Showing papers on "Epoxide published in 2006"
TL;DR: Both the enantiomorphic site effect resulting from the chiral electrophile and the polymer chain end effect mainly from the bulky nucleophile cooperatively control the stereochemistry of the CO(2)/epoxide copolymerization.
Abstract: Asymmetric, regio- and stereoselective alternating copolymerization of CO(2) and racemic aliphatic epoxides proceeds effectively under mild temperature and pressure by using a binary catalyst system of a chiral tetradentate Schiff base cobalt complex [SalenCo(III)X] as the electrophile in conjunction with an ionic organic ammonium salt or a sterically hindered strong organic base as the nucleophile. The substituent groups on the aromatic rings, chiral diamine backbone, and axial X group of the electrophile, as well as the nucleophilicity, leaving ability, and coordination ability of the nucleophile, all significantly affect the catalyst activity, polymer selectivity, enantioselectivity, and stereochemistry. A bulky chiral cyclohexenediimine backbone complex [SalcyCo(III)X] with an axial X group of poor leaving ability as the electrophile, combined with a bulky nuclephile with poor leaving ability and low coordination ability, is an ideal binary catalyst system for the copolymerization of CO(2) and a racemic aliphatic epoxide to selectively produce polycarbonates with relatively high enantioselectivity, >95% head-to-tail connectivity, and >99% carbonate linkages. A fast copolymerization of CO(2) and epoxides was observed when the concentration of the electrophile or/and the nucleophile was increased, and the number of polycarbonate chains was proportional to the concentration of the nucleophile. Electrospray ionization mass spectrometry, in combination with a kinetic study, showed that the copolymerization involved the coordination activation of the monomer by the electrophile and polymer chain growth predominately occurring in the nucleophile. Both the enantiomorphic site effect resulting from the chiral electrophile and the polymer chain end effect mainly from the bulky nucleophile cooperatively control the stereochemistry of the CO(2)/epoxide copolymerization.
383 citations
TL;DR: In this paper, a liquid phase catalytic epoxidation of styrene to styrene epoxide was carried out at 373 K using molecular oxygen at atmospheric pressure, in the presence of Co(II)-exchanged zeolite X.
169 citations
TL;DR: An efficient Lewis acid/base catalyst composed of ZnCl2 and phosphonium halide was developed and showed high activity and selectivity for the coupling reaction of CO2 and epoxide under the mild conditions as mentioned in this paper.
Abstract: An efficient Lewis acid/base catalyst composed of ZnCl2 and phosphonium halide ([PR1R2R3R4]X-+(-); X = Cl, Br, I) was developed and showed high activity and selectivity for the coupling reaction of CO2 and epoxide under the mild conditions. The effects of reaction temperature, CO2 pressure, various compositions of the catalysts have been investigated systematically. It was found that a 96.0% conversion of propylene oxide (PO) and high turnover frequency (TOF) value (4718.4h(-1)) could be achieved in the presence of ZnCl2/PPh3C6H13Br (molar ratio = 1:6) at a low constant pressure of CO2 (1.5 MPa) and mild temperature (120 degrees C) without any organic solvents, the catalyst was also proved to be applicable to other terminal epoxides. Additionally, the catalyst could be reused with little loss of catalytic activity after five times. (c) 2006 Elsevier B.V. All rights reserved.
149 citations
122 citations
TL;DR: Screening saturation mutagenesis libraries, in which mutations were introduced in the active site of an engineered P450, followed by recombination of beneficial mutations generated two P450 BM-3 variants that convert a range of terminal alkenes to either (R)- or (S)-epoxide with high catalytic turnovers and high epoxidation selectivities.
Abstract: Cytochrome P450 BM-3 from Bacillus megaterium was engineered for enantioselective epoxidation of simple terminal alkenes. Screening saturation mutagenesis libraries, in which mutations were introduced in the active site of an engineered P450, followed by recombination of beneficial mutations generated two P450 BM-3 variants that convert a range of terminal alkenes to either (R)- or (S)-epoxide (up to 83 % ee) with high catalytic turnovers (up to 1370) and high epoxidation selectivities (up to 95 %). A biocatalytic system using E. coli lysates containing P450 variants as the epoxidation catalysts and in vitro NADPH regeneration by the alcohol dehydrogenase from Thermoanaerobium brockii generates each of the epoxide enantiomers, without additional cofactor.
116 citations
TL;DR: The results show that the expression of epoxide hydrolase genes that are detected by analyses of genomic databases is a useful strategy for obtaining new biocatalysts.
Abstract: Epoxide hydrolases play an important role in the biodegradation of organic compounds and are potentially useful in enantioselective biocatalysis. An analysis of various genomic databases revealed that about 20% of sequenced organisms contain one or more putative epoxide hydrolase genes. They were found in all domains of life, and many fungi and actinobacteria contain several putative epoxide hydrolase-encoding genes. Multiple sequence alignments of epoxide hydrolases with other known and putative alpha/beta-hydrolase fold enzymes that possess a nucleophilic aspartate revealed that these enzymes can be classified into eight phylogenetic groups that all contain putative epoxide hydrolases. To determine their catalytic activities, 10 putative bacterial epoxide hydrolase genes and 2 known bacterial epoxide hydrolase genes were cloned and overexpressed in Escherichia coli. The production of active enzyme was strongly improved by fusion to the maltose binding protein (MalE), which prevented inclusion body formation and facilitated protein purification. Eight of the 12 fusion proteins were active toward one or more of the 21 epoxides that were tested, and they converted both terminal and nonterminal epoxides. Four of the new epoxide hydrolases showed an uncommon enantiopreference for meso-epoxides and/or terminal aromatic epoxides, which made them suitable for the production of enantiopure (S,S)-diols and (R)-epoxides. The results show that the expression of epoxide hydrolase genes that are detected by analyses of genomic databases is a useful strategy for obtaining new biocatalysts.
112 citations
TL;DR: A convergent and highly stereocontrolled synthesis of amphidinolide E (1) has been accomplished and features a highly diastereoselective (>20:1) BF3·Et2O promoted [3+2] annulation reaction between aldehyde 3 and allylsilane 4 to afford substituted tetrahydrofuran 2.
Abstract: A concise total synthesis of the cytotoxic marine natural product amphidinolide X (1) is described. A key step of the highly convergent route to this structurally rather unusual macrodiolide derivative consists of a newly developed, highly syn selective formation of allenol 6 by an iron-catalyzed ring opening reaction of the enantioenriched propargyl epoxide 5 (derived from a Sharpless epoxidation) with a Grignard reagent. Allenol 6 was then cyclized with the aid of Ag(I) to give dihydrofuran 7 containing the (R)-configured quarternary sp3 chiral center at C19 of the target. The anti-configured chiral centers at C10 and C11 were formed by the palladium-catalyzed, Et2Zn-promoted addition of propargyl mesylate 12 to the functionalized aldehyde 11. The key fragment coupling at the C13−C14 bond was achieved by the “9-MeO-9-BBN” variant of the alkyl-Suzuki reaction. Finally, the 16-membered macrodiolide ring was formed by a Yamaguchi esterification/lactonization strategy.
111 citations
TL;DR: Amine-functionalized silica catalysts (NH 2 /SiO 2, NH(CH 2 ) 2 NH 2 / SiO 2 and 1,5,7-triazabicyclo[4,4,0]dec-5-ene/SiO2 (TBD/Si O 2 )), which were characterized by 29 Si NMR, elemental analysis, N 2 adsorption-desorption method and indicator dye adsoreption, were prepared by ultrasonic technique under mild conditions as discussed by the authors.
104 citations
TL;DR: Density functional calculations at the B3LYP level of theory were carried out to understand the structural and energetic aspects of the two possible reaction pathways for the coupling reactions and indicate that Route II is favored over Route I.
Abstract: The heterobimetallic complexes [(η 5 -C 5 H 5 )Ru(CO)(μ-dppm)Mn(CO) 4 ] and [(η 5 -C 5 Me 5 )Ru(μ-dppm)(μ-CO) 2 Mn(CO) 3 ] (dppm= bisdiphenylphosphinomethane) have been prepared by reacting the hydridic complexes [(η 5 -C 5 H 5 )Ru(dppm)H] and [(η 5 -C 5 Me 5 )Ru(dppm)H], respectively, with the protonic [HMn(CO) 5 ] complex. The bimetallic complexes can also be synthesized through metathetical reactions between [(η 5 -C 5 R 5 )Ru-(dppm)Cl] (R=H or Me) and Li + [Mn(CO) 5 ] - . Although the complexes fail to catalyze the hydrogenation of CO 2 to formic acid, they catalyze the coupling reactions of epoxides with carbon dioxide to yield cyclic carbonates. Two possible reaction pathways for the coupling reactions have been proposed. Both routes begin with heterolytic cleavage of the Ru-Mn bond and coordination of an epoxide molecule to the Lewis acidic ruthenium center. In Route I, the Lewis basic manganese center activates the CO 2 by forming the metallocarboxylate anion which then ring-opens the epoxide; subsequent ring-closure gives the cyclic carbonate. In Route II, the nucleophilic manganese center ring-opens the ruthenium-attached epoxide to afford an alkoxide intermediate; CO 2 insertion into the Ru-O bond followed by ring-closure yields the product. Density functional calculations at the B3LYP level of theory were carried out to understand the structural and energetic aspects of the two possible reaction pathways. The results of the calculations indicate that Route II is favored over Route I.
103 citations
TL;DR: In this article, the carboxylic group reacted with epichlorohydrin, and the ring opening of benzoxazine and epoxy took place simultaneously with no catalyst for both monomers, and both ring-opening polymerizations took place when boron trifluoride monoethylamine or 4-(N,N-dimethylamino)pyridine was used as a catalyst for M-1.
Abstract: Benzoxazines derived from aniline and 4-hydroxybenzoic acid and from phenol and 4-aminobenzoic acid were prepared with two different synthetic approaches. When the carboxylic group reacted with epichlorohydrin, glycidylic derivatives M-1 and M-2, respectively, were obtained. The ring opening of benzoxazine and epoxy took place simultaneously with no catalyst for both monomers. Likewise, both ring-opening polymerizations took place when boron trifluoride monoethylamine (BF3·MEA) or 4-(N,N-dimethylamino)pyridine was used as a catalyst for M-1. However, for M-2, when BF3·MEA was used as a catalyst, the epoxy and benzoxazine ring openings could be distinguished, and a polyether intermediate containing benzoxazine side chains could be obtained. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1529–1540, 2006
100 citations
TL;DR: In this article, a readily prepared bimetallic catalyst is capable of effecting epoxide carbonylation to produce β-lactones at substantially lower CO pressures than previously reported catalyst systems.
TL;DR: In this article, double metal cyanide (DMC) catalysts were used for copolymerization of carbon dioxide with various epoxides, such as cyclohexene oxide, cyclopentene oxide and propylene oxide.
TL;DR: In this paper, eight block copolymers with PG contents from 20 to 84 wt % and fixed molecular weight of the middle PPO block of 2000 were prepared by anionic polymerization of ethoxyethyl glycidyl ether (protected glycidol) followed by cleavage of the protective groups.
Abstract: Eight well-defined poly(glycidol)−poly(propylene oxide)−poly(glycidol) (PG−PPO−PG) block copolymers with PG contents from 20 to 84 wt % and fixed molecular weight of the middle PPO block of 2000 were prepared. The copolymers are considered as analogues to the commercially available Pluronic, poly(ethylene oxide)−poly(propylene oxide)−poly(ethylene oxide) (PEO−PPO−PEO), block copolymers in which the PEO blocks are substituted by blocks of linear PG. They were prepared by means of anionic polymerization of ethoxyethyl glycidyl ether (protected glycidol) followed by cleavage of the protective groups. The resulting products bear hydroxyl groups in each repeating glycidol unit. In aqueous solution all studied copolymers were found to self-associate above a certain critical concentration (cmc), which depends on the PG content and temperature. The cmc values were found to decrease with increasing temperature and decreasing the content of the hydrophilic component less strongly than those of the corresponding Plu...
TL;DR: A detailed mechanistic investigation of epoxide carbonylation by the catalyst [(salph)Al(THF)2]+ [Co(CO)4]- (1, salph = N,N'-o-phenylenebis(3,5-di-tert-butylsalicylideneimine), THF = tetrahydrofuran) is reported.
Abstract: A detailed mechanistic investigation of epoxide carbonylation by the catalyst [(salph)Al(THF)2]+ [Co(CO)4]- (1, salph = N,N‘-o-phenylenebis(3,5-di-tert-butylsalicylideneimine), THF = tetrahydrofuran) is reported. When the carbonylation of 1,2-epoxybutane (EB) to β-valerolactone is performed in 1,2-dimethoxyethane solution, the reaction rate is independent of the epoxide concentration and the carbon monoxide pressure but first order in 1. The rate of lactone formation varies considerably in different solvents and depends primarily on the coordinating ability of the solvent. In mixtures of THF and cis/trans-2,5-dimethyltetrahydrofuran, the reaction is first order in THF. From spectroscopic and kinetic data, the catalyst resting state was assigned to be the neutral (β-aluminoxy)acylcobalt species (salph)AlOCH(Et)CH2COCo(CO)4 (3a), which was successfully trapped with isocyanates. As the formation of 3a from EB, CO, and 1 is rapid, lactone ring closing is rate-determining. The favorable impact of donating solv...
TL;DR: DFT calculations suggest that the catalytic epoxidation of olefins by Mo(vi) complexes, modeled by MoO2Br2(MeN=C(H)-C( H)=NMe), in the presence of MeOOH, starts with a hydrogen transfer from the peroxide to one of the terminal Mo=O oxygen atoms and the remaining MeOO anion binds as a seventh ligand.
Abstract: DFT calculations suggest that the catalytic epoxidation of olefins by Mo(vi) complexes, modeled by MoO2Br2(MeN=C(H)-C(H)=NMe), in the presence of MeOOH, the model for tert-butyl hydroperoxide, starts with a hydrogen transfer from the peroxide to one of the terminal Mo=O oxygen atoms and the remaining MeOO anion binds as a seventh ligand, forming a five-membered Mo-O(alpha)-O(beta)(Me)...H-O-Mo ring held together by a hydrogen bond. In the second step, a concerted approach of ethylene to the Mo-O(alpha) bond gives rise to an intermediate containing a seven-membered Mo-C-C-O(alpha)-O(beta)(Me)...H-O-Mo ring. In the final step, decomposition of the intermediate leads to the starting complex, alcohol and the epoxide. The activation energy for the addition of the olefin (second step) is the highest one, in agreement with available kinetic studies showing that the catalyst formation is not always a rate-limiting step. DFT calculations also show that the alcohol by-product (MeOH) can react with the starting complex, competing with ROOH and hence leading to the progressive catalyst poisoning, which has been observed experimentally.
TL;DR: An experimentally simple and inexpensive catalyst system based on hexabutylguanidinium/ZnBr2 has been developed for the coupling of carbon dioxide and epoxides to form cyclic carbonates with significant catalytic activity under mild reaction conditions without using additional organic solvents as mentioned in this paper.
Abstract: An experimentally simple and inexpensive catalyst system based on hexabutylguanidinium/ZnBr2 has been developed for the coupling of carbon dioxide and epoxides to form cyclic carbonates with significant catalytic activity under mild reaction conditions without using additional organic solvents (e.g. the turnover frequencies (TOF, h−1) values as high as 6.6 × 103 h−1 for styrene oxide and 1.01 × 104 h−1 for epichlorohydrin). This catalyst system also offers the advantages of recyclability and reusability. Therefore, it is a very effective, environmentally benign, and simple catalytic process. The special steric and electrophilic characteristics of hexabutylguanidinium bromide ionic liquid result in the prominent performance of this novel catalyst system.
TL;DR: A Pt-catalyzed pentannulation of propargylic esters containing an epoxide moiety achieves the formation of cyclopentenone products as single diastereomers in good yields.
Abstract: A Pt-catalyzed pentannulation of propargylic esters containing an epoxide moiety has been developed. The present transformation achieves the formation of cyclopentenone products as single diastereomers in good yields. The observed products likely form from pyran intermediates that undergo an oxa-6π electrocyclic ring opening to a functionalized dienone, followed by ring closure with an accompanying acyl shift.
TL;DR: In this paper, the authors used manganese(III) salen complexes as catalysts in homogeneous media and encapsulated in NaX and NaY zeolites using tert-butylhydroperoxide as the oxygen source.
Abstract: Styrene oxidation was investigated using manganese(III) salen complexes as catalysts in homogeneous media and encapsulated in NaX and NaY zeolites using tert-butylhydroperoxide as the oxygen source. The oxidation of styrene led to formation of benzaldehyde, styrene oxide and phenylacetaldehyde, with minor amounts of phenyl-1,2-ethane-diol; some polymer formation was also observed. In homogeneous media, both complexes with ligands functionalized with methoxyl groups showed similar styrene conversions and chemoselectivities in styrene epoxide and benzaldehyde. Upon immobilization, [Mn(3-MeOsalen)]@X showed the highest catalytic activity at 60 °C and atmospheric pressure, and the highest chemoselectivities in styrene epoxide and benzaldehyde. No leaching of the encapsulated metal complexes was observed during the catalytic reactions and support for the entrapment of metal complexes inside the zeolite cages and not on their external surfaces was provided by nitrogen adsorption analyses and SEM micrographs. The reaction mechanism was investigated using the most active heterogeneous catalyst, [Mn(3-MeOsalen)]@X, and the influence of various reaction parameters, such as substrate, catalyst concentration and temperature, on alkene conversion and styrene epoxide yield have been studied. Furthermore, the homogeneous oxidation of styrene using [Mn(3-MeOsalen)CH3COO] was monitored by electronic spectroscopy, the results suggesting a free radical mechanism involving a peroxomanganese species as the catalyst active intermediate.
TL;DR: It is demonstrated that previously assumed "pure" SWNTs are actually "doped" to a level of at least 1 oxygen per 250 carbon atoms, which may be used to determine that wet air oxidation is preferable to either acid or O2/SF6 purification.
Abstract: Epoxidation of single-walled carbon nanotubes (SWNTs) may be carried out by the reaction of SWNTs with either trifluorodimethyldioxirane or 3-chloroperoxybenzoic acid; the resulting O−SWNTs are spectroscopically similar to those formed by ozonolysis. Quantification of the epoxide substituents is possible through the catalytic de-epoxidation reaction using MeReO3/PPh3 and the 31P NMR spectroscopy. The de-epoxidation methodology may be used to determine that wet air oxidation is preferable to either acid or O2/SF6 purification. We have demonstrated that previously assumed “pure” SWNTs are actually “doped” to a level of at least 1 oxygen per 250 carbon atoms.
TL;DR: In this paper, 2.2-pyridylbenzimidazole (2-pybmz) and 3-pymz have been covalently anchored to chloromethylated polystyrene crossed-linked with 5% divinylbenzene in DMF in presence of triethylamine in ethylacetate.
Abstract: 2-(2-pyridyl)benzimidazole (2-pybmz) and 2-(3-pyridyl)benzimidazole (3-pybmz) have been covalently anchored to chloromethylated polystyrene crossed-linked with 5% divinylbenzene in DMF in presence of triethylamine in ethylacetate. These chelating resins readily react with non-isolable metal peroxo species, prepared in situ by stirring V2O5 (in presence of aqueous KOH), MoO3 or WO3 · H2O with an excess of 30% H2O2, to give the corresponding oxodiperoxo complexes, PS-K[VO(O2)2(L)] (L = 2-pybmz: 1, L = 3-pybmz: 4), PS-[MoO(O2)2(L)] (L = 2-pybmz: 2, L = 3-pybmz: 5) and PS-[WO(O2)2(L)] (L = 2-pybmz: 3, L = 3-pybmz: 6). PS-2-pybmz and PS-3-pybmz represent polymer-anchored ligands. Structures of these complexes have been established on the basis of spectroscopic (IR and electronic), thermogravimetric studies and elemental analyses. Oxidation of phenol or styrene with selected catalysts was tested using H2O2 as an oxidant. Oxidation of phenol with ca. 35% conversion gave a mixture of catechol and p-hydroquinone where selectivity towards catechol is ca. 62%. Oxidation of styrene gave five products, styrene epoxide, benzaldehyde, benzoic acid, phenylacetaldehyde and 1-phenylethane-1,2-diol. Various parameters such as amount of oxidant and catalyst, and temperature of the reaction mixture have been taken into consideration for the maximum conversion of substrates. These catalysts are recyclable without considerable loss in their catalytic activities. IR spectral data of both freshly prepared and recovered catalysts are also identical, which indicate that the metal complex moiety is intact during the catalytic reaction.
TL;DR: The first structure of a plant epoxide hydrolase, one of the four homologous enzymes found in potato, is reported and comparisons with mammalian and fungal epoxidehydrolase structures reported earlier show the basis of differing substrate specificities in the various epoxide Hydrolase subfamilies.
Abstract: Epoxide hydrolases catalyze the conversion of epoxides to diols. The known functions of such enzymes include detoxification of xenobiotics, drug metabolism, synthesis of signaling compounds, and intermediary metabolism. In plants, epoxide hydrolases are thought to participate in general defense systems. In the present study, we report the first structure of a plant epoxide hydrolase, one of the four homologous enzymes found in potato. The structure was solved by molecular replacement and refined to a resolution of 1.95 A. Analysis of the structure allows a better understanding of the observed substrate specificities and activity. Further, comparisons with mammalian and fungal epoxide hydrolase structures reported earlier show the basis of differing substrate specificities in the various epoxide hydrolase subfamilies. Most plant enzymes, like the potato epoxide hydrolase, are expected to be monomers with a preference for substrates with long lipid-like substituents of the epoxide ring. The significance of these results in the context of biological roles and industrial applications is discussed.
TL;DR: A 1, 4-bis(phenyl)-1,4-dihydro[60]fullerene resulting from an efficient nucleophilic substitution has been obtained by reaction of a fullerene epoxide, C60O, with nucleophile aromatic compounds in the presence of boron trifluoride etherate as a Lewis acid.
15 May 2006
TL;DR: In this paper, the use of 2,3-anhydro-d-gulofuranosyl thioglycosides and glycosyl sulfoxides in the synthesis of α-d -galactofuranidic bonds was reported.
Abstract: We report here the use of 2,3-anhydro-d-gulofuranosyl thioglycosides and glycosyl sulfoxides in the synthesis of α-d-galactofuranosidic bonds, which are present in a range of bacterial and fungal glycoconjugates. This two-step method involves a stereoselective glycosylation in which a 2,3-anhydro-α-d-gulofuranoside is obtained either as the sole or as the major product, followed by a regioselective opening of the epoxide ring using lithium benzylate in the presence of (−)-sparteine. In exploring the scope of the method, donors protected at O5 and O6 with an isopropylidene acetal, benzyl ethers, or benzoate esters were studied. Overall, the glycosyl sulfoxides provided the products in slightly higher yields and selectivity, with the best results being obtained with benzylated and benzoylated substrates. In the epoxide ring-opening reactions, the acetal- and ether-protected donors afforded poor to modest regioselectivity, whereas the benzoylated products gave good yields of the desired α-d-galactofuranoside...
TL;DR: The energetics of the reaction mechanism as well as the optimized intermediates and transition-state structures are presented and full potential energy curves for the reactions involving nucleophilic attack at either the benzylic or the homo-benzylic carbon atom of (1S,2S)-beta-methylstyrene oxide have been computed.
Abstract: The complete reaction mechanism of soluble epoxide hydrolase (sEH) has been investigated by using the B3LYP density functional theory method. Epoxide hydrolases catalyze the conversion of epoxides to their corresponding vicinal diols. In our theoretical study, the sEH active site is represented by quantum-chemical models that are based on the X-ray crystal structure of human soluble epoxide hydrolase. The trans-substituted epoxide (1S,2S)-beta-methylstyrene oxide has been used as a substrate in the theoretical investigation of the sEH reaction mechanism. Both the alkylation and the hydrolytic half-reactions have been studied in detail. We present the energetics of the reaction mechanism as well as the optimized intermediates and transition-state structures. Full potential energy curves for the reactions involving nucleophilic attack at either the benzylic or the homo-benzylic carbon atom of (1S,2S)-beta-methylstyrene oxide have been computed. The regioselectivity of epoxide opening has been addressed for the two substrates (1S,2S)-beta-methylstyrene oxide and (S)-styrene oxide.
TL;DR: This paper describes the development of green, efficient H(2)O(2)-based epoxidation systems with three kinds of polyoxometalates, and the system with III shows unique stereospecificity, diastereoselectivity, and regioselectivities for the epoxidated of cis/trans olefins, 3-substituted cyclohexenes, and nonconjugated dienes, respectively.
Abstract: This paper describes the development of green, efficient H2O2-based epoxidation systems with three kinds of polyoxometalates: (i) a dinuclear peroxotungstate [W2O3(O2)4(H2O)2]2− (I), (ii) a divacant lacunary polyoxotungstate [γ-SiW10O34(H2O)2]4 (II), (iii) and a divanadium-substituted polyoxotungstate [γ-1,2-H2SiV2W10O40]4− (III). The highly chemo-, regio-, and diastereoselective epoxidation of various allylic alcohols with only 1 equiv H2O2 in water can be efficiently catalyzed by potassium salt of I (K-I). The catalyst K-I can be recycled with the retention of the catalytic performance. Protonation of a divacant lacunary polyoxotungstate [γ-SiW10O36]8− gives [γ-SiW10O34(H2O)2]4− (II) with two aquo ligands. The tetra-n-butylammonium salt of II (TBA-II) catalyzes epoxidation of common olefins including propylene with ≥99% selectivity to epoxide and ≥99% efficiency of H2O2 utilization. The bis(μ-hydroxo)bridged dioxovanadium site in [γ-1,2-H2SiV2W10O40]4− (III) can also efficiently catalyze epoxidation of a variety of olefins with 1 equiv H2O2. Notably, the system with III shows unique stereospecificity, diastereoselectivity, and regioselectivity for the epoxidation of cis/trans olefins, 3-substituted cyclohexenes, and nonconjugated dienes, respectively, which are quite different from those reported for epoxidation systems up to now. Furthermore, the heterogenization of the mentioned polyoxometalates can be achieved by using ionic liquid-modified SiO2 as a support without loss of catalytic performance. © 2006 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 6: 12–22; 2006: Published online in Wiley InterScience (www.interscience.wiley.com) DOI 10.1002/tcr.20067
TL;DR: In this article, a homogeneous binary catalyst system, n-Bu4NBr/n-Bu3N, was found to be active for the synthesis of dimethyl carbonate from styrene oxide (SO), methanol, and supercritical CO2.
Abstract: A homogeneous binary catalyst system, n-Bu4NBr/n-Bu3N, was found to be active for the synthesis of dimethyl carbonate from styrene oxide (SO), methanol, and supercritical CO2 Under the optimized conditions, the dimethyl carbonate yield could reach 84% at SO conversion of 98% Several parameters were studied, ie catalyst precursors, reaction time and temperature, methanol/epoxide feed ratio in moles, and CO2 pressure The best compromise for the one-pot synthesis was achieved with an equimolar amount of n-Bu4NBr/n-Bu3N A possible mechanism for the present n-Bu4NBr/n-Bu3N-catalyzed one-pot synthesis of dimethyl carbonate was proposed
TL;DR: In this paper, the epoxide conversion was carried out over a sodium-promoted aluminum oxide, and the oxidized form of anthraquinone also underwent a disproportion reaction.
Abstract: Epoxided alkyltetrahydroanthraquinone (epoxide) is formed as a byproduct in the oxidation of hydrogenated alkyltetrahydroanthraquinone in hydrogen peroxide production Because epoxide is unable to form hydrogen peroxide it has to be converted back to active quinones The epoxide conversion was carried out over a sodium-promoted aluminum oxide Besides the main reaction, the oxidized form of anthraquinone also underwent a disproportion reaction The conversion kinetics was registered in an isothermal slurry reactor operating batchwise at 60−80 °C A kinetic model based on plausible surface reaction steps was derived, and the parameters included in the model were estimated by nonlinear regression analysis The model explained the experimental data with satisfactory accuracy and can be used to predict the epoxide conversion kinetics over commercial catalysts
TL;DR: Both the individual catalytic action of the peptide strands and the helical conformation as the catalytically competent state of the synthesised peptide catalysts were further supported.
Abstract: In the presence of short solid-phase bound peptide catalysts, the Julia-Colonna epoxidation of enones (such as chalcone) with hydrogen peroxide can be performed with high enantiomeric excess (> or = 95% ee). It was proposed earlier (A. Berkessel, N. Gasch, K. Glaubitz, C. Koch, Organic Letters, 2001, Vol. 3, pp. 3839-3842) that this remarkable catalysis is governed by the N-terminus of individual and helical peptide strands. This mechanistic proposal was scrutinized further. (i) Nonaggregation of the peptide catalysts: five solid-phase bound statistic mixtures (0/100; 30/70; 50/50; 70/30; 100/0) of D-Leu and L-Leu heptamers were generated and assayed as catalysts. A linear dependence of the epoxide ee on the enantiomeric composition of the catalysts resulted. (ii) Catalyst helicity [introduction of the helix-stabilizing C(alpha)-methyl-L-leucine, L-(alphaMe)Leu]: solid-phase bound Leu/(alphaMe)Leu-pentamers of composition TentaGel-NH-[(alphaMe)-L-Leu]n-(L-Leu)m-H (n = 0-4; m = 5-n) were prepared and assayed as catalysts. The introduction of up to two (alphaMe)-L-Leu residues (n = 1, 2) significantly enhanced the catalyst activity relative to the L-Leu homopentamer (n = 0). Higher (alphaMe)-L-Leu contents (n = 3, 4) led to a decrease in both catalyst activity and enantiopurity of the product epoxide. In summary, both the individual catalytic action of the peptide strands and the helical conformation as the catalytically competent state of the peptide catalysts were further supported.