- The Stereochemistry of the Cyclic β-Halogeno-ether Synthesis of Olefinic Alcohols
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The (Z)/(E) composition of olefinic alcohols produced by sodium ring-scission of cyclic β-halogeno-ethers can be accounted for by a mechanism involving fast electron transfer and carbanion inversion, with ring-cleavage speedier than conformational inversion.
- Crombie, Leslie,Wyvill, Robert D.
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- STEREOSELECTIVE SYNTHESIS OF CYCLIC ETHERS VIA BROMINE ASSISTED EPOXIDE RING EXPANSION
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9-Oxabicyclonon-4-ene reacts with bromine to give stereoselectively trans,trans-2,6-dibromo-9-oxabicyclononane and trans,trans-2,5-dibromo-9-oxabicyclononane.
- Davies, Stephen G.,Polywka, Mario E. C.,Thomas, Susan E.
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- Mesoporous ZSM-5 zeolite-supported ru nanoparticles as highly efficient catalysts for upgrading phenolic biomolecules
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Zeolite-based catalysts have been widely used in the conversion of biomass recently, but the catalytic yields of the desired products are strongly limited by the relatively small micropores of zeolite. Here, we reported a hierarchically porous ZSM-5 zeolite with micropore and b-axis-aligned mesopore-supported Ru nanoparticles (Ru/HZSM-5-OM) that are highly efficient for the hydrodeoxygenation of both small and bulky phenolic biomolecules to the corresponding alkanes. Compared with the conventional ZSM-5 zeolite-supported Ru catalyst, the high catalytic activities and alkane selectivities over Ru/HZSM-5-OM are attributed to the abundant exposed acidic sites in HZSM-5-OM with open mesopores. This feature is potentially important for future phenolic bio-oil upgrading.
- Wang, Liang,Zhang, Jian,Yi, Xianfeng,Zheng, Anmin,Deng, Feng,Chen, Chunyu,Ji, Yanyan,Liu, Fujian,Meng, Xiangju,Xiao, Feng-Shou
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- Solvomercuration-Demercuration. 8. Oxymercuration-Demercuration of Methoxy-, Hydroxy-, and Acetoxy-Substituted Alkenes
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The oxymercuration-demercuration (OM-DM) of a series of methoxy-, hydroxy-, and acetoxy-substituted alkenes was examined.The systems examined were the allyl, crotyl, 3-buten-1-yl, 4-penten-1-yl, and 5-hexen-1-yl.The methoxyalkenes undergo hydration with very high regioselectivity and almost quantitative yield in all cases.However, a small -I effect is observed in the case of the allylalkene (97.1percent Markovnikov vs. 99.5percent in 1-hexene).Moreover, in the crotyl case, a major directing effect is observed: 97.7percent 3-ol, 2.3percent 2-ol.The other three alkenes undergo the OM reaction with no effect from the methoxy group (99.5percent Markovnikov isomer).In contrast, only allyl-, crotyl-, and 3-buten-1-yl alcohols produce major amounts of hydrated products, the diols.While no hydroxyl group directing effect is observed in the allyl system, a major one is again seen in the case of the crotyl: 93.5percent l,3-diol and 6.5percent l,2-diol.The major products from the 4-penten-1-yl and 5-hexen-1-yl and alcohols are 2-methyltetrahydrofuran and 2-methyltetrahydropyran, respectively, resulting from OH-5 and OH-6 neighboring group participation in the OM stage.The acetoxy alkenes undergo hydration to give diols in ca. 80percent yield with ca. 20percent unreacted starting material.This is the result of a competitive deoxymercuration reaction which is occurring in the DM stage.However, the yield of hydrated products can be increased by varying the amount of base used in the DM.Neighboring-group participation, AcO-5, is observed in the allyl system only, resulting in a 65percent yield of the Markovnikov oxymercurial, by 1H NMR analysis, and a 35percent yield of the acetoxy-exchanged mercurial.Again, a major -I-directing effect of the acetoxy group was observed in the crotyl system but not in the others.In addition to the expected l,2- and l,3-diols, the OM-DM of crotyl acetate also resulted in small amounts of the unexpected 2,3-diol under kinetic conditions.Finally, a modified DM procedure has been developed which is compatible with the acetoxy group.
- Brown, Herbert C.,Lynch, Gary J.
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- CHIMIE ORGANOMETALLIQUE SOUS HAUTE PRESSION: REACTION DES CHLOROCETONES AVEC L'HYDRURE DE TRIBUTYLETAIN
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High-pressure reaction of tributyltin hydride with several chloroketones (3-chloro-2-butanone, 4-chloro-2-butanone, 5-chloro-2-pentanone, 6-chloro-2-hexanone and 7-chloro-2-heptanone) led to the formation of chloroalkoxytins or cyclic ethers.An ionic mechanism, starting with nucleophilic attack at the carbonyl group, is proposed to explain the formation of the reaction products.
- Degueil-Castaing, M.,Maillard, B.,Rahm, A.
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- Strongly Lewis Acidic Metal-Organic Frameworks for Continuous Flow Catalysis
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The synthesis of highly acidic metal-organic frameworks (MOFs) has attracted significant research interest in recent years. We report here the design of a strongly Lewis acidic MOF, ZrOTf-BTC, through two-step transformation of MOF-808 (Zr-BTC) secondary building units (SBUs). Zr-BTC was first treated with 1 M hydrochloric acid solution to afford ZrOH-BTC by replacing each bridging formate group with a pair of hydroxide and water groups. The resultant ZrOH-BTC was further treated with trimethylsilyl triflate (Me3SiOTf) to afford ZrOTf-BTC by taking advantage of the oxophilicity of the Me3Si group. Electron paramagnetic resonance spectra of Zr-bound superoxide and fluorescence spectra of Zr-bound N-methylacridone provided a quantitative measurement of Lewis acidity of ZrOTf-BTC with an energy splitting (?E) of 0.99 eV between the ?x? and ?y? orbitals, which is competitive to the homogeneous benchmark Sc(OTf)3. ZrOTf-BTC was shown to be a highly active solid Lewis acid catalyst for a broad range of important organic transformations under mild conditions, including Diels-Alder reaction, epoxide ring-opening reaction, Friedel-Crafts acylation, and alkene hydroalkoxylation reaction. The MOF catalyst outperformed Sc(OTf)3 in terms of both catalytic activity and catalyst lifetime. Moreover, we developed a ZrOTf-BTC?SiO2 composite as an efficient solid Lewis acid catalyst for continuous flow catalysis. The Zr centers in ZrOTf-BTC?SiO2 feature identical coordination environment to ZrOTf-BTC based on spectroscopic evidence. ZrOTf-BTC?SiO2 displayed exceptionally high turnover numbers (TONs) of 1700 for Diels-Alder reaction, 2700 for epoxide ring-opening reaction, and 326 for Friedel-Crafts acylation under flow conditions. We have thus created strongly Lewis acidic sites in MOFs via triflation and constructed the MOF?SiO2 composite for continuous flow catalysis of important organic transformations.
- Ji, Pengfei,Feng, Xuanyu,Oliveres, Pau,Li, Zhe,Murakami, Akiko,Wang, Cheng,Lin, Wenbin
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supporting information
p. 14878 - 14888
(2019/10/02)
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- Cobalt-Catalyzed Intermolecular Hydrofunctionalization of Alkenes: Evidence for a Bimetallic Pathway
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A functional group tolerant cobalt-catalyzed method for the intermolecular hydrofunctionalization of alkenes with oxygen- and nitrogen-based nucleophiles is reported. This protocol features a strategic use of hypervalent iodine(III) reagents that enables a mechanistic shift from conventional cobalt-hydride catalysis. Key evidence was found supporting a unique bimetallic-mediated rate-limiting step involving two distinct cobalt(III) species, from which a new carbon-heteroatom bond is formed.
- Zhou, Xiao-Le,Yang, Fan,Sun, Han-Li,Yin, Yun-Nian,Ye, Wei-Ting,Zhu, Rong
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supporting information
p. 7250 - 7255
(2019/05/16)
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- Investigation and mechanistic study into intramolecular hydroalkoxylation of unactivated alkenols catalyzed by cationic lanthanide complexes
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Cationic lanthanide complexes of the type [Ln(CH3CN)9]3+[(AlCl4)3]3–·CH3CN (Ln = Pr, Nd, Sm, Gd, Er, Yb, Y) served as effective catalysts for the intramolecular hydroalkoxylation/cyclization of unactivated alkenols to yield the cyclic ethers with Markovnikov regioselectivity under mild conditions. Novel cationic complexes, [AlCl(CH3CN)5]2+[(AlCl4)2]2–·CH3CN and [Nd(CH3CN)9]3+[(FeCl4)3]3–·CH3CN, were synthesized and evaluated for the intramolecular hydroalkoxylation/cyclization of unactivated alkenols for comparison. The active sequence of [Nd(CH3CN)9]3+[(FeCl4)3]3–·CH3CN 3CN)5]2+[(AlCl4)2]2–·CH3CN 3CN)9]3+[(AlCl4)3]3–·CH3CN observed indicated that both the cation and anion have great influence on the activity. Comparative study on the activity of AlCl3and its cationic complex [AlCl(CH3CN)5]2+[(AlCl4)2]2–·CH3CN revealed the formation of the cationic Al center enhanced the activity greatly. The1H NMR studies indicated the activation of hydroxyl and olefin by the cationic Ln3+center were involved in the reaction pathways.
- Zhu, Xuehua,Li, Guoyao,Xu, Fan,Zhang, Yong,Xue, Mingqiang,Shen, Qi
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p. 1451 - 1458
(2017/02/19)
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- Catalytic conversion of sorbitol to gasoline-ranged products without external hydrogen over Pt-modified Ir-ReOx/SiO2
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Deoxygenation of sorbitol was carried out over a Pt-modified Ir-ReOx/SiO2 catalyst in biphasic solvent system (n-decane + H2O) without external hydrogen. Good yield of gasoline-ranged products was obtained including C5-C6 alkanes and C2-C6 mono-functionalized compounds such as ketones, alcohols, cyclic ethers and carboxylic acids. The Pt(3 wt%)-Ir-ReOx/SiO2 catalyst showed the best performance in the production of gasoline-ranged products. The maximum yield of gasoline-ranged products was 42%. The distribution of the products can be tuned by the addition of HZSM-5. The main products were C5-C6 alkanes with addition of HZSM-5 while the main products were C2-C6 mono-functionalized compounds without addition of HZSM-5. Characterizations such as TPR, XRD, TEM, XANES, EXAFS, CO adsorption were performed. The results demonstrated that the Pt-Ir-ReOx/SiO2 catalyst showed the structure of Pt-Ir alloy particles partially covered with ReOx species. The number of surface Pt atoms in Pt(3)-Ir-ReOx/SiO2 was larger than that of Pt/SiO2 or Pt-ReOx/SiO2 because of the small size of Pt-Ir alloy particles. The large number of surface Pt atoms and the synergetic effect of Pt, Ir and ReOx species make the catalyst efficiently generate hydrogen by aqueous phase reforming of sorbitol, and the generated hydrogen is consumed in the hydrogenolysis C-O bonds.
- Liu, Sibao,Okuyama, Yasuyo,Tamura, Masazumi,Nakagawa, Yoshinao,Imai, Akio,Tomishige, Keiichi
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p. 122 - 131
(2016/05/11)
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- Experimental investigation of the low temperature oxidation of the five isomers of hexane
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The low-temperature oxidation of the five hexane isomers (n-hexane, 2-methyl-pentane, 3-methyl-pentane, 2,2-dimethylbutane, and 2,3-dimethylbutane) was studied in a jet-stirred reactor (JSR) at atmospheric pressure under stoichiometric conditions between 550 and 1000 K. The evolution of reactant and product mole fraction profiles were recorded as a function of the temperature using two analytical methods: gas chromatography and synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). Experimental data obtained with both methods were in good agreement for the five fuels. These data were used to compare the reactivity and the nature of the reaction products and their distribution. At low temperature (below 800 K), n-hexane was the most reactive isomer. The two methyl-pentane isomers have about the same reactivity, which was lower than that of n-hexane. 2,2-Dimethylbutane was less reactive than the two methyl-pentane isomers, and 2,3-dimethylbutane was the least reactive isomer. These observations are in good agreement with research octane numbers given in the literature. Cyclic ethers with rings including 3, 4, 5, and 6 atoms have been identified and quantified for the five fuels. While the cyclic ether distribution was notably more detailed than in other literature of JSR studies of branched alkane oxidation, some oxiranes were missing among the cyclic ethers expected from methyl-pentanes. Using SVUV-PIMS, the formation of C 2-C3 monocarboxylic acids, ketohydroperoxides, and species with two carbonyl groups have also been observed, supporting their possible formation from branched reactants. This is in line with what was previously experimentally demonstrated from linear fuels. Possible structures and ways of decomposition of the most probable ketohydroperoxides were discussed. Above 800 K, all five isomers have about the same reactivity, with a larger formation from branched alkanes of some unsaturated species, such as allene and propyne, which are known to be soot precursors.
- Wang, Zhandong,Herbinet, Olivier,Cheng, Zhanjun,Husson, Benoit,Fournet, Rene,Qi, Fei,Battin-Leclerc, Frederique
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p. 5573 - 5594
(2014/08/18)
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- Aqueous-phase hydrogenation and hydrodeoxygenation of biomass-derived oxygenates with bimetallic catalysts
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The reaction rate on a per site basis for aqueous-phase hydrogenation (APH) of propanal, xylose, and furfural was measured over various alumina-supported bimetallic catalysts (Pd-Ni, Pd-Co, Pd-Fe, Ru-Ni, Ru-Co, Ru-Fe, Pt-Ni, Pt-Co, and Pt-Fe) using a high-throughput reactor (HTR). The results in this paper demonstrate that the activity of bimetallic catalysts for hydrogenation of a carbonyl group can be 110 times higher than monometallic catalysts. The addition of Fe to a Pd catalyst increased the activity for hydrogenation of propanal, xylose, and furfural. The Pd1Fe3 catalyst had the highest reaction rate for APH of propanal among all catalysts tested in the HTR. The addition of Fe to the Pd catalyst increased the reaction rate for xylose hydrogenation by a factor of 51, compared to the monometallic Pd catalyst. However, no bimetallic catalyst tested in this study was more active than the monometallic Ru catalyst for hydrogenation of xylose. The Pd1Fe 3 catalyst had the highest reaction rate for APH of furfural, which was 9 times higher than the rate of the Pd catalyst. The Pd1Fe 3/Zr-P, a bimetallic bifunctional catalyst, was 14 times more active on a per site basis than a Pd/Zr-P catalyst for aqueous-phase hydrodeoxygenation (HDO) of sorbitol in a continuous flow reactor. The addition of Fe to the Pd catalyst increased the rate of C-C cleavage reactions and promoted the conversion of sorbitan and isosorbide in HDO of sorbitol. Pd1Fe 3/Zr-P also had a higher yield of gasoline-range products than the Pd/Zr-P catalyst.
- Lee, Jechan,Kim, Yong Tae,Huber, George W.
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supporting information
p. 708 - 718
(2014/02/14)
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- METHOD FOR PREPARING TRIOLS AND DIOLS FROM BIOMASS-DERIVED REACTANTS
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A method to make triols and diols is described. The method includes the steps of performing an aqueous-phase hydrodeoxygenation reaction on a feedstock containing a biomass-derived reactant in aqueous solution. The feedstock is contacted with a heterogeneous metal-containing bifunctional catalyst or a combination of two or more heterogeneous metal-containing catalysts that catalyze cleavage of C—C and C—O bonds, for a time, temperature, pressure, and weight hourly space velocity to yield a product mix comprising triols, diols, or combinations thereof.
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Paragraph 0092; 0093
(2014/09/30)
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- Intramolecular hydroalkoxylation in Bronsted acidic ionic liquids and its application to the synthesis of (±)-centrolobine
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The SO3H-tethered imidazolium and triazolium salts, nonvolatile and recyclable Bronsted acidic ionic liquids, efficiently mediate intramolecular hydroalkoxylations of alkenyl alcohols. They have been successfully employed in the synthesis of (±)-centrolobine.
- Jeong, Yunkyung,Kim, Do-Young,Choi, Yunsil,Ryu, Jae-Sang
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supporting information; experimental part
p. 374 - 378
(2011/02/28)
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- Biocell - Three step process for obtaining a liquid biofuel from cellulose consisting mainly of isomers of methylpyran and methyl tetrahydropyran using N alkyl imidazol chloride as solvent and catalyst
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Three step process for obtaining liquid biofuels consisting mainly of isomers of methylpyran and methyl tetrahydropyran from cellulose, hemicelluloses or starch, using in a first step N alkyl imidazol chloride containing a slight excess of hydrochloric acid 37%. This mixture acts as a solvent and catalyst for dissolution and hydrolysis of cellulose and dehydration of glucose to isomers of hydroxymethyl pyranone (HMP). In a second step the reaction mixture is mixed with a water solution of sodium chloride to increase the ionic strength, and to make possible an extraction with butanol, which otherwise is miscible with the ionic liquid reaction mixture. Butanol was found to be the best solvent for HMP under these conditions. In a third step, the butanol extract is concentrated by distillation, and the concentrate is submitted to hydrogenation with gaseous hydrogen using palladium or platin catalysts on carbon or alumina. The final mixture is separated from butanol by distillation. By GCMS only one relevant product was obtained representing about 90% of final products. It was identified by NMR as 2-methyl pyran.
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Page/Page column 2
(2010/12/29)
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- Efficient intramolecular hydroalkoxylation of unactivated alkenols mediated by recyclable lanthanide lriflate ionic liquids: Scope and mechanism
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Lanthanide triflate complexes of the type [Ln(OTf)3] (Ln = La, Sm, Nd, Yb, Lu) serve as effective, recyclable catalysts for the rapid intramolecular hydroalkoxylation (HO)/cyclization of primary/secondary and aliphatic/aromatic hydroxyalkenes in imidazolium-based room-temperature ionic liquids (RTILs) to yield the corresponding furan, pyran, spirobicyclic furan, spirobicyclic furan/pyran, benzofuran, and isochroman derivatives. Products are straightforwardly isolated from the catalytic solution, conversions exhibit Markovnikov regioselectivity, and turnover frequencies are as high as 47 h -1 at 120°C. The ring-size rate dependence of the primary alkenol cyclizations is 5>6, consistent with a sterically controlled transition state. The hydroalkoxylation/cyclization rates of terminal alkenols are slightly more rapid than those of internal alkenols, which suggests modest steric demands in the cyclic transition state. Cyclization rates of aryl-functionalized hydroxyalkenes are more rapid than those of the linear alkenols, whereas five- and five/six-membered spirobicyclic skeletons are also regioselectively closed. In cyclization of primary, sterically encumbered alkenols, turnoverfrequency dependence on metal-ionic radius decreases by approximately 80fold on going from La3+ (1.160 A) to Lu3+ (0.977 A), presumably reflecting steric impediments along the reaction coordinate. The overall rate law for alkenol hydroalkoxylation/cyclization is v≈[catalys] 1[alkenol]1. An observed ROH/ROD kinetic isotope effect of 2.48 (9) is suggestive of a catalytic pathway that involves kinetically significant intramolecular proton transfer. The present activation parameters-enthalpy (ΔH≠) = 18.2 (9) Kcal mol-1, entropy (ΔS≠) = -17.0 (1.4) eu, and energy (E,) = 18.2 (8) kcal mol-1-suggest a highly organized transition state. Proton scavenging and coordinative probing results suggest that the lanthanide inflates are not simply precursors of free triflic acid. Based on the kinetic and mechanistic evidence, the proposed catalytic pathway invokes hydroxyl and olefin activation by the electron-deficient Ln3+ center, and intramolecular H+ transfer, followed by alkoxide nucleophilic attack with ring closure.
- Dzudza, Alma,Marks, Tobin J.
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supporting information; experimental part
p. 3403 - 3422
(2010/06/21)
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- Efficient intramolecular hydroalkoxylation/cyclization of unactivated alkenols mediated by lanthanide triflate ionic liquids
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Lanthanide triflates, Ln(OTf)3, serve as efficient catalysts for the intramolecular hydroalkoxylation (HO)/cyclization of primary/secondary and aliphatic/aromatic hydroxyalkenes in room temperature ionic liquids (RTlLs). Cyclizations are effective in the formation of five-and sixmembered oxygen heterocycles with Markovnikov-type selectivity. Reaction rates exhibit first-order dependence on [Ln3+] and [substrate].
- Dzudza, Alma,Marks, Tobin J.
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supporting information; experimental part
p. 1523 - 1526
(2009/09/06)
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- Catalytic synthesis of oxygenate from alcohol
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The present invention discloses a method for catalytic synthesis of oxygenate from alcohol. At first, a feeding material comprising at least one alcohol is provided. Next, a copper-containing catalyst is provided and the catalyst further comprises at least one metal element selected from the group consisting of the following: zinc, magnesium, and aluminum elements. Following that, a catalytic reaction of the feeding material over the copper-containing catalyst is carried out to synthesize at least one oxygenate.
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Page/Page column 4-8
(2008/06/13)
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- Rule of five cyclizations in 5-hexenyl radicals and photocycloadditions of 1,5-hexadienes: Effect of 4-oxa substitution
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The effect of 4-oxa substitution on the regiochemistry and rate of 5-hexenyl radical cyclizations was investigated, as a potential model for [2 + 2] photocycloadditions of 2-acyl-4-oxa-1,5-hexadienes. Increasing the electron density in the alkene decreases the rate of cyclization in the 4-oxa-hexejiyl radicals, relative to the all carbon analogs, but has little effect on the regioselectivity of the cyclization. The radical model does not reproduce the high degree of 1,6 closure, observed in the [2 + 2] photocycloadditions for 4-oxa-1,5-hexadiene la. However, the radical model does reinforce the interpretation that ground state conformational effects, engendered by substitution remote from the reacting centers have important rate consequences for cyclization reactions. Copyright
- Matlin, Albert R.,Brinton, Karen Feit,Nivaggioli, Belinda Tsao
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- Catalytic formation of C-O bonds by alkene activation: Lewis acid-cycloisomerisation of olefinic alcohols
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Tin(IV) trifluoromethanesulfonate has been found to be an excellent catalyst for the cycloisomerisation of non-activated and differently substituted olefinic alcohols to cyclic ethers. The Royal Society of Chemistry 2005.
- Coulombel, Lydie,Favier, Isabelle,Dunach, Elisabet
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p. 2286 - 2288
(2007/10/03)
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- Platinum-catalyzed intramolecular hydroalkoxylation of γ- and δ-hydroxy olefins to form cyclic ethers
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Reaction of 2,2-diphenyl-4-penten-1-ol with a catalytic mixture of [PtCl2(H2C=CH2)]2 (1 mol %) and P(4-C6H4CF3)3 (2 mol %) at 70 °C for 24 h led to the isolation of 2-
- Qian, Hua,Han, Xiaoqing,Widenhoefer, Ross A.
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p. 9536 - 9537
(2007/10/03)
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- Neighbouring Hydroxy Group Participation in the acid-catalyzed Reaction of Pentane-1,4-diol and -1,2,5-triol and Hexane-1,5-diol and -1,2,6-triol
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Rate constants have been determined and product analysis made for the acid-catalysed reaction in aqueous solution of ω-primary-secondary polyols HO(CH2)nCHOHX; n = 3 or 4, X = CH3 or CH2OH.The pentane series, n = 3, give five membered cyclic ethers, O-5 ethers in > 90percent yield, while the hexane series, n = 4, give O-6 ethers in ca.35 to 40percent yield.For the pentane series it is known that the cyclisation results from reaction at the hindered primary reaction site, and it is shown here that this is occuring with anchimeric assistance.For the hexane series inter- andintramolecular processes are competitive and anchimeric assistance is slight or absent, while cyclisation at the secondary as well as the pimary reaction site is feasible.It is proposed that neighbouring hydroxy group participation in the reaction of all these polyols is a concerted process.
- Herlihy, Kevin P.,Sagatys, Dalius S.
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p. 501 - 524
(2007/10/03)
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- Ring closure reactions of substituted 4-pentenyl-1-oxy radicals. The stereoselective synthesis of functionalized disubstituted tetrahydrofurans
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N-(Alkyloxy)pyridine-2(1H)-thiones 3 and benzenesulfenic acid O-esters 5 have been synthesized from substituted 4-pentenols 1 or the derived tosylates. Compounds 3 and 5 are efficient sources of free alkoxy radicals 6 which undergo synthetically useful fast ring closure reactions 6 → 8 [k(exo) = (2 ± 1) x 108 s-1 to (6 ± 2) x 109 s-1 (T = 30 ± 0.2°C)]. Tetrahydrofurfuryl radicals 8 can be trapped with, e.g., hydrogen or chlorine atom donors to afford either trans- or cis-disubstituted tetrahydrofurans 10 or 12 depending on the substitution pattern of the 4-pentenyloxy radical. Substituted tetrahydropyrans 11 or 13 are formed in the minor 6-endo-trig cyclization. According to the data of competition kinetics, the observed stereoselectivities in free alkoxy radical cyclizations arise from steric interactions between the substituents in the transition state of the ring closure reactions. Alkyl substituents cause small differences in the measured relative rate constants of 5-exo cyclizations which are reminiscent of the data obtained from the rearrangements of alkyl-substituted 5-hexenyl radicals. Likewise, a stereochemical model for oxygen radical cyclization is proposed where the pentenyloxy chain adopts a six-membered, chairlike transition state with the alkyl substituents preferentially situated in the pseudoequatorial positions leading to 2,5-trans-, 2,4-cis-, and 2,3-trans-substituted tetrahydrofurfuryl radicals 8 as the major intermediates.
- Hartung,Gallou
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p. 6706 - 6716
(2007/10/03)
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- ORGANOTINS AS ETHERIFICATION CATALYSTS. II. CATALYTIC CONVERSION OF ALCOHOLS TO OPEN-CHAIN AND CYCLIC ETHERS BY ORGANOTIN TRICHLORIDES
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1,5-Heptadien-4-ol and various 1,4- and 1,5-glycols are catalytically converted to open-chain and cyclic ethers respectively in the presence of butyltin trichloride.The dehydration of alcohols is mediated by formation of organoalkoxytin dihalides BuSn(OR)Cl2.A mechanism for the formation of these ethers is proposed.The catalytic activity of ether organotins together with SnCl4 has been examined for the conversion of 1,5-pentanediol to THP; the scale of the catalytic efficiency is: MeSnCl3 >= PhSnCl3 > SnCl4 > BuSnCl3 > Me2SnCl2 > Bu2SnCl2 >> (Bu2SnCl)2O.
- Tagliavini, Giuseppe,Marton, Daniele,Furlani, Donatella
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p. 1187 - 1196
(2007/10/02)
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- Synthesis of Cyclic Ethers via Bromine Assisted Epoxide Ring Expansion
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Neighbouring group participation by epoxide oxygen in the opening of bromonium ions results in the stereoselective synthesis of cyclic ethers. 9-Oxabicyclonon-4-ene gives trans, trans-2,6-dibromo-9-oxabicyclononane and trans,trans-2,5-dibromo-9-oxabicyclononane.Sequential bromination and Bu3SnH reduction converts 1,2-epoxyhex-5-ene into cis- and trans-2,5-dimethyltetrahydrofuran and 2-methyltetrahydropyran while (+)-cis-limonene oxide is converted into non-chiral cineole.
- Davies, Stephen G.,Polywka, Mario E. C.,Thomas, Susan E.
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p. 1277 - 1282
(2007/10/02)
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- Studies on the Conversions of Diols and Cyclic Ethers. Dehydration of Alcohols and Diols on the Action of Dimethylsulfoxide
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The transformations of 13 alcohols and 13 diols in the presence of a small amount dimethylsulfoxide (1/16 mol) were studied.Relationships were found between the type of the hydroxy compound and the selectivity of the transformation, and conclusions were drawn regarding the transformation mechanism.The ether formation observed with certain alcohols proceeds via a carbenium cation.The reaction conditions applied were found suitable for including water elimination from the ditertiary 1,2- and 1,3-diols (pinacol rearrangement, 1,2-elimination).From the 1,4- and 1,5-diols the corresponding oxacycloalkanes can be obtained in good yield.Cyclodehydration occurs by intramolecular nucleophilic substitution, via a concerted mechanism.The effect of DMSO is excerted directly, and proton-catalysis occurs simultaneously.
- Molnar, Arpad,Bartok, Mihaly
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p. 389 - 398
(2007/10/02)
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