on the structural requirements of the chiral ketone catalysts, how structural changes effect the reactivity and selectivity of the catalyst, reaction conditions, and transition states.

Allylic Alcohols via Catalytic Asymmetric Epoxide Rearrangement

Abstract: Optically active allylic alcohols can be prepared via rearrangement of epoxides using chiral lithium amides, but other than for a small subset of meso-epoxides, insufficient reactivity and enantios...

Toxicity of Epoxy Fatty Acids and Related Compounds to Cells Expressing Human Soluble Epoxide Hydrolase

Abstract: Soluble epoxide hydrolase (sEH) is suggested to alter the mode of action and increase the toxic potency of fatty acid epoxides. To characterize the structural features necessary for sEH-dependent epoxy fatty acid toxicity, 75 aliphatic compounds were assayed for cytotoxicity in the presence and absence of sEH. Three groups of aliphatic epoxide−diol pairs were described by their observed differential toxicity. Group I compounds were typified by terminal epoxides whose toxicity was reduced in the presence of sEH. Group II compounds were toxic in either their epoxide or diol form, but toxicity was unaffected by sEH. Group III compounds exhibited sEH-dependent toxicity and were therefore used to investigate the structural elements required for cytotoxicity in this study. The optimal structure for group III compounds appeared to be a fatty acid 18−20 atoms long (e.g., a carbon backbone plus a terminal heteroatom) with an epoxide positioned between C-7 and C-12. In the absence of sEH, replacement of epoxides wi...

Alcoholysis of ester and epoxide catalyzed by solid bases

Abstract: Reactions of alcohols with ethyl acetate (transesterification) and propylene oxide were investigated by use of a variety of solid base catalysts to elucidate the activity determining factors of the catalyst in relation with type of alcohol. Solid base catalysts examined were alkaline earth oxides, hydroxides, and carbonates, alumina supported KF and KOH, rare earth oxide, zirconium oxide, etc. Reaction rate of the alcoholysis of ethyl acetate varies with the combination of type of alcohol and basic strength of solid base catalyst. Over strongly basic catalysts such as CaO, SrO, BaO, 2-propanol reacted much faster than methanol. On the other hand, over weakly basic catalysts such as alkaline earth hydroxides, methanol reacted faster than 2-propanol. 2-Methyl-2-propanol reacted only over strongly basic catalysts, and much slower than methanol and 2-propanol. Alcoholysis with propylene oxide was catalyzed only by strongly basic catalysts such as alkaline earth oxides, and KF/alumina, alkaline earth hydroxides scarcely showed activity. γ-Alumina, however, showed a high activity, though the selectivity of products was different from those for alkaline earth oxides. Reactives of alcohols with propylene oxide were in the order, methanol>ethanol>2-propanol>2-methyl-2-propanol, regardless of the type of catalyst. One of the characteristic features observed for both alcoholyses is that the catalysts are tolerant to air exposure, which is caused by strong adsorptivity of alcohol competitive to that of carbon dioxide and water.

Enantioselective hydrolysis of p-nitrostyrene oxide by an epoxide hydrolase preparation from Aspergillus niger.

Abstract: The epoxide hydrolase activity of Aspergillus niger was synthesized during growth of the fungus and was shown to be associated with the soluble cell fraction. An enzyme preparation was worked out which could be used in place of the whole mycelium as biocatalyst for the hydrolysis of epoxides. The effect of four different cosolvents on enzyme activity was investigated. Consequently, dimethylsulfoxide (DMSO) was selected for epoxide solubilization. The effect of temperature on both reaction rate and enzyme stability was studied in the presence of DMSO (0.2 volume ratio). A temperature of 25 degrees C was selected for the reaction of bioconversion. With a substrate concentration of 4.5 mM a batch reactor showed that the enzyme preparation hydrolyzed para-nitrostyrene oxide with very high enantioselectivity. The (S) enantiomer of the epoxide remained in the reaction mixture and showed an enantiomeric excess higher than 99%. The substrate concentration could be increased to 20 mM without affecting the enantiomeric excess and degree of conversion. Therefore, the method is potentially useful for the preparative resolution of epoxides. Application are in the field of chiral synthons which are important building blocks in organic synthesis.

Toluene monooxygenase-catalyzed epoxidation of alkenes.

Abstract: Several toluene monooxygenase-producing organisms were tested for their ability to oxidize linear alkenes and chloroalkenes three to eight carbons long. Each of the wild-type organisms degraded all of the alkenes that were tested. Epoxides were produced during the oxidation of butene, butadiene, and pentene but not hexene or octadiene. A strain of Escherichia coli expressing the cloned toluene-4-monooxygenase (T4MO) of Pseudomonas mendocina KR1 was able to oxidize butene, butadiene, pentene, and hexene but not octadiene, producing epoxides from all of the substrates that were oxidized. A T4MO-deficient variant of P. mendocina KR1 oxidized alkenes that were five to eight carbons long, but no epoxides were detected, suggesting the presence of multiple alkene-degrading enzymes in this organism. The alkene oxidation rates varied widely (ranging from 0.01 to 0.33 μmol of substrate/min/mg of cell protein) and were specific for each organism-substrate pair. The enantiomeric purity of the epoxide products also varied widely, ranging from 54 to >90% of a single epoxide enantiomer. In the absence of more preferred substrates, such as toluene or alkenes, the epoxides underwent further toluene monooxygenase-catalyzed transformations, forming products that were not identified.

TaCl5-Catalyzed Cleavage of Epoxides with Aromatic Amines

Abstract: Tantalum(V) chloride has been demonstrated as an efficient catalyst in ring opening of epoxides with aromatic amines for the synthesis of β-amino alcohols for the first time.

Copolymerization of carbon dioxide and propylene oxide using various zinc glutarate derivatives as catalysts

Abstract: Various zinc dicarboxylate catalysts were synthesized by the reaction of zinc oxide with eleven different glutaric acid derivatives, and their coordination characteristics were investigated by infrared spectroscopy. The electronic nature and steric hinderance of substituents influenced the coordination of the carboxylate and the zinc metal ion. The coordination characteristics were classified into three categories: (i) compounds exhibiting bridging bidentate coordinating bonding modes, such as syn-anti and syn-syn bridging; (ii) compounds with only unidentate coordination; and (iii) compounds with mixed coordinations of unidentate and bridging bidentate. All the zinc carboxylate catalysts produced poly(propylene carbonate)s (PPCs) by the copolymerization of carbon dioxide and propylene oxide. The first category of catalysts produced relatively higher yields than the other categories. Zinc glutarate without any substituent, which is a catalyst in the first category, produced PPC with the highest yield and the highest molecular weight. The catalytic activity of zinc glutarate was suppressed by incorporation of substituents. The suppression of the catalytic activity might be due to the variation in the Lewis acidity of the zinc site as well as changes in the morphological structure caused by the substituents. Methylaluminoxane was also evaluated as a catalyst for the copolymerization, but it produced copolymers containing a large amount of ether linkages.

Epoxidation of styrene on Si/Ti/SiO2 catalysts prepared by chemical grafting

Abstract: Titania–silica (Ti/SiO2) and silica–titania–silica (Si/Ti/SiO2) catalysts were prepared by chemical grafting using TiCl4 and tetraethyl orthosilicate (TEOS) as precursors and SiO2 as support The prepared catalysts were characterized by UV Raman and visible Raman spectroscopies, XRD and the epoxidation of styrene Ti/SiO2 catalyst grafted with only titanium species is not very active for epoxidation using H2O2 (30%), but is active and selective when one uses tert-butyl hydroperoxide (TBHP) The catalyst grafted at high temperatures shows better epoxide selectivity Si/Ti/SiO2 catalyst, the titanium-silica grafted further with TEOS, is active and selective for the epoxidation of styrene using either dilute H2O2 or TBHP, possibly due to the fact that the grafting of Ti/SiO2 with TEOS modifies the coordination structure of titanium and makes the titanium sites of Si–O–Ti–O–Si species less hydrophilic A characteristic band at 1085 cm−1 due to Ti–O–Si species is detected for the grafted catalysts by UV resonance Raman spectroscopy Reaction between TiCl4 and SiO2 at high temperatures favors the formation of Ti–O–Si species Better activity and selectivity to epoxide is found for the catalysts with more Ti–O–Si species It is assumed that the active sites are the highly isolated Ti–O–Si species For Si/Ti/SiO2 catalyst, the gas phase O2 can participate in the catalytic oxidation of styrene when H2O2 is present and it causes the formation of benzaldehyde

Highly chemoselective hydrogenation with retention of the epoxide function using a heterogeneous Pd/C-ethylenediamine catalyst and THF

Abstract: In general, palladium-carbon (Pd/C) catalyzed hydrogenation of epoxides affords the corresponding primary and secondary alcohols as a mixture. It has been found that the catalytic activity of a Pd/C -ethylenediamine complex catalyst [Pd/C(en)] in the hydrogenolysis of epoxide functions is drastically reduced. Herein we describe a mild and chemoselective method for the hydrogenation of olefin, nitro, and azide functions with retention of the epoxide function. The chemoselectivity was accomplished by using a combination of 5% Pd/C(en) and THF as solvent. A significant drop in the chemoselectivity of the hydrogenation is observed with 5% Pd/C(en) in MeOH. These results reinforce the utility of epoxides as important precursors of alcohols in synthetic chemistry.

1H NMR and EPR spectroscopic monitoring of the reactive intermediates of (Salen)MnIII catalyzed olefin epoxidation

Abstract: Using 1H NMR and EPR spectroscopy, manganese species formed in the catalytic systems 1+iodosobenzene (PhIO) and 1+meta-chloroperoxybenzoic acid (m-CPBA), where 1 is (R,R)-(−)-N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamino-manganese(III) chloride ([(Salen)MnIII]) were studied. Three types of manganese complexes were characterized in the catalytic system 1+PhIO (4–6). Complex 4 is very unstable and reacts with styrene at −20°C to afford styrene oxide. It exhibits three signals of tBu groups at 1.68, 1.64 and 1.42 ppm. This pattern closely resembles that for a model complex [(Salen)MnV≡N]. Based on these data, 4 was identified as d2 low-spin oxomanganese(V) complex [(Salen)MnV=O]+. Complexes 5 and 6 are relatively stable at −20°C and poorly reactive towards styrene at this temperature. They display 1H NMR spectra characteristic for antiferromagnetically coupled μ-oxo-dinuclear MnIV species and are identified as dinuclear complexes [(Salen)LMnIV-O-MnIV(Salen)L′] with L, L′=Cl− and PhIO. It was found by EPR that the acylperoxo complex (Salen)MnIII(OOCOAr) (7) was formed at the first stage of the interaction of 1 with m-CPBA in CH2Cl2. Complex 7 is unstable and converts into manganese(IV) oxo complex [(Salen)MnIV(O)] (8). The evaluated first order rate constant of this conversion is 0.25±0.08 min−1 at −70°C. Complex 7 reacts with styrene with the rate constant 1.1±0.4 M−1 min−1 at −70°C to give epoxide and restore 1. Complex 8 is inert towards styrene at low temperature. The effect of donor ligand N-methylmorpholine-N-oxide (NMO) on the epoxidation of styrene by the system 1+m-CPBA was studied. Addition of NMO (2–5 equiv.) to the solution of 1 in CH2Cl2 before interaction with m-CPBA was found to dramatically increase the rate of undesirable transformation of the reactive acylperoxo complex 7-NMO into relatively inert oxo complex-8-NMO. However, in the presence of styrene, such undesirable conversion is entirely suppressed by very rapid reaction of 8-NMO with styrene to afford styrene oxide and restore 1-NMO.

Enantioselective epoxidation of (Z)-stilbene using a chiral Mn(III)-salen complex: effect of immobilisation on MCM-41 on product selectivity

Abstract: Manganese-exchanged Al-MCM-41 modified by the chiral salen ligand [(R,R)-(−)-N,N′-bis(3,5-di-tert-butylsalicylidene)cyclohexane-1,2-diamine] has been investigated as a heterogeneous catalyst for the enantioselective epoxidation of (Z)-stilbene using iodosylbenzene as oxygen donor, with particular interest in the effect of reaction conditions on the cis∶trans ratio of the epoxide product. Immobilisation of the chiral Mn–salen complex in Al-MCM-41 increases the cis∶trans ratio of the epoxide product when compared to the non-immobilised complex under the same conditions. Increasing the level of Mn-exchange in the Al-MCM-41 increases the amount of trans-epoxide, whereas increasing the iodosylbenzene∶substrate ratio increases the amount of cis product formed. Increasing the reaction temperature also increases the amount of trans-epoxide for the homogeneous Mn-complex under the same conditions. A series of experiments is described in which the external ion-exchange sites on Al-MCM-41 are preferentially silanised, which enables the cis/trans selectivity for external and internal sites to be determined. Mn–salen immobilised on the external surface of Al-MCM-41 gives the same cis∶trans ratio as that observed with the non-immobilised Mn–salen complex in solution, whereas Mn–salen immobilised within the pores gives the cis-epoxide preferentially.The enantioselection of the immobilised chiral Mn–salen complex is shown to decrease with reaction time at −10 °C, but the cis∶trans epoxide ratio remains unchanged; whereas for the non-immobilised complex in solution the enantioselection is independent of reaction time. Iodobenzene, a decomposition product formed from iodosylbenzene, is found to act as a poison for the immobilised catalyst, leading to a slower reaction and lower enantioselection.

NaY zeolite as host for the selective heterogeneous oxidation of silanes and olefins with hydrogen peroxide catalyzed by methyltrioxorhenium

Abstract: The methyltrioxorhenium(MTO)-catalyzed oxidation of silanes to silanols and the epoxidation of various olefins by aqueous 85% H2O2 proceed in high yields and excellent product selectivities (no disiloxanes, diols) in the presence of the zeolite NaY. The oxidative species is located inside the 12-A supercages. This prevents the bimolecular condensation of the silanol to disiloxane by steric means and the Lewis-acid assisted hydrolysis of the epoxide to the diol.

Synthesis of cryptophycin 52 using the sharpless asymmetric dihydroxylation: diol to epoxide transformation optimized for a base-sensitive substrate.

Abstract: A synthesis of cryptophycin 52 (2) is reported using a Sharpless asymmetric dihydroxylation (AD) strategy to install the epoxide moiety. The high stereoselectivity of the AD reaction that allows for an efficient means of preparing the epoxide is in contrast to the standard direct epoxidation of cryptophycin substrates, which proceeds with poor diastereoselectivity. Methodology for conversion of the diol AD product to the requisite epoxide is disclosed. The transformation has been optimized to proceed in high yield in the presence of base sensitive functionality.

Ultra-selective epoxidation of styrene on pure Cu{111} and the effects of Cs promotion

Abstract: Styrene undergoes efficient epoxidation to styrene epoxide on the Cu{111} surface. At the optimum condition (Θo = 0.03 ML) ∼20% of the styrene is converted to the epoxide with almost 100% selectivity. Comparison with Ag{111} shows that the epoxidation activity and selectivity of Cu greatly exceed those of Ag. Incipient oxidation of the Cu{111} surface does not suppress the adsorption of styrene, but the oxidised metal is catalytically inert. Submonolayer amounts of Cs enhance styrene uptake and increase conversion to the epoxide without adversely affecting epoxidation selectivity. This effect is due to inhibition of Cu oxidation by Cs. Our findings are discussed in the light of current understanding of Ag‐catalysed alkene epoxidation.