- INTRAMOLECULAR CYCLIZATIONS OF ORGANOMETALLIC COMPOUNDS IV. HETEROATOM INFLUENCE ON CYCLIZATIONS OF ORGANOLITHIUM COMPOUNDS
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Alkoxyl groups in alkenyllithiums can influence the stereochemistry of cyclization.The presence of n-butyllithium increases the stereoselectivity such that only one stereochemistry results; the presence of TMEDA negates the heteroatom's influence so that only the other stereochemistry results.Yields are 33percent to 44percent.
- Smith, Michael J.,Wilson, Stanley E.
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- INTRAMOLECULAR CYCLIZATIONS OF ORGANOMETALLIC COMPOUNDS V. 6,6-BIS(DIETHYLALUMINUM)-1-HEXENES TO SUBSTITUTED CYCLOPENTANES.
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3-Substituted-hex-1-en-5-ynes were cyclized by treatment with diethylaluminum hydride; in hydrocarbon/ether solvent, trans-2-substituted-methylcyclopentane is produced, while in the absence of ether, the product is 2-substituted-methylenecyclopentane.Yields are 69percent to 76percent.
- Smith, Michael J.,Wilson, Stanley E.
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- Insights into the Major Reaction Pathways of Vapor-Phase Hydrodeoxygenation of m-Cresol on a Pt/HBeta Catalyst
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Conversion of m-cresol was studied on a Pt/HBeta catalyst at 225-350°C and ambient hydrogen pressure. At 250°C, the reaction proceeds through two major reaction pathways: (1) direct deoxygenation to toluene (DDO path); (2) hydrogenation of m-cresol to methylcyclohexanone and methylcyclohexanol on Pt, followed by fast dehydration on Br?nsted acid sites (BAS) to methylcyclohexene, which is either hydrogenated to methylcyclohexane on Pt or ring-contracted to dimethylcyclopentanes and ethylcyclopentane on BAS (HYD path). The initial hydrogenation is the rate-determining step of the HYD path as its rate is significantly lower than those of subsequent steps. The apparent activation energy of the DDO path is 49.7 kJ mol-1 but the activation energy is negative for the HYD path. Therefore, higher temperatures lead to the DDO path becoming the dominant path to toluene, whereas the HYD path, followed by fast equilibration to toluene, is less dominant, owing to the inhibition of the initial hydrogenation of m-cresol.
- Sun, Qianqian,Chen, Guanyi,Wang, Hua,Liu, Xiao,Han, Jinyu,Ge, Qingfeng,Zhu, Xinli
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p. 551 - 561
(2016/02/20)
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- Catalytic activity of Mo oxide before and after alkali metal addition for methylcyclohexane and methylcyclopentane compounds
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Abstract Different catalytic reactions of methylcyclohexane MCH are performed depending on the nature of the catalytic active site (s) and experimental conditions. Ring contraction RC catalytic processes, producing dimethylcyclopenanes DMCP's of high octane numbers as compared to MCH are catalysed by acidic function of zeolites systems such as HY. Better activity, selectivity and stability concerning these RC reactions were obtained using Pt/HY catalyst. At higher reaction temperature, dehydrogenation of MCH to toluene and hydrocracking reactions are catalyzed by Pt. Comparable catalytic behavior is obtained using a bifunctional (metal-acid) MoO2-x(OH)y/TiO2 (MoTi) system. Different metallic character strength is observed following the suppression of the Br?nsted acid MoOH function(s) to MoO2-x(OA)y/TiO2 (A = Na, K, Rb) by the addition of small amount of alkali metal A. Rubidium addition seems to be the most performant in the dehydrogenation of MCH to toluene. The metallic functions in MoTi and modified AMoTi are not efficient for RO in MCP. In-situ characterization of the different oxidation states of Mo at different experimental conditions were conducted using in-situ XPS-UPS techniques.
- Al-Kandari,Mohamed,Al-Kharafi,Katrib
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p. 189 - 193
(2015/07/27)
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- Synthesis, reactivity, and catalytic application of a nickel pincer hydride complex
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The nickel(II) hydride complex [(MeN2N)Ni-H] (2) was synthesized by the reaction of [(MeN2N)Ni-OMe] (6) with Ph2SiH2 and was characterized by NMR and IR spectroscopy as well as X-ray crystallography. 2 was unstable in solution, and it decomposed via two reaction pathways. The first pathway was intramolecular N-H reductive elimination to give MeN2NH and nickel particles. The second pathway was intermolecular, with H2, nickel particles, and a five-coordinate Ni(II) complex [(MeN2N)2Ni] (8) as the products. 2 reacted with acetone and ethylene, forming [( MeN2N)Ni-OiPr] (9) and [(MeN 2N)Ni-Et] (10), respectively. 2 also reacted with alkyl halides, yielding nickel halide complexes and alkanes. The reduction of alkyl halides was rendered catalytically, using [(MeN2N)Ni-Cl] (1) as catalyst, NaOiPr or NaOMe as base, and Ph2SiH2 or Me(EtO)2SiH as the hydride source. The catalysis appears to operate via a radical mechanism.
- Breitenfeld, Jan,Scopelliti, Rosario,Hu, Xile
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experimental part
p. 2128 - 2136
(2012/06/01)
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- Study of Ir/WO3/ZrO2-SiO2 ring-opening catalysts: Part II. Reaction network, kinetic studies and structure-activity correlation
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The present paper is the second part of a systematic study of the influence of W and Ir loading on the activity of Ir/WO3/ZrO2-SiO2 catalysts for the ring-opening reaction of naphthenic molecules using methylcyclohexane (MCH) as a model compound. A series of Si-stabilized tungstated zirconias, WOx/ZrO2-SiO2, containing up to 3.5 atom W/nm2, was prepared. Ir-based catalysts containing up to 1.2 wt% were obtained by impregnation of these solids. Characterization of the metal dispersion and catalyst acidity was described in a previous article. The objective of the present study was to determine the best metal/acid balance for optimal performance of Ir/WOx/ZrO2-SiO2 catalysts in the ring-opening reaction of MCH. Monofunctional (acid WOx/ZrO2-SiO2 or metal Ir/ZrO2-SiO2) and bifunctional (Ir/WO3/ZrO2-SiO2) catalysts were examined. Based on the analysis of the yields and products distributions, a reaction network was proposed, and kinetic data (e.g., activation energies, initial rates) were calculated. Correlations between characterization results obtained earlier (e.g., acidity, dispersion) and catalytic performance are also reported. The monofunctional acid catalysts WOx/ZrO2-SiO2 showed a low selectivity for ring opening. The ring-contraction activity developed for W surface density above a threshold value of 1 atom W/nm2. This was attributed to the appearance and the development of a relatively strong Broensted acidity monitored by infrared measurements. MCH ring contraction and C5 naphthene ring opening occur according to a classic acid mechanism. For low conversions, the monofunctional metal catalysts Ir/ZrO2-SiO2 exhibited significant selectivity for ring opening that decreased with increasing conversion. Because of the lack of ring-contraction products, the observed activity was attributed to the direct ring opening of the MCH. Ring opening and cracking occur according to a dicarbene mechanism. The study of MCH conversion on Ir/WOx/ZrO2-SiO2 catalysts indicated that MCH ring contraction to alkylcyclopentanes occurs before ring opening. The best yields for ring opening were obtained with the 1.2% Ir/WOx/ZrO2 (1.5 atom of W/nm2). Further increases in W surface density led to a decrease in the indirect ring-opening yield, attributed to a decrease in Ir dispersion. For bifunctional metal/acid catalysts, analysis of the mechanism is less straightforward. The activation energy for C6 ring contraction and indirect C6 ring opening is a function of the metal/acid ratio. For high ratios, indirect ring opening occurs essentially over metallic sites. A decrease in the metal/acid ratio enhances the contribution of acid mechanism.
- Lecarpentier, Sebastien,van Gestel, Jacob,Thomas, Karine,Gilson, Jean-Pierre,Houalla, Marwan
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- Avoiding olefin isomerization during decyanation of alkylcyano α,ω-dienes: A deuterium labeling and structural study of mechanism
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(Chemical Equation Presented) A two-step synthetic pathway involving decyanation chemistry for the synthesis of pure alkyl α,ω-dienes in quantitative yields is presented. Prior methodologies for the preparation of such compounds required 6-9 steps, sometimes leading to product mixtures resulting from olefin isomerization chemistry. This isomerization chemistry has been eliminated. Deuteration labeling and structural mechanistic investigations were completed to decipher this chemistry. Deuterium labeling experiments reveal the precise nature of this radical decyanation chemistry, where an alcohol plays the role of hydrogen donor. The correct molecular design to avoid competing intramolecular cyclization, and the necessary reaction conditions to avoid olefin isomerization during the decyanation process are reported herein.
- Rojas, Giovanni,Wagener, Kenneth B.
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p. 4962 - 4970
(2008/12/20)
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- A novel reduction of polycarboxylic acids into their corresponding alkanes using n-butylsilane or diethylsilane as the reducing agent
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A convenient one-pot reaction has been developed for the reduction of polycarboxylic acids on aliphatic and aromatic systems to their corresponding alkanes. The reduction utilises either diethylsilane or n-butylsilane as the reducing agent in the presence of the Lewis acid catalyst tris(pentafluorophenyl)borane.
- Nimmagadda, Rama D.,McRae, Christopher
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p. 3505 - 3508
(2007/10/03)
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- Methylcyclohexane ring-contraction: A sensitive solid acidity and shape selectivity probe reaction
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In this paper we describe the utility of an acid-catalyzed isomerization reaction, specifically, ring-contraction of methylcyclohexane to an isomeric mixture of alkylcyclopentanes as a tool for characterizing the acidic properties of a wide range of platinum-loaded solid acids. Methylcyclohexane isomerization is particularly useful as a solid acidity probe reaction since it is a simple molecule containing one six-membered ring and a single methyl group substituent. As a solid acidity probe molecule methylcyclohexane has a number of advantages over cyclohexane. Ring-contraction of cyclohexane produces a single product, methlycyclopentane. Methylcyclohexane ring-contraction, in contrast, yields a richer and thus more informative product mixture including ethylcyclopentane, and five isomeric dimethylcyclopentanes. For the first time it will be shown that variations in the three primary descriptors of solid acids, acid site density, acid site strength, and shape selectivity, within a wide range of amorphous and crystalline solid acids can be simultaneously ranked using a single component probe reaction, namely, methylcyclohexane ring-contraction.
- Mcvicker, Gary B.,Feeley, Owen C.,Ziemiak, John J.,Vaughan, David E. W.,Strohmaier, Karl C.,Kliewer, Wayne R.,Leta, Daniel P.
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p. 2222 - 2226
(2007/10/03)
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- Isomerization of cycloheptane, cyclooctane, and cyclodecane catalyzed by sulfated zirconia - Comparison with open-chain alkanes
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The skeletal isomerization of cycloalkanes with the number of carbons greater than six, cycloheptane, cyclooctane, cyclodecane, and cyclododecane, was performed over sulfated zirconia in liquid phase at 50°C. A main product of methylcyclohexane was formed from cycloheptane via a protonated cyclopropane intermediate, protonated [4.1.0]bicycloheptane, together with small amounts of trans-1,2-dimethylcyclopentane, as- and trans-1,3- dimethylcyclopentanes, 1,1-dimethylcyclopentane, and ethylcyclopentane. A major product from cyclooctane was ethylcyclohexane via a protonated cyclobutane intermediate, protonated [4.2.0]bicyclooctane, followed by cis-1,3- dimethylcyclohexane in addition to small amounts of trans-1,2-, -1,3-, -1,4-dimethylcyclohexanes, 1,1-dimethylcyclohexane, and methylcycloheptane. The detailed reaction-paths for cycloheptane and cyclooctane were shown after additional examinations in reactions of methylcyclohexane, ethylcyclopentane, ethylcyclohexane, and 1,2-dimethylcyclohexane. Cyclodecane was dehydrogenated into cis- or trans-decaline with the evolution of a dihydrogen. Cyclododecane was converted into lots of products, more than 30 species.
- Satoh, Daishi,Matsuhashi, Hiromi,Nakamura, Hideo,Arata, Kazushi
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p. 4343 - 4349
(2007/10/03)
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- Catalytic cycloisomerization of unsaturated organoiodides
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Catalytic quantities of phenyllithium (PhLi) have been found to initiate novel 5-exo cycloisomerization of a variety of structurally diverse unsaturated organoiodides. The isomerization reaction appears to be a process of broad synthetic utility for the preparation of iodomethyl-substituted five-membered rings. Primary, secondary, tertiary, or aryl iodides tethered to a suitably positioned carbon-carbon π-bond are converted cleanly to their cyclic isomers in good to excellent yield (i.e., 70-90%) by simply allowing a hydrocarbon-MTBE solution of the iodide to stand in the presence of a small quantity of PhLi at an appropriate temperature. The mechanism of the cycloisomerization was found to be substrate dependent: unsaturated aryl and primary alkyl iodides undergo isomerization via a three-step cascades (eqs 1- 3) mediated by two reversible lithium-iodine exchange reactions bracketing an irreversible 5-exo cyclization of an unsaturated organolithium; unsaturated secondary and tertiary alkyl iodides apparently isomerize via a radical- mediated atom transfer process initiated by homolytic fragmentation of the ate-complex generated upon attack of PhLi on the iodine atom of the substrate.
- Bailey, William F.,Carson, Matthew W.
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p. 9960 - 9967
(2007/10/03)
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- Pyrolysis of Alkyl Acetates. A Radical Pathway for the Formation of Minor Products
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Decyl acetate and decene pyrolysis under the same reaction conditions show that the formation of the minor products during the pyrolysis of esters occurs by a parallel, and not a secondary, reaction.The radioactivity observed in the CO2 and CO produced during the pyrolysis of 1-hexyl acetate-1-14C and the radioactivity in the methane from the pyrolysis of 1-hexyl acetate-2-14C strongly suggested a radical reaction pathway for the formation of the minor products.Considering the above results together with the radioactivity data of gas samples from the pyrolysis of 1-decyl-1-14C acetate, 1-hexyl-1-14C acetate, and 1-heptadecyl-1-14C acetate leads to the conclusion that the mechanism of pyrolysis of esters may be viewed as a cyclo-DEDNAN mechanism for the formation of the major alkene product and a radical mechanism for the formation of minor products.A useful synthetic procedure for the preparation of high carbon number alkenes (>10) also results from this study.
- Shi, Buchang,Ji, Ying,Dabbagh, Hossein A.,Davis, Burtron H.
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p. 845 - 849
(2007/10/02)
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- Stereoselective Cyclomagnesiation of 1,6-Heptadienes Catalyzed by Zirconocenes: Effect of Substituents, Solvent and Magnesium Reagents
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Zirconocene dichloride acts as a catalyst precursor for the stereoselective cyclomagnesiation of an array of substituted 1,6-heptadienes in the presence of butylmagnesium chloride to form trans 1,2-(bis)methylmagnesium substituted carbocycles.
- Wischmeyer, Ulrike,Knight, Kyle S.,Waymouth, Robert M.
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p. 7735 - 7738
(2007/10/02)
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- Rate Constants and Arrhenius Parameters for the Reactions of Some Carbon-Centered Radicals with Tris(trimethylsilyl)silane
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Rate constants for the reactions of some carbon-centered radicals with (Me3Si)3SiH have been measured over a range of temperatures by using competing unimolecular radical reactions as timing devices.For example, the rate constants (at 298 K) are 3.7, 1.4, and 2.6 x 1E5 M-1 s-1 from primary, secondary, and tertiary alkyl radicals, respectively.Comparison of the radical trapping abilities of tri-n-butylstannane and tris(trimethylsilyl)silane is discussed.The use of 1,1-dimethyl-5-hexenyl cyclization as a radical clock has been recalibrated by using new data and data from the literature.
- Chatgilialoglu, C.,Dickhaut, J.,Giese, B.
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p. 6399 - 6403
(2007/10/02)
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- Catalysis with Palladium Deposited on Rare Earth Oxides: Influence of the Support on Reforming and Syngas Activity and Selectivity
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The influence of the support has been tested on the reactivity of Pd/rare earth oxides catalysts (La2O3, CeO2, Pr6O11, Nd2O3, Tb4O7).According to BET surface area, chemisorption, temperature-programmed reduction (TPR) and oxidation (TPO), X-ray diffraction (XRD) and X-ray photoemission (XPS) characterizations, these catalysts have been classified into threeclasses according to their ability to create anion vacancies: (i) oxides of the type Re2O3 which are unreducible, (ii) CeO2 where anion vacancies can be created extrinsically by the reduction process, and (iii) Pr6O11 and Tb4O7 where anion vacancies exist due to the nonstoichiometric nature of these oxides.We emphasize also the role of chlorine, coming from the palladium precursor salt, which reacts with the support to form a stable oxychloride phase surrounding the metallic particle and interacting with it.Concerning the catalytic activity, (i) the active site is purely metallic in methylcyclopentane hydrogenolysis, with small selectivity changes on fluorite oxides as compared to Pd/Al2O3 catalysts due to some electronic interaction with the support, but (ii) the mechanism is found to be partly bifunctional in 3-methylhexane aromatization with a large increase in aromatization on Pr6O11 and Tb4O7 supports, and (iii) in syngas conversion, production of high alcohols occurs at the metal-support interface and is favored by the presence of intrinsic anion vacancies on Pr6O11 and Tb4O7 supports.A correlation is found between the density of anion vacancies on these supports and the chain growth probability deduced from the Anderson-Schulz-Flory plot.
- Normand, F. Le,Barrault, J.,Breault, R.,Hilaire, L.,Kiennemann, A.
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p. 257 - 269
(2007/10/02)
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- Kinetics for the Reaction of a Secondary Alkyl Radical with Tri-n-butylgermanium Hydride and Calibration of a Secondary Alkyl Radical Clock Reaction
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Arrhenius parameters for the reaction of a secondary alkyl radical with tri-n-butylgermanium hydride have been measured by using the cyclization of 1-methyl-5-hexenyl radical as a "clock" reaction.At 298 K the rate constant is 1.8*104 M-1s-1, which makes the secondary alkyl radical/n-Bu3GeH reaction about 80 times slower than the corresponding reaction with tri-n-butyltin hydride.The secondary alkyl radical clock reaction has been rather precisely calibrated by using new data and data from the literature.At attempt to carry out similar experiments with 1,1-dimethyl-5-hexenyl yielded much less precise data for the cyclization o f this tertiary alkyl radical.Reliable kinetic data for hydrogen abstraction from n-Bu3GeH by tertiary alkyl radicals could not be obtained by using either the parent bromide or appropriate N-hydroxypyridine-2-thione esters as alkyl radical sources.
- Lusztyk, J.,Maillard, B.,Deycard, S.,Lindsay, D. A.,Ingold, K. U.
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p. 3509 - 3514
(2007/10/02)
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- Single Electron Transfer in Metal-Halogen Exchange. The Reaction of Organolithium Compounds with Alkyl Halides
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Halogen-metal exchange has been studied by allowing tert-butyllithium to react with a series of alkyl halides containing a cyclizable radical probe in order to evaluate the occurrence of a radical intermediate in the reaction.It was found that radical intermediates, formed via a single-electron-transfer pathway, are involved in the reactions of t-BuLi with the radical probe, endo-5-(2-bromoethyl)-2-norbornene in pentane/Et2O at -78 deg C, since cyclized hydrocarbons were formed during the reaction.However, there was no evidence to support an electron-transfer pathway in reactions of the corresponding iodide and chloride with t-BuLi under the same conditions since only the straight-chain organolithium compound was formed.However, evidence indicative of a radical intermediate was obtained from reactions of the iodo compound with t-BuLi in pentane/Et2O at higher temperatures (-45 deg C and -23 deg C) and in pure pentane at -78 deg C and -23 deg C in which stable cyclized hydrocarbon product was obtained.Lithium complexing agents such as TMEDA, HMPA, and 18-crown-6 were employed in these reactions in order to increase the carbanionic nature of the organolithium products.It was found that cyclization of the straight-chain organolithium compound to the corresponding cyclized organolithium compound in the presence of such complexing agents is relatively slow.On the other hand, the effectiveness of the complexing agents to increase carbanion character of the straight-chain organolithium compound was demonstrated by a significant lowering of the deuterium content of the straight-chain product due to ether cleavage by the straight-chain organolithium compound.Reactions of the 6-halo-1-heptenes with t-BuLi were also examined.It was shown that 6-bromo-1-heptene reacted with t-BuLi via SET pathway in both pentane/Et2O and pentane at -78 deg C since cyclized product with a high cis/trans ratio was the major product in these reactions.On the other hand, the reaction of the corresponding iodide with t-BuLi in pentane/Et2O at -78 deg C produced a high yield of cyclized product with a low cis/trans ratio (1.7).The reaction of the alkyl iodide with t-BuLi in pure pentane produced the cyclized product with a cis/trans ratio (4.0) indicative of a radical intermediate.Lithium complexing agents were also added to these reaction mixtures, in which the product ratio remained constant although the deuterium concentration decreased.Hence, the presence of the cyclized products with a high cis/trans ratio in the presence of cation complexing agents is indicative of the formation of radical intermediates both in the presence and absence of such intermediates.
- Ashby, E. C.,Pham, Tung N.
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p. 1291 - 1300
(2007/10/02)
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- NIOBIUM SULFUR SYSTEM: INFLUENCE OF S2-- GROUPS ON THE CATALYTIC PROPERTIES IN HYDROGENATION REACTIONS
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Unsupported NbS3 was prepared by direct combination of the elements and "NbS2" or Nb1.12S2 were obtained by thermal decomposition of the former.These samples were characterized by XRD and XPS and their activity was measured in the hydrogenation of biphenyl under medium pressure conditions.Compared to WS2, all these samples present higher activity and a peculiar ability to perform cracking reaction.These differences have been explained by the active site occurrence probability which is higher in NbS3 due to the reducibility of S2-- pairs.Such properties of niobium sulfides have been also evidenced in other hydrogenation reactions and have shown the potentiality of niobium sulfur system as catalyst for reactions involving hydrogenation and C-C bond cleavage.
- Vrinat, M.,Guillard, C.,Lacroix, M.,Breysse, M.,Kurdi, M.,Danot, M.
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p. 1017 - 1025
(2007/10/02)
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- SUPPRESSING THE CYCLIZATION OF (1-METHYL-5-HEXENYL)SODIUM
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In reactions of 1-methyl-5-hexenyl chloride and bromide with sodium metal and sodium naphthalene in DME and THF, the cyclization of (1-methyl-5-hexenyl)sodium is suppressed by added tert-butylamine.Since the cyclization of 1-methyl-5-hexenyl radical does not appear to be affected, this demonstrates the practicality of using the 1-methyl-5-hexenyl group as a probe for radical intermediates in the presence of tert-butylamine.
- Garst, John F.,Hines, John B.,Bruhnke, John D.
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p. 1963 - 1966
(2007/10/02)
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- EVIDENCE FOR SINGLE ELECTRON TRANSFER IN METAL-HALOGEN EXCHANGE. THE REACTION OF ORGANOLITHIUM COMPOUNDS WITH ALKYL HALIDES.
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The reaction of t-BuLi with cyclizable 1 and 2 alkyl halide radical probes at low temperature produced stable cyclized and uncyclized organolithium products as well as cyclized hydrocarbons which clearly indicate the presence of radical intermrdiates during the course of these reactions.
- Ashby, E. C.,Pham, Tung N.,Park, Bongjin
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p. 4691 - 4694
(2007/10/02)
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- Hydrogenolysis of Small Cycloalkanes, X. - Catalytic Hydrogenation of Bicycloalkanes
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Dependent on n different products are obtained from bicycloalkanes by hydrogenation on Pt and Pd/C catalysts: from n = 5 onward only methylcycloalkanes of the same ring size; with n = 4 additionally 2-7 percent of cycloheptane is formed; with n = 3 ring enlargement increases to 5-20 percent and with n = 2 cyclopentane is the only product.Mainly butane is formed from bicyclobutane and no intermediate could be detected.Explanations are attempted.The expected products are produced on hydrogenation of methyl-substituted derivatives and spiroalkanes.
- Stahl, Karl-Johannes,Hertzsch, Winfried,Musso, Hans
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p. 1474 - 1484
(2007/10/02)
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- Evidence for Single Electron Transfer in the Reduction of Organic Halides by Lithium Triethylborohydride
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Product studies involving the reduction of cyclizable alkyl iodides and bromides, trapping of intermediate radicals by dicyclohexylphosphine, and direct EPR observation of radicals have been used to detect the occurence of a single electron transfer pathway in the reduction of these halides by lithium triethylborohydride.
- Ashby, E.C.,Wenderoth, Bernd,Pham, Tung N.,Park, Won-Suh
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p. 4505 - 4509
(2007/10/02)
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- Electron transfer in the reactions of alkyl halides with sodium trimethyltin
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The reaction of sodium trimethyltin with alkyl halides has been studied in detail by using radical probes, stereochemical probes, and radical traps. All evidence indicates that the reaction proceeds by an electron-transfer process involving radical intermediates for the systems studied.
- Ashby,DePriest,Su, Wei-Yang
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p. 1718 - 1727
(2008/10/08)
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- Occurrence of Electron Transfer in the Reduction of Organic Halides by LiAlH4 and AlH3
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A variety of methods have been utilized to detect the occurrence of a single electron transfer pathway in the reduction of alkyl halides by LiAlH4 and AlH3, i.e., (1) product studies of reduction of cyclizable alkyl halides containing the 5-hexenyl group, (2) trapping of intermediate radicals by dicyclohexylphosphine and other trapping agents, (3) direct EPR observation of the trityl radical in the reduction of trityl bromide, and (4) stereochemical studies of the reduction of secondary halides by lithium aluminum deuteride.The extent of electron transfer was found to be a function of the solvent, the substrate, the leaving group, and the hydride reagent.For alkyl iodides, and to a lesser extent bromides, electron transfer was found to be the major reaction pathway; however, no evidence for electron transfer was found for the corresponding chlorides or tosylates.Reduction of (+)-2-octyl iodide by LiAlD4 was found to be much less stereospecific than the corresponding reduction of bromide, chloride, or tosylate, indicating intermediate radical formation in the reduction of the secondary iodide.
- Ashby, E. C.,DePriest, R. N.,Goel, A. B.,Wenderoth, Bernd,Pham, Tung N.
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p. 3545 - 3556
(2007/10/02)
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- CONCERNING THE REDUCTION OF ALKYL HALIDES BY LiAlH4. EVIDENCE THAT AlH3 PRODUCED IN SITU IS THE ONE ELECTRON TRANSFER AGENT.
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The reduction of 10 and 20 alkyl iodides by LiAlH4 has been shown to involve a radical intermediate formed by the reaction of the alkyl iodide with the AlH3 and LiI produced in situ in conjunction with LiAlH4 rather than by LiAlH4 alone, as evidenced by cyclized products in the reduction of 6-iodo-1-heptene, by the trapping of the radical and by stereochemical studies of the 2-halooctanes.
- Ashby, E. C.,DePriest, R. N.,Pham, T. N.
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p. 2825 - 2828
(2007/10/02)
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- EVIDENCE FOR A SINGLE ELECTRON TRANSFER MECHANISM IN REACTIONS OF LITHIUM DIORGANOCUPRATES WITH ORGANIC HALIDES
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It has been demonstrated by means of spectroscopic studies involving cyclizable alkyl halides that lithium dimethylcuprate can react with organic halides by a single electron transfer pathway.
- Ashby, E. C.,DePriest, R. N.,Tuncay, A.,Srivastava, Sushil
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p. 5251 - 5254
(2007/10/02)
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- CYCLIZATION OF C7-ALKANES OVER Pt BLACK CATALYST
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C6-and C5-cyclization of heptane isomers (and also, olefin formation as a related process) over Pt-black have been studied in pulse and circulation systems.Hydrogendeficient conditions favour aromatization, via presumably terminal olefins.C5-Cyclization in the presence of more hydrogen is accompanied by internal olefin formation.Relative reactivities of all heptane isomers have been measured; this shows that cyclization is easier between terminal methyl groups.Optimum hydrogen pressures for both types of cyclization have been determined (and compared with hydrogenolysis, too).Earlier mechanism suggestion for aromatization and cyclopentane formation have been confirmed; the distinction between two types of bond shift mechanisms producing aromatics (from substituted pentanes) and saturated isomers, respectively, has recieved additional support facilitating the identification of these two reactions with mechanisms proposed in the literature.
- Zimmer, H.,Paal, Z.,Tetenyi, P.
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p. 513 - 532
(2007/10/02)
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- Mechanism of Isomerization of Hydrocarbons on Metals. Part 11.-Isomerization and Dehydrocyclization of (13)C-labelled 3-Methylhexanes on Pt-Al2O3 Catalysts
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The isomerization, dehydrocyclization and hydrogenolysis of 3-methylhexane have been studied at 320-380 deg C over a series of Pt-Al2O3 catalysts with a metal dispersion extending from 0.05 to 1.The use of five labelled compounds, 3-methyl(1-(13)C), (2-(13)C), (3-(13)C), (6-(13)C)hexanes and 3-methyl((13)C)hexane, alloved distinction between the various parallel pathways.On all catalysts the predominant reaction was the isomerization according to a cyclic mechanism involving either 1,3-dimethyl-, 1,2-dimethyl- or ethyl-cyclopentane intermediates with a relative contribution of 60, 40 and 20 percent, respectively.These results are consistent wiith a dehydrocyclization scheme involving a metallocarbene as precursor and dicarbene or dicarbyne recombination as the rate-determining step.
- Amir-Ebrahimi, Valia,Gault, Francois G.
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p. 1735 - 1756
(2007/10/02)
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- Mechanism of Isomerization of Hydrocarbons on Metals. Part 9.-Isomerization and Dehydrocyclization of 2,3-Dimethyl(2-(13)C)pentane on a 10percent Pt-Al2O3 Catalyst
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The isomerization, dehydrocyclization and hydrogenolysis of 2,3-dimethyl(2-(13)C)pentane have been studied at 260 deg C over a 10percent Pt-Al2O3 catalyst of low dispersion, under various hydrocarbon and hydrogen pressures.Most of the isomerization products are accounted for either by a bond-shift mechanism or by a cyclic mechanism involving 1,2-dimethylcyclopentane intermediate.The absence of significant scrambling of the label suggests that the rate-determining step in isomerization is the skeletal rearrangement of highly dehydrogenated species.The positive order as a function of hydrogen which is found (0.8-1.2) cannot then be taken as evidence that desorption is rate-determining.It is best explained by assuming multisite adsorption of hydrocarbon and competition with hydrogen for chemisorption on the same sites.The results provide argument in favour of a reactive rather than dissociative-type adsorption step.
- Parayre, Patrick,Amir-Ebrahimi, Valia,Gault, Francois G.,Frennet, Alfred
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p. 1704 - 1722
(2007/10/02)
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- Stereoselectivity of Ring Closure of Substituted Hex-5-enyl Radicals
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1,5-Ring closure of 1- or 3-substituted hex-5-enyl radicals affords mainly cis-disubstituted cyclic products, whereas 2- or 4-substituted species give mainly trans-products; the significance of this stereoselectivity is demonstrated in the formation of the norbornane system from acyclic precursors.
- Beckwith, Athelstan L. J.,Lawrence, Tony,Serelis, Algirdas K.
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p. 484 - 485
(2007/10/02)
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