- Development of a well-defined silica-supported tungstenocarbyne complex as efficient heterogeneous catalyst for alkyne metathesis
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The interaction of [W({triple bond, long}C-tBu)(CH2-tBu)(OAr)2] (Ar = 2,6-iPr2C6H3) (1), with the hydroxyl groups of a silica dehydroxylated at 700 °C leads to [({triple bond, long}SiO)W(OAr)2({triple bond, long}C-tBu)] (2) which was characterized by IR, solid-state NMR and mass balance analysis. This well-defined surface species is an efficient catalyst for the metathesis of pent-2-yne.
- Merle, Nicolas,Taoufik, Mostafa,Nayer, Morad,Baudouin, Anne,Roux, Erwan Le,Gauvin, Régis M.,Lefebvre, Frédéric,Thivolle-Cazat, Jean,Basset, Jean-Marie
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Read Online
- REACTIONS OF DIMETHYLSELENADIAZOLE AND ALKYLTIN DERIVATIVES
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Reactions of 4,5-dimethyl-1,2,3-selenadiazole with hexaalkylditins or trialkyltin anion followed by quenching with trialkyltin chloride affords only dimethylacetylene and bis(trimethyltin) selenide.Implications of these results in relation to possible syntheses of selenatellurafulvalenes are discussed.
- Arad-Yellin, Rina,Wudl, Fred
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Read Online
- Use of but-1-yne as a probe for the characterization of the basicity of alkali-exchanged zeolites
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But-1-yne has been adsorbed at room temperature on a series of LiNa, Na and CsNaX and Y zeolites and also on CsNaX,9Cs and CsNaY,9Cs samples containing nine Cs atoms occluded by a unit cell. An IR study of the 3000-2800 cm-1 frequency range clearly showed that but-1-yne isomerized into but-2-yne on CsNaX,9Cs whereas the observation of a band near 1950 cm-1 in the case of CsNaY,9Cs characterized the formation of buta-1,2-diene. Such partial transformation of but-1-yne to isomers did not occur on LiNa and Na samples, allowing one to study the basicity of such zeolites from the v(≡CH) shift which decreases in the following order: NaX > LiNaX > NaY > LiNaY. The main feature is the observation of at least two perturbed v(≡CH) bands for the NaX and NaY samples, revealing the heterogeneity of the basic sites. This result is discussed taking into account the presence of cations in different positions.
- Lavalley, Jean-Claude,Lamotte, Jean,Travert, Arnaud,Czyzniewska, Jolanta,Ziolek, Maria
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Read Online
- "canopy Catalysts" for Alkyne Metathesis: Molybdenum Alkylidyne Complexes with a Tripodal Ligand Framework
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A new family of structurally well-defined molybdenum alkylidyne catalysts for alkyne metathesis, which is distinguished by a tripodal trisilanolate ligand architecture, is presented. Complexes of type 1 combine the virtues of previous generations of silanolate-based catalysts with a significantly improved functional group tolerance. They are easy to prepare on scale; the modularity of the ligand synthesis allows the steric and electronic properties to be fine-tuned and hence the application profile of the catalysts to be optimized. This opportunity is manifested in the development of catalyst 1f, which is as reactive as the best ancestors but exhibits an unrivaled scope. The new catalysts work well in the presence of unprotected alcohols and various other protic groups. The chelate effect entails even a certain stability toward water, which marks a big leap forward in metal alkylidyne chemistry in general. At the same time, they tolerate many donor sites, including basic nitrogen and numerous heterocycles. This aspect is substantiated by applications to polyfunctional (natural) products. A combined spectroscopic, crystallographic, and computational study provides insights into structure and electronic character of complexes of type 1. Particularly informative are a density functional theory (DFT)-based chemical shift tensor analysis of the alkylidyne carbon atom and 95Mo NMR spectroscopy; this analytical tool had been rarely used in organometallic chemistry before but turns out to be a sensitive probe that deserves more attention. The data show that the podand ligands render a Mo-alkylidyne a priori more electrophilic than analogous monodentate triarylsilanols; proper ligand tuning, however, allows the Lewis acidity as well as the steric demand about the central atom to be adjusted to the point that excellent performance of the catalyst is ensured.
- Copéret, Christophe,Fürstner, Alois,Gordon, Christopher P.,Hillenbrand, Julius,Leutzsch, Markus,N?thling, Nils,Wille, Christian,Yiannakas, Ektoras
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supporting information
p. 11279 - 11294
(2020/07/13)
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- Quantifying Error Correction through a Rule-Based Model of Strand Escape from an [ n]-Rung Ladder
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The rational design of 3D structures (MOFs, COFs, etc.) is presently limited by our understanding of how the molecular constituents assemble. The common approach of using reversible interactions (covalent or noncovalent) becomes challenging, especially when the target is made from multivalent building blocks and/or under conditions of slow exchange, as kinetic traps and nonequilibrium product distributions are possible. Modeling the time course of the assembly process is difficult because the reaction networks include many possible pathways and intermediates. Here we show that rule-based kinetic simulations efficiently model dynamic reactions involving multivalent building blocks. We studied "strand escape from an [n]-rung ladder" as an example of a dynamic process characterized by a complex reaction network. The strand escape problem is important in that it predicts the time a dynamic system needs to backtrack from errors involving [n]-misconnections. We quantify the time needed for error correction as a function of the dissociation rate coefficient, strand valency, and seed species. We discuss the simulation results in relation to a simple probabilistic framework that captures the power law dependence on the strand's valency, and the inverse relationship to the rung-opening rate coefficient. The model also tests the synthetic utility of a one-rung (i.e., hairpin) seed species, which, at intermediate times, bifurcates to a long-lived, fully formed [n]-rung ladder and a pair of separated strands. Rule-based models thus give guidance to the planning of a dynamic covalent synthesis by predicting time to maximum yield of persistent intermediates for a particular set of rate coefficients and valency.
- Cencer, Morgan M.,Greenlee, Andrew J.,Moore, Jeffrey S.
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supporting information
p. 162 - 168
(2020/01/03)
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- Molybdenum Alkylidyne Complexes with Tripodal Silanolate Ligands: The Next Generation of Alkyne Metathesis Catalysts
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A new type of molybdenum alkylidyne catalysts for alkyne metathesis is described, which is distinguished by an unconventional podand topology. These structurally well-defined complexes are easy to make on scale and proved to be tolerant toward numerous functional groups; even certain protic substituents were found to be compatible. The new catalysts were characterized by X-ray crystallography and by spectroscopic means, including 95Mo NMR.
- Hillenbrand, Julius,Leutzsch, Markus,Fürstner, Alois
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supporting information
p. 15690 - 15696
(2019/10/28)
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- Molecular and Silica-Supported Molybdenum Alkyne Metathesis Catalysts: Influence of Electronics and Dynamics on Activity Revealed by Kinetics, Solid-State NMR, and Chemical Shift Analysis
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Molybdenum-based molecular alkylidyne complexes of the type [MesC≡Mo{OC(CH3)3-x(CF3)x}3] (MoF0, x = 0; MoF3, x = 1; MoF6, x = 2; MoF9, x = 3; Mes = 2,4,6-trimethylphenyl) and their silica-supported analogues are prepared and characterized at the molecular level, in particular by solid-state NMR, and their alkyne metathesis catalytic activity is evaluated. The 13C NMR chemical shift of the alkylidyne carbon increases with increasing number of fluorine atoms on the alkoxide ligands for both molecular and supported catalysts but with more shielded values for the supported complexes. The activity of these catalysts increases in the order MoF0 3 6 before sharply decreasing for MoF9, with a similar effect for the supported systems (MoF0 ≈ MoF9 6 3). This is consistent with the different kinetic behavior (zeroth order in alkyne for MoF9 derivatives instead of first order for the others) and the isolation of stable metallacyclobutadiene intermediates of MoF9 for both molecular and supported species. Detailed solid-state NMR analysis of molecular and silica-supported metal alkylidyne catalysts coupled with DFT/ZORA calculations rationalize the NMR spectroscopic signatures and discernible activity trends at the frontier orbital level: (1) increasing the number of fluorine atoms lowers the energy of the π?(M≡C) orbital, explaining the more deshielded chemical shift values; it also leads to an increased electrophilicity and higher reactivity for catalysts up to MoF6, prior to a sharp decrease in reactivity for MoF9 due to the formation of stable metallacyclobutadiene intermediates; (2) the silica-supported catalysts are less active than their molecular analogues because they are less electrophilic and dynamic, as revealed by their 13C NMR chemical shift tensors.
- Estes, Deven P.,Gordon, Christopher P.,Fedorov, Alexey,Liao, Wei-Chih,Ehrhorn, Henrike,Bittner, Celine,Zier, Manuel Luca,Bockfeld, Dirk,Chan, Ka Wing,Eisenstein, Odile,Raynaud, Christophe,Tamm, Matthias,Copéret, Christophe
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supporting information
p. 17597 - 17607
(2017/12/15)
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- An acelylenically of a diene compound and/or method of manufacturing
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Provided is a novel method for producing a compound having acetylene bonds and/or a diene. The method for producing a compound having acetylene bonds and/or a diene is characterized in that at least one selected from the group consisting of ketone compound (I), ketone compound (II), aldehyde compound (III), aldehyde compound (IV), and aldehyde compound (V) is dehydrated in the presence of a catalyst wherein a carrier containing silica supports at least one selected from the group consisting of compounds containing group 1 metal elements, compounds containing group 2 metal elements, group 1 metal elements, and group 2 metal elements.
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Paragraph 0427; 0428; 0437-0445
(2017/03/28)
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- Catalysts for the alkyne metathesis
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Organometallic compounds of the general formula (I), in which M=Mo, W, are claimed.
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Page/Page column 25; 26; 34
(2015/04/15)
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- The elusive ethenediselone, Se=C=C=Se
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The neutral ethenediselone, Se=C=C=Se, has been characterised by neutralisation-reionisation mass spectrometry, which implies a minimum lifetime of the order of microseconds. Tetraselenafulvalene 1 and tetramethyltetraselenafulvalene 2 were used as precursor molecules. Flash vacuum thermolysis (FVT) of these compounds with isolation of the products in Ar matrices permitted the identification of ethyne, 2-butyne, CSe2, and selenoketene, H2C=C=Se, but at best traces of Se=C=C=Se survived the FVT/matrix isolation experiment. Multiconfigurational calculations indicate that Se=C=C=Se is a ground state triplet molecule with a very small singlet-triplet gap.
- Pedersen, Carl Th.,Wong, Ming Wah,Takimiya, Kazuo,Gerbaux, Pascal,Flammang, Robert
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p. 1195 - 1200
(2014/11/07)
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- Well-defined silica-supported molybdenum nitride species: Silica grafting triggers alkyne metathesis activity
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The grafting of [Mo(N)(NR2)(OR)2(pyr)] (R = SiMe 3, pyr = pyridine) onto highly dehydroxylated silica proceeds to the formation of well-defined silica-supported species. Whereas the molecular precursor is inactive towards
- Genelot, Marie,Cheval, Nicolas P.,Vitorino, Marta,Berrier, Elise,Weibel, Jean-Marc,Pale, Patrick,Mortreux, Andre,Gauvin, Regis M.
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p. 2680 - 2685
(2013/07/19)
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- Rhodium-carbon bond energies in Tp′Rh(CNneopentyl)(CH2X)H: Quantifying stabilization effects in M-C bonds
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A series of substituted methyl derivatives of the type Tp′ Rh(CNneopentyl)(CH2X)H (CH2X = CH2C(i - O)CH3, CH2Ci - CCH3, CH 2O-t-Bu, CH2CF3, CH2F, CHF 2) was synthesized either by photolysis of Tp′Rh(CNneopentyl) (PhNCNneopentyl) in neat CH3X or by exchange with the labile hydrocarbon in Tp′Rh(CNneopentyl)(n-pentyl)H or Tp′Rh(CNneopentyl) (CH3)H. Only a single product was observed in each case. Clean reductive elimination was observed for all compounds in C6D 6. Structures of these complexes and their corresponding chlorinated derivatives have been characterized by NMR spectroscopy, elemental analysis, and X-ray crystallography. Relative Rh-C bond energies are calculated using previously established kinetic techniques, and two separate linear correlations are observed versus known C-H bond strengths, one for the parent hydrocarbons, and one for the substituted hydrocarbons. Both correlations have slopes of 1.4, and are separated vertically by 7.5 kcal mol-1 (-CH2X above -CxHy). In addition, it is now clear that preferences for linear vs branched olefin insertion products in substituted derivatives can be predicted on the basis of the strengths of the β-C-H bonds. The DFT calculations of the metal-carbon bond strengths in these Rh-CH2X derivatives with α-substitution show a trend that is in good agreement with the experimental results.
- Jiao, Yunzhe,Evans, Meagan E.,Morris, James,Brennessel, William W.,Jones, William D.
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p. 6994 - 7004
(2013/06/27)
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- Multiple Si-H bond activations by tBu2PCH 2CH2PtBu2 and tBu 2PCH2PtBu2 Di(phosphine) complexes of rhodium and iridium
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Reactions of the di(tert-butylphosphino)ethane complex (dtbpe)Rh(CH 2Ph) with Ph2SiH2 and Et2SiH 2 resulted in isolation of (dtbpe)Rh(H)2(SiBnPh 2) (1; Bn = CH2Ph) and (dtbpe)Rh(H) 2(SiBnEt2) (2), respectively. Both 1 and 2 feature strong interactions between the rhodium hydride and silyl ligands, as indicated by large 2JSiH values (44.4 and 52.1 Hz). The reaction of (dtbpm)Rh(CH2Ph) (dtbpm = di(tert-butylphosphino)methane) with Mes2SiH2 gave the pseudo-three-coordinate Rh complex (dtbpm)Rh(SiHMes2) (3), which is stabilized in the solid state by agostic interactions between the rhodium center and two C-H bonds of a methyl substituent on the mesityl group. The analogous germanium compound (dtbpm)Rh(GeHMes2) (4) is also accessible. Complex 3 readily undergoes reactions with diphenylacetylene, phenylacetylene, and 2-butyne to give the silaallyl complexes (dtbpm)Rh[Si(CPh=CHPh)Mes2] (5), (dtbpm)Rh[Si(CH=CHPh)Mes2] (7), and (dtbpm)Rh(Si(CMe=CHMe)Mes 2) (8) via net insertions into the Si-H bond. The germaallyl complexes (dtbpm)Rh[Ge(CPh=CHPh)Mes2] (6) and (dtbpm)Rh[Ge(CMe=CHMe) Mes2] (9) were synthesized under identical conditions starting from 4. The reaction of (dtbpm)Rh(CH2Ph) with 1 equiv of TripPhSiH 2 yielded (dtbpm)Rh(H)2[5,7-diisopropyl-3-methyl-1-phenyl- 2,3-dihydro-1H-silaindenyl-κSi] (11), and catalytic investigations indicate that both (dtbpm)Rh(CH2Ph) and 11 are competent catalysts for the conversion of TripPhSiH2 to 5,7-diisopropyl-3-methyl-1- phenyl-2,3-dihydro-1H-silaindole. A dtbpm-supported Ir complex, [(dtbpm)IrCl]2, was used to access the dinuclear bridging silylene complexes [(dtbpm)IrH](μ-SiPh2)(μ-Cl)2[(dtbpm)IrH] (12) and [(dtbpm)IrH](μ-SiMesCl)(μ-Cl)(μ-H)[(dtbpm)IrH] (13). The reaction of [(dtbpm)IrCl]2 with a sterically bulky primary silane, (dmp)SiH3 (dmp = 2,6-dimesitylphenyl), allowed isolation of the mononuclear complex (dtbpm)Ir(H)4(10-chloro-1-mesityl-5,7-dimethyl-9, 10-dihydrosilaphenanthrene-κSi), in which the dmp substituent has undergone C-H activation.
- Fasulo, Meg E.,Calimano, Elisa,Buchanan, J. Matthew,Tilley, T. Don
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p. 1016 - 1028
(2013/04/23)
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- Metal ions do not play a direct role in the formation of carbon-carbon triple bonds during reduction of trihaloalkyls by CrII or V II
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Carbyne radicals: Reactions of trihaloalkyl compounds with Cr2+ or V2+ in aqueous solutions yield alkynes and other products. Stepwise halogen abstractions from the trihaloalkyls form alkyl carbyne triradicals in solution. These radicals undergo coupling reactions, producing triply bonded alkyne molecules (see scheme). This process is not metal-assisted and does not occur in the coordination sphere of the metal ions.
- Levy, Ophir,Bino, Avi
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supporting information
p. 15944 - 15947
(2013/02/23)
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- Introducing a podand motif to alkyne metathesis catalyst design: A highly active multidentate molybdenum(VI) catalyst that resists alkyne polymerization
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Podand prevents polymers: The molybdenum(VI) propylidyne catalyst 1 with a podand triphenolamine ligand shows high activity in the metathesis of a variety of alkyne substrates, including heterocycles that contain donor moieties. With one substrate-binding site blocked by the multidentate ligand, the undesired polymerization of small alkynes that occurs with non-podand-ligand complex 2 is completely inhibited. Copyright
- Jyothish, Kuthanapillil,Zhang, Wei
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supporting information; experimental part
p. 3435 - 3438
(2011/05/04)
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- Nucleophilic reactivity of 1-zirconacyclopent-3-ynes: Carbon-carbon bond formation with aldehydes
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Nucleophilic reactions of 1,1-bis(η5-cyclopentadienyl)-1-zirconacyclopent-3-yne (1) with proton and aldehydes were studied. The reaction with HCl gave a mixture of 2-butyne and 1,2-butadiene. Complex 1 reacted with benzaldehyde to give 1-phenyl-2-methyl-2,3-butadien-1-ol (3) in moderate yields in the presence of a proton source such as triethylammonium hydrochloride, while it gave 2-methylene-1-phenyl-3-buten-1-ol (4) on using triethylammonium tetraphenylborate.
- Suzuki, Noriyuki,Watanabe, Takaaki,Hirose, Takuji,Chihara, Teiji
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p. 5317 - 5321
(2008/03/12)
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- A highly active, heterogeneous catalyst for alkyne metathesis
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(Chemical Equation Presented) An alkylidyne molybdenum amide complex is attached to nontoxic, amorphous silica to form a highly active, recyclable heterogeneous catalyst for alkyne metathesis. The catalyst does not undergo alkyne polymerization, can be utilized at a loading of 1 mol% at room temperature, and has shown unprecedented metathesis activity for the homodimerization of 2-propynylthiophene, a substrate that was previously problematic for alkyne metathesis.
- Weissman, Haim,Plunkett, Kyle N.,Moore, Jeffrey S.
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p. 585 - 588
(2007/10/03)
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- HETEROGENEOUS ALKYNE METATHESIS
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The present invention provides heterogeneous organometallic catalysts for alkyne metathesis, including the metathesis of internal alkynes. Organometallic precursors are covalently bonded to the oxygen atoms of metal oxide supports to form catalysts having carbyne functionality. The heterogeneous catalysts provide improved turn-over frequencies at lower reaction temperatures than conventional catalysts.
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Page/Page column 7
(2008/06/13)
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- Highly active molybdenum-alkylidyne catalysts for alkyne metathesis: Synthesis from the nitrides by metathesis with alkynes
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Terminal nitrido complexes N≡Mo(OC(CF3)2Me)3 (4), N≡Mo(OC(CF3)2Me)3(NCMe) (4-NCMe), and NMo(OC(CF3)3)3(NCMe) (5-NCMe) react irreversibly with 3-hexyne at elevated temperature in hydrocarbon solution to form the corresponding propylidyne complexes EtC≡Mo(OC(CF3)2Me)3 (3) and EtC≡Mo(OC(CF3)3)3 (6), long known as exceptionally active catalysts for alkyne metathesis. The propylidyne complexes are isolated as the more readily crystallized 1,2-dimethoxyethane (DME) adducts for convenience; 3-DME is isolated in 61% yield on a multigram scale. Copyright
- Gdula, Robyn L.,Johnson, Marc J. A.
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p. 9614 - 9615
(2007/10/03)
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- A Mo(VI) Alkylidyne complex with polyhedral oligomeric silsesquioxane ligands: Homogeneous analogue of a silica-supported alkyne metathesis catalyst
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A highly active alkyne metathesis catalyst is realized by replacing the amide ligands of a molybdenum(VI) trisamide alkylidyne complex with silanol groups from incompletely condensed POSS (polyhedral oligomeric silsesquioxane) ligands. This catalyst serves as an effective homogeneous mimic of an amorphous silica-supported catalyst. Reactivities of various catalytic mixtures are reported along with an X-ray structure of the aniline-coordinated amidodisiloxymolybdenum(VI) alkylidyne complex. Copyright
- Hyeon, Mo Cho,Weissman, Haim,Wilson, Scott R.,Moore, Jeffrey S.
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p. 14742 - 14743
(2008/02/04)
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- Experimental and theoretical characterization of the valence isomerization of Bi-2H-azirin-2-yls to diazabenzenes
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3,4-Diazidocyclobutenes 16 were prepared from the corresponding dihalides. Some of these diazides, such as parent compound 16 d and phenyl-substituted derivatives 16c,f, underwent spontaneous stereoselective electrocyclic ring opening below room temperature, whereas the tetraalkyl derivatives of 16 had to be heated to force the same reaction. In most cases, the resulting 1,4-diazidobuta-1,3-dienes 8 were isolated to study their photochemical transformation into bi-2Hazirin-2-yls 9 via intermediate monoazirines 17. Except for starting materials with a low number of substituents such as 9d and 9f, title compounds 9 underwent a thermal valence isomerization which led exclusively to pyridazines 18 at surprisingly low temperatures. Based on quantum-chemical calculations for the parent bi-2H-azirinyl 2-yl 9d at the UB3LYP/6-31+G(d) and MR-MP2/TZV(2df,2p) levels, the valence isomerization process is best explained by simultaneous homolytic cleavage of both C-N single bonds of 9 to generate energetically favorable N,N′ diradicals 26, which cyclize to 18. The theoretical studies indicate also that one stereoisomer of 9, namely, the rac compound, should undergo valence isomerization more easily than the other, which is in conformity with different rates of these rearrangement reactions found experimentally. For the tetramethyl-bi-2H-azirin-2-yls 9g, which are better models for the experimentally studied compounds, simultaneous homolytic cleavage of both C-N single bonds is also predicted by the calculations, although the intermediate diradicals 26 g are significantly higher in energy than those of the parent system 9d.
- Banert, Klaus,Grimme, Stefan,Herges, Rainer,Hess, Kirsten,Koehler, Frank,Mueck-Lichtenfeld, Christian,Wuerthwein, Ernst-Ulrich
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p. 7467 - 7481
(2007/10/03)
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- Bi-3H-diazirin-3-yls as precursors of highly strained cycloalkynes
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(Chemical Equation Presented) No reagents and very mild conditions are required for the transformation of isolable precursors 3 into highly strained cycloalkynes 4 (cycloheptyne, cyclohexyne, norbornyne), which can be trapped by cycloaddition reactions.
- Al-Omari, Mohammad,Banert, Klaus,Hagedorn, Manfred
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p. 309 - 311
(2007/10/03)
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- Complete study of the pyrolysis and gasification of scrap tires in a pilot plant reactor
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The pyrolysis and gasification of tires was investigated in a pilot plant reactor provided with a system for condensation of semivolatile matter. The study comprised experiments at 450°, 750°, and 1000°C both in nitrogen and 10% oxygen atmospheres. In the gas phase, only methane and benzene yields increased with temperature until 1000°C. In the liquids, the main components were styrene, limonene, and isoprene. The solid fraction (including soot) increased with temperature. Zinc content of the char decreased with increasing temperature. Analysis of the surface area of the solids showed that the area was similar in all cases to that of a commercial carbon black. The higher surface of the soot with respect to the chars was observed. The results coincided with published findings, i.e., kinetic severity function values would produce 0.2% of methane at 450°C and 4.5% at 750°-1000°C.
- Conesa, Juan A.,Martin-Gullon,Font,Jauhiainen
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p. 3189 - 3194
(2008/12/21)
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- Semivolatile and volatile compounds in combustion of polyethylene
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The evolution of semivolatile and volatile compounds in the combustion of polyethylene (PE) was studied at different operating conditions in a horizontal quartz reactor. Four combustion runs at 500 and 850°C with two different sample mass/air flow ratios and two pyrolytic runs at the same temperatures were carried out. Thermal behavior of different compounds was analyzed and the data obtained were compared with those of literature. It was observed that α,ω-olefins, α-olefins and n-paraffins were formed from the pyrolytic decomposition at low temperatures. On the other hand, oxygenated compounds such as aldehydes were also formed in the presence of oxygen. High yields were obtained of carbon oxides and light hydrocarbons, too. At high temperatures, the formation of polycyclic aromatic hydrocarbons (PAHs) took place. These compounds are harmful and their presence in the combustion processes is related with the evolution of pyrolytic puffs inside the combustion chamber with a poor mixture of semivolatile compounds evolved with oxygen. Altogether, the yields of more than 200 compounds were determined. The collection of the semivolatile compounds was carried out with XAD-2 adsorbent and were analyzed by GC-MS, whereas volatile compounds and gases were collected in a Tedlar bag and analyzed by GC with thermal conductivity and flame ionization detectors.
- Font, Rafael,Aracil, Ignacio,Fullana, Andrés,Conesa, Juan A.
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p. 615 - 627
(2007/10/03)
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- Thermal degradation processes in polysulfide copolymers investigated by direct pyrolysis mass spectrometry and flash pyrolysis - Gas chromatography/mass spectrometry
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This is the first report on the analysis of random block polysulfide copolymers containing different amounts of repeating units in the copolymer backbone, which has been studied by direct pyrolysis mass spectrometry (DPMS) and by pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). The homopolymers such as poly(ethylene sulfide) (PES), poly(styrene sulfide) (PSS), and two random copolymers, viz., poly(ethylene sulfidex-co-styrene sulfidey) [copolymer I (x = y = 0.5) and copolymer II (x = 0.74, y = 0.26)] were investigated by both DPMS and Py-GC/MS (except copolymer II) techniques. In the case of copolymer I, the thermal degradation products of SE1, SE2, S2, and S2E (S = styrene sulfide, E = ethylene sulfide) were detected in DPMS, whereas the formation of SE1 and SE2 were observed by Py-GC/MS technique. However, for copolymer II, SE3 was also found along with SE1, SE2, S2, and S2E in DPMS. The formation of additional product (SE3) observed in copolymer II could be due to an increase in the block length formed during copolymerization. Further, a comparative study on thermal degradation of PES, poly(ethylene disulfide) (PEDS), and poly(ethylene tetrasulfide) (PETS) were investigated by Py-GC/MS. The pyrolysis products detected by both DPMS and Py-GC/MS indicates that the thermal decomposition of these polymers yield cyclic sulfides through an intramolecular exchange or by backbiting processes. The linear products with thiol and vinyl groups were also observed by Py-GC/MS along with the cyclic products via carbon hydrogen transfer reaction.
- Sundarrajan,Surianarayanan,Srinivasan,Kishore
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p. 3331 - 3337
(2007/10/03)
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- Conrotatory photochemical ring opening of alkylcyclobutenes in solution. A test of the hot ground-state mechanism
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Quantum yields for photochemical ring opening of six alkylcyclobutenes have been measured in hexane solution using 228-nm excitation, which selectively populates the lowest π,R(3s) excited singlet states of these molecules and has been shown previously to lead to ring opening with clean conrotatory stereochemistry. The compounds studied in this work - 1,2-dimethylcyclobutene (1), cis- and trans-1,2,3,4-tetramethylcyclobutene (cis- and trans-5), hexamethylcyclobutene (8), and cis- and trans-tricyclo[6.4.0.02,7]dodec-12-ene (cis- and trans-9) - were selected so as to span a broad range in molecular weight and as broad a range as possible in Arrhenius parameters for thermal (ground-state) ring opening. RRKM calculations have been carried out to provide estimates of the rate constants for ground-state ring opening of each of the compounds over a range of thermal energies from 20 00O to 49 000 cm-1. These have been used to estimate upper limits for the quantum yields of ring opening via a hot ground-state mechanism, assuming a value of kdeact = 1011 s-1 for the rate constant for collisional deactivation by the solvent, that internal conversion to the ground state from the lowest Rydberg state occurs with close to unit efficiency, and that ergodic behavior is followed. The calculated quantum yields are significantly lower than the experimental values in all cases but one (1). This suggests that the Rydberg-derived ring opening of alkylcyclobutenes is a true excited-state process and rules out the hot ground-state mechanism for the reaction.
- Cook,Leigh,Walsh
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p. 5188 - 5193
(2007/10/03)
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- Stereospecific (Conrotatory) Photochemical Ring Opening of Alkylcyclobutenes in the Gas Phase and in Solution. Ring Opening from the Rydberg Excited State or by Hot Ground State Reaction?
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The photochemistry of 1,2-dimethylcyclobutene and cis- and trans-1,2,3,4-tetramethylcyclobutene has been studied in the gas phase (1 atm; SF6 buffer) and in hydrocarbon solvents with 193-, 214-, and 228-nm light sources. The major products are the isomeric dienes from electrocyclic ring opening and 2-butyne + alkene (ethylene or E-/Z- 2-butene) due to formal [2+2]-cycloreversion. The total yields of dienes relative to 2-butyne are generally higher in the gas phase than in solution but decrease with increasing excitation wavelength under both sets of conditions. In the case of cis-1,2,3,4-tetramethylcyclobutene, 228-nm photolysis results in the stereospecific formation of E,Z-3,4-dimethyl-2,4-hexadiene - the isomer corresponding to ring opening by the thermally allowed (conrotatory) electrocyclic pathway - in both the gas phase and solution. All three diene isomers are obtained upon 228-nm photolysis of trans-1,2,3,4-tetramethylcyclobutene, but control experiments suggest that the thermally allowed isomers (E,E- and Z,Z-3,4-dimethyl-2,3-hexadiene) are probably the primary products in this case as well. The results are consistent with cycloreversion resulting from excitation of the low-lying π,R(3s) singlet state and with ring opening proceeding by at least two different mechanisms depending on excitation wavelength. The first, which dominates at short wavelengths, is thought to involve direct reaction of the second excited singlet (π,π*) state of the cyclobutene. The second mechanism, which dominates at long wavelengths, is proposed to ensue either directly from the lowest energy (Rydberg) state or from upper vibrational levels of the ground state, populated by internal conversion from this excited state.
- Leigh, William J.,Cook, Bruce H. O.
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p. 5256 - 5263
(2007/10/03)
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- Synthesis of Dicarbonyl(η4-tricarbonylcobaltacyclopentadiene)cobalt Complexes from Co2(CO)8. A General Route to Intermediates in Cobalt Carbonyl Mediated Alkyne Trimerization
-
We have shown that cobaltacyclopentadiene complexes, derived from cobalt carbonyl and two alkyne molecules and long believed to be intermediates in cyclotrimerization as well as in the formation of the (three-alkyne-derived) flyover complexes, can be isolated in selected cases if the mono(alkyne)-Co2(CO)6 complexes are treated at room temperature with just 1 equiv of alkyne in the presence of trimethylamine N-oxide. The complex (PhC=CHCMe= CMe)Co2(CO)5 has been characterized by X-ray crystallography.
- Baxter, R. John,Knox, Graham R.,Pauson, Peter L.,Spicer, Mark D.
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p. 197 - 205
(2008/10/08)
-
- Thermal decomposition of 2,5-dimethylfuran. Experimental results and computer modeling
-
The thermal reactions of 2,5-dimethylfuran were studied behind reflected shock waves in a pressurized driver single pulse shock tube over the temperature range 1070-1370 K and overall densities of ～3 × 10-5 mol/ cm3. A large number of products resulting from unimolecular cleavage of the ring and consecutive free radical reactions were obtained under shock heating. A methyl group migration from C(2) to C(3) in the ring with the elimination of CO produces four isomers of C5H8 in unimolecular processes. An additional unimolecular process is the decomposition of 2,5-dimethylfuran to CH3CO and C4H5 which is an important initiator of free radical reactions. Ejection of a hydrogen atom from the methyl group in the molecule is another channel for initiation of free radical reactions in the system. The 2,5-dimethylfuryl radical, which is obtained in the process of H-atom ejection, decomposes in channels similar to those of 2,5-dimethylfuran to produce, among other products, C5H7, which is the precursor of cyclopentadiene. The major decomposition product found in the post shock mixtures is carbon monoxide. The rate constant of its overall formation is estimated as kCO = 1015.81exp(-75.1 × 103/RT) s-1 where R is expressed in units of cal/(K mol). Other products that were found in the postshock samples in decreasing order of abundance were C4H4, C2H2, and CH4 in roughly the same abundance, C2H4, C2H6, CH2=CH-CH=CH2, cyclopentadiene p-C3H4, and a-C3H4 and 2-methylfuran. Other isomers of C4H6, C5H6 and C5H8, and some additional products were found in very small quantities. The total decomposition of 2,5-dimethylfuran in terms of a first-order rate constant is given by: ktotal = 1016.22exp(-77.5 × 103/RT)s-1. An oxygen-carbon mass balance among the decomposition products is obtained. A reaction scheme composed of 50 species and some 180 elementary reactions accounts for the product distribution over the temperature range covered in this study. First-order Arrhenius rate parameters for the formation of the various reaction products are given, a reaction scheme is suggested, and results of computer simulation and sensitivity analysis are shown. Differences and similarities among the reactions of furan, 2-methylfuran, and 2,5-dimethylfuran are discussed.
- Lifshitz, Assa,Tamburu, Carmen,Shashua, Ronen
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p. 10655 - 10670
(2007/10/03)
-
- Decomposition of 2-methylfuran. Experimental and modeling study
-
The thermal reactions of 2-methylfuran were studied behind reflected shock waves in a pressurized driver single pulse shock tube over the temperature range 1100-1400 K and with overall densities of approx. 3 × 10-5 mol/cm3. A large number of products resulting from unimolecular cleavage of the ring and consecutive free radical reactions were obtained under shock heating. The unimolecular decomposition is initiated by two parallel channels: (1) 1,2-hydrogen atom migration from C(5) to C(4) and (2) a methyl group migration from C(2) to C(3) in the ring. Each channel is followed by two parallel modes of ring cleavage. In the first channel, breaking the O - C(2) and the C(4) - C(5) bonds in the ring yields CO and different isomers of C4H6, whereas breaking of the O - C(2) and the C(3) - C(4) bonds yields CH2CO and two isomers C3H4. In the second channel, breaking the O - C(5), and C(2) - C(3) bonds in the ring yields again CO and isomers of C4H6, whereas in the second mode O - C(5), C(2) - C(3), and C(3) - C(4) are broken to yield CO, C2H2, and C2H4. The four C4H6 isomers in decreasing order of abundance were 1,3-butadiene, 1-butyne, 1,2-butadiene, and 2-butyne. The major decomposition product is carbon monoxide. The rate constant for its overall formation is estimated to be kCO = 1015.88 exp(-78.3 × 103/RT) s-1, where R is expressed in units of cal/(K mol). Other products that were found in the postshock samples in decreasing order of abundance were C4H4, C2H2, CH4, p-C3H4, C2H6, C2H4, a-C3H4, C6H6, C4H4O, C3H6, and C4H2. The total decomposition of 2-methylfuran in terms of a first order rate constant is given by ktotal = 1014.78 exp(-71.8 × 103/RT) s-1. This rate and the production rate of carbon monoxide are slightly higher than the ones found in the decomposition of furan. An oxygen-carbon mass balance among the decomposition products was obtained. A reaction scheme composed of 36 species and some 100 elementary reactions accounts for the product distribution over the temperature range covered in this study. First order Arrhenius rate parameters for the formation of the various reaction products are given, a reaction scheme is suggested, and results of computer simulation and sensitivity analysis are shown. Differences and similarities in the reactions of furan and 2-methylfuran are discussed.
- Lifshitz,Tamburu,Shashua
-
p. 1018 - 1029
(2007/10/03)
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- Vibrational overtone activation of methylcyclopropene
-
Laser vibrational overtone activation has been used to investigate the reaction channel competition in the isomerization of 1-methylcyclopropene (MCPene). The vibrational overtone activation of three types of CH stretches (methyl, methylenic, and olefinic) in the 5vCH and 6vCH transitions initiated the isomerization and all three products (2-butyne, 1,3-butadiene, and 1,2-butadiene) were detected by gas chromatography. Stern-Volmer plots were constructed for the appearance of each individual product and the derived experimental specific rate coefficients were compared to those of the Rice-Ramsperger-Kassel-Marcus (RRKM) theory. The rate coefficients for the 6vCH transitions were in good agreement with the predicted values but those for the 5vCH transition were as much as a factor of 5 too large. Product ratios of 1,3-butadiene to 2-butyne and 1,2-butadiene to 2-butyne were independent of pressure. In general, these ratios were lower than the RRKM predicted ratios due to collisional deactivation. No evidence of mode specific behavior was observed in these product yield ratios.
- Snavely,Grinevich,Hassoon,Snavely
-
p. 5845 - 5851
(2007/10/03)
-
- INTERACTION OF ALKYLIDENE CARBENES WITH NITROSO COMPOUNDS
-
The reaction of an alkylidenecarbene with 2-methyl-2-nitrosopropane results in various amides via the intermediacy of keteneimine N-oxide and aziridinone.
- Stang, Peter J.,Anderson, Gary H.
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p. 329 - 332
(2007/10/02)
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- Thermal Isomerization and Decomposition of 1,2-Butadiene in Shock Waves
-
1,2-Butadiene diluted with Ar was heated behind reflected shock waves over the temperature and the total density ranges of 1100 - 1600 K and 1.36*10-5 - 1.75*10-5 mol/cm3.The major products were 1,3-butadiene, 1-butyne, 2-butyne, vinylacetylene, diacetylene, allene, propyne, C2H6, C2H4, C2H2, CH4, and benzene, which were analyzed by gas-chromatography.The UV kinetic absorption spectroscopy at 230 nm showed that 1,2-butadiene rapidly isomerizes to 1,3-butadiene from the initial stage of the reaction above 1200 K.In order to interpret the formation of 1,3-butadiene, 1-butyne, and 2-butyne, it was necessary to include the parallel isomerizations of 1,2-butadiene to these isomers.The present data were successfully modeled with a 82 reaction mechanism.From the modeling, rate constant expressions were derived for the isomerization 1,2-butadiene = 1,3-butadiene to be k3 = 2.5*1013 exp(-63 kcal/RT) s-1 and for the decomposition 1,2-butadiene = C3H3 + CH3 to be k6 = 2.0*1015 exp(-75 kcal/RT) s-1, where the activation energies, 63 kcal/mol and 75 kcal/mol, were assumed.These rate constants are only applicable under the present experimental conditions, 1100 - 1600 K and 1.23 - 2.30 atm.
- Hidaka, Yoshiaki,Higashihara, Tetsuo,Ninomiya, Natsuhiko,Oki, Takashi,Kawano, Hiroyuki
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p. 331 - 342
(2007/10/02)
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- Cyclobutene photochemistry. Steric effects on the photochemical ring opening of alkylcyclobutenes
-
Quantum yields for photochemical ring opening and cycloreversion in hydrocarbon solution have been determined for the direct photolysis (214 nm) of six 1,2-dimethylcyclobutene derivatives which contain methyl groups at C3, and C4 in numbers varying from zero to four. As the hydrogens on C3/C4 of the parent compound (1,2-dimethylcyclobutene) are replaced with increasing numbers of methyl groups, the total quantum yield for ring opening increases to a maximum of ～0.3 and then decreases with further methyl substitution. The quantum yields for ring opening (φtotal) of hexamethylcyclobutene and 1,2-dimethylcyclobutene are nearly the same, and the lowest in the series. The maximum occurs with trans-1,2,3,4-tetramethylcyclobutene; φtotal for the cis-isomer is significantly lower, but both yield an approximate 1:1 mixture of formally allowed and forbidden diene isomers. A similar trend is observed in the relative quantum yields for ring opening and cycloreversion throughout the series. The results are interpreted in terms of a combination of bond strength and steric effects on the efficiency of the ring-opening process. Increasing methyl substitution causes an increase in φtotal through the first three members of the series owing to progressive weakening of the C3-C4 bond. Compounds containing cis-dimethyl substitution exhibit substantially reduced quantum yields for ring opening, relative to what would be expected based on bond strength effects alone. This is proposed to be due to steric effects on the efficiency of the process, suggesting that the initial stages of the photochemical ring opening of cyclobutene involve disrotatory motions on the excited singlet state potential energy surface.
- Leigh, William J.,Postigo, J. Alberto
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p. 1688 - 1694
(2007/10/02)
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- Flash Vacuum Pyrolysis of Stabilised Phosphorus Ylides. Part 1. Preparation of Aliphatic and Therminal Alkynes
-
Thermal extrusion of Ph3PO from β-oxoalkylidenetriphenylphosphoranes 4 to give the alkynes 5, which under conventional pyrolysis conditions is restricted to cases in which R1 is an electron withdrawing group, has been successfully achieved for R1=H or alkyl by using FVP.The method allows convenient construction of multigram quantities of the alkynes 5 from alkyl halides 1 and allows convenient construction of multigram quantities of the alkynes 5 from alkyl halides 1 and acid chlorides 3 in three steps with good overall yields.Under the conditions used the ylides with R2 = cyclobutyl also undergo less of ethene to provide convenient access to the vinylalkynes 6.
- Aitken, R. Alan,Atherton, J. Ian
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p. 1281 - 1284
(2007/10/02)
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- Stereochemical Consequences of Halogen Atom Substitution. 1. Rotational Conformer Effects in Gaseous Diastereomeric 2,3-Dihalobutanes
-
The stereochemical consequences of translationally excited 38Cl-for-X (X = F, Cl) substitution in (2S,3R)-meso- and (2S,3S)-dl-difluorobutane, (2S,3R)-dl- and (2S,3R)-dl-chlorofluorobutane, and (2S,3R)-meso- and (2S,3S)-dl-dichlorobutane were studied in the gas phase.Although retention of configuration was determined to be the dominant substitution pathway, substantial inversion product yields were observed in all cases.A comparison of these yields revealed that the (S,R) configuration within each set always gave a smaller yield of the inverted product than the (S,S) configuration.This observation was consistent with the hypothesis that back-side substitution was a direct mechanism and that steric hindrance to such attack was subject to differences in the rotational conformer populations within each substrate configuration.In contrast, the measured yields of retention products were insensitive to substrate configuration.
- Sharma, Ram B.,Ferrieri, Richard A.,Meyer, Richard J.,Rack, Edward P.,Wolf, Alfred P.
-
p. 2316 - 2320
(2007/10/02)
-
- Conformation in 2,3-Difluorobutanes
-
A conformational analysis for meso- and d,l-2,3-difluorobutanes has been carried out, employing 1H and 19F NMR and theoretical calculations.The real configurations of the two isomers were assigned by specific optical rotation measurements of the products coming from an optically active precursor.Gauche conformations were predominant for meso (E) isomer, while the d,l (T) isomer showed all possible staggered rotamers almost equally populated.Finally remarks for NMR peak assignment of homo- and copolymers partially fluorinated by using gauche additive effects are given.
- Angelini, G.,Gavuzzo, E.,Segre, A. L.,Speranza, M.
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p. 8762 - 8766
(2007/10/02)
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- Flash vacuum pyrolysis of 2-diazo-8-oxabicyclo[3.2.1]oct-6-en-3-ones. A new method for the preparation of propadienones
-
When diazoketones 3a and 3b are pyrolysed at 430°C and 10-4 torr, they undergo loss of N2 followed by Wolff rearrangement and loss of furan by retro Diels-Alder reaction to yield the desired propadienones 1(a,b). If argon is added to the pyrolysate mixture, these reactive compounds can be trapped under matrix isolation conditions at 22 K and observed by FTIR. Upon photolysis, 1b yields carbon monoxide and 2-butyne.
- Brahms,Dailey
-
p. 1381 - 1384
(2007/10/02)
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- Kinetic Investigation of the Reactions of Methyl Radicals with But-2-yne
-
The reactions of methyl radicals with but-2-yne have been studied in a greaseless static vessel in the temperature range 543...583 K.The methyl radicals were generated by thermolysis of azomethane (initial pressures 2.66 kPa and 3.19 kPa) in mixtures with the alkyne (initial pressures 0...7.8 kPa).The primary steps are H-abstractions producing 3-methylpropargyl radicals and additions to the triple bond forming trimethylvinyl radicals.Both radicals were stabilized by hydrogen abstraction from the parent compounds and by combination processes. The temperature dependence of the rate constant k2 for the addition reactio n can be expressed by the equation k2 = 108.65+/-0.20exp(-40300 +/- 1300)Jmol-1/RT M-1s-1 For the rate constant of hydrogen abstraction a value of k1 = 1.65 . 105 M-1s-1 at 573 K was estimated.Therefore the H-abstraction proceeds about two times faster than the addition reaction.
- Claus, P.,Marquardt, M.,Trung, Tran Kim,Scherzer, K.
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p. 533 - 539
(2007/10/02)
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- Mechanism of CH2.+ Transfer from Distonic Ions X-CH2.+ (X=CH2O, CH2CH2) to ?- and n-Electron Bases in the Gas Phase. A Fourier Transform Ion Cyclotron Resonance (FTICR) Study Supplemented by ab initio MO Calculations
-
The reactions of acetonitrile, propyne, acetylene, trimethylsilylacetylene, and tetramethylsilane, with distonic ions CH2XCH2.+ are studied in the gas phase using Fourier Transform Ion Cyclotron Resonance (FTICR) mass spectrometry.In line with previous studies, CH2.+ is transferred to the electron lone-pair of the nitrogen atom of CH3CN to generate CH3CN-CH2.+ (4); upon collisional excitation, this ion undergoes competitive losses of H. and CH..While both neutrals originate from the "methylene" unit of 4, detailed studies employing labeledsubstrates and using various types of collision experiments reveal an intriguing dissociation pattern in that the dissociations are preceded by two intramolecular hydrogen migrations giving rise to CH3C(H)=NCH.+ (6) and CH3C=N(H)CH.+ (7).While 6 serves as intermediate en route to loss of H. from the "CH" moiety, 7 is the actual precursor to generate, by loss of CH., protonated acetonitrile, CH3CNH+ (12) (Scheme 5).In addition, 12 is formed by bimolecular proton transfer.In this reaction, translationally excited CX3CN-CY2.+* (X, Y = H, D) transfers X+ to neutral CX3CN to generate CX3CNX+ (Scheme 4).The bimolecular proton transfer as well as the intramolecular isomerizations of 4 to 6 and 7 are subject to very large kinetic isotope effects.In the transfer of CH2.+ to CH3CCH two products are formed .+ (16) and CH2=C=CHCH3.+ (17) presumably via intermediate 18 (Scheme 6)>; the latter is formed by addition of CH2.+ to the less hindered carbon atom of HCCCH3 reflecting the higher stability of the so-formed intermediate compared with addition to C-2.Reactions of 2 and 3 with HCCH do not result in the formation of a detectable CH2.+-transfer product.When using CH2CH2CH2.+ (2) the reaction is prohibited by the endothermicity to generate the initial complex (structurally related to 18).On the other hand, when CH2OCH2.+ (3) is employed, the intermediate of CH2.+ transfer is formed with sufficient energy to split off a hydrogen atom.Preliminary experiments with silicon-containing molecules, like Si(CH3)4 or HCCSi(CH3)3, demonstrate that the favored processes of these neutrals with 2 and 3 are due to charge transfer (in the form of an electron or an anion like CH3- or C2H-) from the silicon-containing molecule to the distonic ions.The experimental results obtained for CH3CN/CH2.+ system are supported by ab initio MO calculations (3-21G/3-21G + ZPVE).
- Rusli, Ridwan D.,Schwarz, Helmut
-
p. 535 - 540
(2007/10/02)
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- Kinetics of the Reaction of CH2(a1A1) with CH3C2H, HCN, CO2, N2O and COS
-
The reactions of CH2(a1A1) with CH3C2H, HCN, CO2, N2O and COS are investigated at room temperature.CH2(a1A1) is generated by pulsed laser photolysis of CH2CO.Overall removal rate constants are derived from concentration profiles under first order reaction conditions using direct, time resolved LIF detection of CH2(a1A1).The second order rate constants are found in units of 1E13 cm2/mol s to be 24, 18, 2.0, 3.8 and 20, respectively. - The contribution of physical quenching to the removal of CH2(a1A1) are determined by monitoring directyl the formation of CH2(X3B1) with LMR absorption technique.The branching ratios of collision induced intersystem crossing versus total consumption of 1CH2 are 0.14, 0.32, 0.67, 0.46 and 0.29 for the five reactants.
- Koch, M.,Temps, F.,Wagener, R.,Wagner, H. Gg.
-
p. 645 - 650
(2007/10/02)
-
- Cyclobutene photochemistry. Substituent and wavelenght effects on the photochemical ring opening of monocyclic alkylcyclobutenes
-
The photochemical ring opening of cis- and trans-3,4-dimethyl-, 1,3,4-trimethyl-, and 1,2,3,4-tetramethylcyclobutene (1,3, and 4, respectively) has been investigated in hydrocarbon solution with 193 nm and 214 nm light sources.Ring opening is non-stereospecific in all cases at both wavelenghts.The ratio of dienes formed by the formally allowed to formally forbidden pathways in the photolysis of these compounds is highest (ca. 2) for the trimethylcyclobutenes, and approximately 1 for both cis and trans isomers of the di- and tetramethylcyclobutenes with 193 nm excitation.The diene distributions from photolysis of all compounds but cis-3 show slight wavelength dependence.Gas- and solution-phase UV absorption spectra are reported for 3 and 4, and indicate that there are at least three singlet excited states accessible in the 185-230 nm region in these molecules.The ?,R(3s) state is the lowest energy state in the gas phase in 3 and 4.The results verify that orbital symmetry factors do not play a role ( or a consistent one, at least) in controlling the stereochemistry of the reaction, but they do not allow a firm assignment of the excited state(s) responsible for ring opening.Direct photolysis of these compounds also results in fragmentation to yield Z-2-butene (from cis-3 and 4) or E-2-butene (from trans-3 and 4) in addition to propyne or 2-butyne.The 2-butenes are formed with greater than 90percent stereospecifity in all cases.The structures of the four 3-methyl-2,4-hexadiene isomers obtained from photolysis of 3 have been assigned on the basis of 1H NMR spectroscopy and the results of thermolysis of the two cyclobutene isomers.
- Leigh, William J.,Zheng, Kangcheng,Clark, K. Brady
-
p. 1988 - 1997
(2007/10/02)
-
- Selective Lanthanide-catalysed Reactions. Catalytic Properties of Sm and Yb Metal Vapour Deposition Products
-
The characteristics of lanthanide catalysts obtained when Sm and Yb were vaporized into a frozen organic (tetrahydrofuran, benzene and methylcyclohexane) matrix (77 K) were investigated.These low-valent, highly dispersed lanthanide particles (indicated as Sm/THF, Sm/benzene, Yb/THF, Yb/benzene etc.) were catalytically active and selective for hydrogenation and isomerization.Samarium usually showed a greater activity than ytterbium.Olefin hydrogenation obeyed the rate law v=kPH, suggesting that the reaction is controlled by catalytic activation of hydrogen.The molecular isotopic identity of hydrogen was conserved during the hydrogenation.Yb/THF and Yb/benzene were active for partial hydrogenation of benzene to cyclohexene.For the hydrogenation of olefins and acetylenes the substrate specificity was high; thus C-C double bonds were more readily reduced than triple bonds.The samarium and ytterbium catalysts discriminate between terminal and internal C-C triple bonds, only internal CC bonds (but-2-yne and pent-2-yne) being reduced very selectively in contrast to acetylene, methylacetylene and but-1-yne.Solid base character of the lanthanide provides a cause for these differences in catalytic properties.
- Imamura, Hayao,Kitajima, Keiji,Tsuchiya, Susumu
-
p. 1647 - 1654
(2007/10/02)
-
- The isomerization of 1-methylcyclopropene: A multichannel unimolecular reaction induced by overtone excitation
-
The distribution of products resulting from the isomerization of gas phase 1-methylcyclopropene (1) induced by C-H stretch overtone excitation has been studied in a search for mode specific effects.Quantum yields for the disappearance of 1 have been determined as a function of the total pressure of a mixture of 1 and methylenecyclopropane and of the added bath gases SF6 and Ar using a combination of intracavity dye laser photolysis and end product analysis by gas chromatography.These data, together with the ratios of the yields of the major isomerization products, are adequately reproduced by RRKM theory and a weak collisional energy transfer model: no evidence for mode specifity is obtained.The experimental data demonstrate the importance of inefficient collisional energy transfer but allow some conclusions to be drawn concerning the applicability of RRKM theory which are not strongly dependent upon the details of the energy transfer model.
- Baggott, J. E.,Law, D. W.
-
p. 900 - 906
(2007/10/02)
-
- Low-valent rhenium-oxo-alkoxide complexes: Synthesis, characterization, structure, and ligand exchange and carbon monoxide insertion reactions
-
A series of rhenium(III)-oxo-alkoxide-bis(acetylene) complexes of the form Re(O)(OR)(R′C≡CR′)2 have been prepared by reaction of the iodide derivatives Re(O)I(R′C≡CR′)2 with thallium alkoxides. An X-ray crystal structure of the phenoxide complex 7 a shows the pseudotetrahedral coordination geometry typical of Re(O)X(RC≡CR)2 compounds, with a short rhenium-oxo bond of 1.712 (13) ? and a longer Re-OPh distance of 1.966 (14) ?. The alkoxide complexes decompose in solution at less than 100°C by a number of pathways including β-hydrogen elimination. The complexes react rapidly with protic reagents such as alcohols, water, amines, acids, etc. with displacement of alcohol. Reactions of the tert-butoxide complex with ammonia or methylamine yield the corresponding amide complexes, and H2S gives the hydrosulfide species. Many of these ligand exchange reactions give equilibrium mixtures, indicating that the Re-X bond strengths in general parallel H-X bond strengths. The methylamide complex is fluxional on the NMR time scale, with a ground state that places the methyl group pointing at one of the acetylene ligands. The phenoxide ligand in the X-ray structure is approximately in the same sterically encumbered orientation. This orientation is preferred because it favors π-bonding and minimizes π-antibonding interactions of the π-donor orbital of the amide or phenoxide ligand. The ethoxide complex readily inserts carbon monoxide and isopropyl isocyanide to give Re(O)[C(O)OEt](MeC≡CMe)2 and Re(O)[C(N-i-Pr)OEt](MeC≡CMe)2, respectively. Crystal data for 7a: Pna21, a = 18.620 (7) ?, b = 7.2389 (9) ?, c = 10.552 (3) ?, Z = 4, refined to R = 0.044, Rw = 0.044.
- Erikson, Torsten K. G.,Bryan, Jeffrey C.,Mayer, James M.
-
p. 1930 - 1938
(2008/10/08)
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- Isomerisation des radicaux insatures. IV. Les radicaux methyl-1-, methyl-2- et dimethyl-1,2-propene-1 yles produit dans la photolyse des tri- et tetramethylethylenes a 147 nm
-
Direct photolysis at 147 nm of tri- and tetramethylethylenes has been used to produce highly excited vinylic 1-methyl- and 2-methyl-1-propene-1-yl radicals (C4H7), as well as the 1,2-dimethyl-1-propen-1-yl radical (C5H9).In both cases, the primary fragmentation of the photoexcited molecules involved the split of an α(C-C) bond with high yields.The excess energy is sufficiently important that the majority of the vinylic radicals are very hot, and are able to isomerize towards an allylic structure before the occurence of fragmentation.At a pressure of a few Torr, most of the vinylic C4H7 radicals (ca. 70percent) isomerize to an allylic structure, and its lifetime is estimated to be less than one nanosecond under the present conditions.Although the mechanism is less simple, the excited vinylic C5H9 radical exhibits similar behaviour.
- Collin, Guy J.,Deslauriers, Helene,Mare, George R. De
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p. 391 - 396
(2007/10/02)
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- Gezielte Synthese von Kohlenwasserstoff-verbrueckten Komplexen. Nucleophile Addition von Pentacarbonylrhenat und -manganat an ?-gebundene Dien-, Cyclodien-, Trimethylenmethan-, Cycloheptatrienyl- und Alkin-Liganden
-
Pentacarbonyl-rhenate and -manganate react with the cationic complexes +, +, +, +, 7-C
- Mueller, Hans-Joachim,Beck, Wolfgang
-
p. C13 - C16
(2007/10/02)
-
- Thermal Rearrangements, part XIV. Gas Phase Kinetics of Pyrolysis of 1-Methyl-1-cyclopropene
-
The title study has been carried out in the temperature range 210-250 deg C.The isomerisation products are butyne (91-94percent), 1,3-butadiene (5-8percent), and 1,2-butadiene (1-2percent).No other products were detected and material recovery was complete within experimental error (+/- 5percent).The reaction obeyed first-order kinetics, with the products formed by parallel pathways, and appeared to both unimolecular and homogeneous although some surface sensitivity was detected in packed vessels.Studies of pressure dependence showed characteristic rate constant "fall-off" below 20 Torr (SF6) and 100 Torr (N2).Within at least 2percent of the high pressure limit, the following Arrhenius equation for overall decomposition was found: log(k/s-1) = (12.91 +/- 0.15) - (160 +/- 1.5 kJ mol-1)/RTln 10.Arrhenius equations were similarly found for the individual pathways.The figures are discussed in the light of Transition State Theory and represent a significant improvement over a previous study.It is argued that all products arise via 1,2 shifts in a diradical-like intermediate and the propensities for different H shifts are discussed.A 1-methyl substituent, rather surprisingly, deactivates cyclopropene in its isomerisation reactions.
- Hopf, Henning,Wachholz, Gerhard,Walsh, Robin
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p. 3579 - 3587
(2007/10/02)
-
