- Nuclear spin isomers of ethylene: Enrichment by chemical synthesis and application for NMR signal enhancement
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Taking CH2CH2 for a spin: Enrichment of the nuclear spin isomers of ethylene was achieved by a catalytic reaction of acetylene with parahydrogen (see scheme). The spin isomers were used for NMR signal enhancement, both with and without subjecting them to a chemical reaction. The interconversion times of the nuclear spin isomers were evaluated at ambient pressure, thus revealing extremely long-lived spin states with lifetimes of (1000±400)s.
- Zhivonitko, Vladimir V.,Kovtunov, Kirill V.,Chapovsky, Pavel L.,Koptyug, Igor V.
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- Ethylene Formation from 1-Aminocyclopropanecarboxylic Acid by the Reaction of Molecular Oxygen and Dihydropyperidine Mediated by Flavin Mononucleotide and Mn(II) ion
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Oxidation of 1-aminocyclopropanecarboxylic acid by O2 in the presence of 1-benzyl-3-carbamoyl-1,4-dihydropyridine, Mn(II) ion, and flavin mononucleotide reproduced the biological ethylene forming reaction in plant tissues with respect to products, stereoc
- Okamoto, Tadashi,Shimada, Mikio,Oka, Shinzaburo
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- Stereospecificity of acetylene reduction catalyzed by nitrogenase
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In addition to catalyzing the reduction of dinitrogen to ammonia, the metalloenzyme nitrogenase catalyzes the reduction of a number of alternative substrates, including acetylene (C2H2) to ethylene (C2H4) and, in certain cases, to ethane (C2H6). The stereochemistry of proton addition for C2D2 reduction to C2D2H2 catalyzed by the Mo-dependent nitrogenase has been used to probe substrate binding and proton addition mechanisms. In the present work, the C2D2 reduction stereospecificity of altered MoFe proteins having amino acid substitutions within the active site FeMo-cofactor environment was examined by Fourier transform infrared (FTIR) spectroscopy. Altered MoFe proteins examined included those having the α-subunit 96Arg residue substituted by Gln, Leu, or Ala, the α-subunit 69Gly residue substituted by Ser, and the α-subunit 195His residue substituted by Asn. The stereochemistry of proton addition to C2D2 does not correlate with the measured Km values for C2H2 reduction, or with the ability of the enzyme to reduce C2H2 by four electrons to yield C2H6. Instead, the electron flux through nitrogenase was observed to significantly influence the ratio of cis- to trans- 1,2-C2H2D2 formed. Finally, the product distribution observed for reduction of C2H2 in D2O is not consistent with an earlier proposed enzyme-bound intermediate. An alternative model that accounts for the stereochemistry of C2H2 reduction by nitrogenase based on a branched reaction pathway and an enzyme-bound η2-vinyl intermediate is proposed.
- Benton,Christiansen,Dean,Seefeldt
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- Concerted and nonconcerted pathways for thermal conversions of deuterium-labeled cyclohexenes to butadienes and ethylenes
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The retro Diels-Alder reactions of cyclohexene-3,3,6,6-d4 and the 4,5-cis isomers of cyclohexene-1,2,3,4,5,6-d6 have been followed using single-pulse shock tube kinetic and tunable diode laser infrared spectroscopic techniques. At te
- Lewis, David K.,Brandt, Benjamin,Crockford, Lisa,Glenar, David A.,Rauscher, Grant,Rodriguez, Julio,Baldwin, John E.
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- The Stereochemistry of Oxidation of 1-Amino-cyclopropanecarboxylic Acid
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Whereas hypochlorite oxidation of 1-amino-cis-cyclopropane carboxylic acid yields ethylene with retention of stereochemistry the use of transition metal oxidants, such as copper(II), permanganate, and ferrate ions gives completely scrambled cis- and trans-ethylene as is found in the biosynthethic process.
- Baldwin, Jack E.,Jackson, David A.,Adlington, Robert M.,Rawlings, Bernard J.
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- A STEREOCHEMICAL STUDY OF THE THERMOLYSIS OF CYCLOBUTANE-1,2-DIONE
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The thermolysis of both dl and meso-3,4-dideuterio-cyclobutane-1,2-dione at 250 oC yields ethylene and carbon monoxide in wich 79percent of the stereochemistry of reactant is retained in the ethylene.
- Chickos, J. S.,Al-Nawwar, K.
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- Catalytic behavior of a polynuclear Mg-Mo complex and nitrogenase active site (FeMoco) isolated from the enzyme in reactions with C2H 2, N2, and CO: A comparative study
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In order to identify common and distinctive features in the catalytic behavior of natural and artificial nitrogen-fixation clusters, the kinetics of the catalytic reduction of C2H2 in the presence of Mg-Mo-cluster (1) was investigated and compared with the kinetics of acetylene reduction catalyzed by the cluster FeMoco (2) isolated from the enzyme nitrogenase we studied previously. The reactions were conducted in the presence of Zn/Hg and Eu/Hg as reducing agents and PhSH and C6F5SH as proton donors, i.e., under the same conditions as had been used in the case of 2. Both polynuclear Mg-Mo-complex and the europium amalgam-reduced FeMoco have multiple interdependent binding sites for substrates and/or inhibitors. Carbon monoxide inhibits the acetylene reduction much less efficiently in systems with cluster 1 than in systems with cluster 2, although the type of inhibition is mixed in both systems: CO binds to multiple sites of the cluster and affects both C2H2 complexation to the reduced cluster and decomposition of the catalyst-substrate complex to give the products. Unlike isolated FeMoco, the Mg-Mo-cluster efficiently catalyzes the reduction of molecular nitrogen. The reaction is greatly inhibited by acetylene, while no inhibiting effect of N2 is observed in acetylene reduction, as was found earlier for a system with the natural cluster as the catalyst. Springer Science+Business Media, Inc. 2006.
- Bardina,Bazhenova,Petrova,Shilova,Shilov
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p. 793 - 801
(2007/10/03)
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- Stereochemistry of the thermal conversion of 1-vinyl-2,3-cis-dideuteriocyclobutane to butadiene and 1,2-dideuterioethylenes
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This study has examined the stereochemistry of the decomposition at 900-1000 K in a single-pulse shock tube of 1-vinyl-2,3-cis-dideuteriocyclobutane to ethylenes and buta-1,3-dienes. The deuterated ethylenes formed during the decomposition, CHD=CH2, (E)-CHD=CHD, and (Z)-CHD=CHD, were quantified via two independent techniques, FTIR and IR absorption spectroscopy using a tunable diode laser spectrometer. The results of both analyses indicated that equal amounts of (E)-CHD=CHD and (Z)-CHD=CHD were formed from the cis-labeled reactant. In an earlier shock tube study of the decomposition at 1000-1200 K of two deuterium-labeled cyclohexenes to ethylenes plus buta-1,3-dienes, a pathway via a vinylcyclobutane intermediate was implicated in a significant fraction of the decomposition events; the measured ratios of (E)-CHD=CHD to (Z)-CHD=CHD in that study were consistent with a complex mechanistic model in which the stereochemistry of deuterium labels in the cyclohexene reactant was lost in ethylene products formed via vinylcyclobutane. The present results provide additional support for that model.
- Lewis, David K.,Hutchinson, Avery,Lever, Steven J.,Spaulding, Eric L.,Bonacorsi Jr., Samuel J.,Baldwin, John E.
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p. 233 - 237
(2007/10/03)
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- Rearrangement and Cycloreversion of Diels-Alder Adducts of Cyclic 1,3-Dienes to Norbornadienes. A Novel homo-Cope Rearrangement. Evidence for Competitive Pericyclic and Diradical Processes
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On thermolysis of endo,endo-5, endo,endo-17, and endo,exo-17 ( the endo,endo and endo,exo Diels-Alder adducts of cyclopentadiene 7 or dimethylfulvene 15 to norbornadiene 8 or 7-isopropylidenenorbornadiene 20), a novel type of homo-Cope rearrangement leading to 6, 21, and 23a, respectively, competes with the retro-Diels-Alder reactions.According to a force-field analysis of the kinetic parameters, the competitive reactions (rearrangement and retro-Diels-Alder reaction) of endo,endo-5 occur in a pericyclic fashion whereas in the isopropylidene-substituted systems stepwise processes compete with the corresponding pericyclic reactions.Indirect experimental evidence for this assumption comes from a stereochemical analysis of the retro-Diels-Alder reaction in the cis-5,6-dideuterionorbornene derivatives exo-36-d2, endo-36-d2 and exo-39-d2, endo-39-d2 occuring stereospecifically in the case of exo-36-d2, endo-36-d2 and non-stereospecifically in the case of exo-39-d2, endo-39-d2.The change in mechanism rationalized by a different (allyl vs. pentadienyl) stabilization of the potential diradical intermediates in the stepwise reactions. - Key Words: Kinetic parameters / Stepwise and pericyclic reactions / Calculations, force field / Calculations of transition-state resonance energy (energy of concert)
- Hochstrate, Dirk,Klaerner, Frank-Gerrit
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p. 745 - 754
(2007/10/02)
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- Reactivity of Cyclopentenyl Anion Analogous Heterocycles: 1,5-Electrocyclization of 2-Oxa-, 2-Thia, 2-Aza- and 2-Phosphabicyclohept-3-ene. A Sigmatropic Carbon Shift
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Carbonyl ylide-like intermediates are involved in the 1,5-electrocyclization of the bicycloheptenes 3a-c.The activation barriers analyzed by the time- and temperature-dependence of the exo endo isomerization of specifically deuterated derivatives or of the racemization of optically active derivatives turned out to be higher by ΔΔG (excit.) >/= 11 kcal/mol than those determined for the corresponding bicyclohexenes 1a-c.This result can be considered as an evidence for the electrocyclic nature of these ring openings due to the diminished Walsh character of cyclobutane bonds compared to cyclopropane bonds.A stereochemical analysis of the fragmentation of 2-oxabicycloheptene 3a to furan and ethene leads to the conclusion that a sigmatropic carbon shift proceeding with inversion of the migarting carbon followed by stereospecific retro-Diels-Alder reaction is the major pathway for this reaction similar to the rearrangement and fragmentation of the corresponding carbocycle 3e. - Key Words: Furans/ Phospholes/ Pyrroles/ Thiophenes/ 1,5-Electrocyclization/ Ylides/ Sigmatropic carbon shift
- Klaerner, Frank-Gerrit,Yaslak, Salih,Drewes, Rolf,Gesenberg, Christoph,Peter, Michael
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p. 203 - 210
(2007/10/02)
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- Synthesis and stuctures of moderately stable metallaxoetanes: (η5-C5Me5)2(CH 3)TaOCHRCH2= (R=H, C6H5) investigations of their decompositon to olefin and (η5-C5Me5)2 Ta(=O)CH3
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Metallaoxetanes Cp*2(CH3)TaOCHRCH2 (Cp* = ((η5-C5Me5); R = H, p-C6H4X; X = H, Cl, CF3, CN, NO2) have been prepared by reaction of Cp*2Ta(=CH2)CH3 with the appropriate aldehyde RCHO, The kinetic products of the 2 + 2 reaction between the benzaldehydes with Cp*2Ta(==CH2)CH3 are O-syn-Cp*2)(CH3)TaCH2)CH(p-C 6)H4X)O, which rearrange in benzene solution at 25°C to the more O-anti-Cp*2(CH3)TaCH2CH(p-C 6H4X)CH2. An X-ray crystal structure determination for O-anti-Cp*2(CH3)TaOCH(C6H 5)CH2 has been carried out (space group P21/c (No. 14) with cell parameters a = 15.677 (10) A?, b = (4) A?, c = 18.315 (18) A?, β= 110.81 (7)°, V= 2550.3 (32) A?3, and Z = 4), a puckered four-membered ring. The distortion from planarity likely arises from close contacts between a pentamethylcyclo-pentadienyl ligand and the phenyl ring. Decomposition of O-anti-Cp*22(CH3)TaOCHRCH2 smoothly at 80°C to afford Cp*2Ta(=O)CH3 and RCH=CH2. The rates for styrene formation depend only slightly on the para substituent and do not correlate with σ orσ=. In a related transformation, Cp*2Ta(=CH2)H reacts with epoxides in THF even at -50°C to yield Cp*2Ta(=O)CH3 and olefin. Significantly, neither Cp*2Ta(=O)CH3 nor O-syn-Cp*2(CH3)-TaCH2CH2O nor O-anti-Cp"2is observed as an intermediate for the deoxygenation of ethylene oxide, and deoxygenation of trans-styrene-d1(CH3) TaOCH2CH2 is observed as an intermediate for the deoxygenation of trans-styrene-d1 oxide, trans-ethylene-d2 oxide with >95% retention of stereochemistry. These result strongly implicate a concerted process and indicate that epoxide deoxygenation and (more importantly) olefin epoxidation with transition-metal derivatives need not involve metallaoxetane intermediates.
- Whinnery Jr., LeRoy L.,Henling, Lawrence M.,Bereaw, John E.
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p. 7575 - 7582
(2007/10/02)
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- Investigation of the stereochemistry of Fe-Cα bond cleavage when phenylcyclopropane is generated by γ-ionization of stereospecifically deuterated C5H5(CO)2 FeCHDCHDCH(OCH3)C6H5 complexes.
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C5H5(CO)2FeCH2CH 2CH(OCH3)C6H5, 4, and stereospecifically deuterium labeled threo-d2-C5H5-(CO) 2FeCHDCHDCH(OCH3)C6H5, 7a,b and erythro-d2-C5H5(CO) 2FeCHDCHDCH(OCH3)C6H5, 8a,b were synthesized. Treatment of compound 4 with trimethylsilyl triflate results in ionization of the γ-methoxy group and formation of phenylcyclopropane in good yields. Ionization of 7a,b gives a 1:1 mixture of cis-2,cis-3-dideuterio- and trans-2,trans-3-dideuterio-r-1-phenylcyclopropane, while ionization of 8a,b gives cis-2,trans-3-dideuterio-r-1-phenylcyclopropane. These results established that the cyclopropane ring is formed by backside attack of electrophilic Cγ on Cα with net inversion of stereochemistry at Cα. These reactions serve as models for the reactions of carbene complexes C5H5(CO)2Fe=CHR+ with alkenes to give cyclopropanes and suggest that in the transfer reactions Fe-Cα is cleaved with inversion.
- Brookhart, Maurice,Liu, Yumin
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p. 939 - 944
(2007/10/02)
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- Intermediates for the Degenerate and Productive Metathesis of Propene Elucidated by the Metathesis Reaction of (Z)-Propene-1-d1
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Intermediates for productive and degenerate metathesis of propene were elucidated by using (Z)-propene-1-d1 on molybdenum oxide catalysts activated with SnMe4.Productive metathesis of (Z)-propene-1-d1 yielded 2-butene with cis:trans 1 and ethylene-1,2-d2 with cis:trans=1 on the catalysts.This result suggests that the mono-methyl-substituted metallacyclobutanes have no preferential configuration, while the 1,2-dimethylmetallacyclobutanes prefer to take the trans configuration.Degenerate metathesis between (Z)-propene-1-d1 and propene-d6 yielded (E)-propene-1-h1 in 80 percent selectivity.On the basis of these results, the ethylidene species is deduced as the dominant intermediate for the degenerate metathesis of propene.It is assumed in a proposed mechanism that (1) the rate-determinig step for the productive metathesis is the reaction of propene with Mo=CH-CH3 and (2) the degenerate and productive metatheses of propene are competitive in the reaction of propene with Mo=CH-CH3, but they bring about no mutual inhibition, because the degenerate metathesis does not change the population of the Mo=CH-CH3 intermediate.
- Tanaka, Katsumi,Tanaka, Ken-ichi,Takeo, Harutoshi,Matsumura, Chi
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p. 2422 - 2425
(2007/10/02)
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- A Stereochemical Study of the Thermolysis of cis-anti- and trans-1,2-Dimethyl-cis-3,4-dideuteriocyclobutane
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The stereochemistry of the fragmentation and isomerization of cis-anti- and trans-1,2-dimethyl-cis-3,4-dideuteriocyclobutane at 510 deg C is reported.The cis-anti-cis isomer undergoes fragmentation to yield cis/trans-propene-d1 (1.5/1, major pathway), cis/trans-2-butene (1.4/1), and cis/trans-ethylene-d2 (1/1, minor pathway).Recovered cis-1,2-dimethylcyclobutane-d2 containing approximately 40percent of the double rotation product relative to the product of single methyl rotation, trans-1,2-dimethylcyclobutane-d2.The trans isomer behaves similarly, yielding cis/trans-propene-d1 (1/1, major pathway), cis/trans-2-butene (1/5), and cis/trans-ethylene-d2 (1/1, minor pathway).Recovered cis-1,2-dimethylcyclobutane-d2 from thermolysis of the trans isomer consists mainly of equal amounts of cis-anti-cis- and cis-syn-cis-1m2-dimethylcyclobutane-d2 as analyzed by NMR.On the basis of product composition, the thermal chemistry of this system can be explained as proceeding through 2,5-hexanediyl (major pathway) and 3-methyl-1,4-pentanediyl (minor pathway).On the basis of the observed stereochemistry, it can be concluded that the lifetimes of both 2,5-hexanediyl and 3-methyl-1,4-pentanediyl are similar and of the same order as bond rotations at a radical center.This suggests that the gauche to trans conformational changes involving carbon-carbon bond rotation at carbon 2 and 3 of 1,4-diyls may not be competitive with fragmentation.
- Wang, Yen-Seine,Chickos, James S.
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p. 4776 - 4781
(2007/10/02)
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- Thermolysis of (1R,2R)-1,2-Dideuteriocyclobutane. An Application of Vibrational Circular Dichroism to Kinetic Analysis
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The relative rates of geometric isomerization to racemization have been studied for the title compound by using a combination of infrared (IR) and vibrational circular dichroism (VCD) spectroscopies, respectively.The results are interpreted with a kinetic and mechanistic scheme which parallels that used by Berson, Pedersen, and Carpenter on a similar study of chiral cyclopropane-d2 thermolysis.Relative rates of isomerization to stereomutation of 1.5+/-0.4 were obtained which can be interpreted to be consistent with a mechanism best described by random methylene rotation in tetramethylene-d2.This is the first application of VCD to kinetic analysis, and the advantages of IR techniques over the more usually employed UV spectroscopies to this type of basic mechanistic problem are illustrated.
- Chickos, James S.,Annamalai, A.,Keiderling, T. A.
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p. 4398 - 4402
(2007/10/02)
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- On the Stereochemistry of Ethylene Biosynthesis
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The conversion of (i) an equal mixture of the two isomers of cis-2,3-dideuterio-1-aminocyclopropanecarboxylate or (ii) (+/-)-trans-2,3-dideuterio-1-aminocyclopropanecarboxylate by apple slices gave in both cases a 1:1 mixture of cis- and trans-1,2-dideuterioethylene; in contrast the chemical oxidation (NaOCl) of these substances proceeded with complete retention of configuration to cis-dideuterioethylene and trans-dideuterioethylene, respectively.
- Adlington, Robert M.,Baldwin, Jack E.,Rawlings, Bernard J.
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p. 290 - 292
(2007/10/02)
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- The Steric Course of the Reaction of Ethylene Oxide with Hydrogen Halides in the Gas Phase
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The steric course of the gas-phase reaction of trans-ethylene oxide with HF, HCl, and HBr was investigated in order to test experimentally a mechanistic proposal based on ab initio calculations involving a concerted syn-opening mechanism.In contrast with this proposal the reactions with HCl and HBr take place entirely with anti-opening of the ring to give erythro-2-chloro- and 2-bromoethanol.The reaction of ethylene oxide with gaseous HF yields only 5percent 2-fluoroethanol, 37percent dioxan, oligomers and polymers being the main products.An inproved method for the conversion of cis- and trans-ethylene into the corresponding epoxides is described.
- Bellucci, Giuseppe,Berti, Giancarlo,Bianchini, Roberto,Ingrosso, Giovanni,Moroni, Antonio
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p. 1336 - 1340
(2007/10/02)
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- Synthesis and Thermal Decomposition of cis-3,4,5,6-Tetrahydropyridazine-3,4-d2. Relative Rates of Rotation, Cleavage, and Closure for Tetramethylene
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The stereospecific syntheses of cis-3,4,5,6-tetrahydropyridazine-3,4-d2 (6) and cis- and trans-cyclobutane-1,2-d2 are reported.The thermal decomposition of cis-3,4,5,6-tetrahydopyridazine (6) (gas phase, 439 deg C) affords 67.1 +/- 0.9percent cis-ethylene-1,2-d2, 16.1 +/- 0.8percent trans-ethylene-1,2-d2, 9.4 +/- 0.4percent cis-cyclobutane-1,2-d2, and 7.4 +/- 0.4percent trans-cyclobutane-1,2-d2.The relative rates of rotation, cleavage, and closure for this 1,2-diazene generated tetramethylene-d2 are k(cleavage)/k(closure) = 2.2 +/- 0.2 and k(rotation)/k(closure) = 12 +/- 3.An extra stereospecific cleavage component (46percent) superimposed on the 1,4-biradical pathway (54percent) from the parent tetrahydropyridazine was found, similar to that observed in the 3,4-dimethyl-3,4,5,6-tetrahydropyridazine thermal reactions.Finally, the experimental data fot the parent 1,4 biradical, tetramethylene, are compared to calculated values in the literature.
- Dervan, Peter B.,Santilli, Donald S.
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p. 3863 - 3870
(2007/10/02)
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- Free Radical Mechanism in Olefin Isomerization Isomerization of (Z)-But-2-ene and (Z)-Ethylene over Siloxene
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Isomerization of (Z)-but-2-ene and (Z)-but-ethylene over siloxene has been studied.The isomerization of (Z)-but-2-ene gives (E)-but-2-ene as sole product and the double bond migration reaction does not proceed.The isomerization of (Z)-ethylene gives (E)-ethylene and scrambling of deuterium atoms does not occur.Both the radical species on siloxene and the butene isomerization activity sharply increase with the heat treatment temperature of siloxene.Free radical mechanisms for isomerizations are proposed.
- Sendoda, Yoko,Ono, Yoshio
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p. 435 - 441
(2007/10/02)
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