- Application of aerosol techniques to study the catalytic formation of methane on gasborne nickel nanoparticles
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A well known reaction, the so-called methanation reaction over a Ni catalyst, i.e., the formation of methane from CO and hydrogen, was studied to demonstrate the possibilities of the aerosol technique. Reaction order and activation energy conformed to generally accepted values from supported Ni catalysts. The turnover rate (TOR) decreased strongly during the first 10 sec as the reaction proceeded toward a steady value. The decrease correlated with a buildup of about 0.3 monolayer equivalents of carbon on the particle surface measured by TGA and a decline in particle photoelectric activity found via measurement by aerosol photoemission spectroscopy. Order-of-magnitude changes were induced in TOR via defined changes in particle morphology induced by aerosol restructuring techniques preceeding exposure to the catalytic reaction. Aerosol catalysis has potential to develop new catalysts and could be an avenue for studying the putative relationship between combustion aerosols and the formation of dioxin.
- Weber,Seipenbusch,Kasper
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Read Online
- Evidence for Retention of the Cyclic C3H5 Structure during Positive-ion Processes in the Gas Phase
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The decomposition of gaseous cyclopropylmethylbromonium ions has yielded the cyclic hydrocarbons cyclopropane and methylcyclopropane, and methyl bromide.
- Colosimo, Marcello,Bucci, Roberto
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Read Online
- Light-Promoted Transfer of an Iridium Hydride in Alkyl Ether Cleavage
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A catalytic, light-promoted hydrosilylative cleavage reaction of alkyl ethers is reported. Initial studies are consistent with a mechanism involving heterolytic silane activation followed by delivery of a photohydride equivalent to a silyloxonium ion generated in situ. The catalyst resting state is a mixture of Cp*Ir(ppy)H (ppy = 2-phenylpyridine-κC,N) and a related hydride-bridged dimer. Trends in selectivity in substrate reduction are consistent with nonradical mechanisms for C-O bond scission. Irradiation of Cp*Ir(ppy)H with blue light is found to increase the rate of hydride delivery to an oxonium ion in a stoichiometric test. A comparable rate enhancement is found in carbonyl hydrosilylation catalysis, which operates through a related mechanism also involving Cp*Ir(ppy)H as the resting state.
- Fast, Caleb D.,Schley, Nathan D.
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supporting information
p. 3291 - 3297
(2021/10/12)
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- Experimental evidence for heavy-atom tunneling in the ring-opening of cyclopropylcarbinyl radical from intramolecular 12C/13C kinetic isotope effects
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The intramolecular 13C kinetic isotope effects for the ring-opening of cyclopropylcarbinyl radical were determined over a broad temperature range. The observed isotope effects are unprecedentedly large, ranging from 1.062 at 80 °C to 1.163 at -100 °C. Semiclassical calculations employing canonical variational transition-state theory drastically underpredict the observed isotope effects, but the predicted isotope effects including tunneling by a small-curvature tunneling model match well with experiment. These results and a curvature in the Arrhenius plot of the isotope effects support the recently predicted importance of heavy-atom tunneling in cyclopropylcarbinyl ring-opening.
- Gonzalez-James, Ollie M.,Zhang, Xue,Datta, Ayan,Hrovat, David A.,Borden, Weston Thatcher,Singleton, Daniel A.
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supporting information; experimental part
p. 12548 - 12549
(2010/11/05)
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- Homolytic C-S bond scission in the desulfurization of aromatic and aliphatic thiols mediated by a Mo/Co/S cluster: Mechanistic aspects relevant HDS catalysis
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The kinetics of the reaction of a series of aromatic and aliphatic thiols with cluster 1 were determined. These reactions form cluster 2 and the arene or alkane corresponding to the thiol: Cp'2Mo2Co2S3(CO)4 (1) + RSH → Cp'2Mo2Co2S4(CO)2 (2) + RH + 2CO. These reactions are first order in thiol and first order in cluster 1 with appreciable negative entropies of activation. These data suggest that the rate determining step of the desulfurization reaction is the initial association of the thiol to the cluster. The more nucleophilic thiolate anions react with 1 at -40°C to form an adduct in which the thiolate anion is bound η1 to the Co atom. At -25°C, the initial adduct rearranges to a fluxional μ2, η1-bound thiolate. The fluxional process is proposed to involve a concerted 'walking' of the thiolate and a μ2-bound sulfide ligand on the surface of the cluster. Near 35°C, the thiolate-cluster adduct undergoes C-S bond homolysis to give the paramagnetic anion of cluster 1 and the phenyl or alkyl radical. The radical nature of the C-S bond cleavage was confirmed by the desulfurization of the radical clock reagents, cyclopropylmethanethiol and -thiolate anion, that form the cyclopropylmethyl radical which rearranged to the butenyl radical. The possible similarity in the C-S bond cleavage mechanism in these desulfurization reactions to those occurring in hydrodesulfurization (HDS) over Co/Mo/S catalysts is discussed.
- Curtis, M. David,Druker, Scott H.
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p. 1027 - 1036
(2007/10/03)
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- Ion and radical rearrangements as a probe of the mechanism of a surface reaction : The desulfurization of cyclopropylmethanethiol and 3-butene-1-thiol on Mo(110)
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Rearrangement reactions were used to probe the transient intermediates in thiol desulfurization induced by Mo(110) by studying cyclopropylmethanethiol and 3-butene-1-thiol. Thiolate intermediates were identified in both cases using vibrational spectroscopy, which indicates facile S-H bond scission on Mo(110). Heterolytic C-S bond scission, leading to a cationic intermediate, is excluded based on the lack of rearrangement products in the reactions of 3-butene-1-thiolate and the absence of cyclobutane or cyclobutene in the reaction of cyclopropylmethyl thiolate on Mo(110). Hydrogenolysis without rearrangement is the primary pathway for both thiols investigated. The lack of rearrangement in the 3-butene-1-thiolate indicates that C-S bond scission and C-H bond formation occur nearly simultaneously. Evidence for the radical pathway is obtained from the production of 1,3-butadiene formed via the rearrangement of cyclopropylmethyl group following C-S bond scission in the cyclopropylmethyl thiolate and by related studies of cyclopropylmethyl bromide. The investigation of the cyclopropylmethyl bromide also demonstrates that trapping of the cyclopropylmethyl radical is favored over selective β-dehydrogenation. This is the first study in which radical rearrangements have been used to obtain detailed information about the nature of extremely short-lived reactions in a surface process.
- Wiegand,Napier,Friend,Uvdal
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p. 2962 - 2968
(2007/10/03)
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- Titanium Catalyzed Reduction of Aromatic Halides by Sodium Borohydride
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The reduction of aryl halides by sodium borohydride is catalyzed by titanium complexes; di(cyclopentadienyl)titanium dichloride (titanocence dichloride) is highly effective.The reaction scope and mechanism are solvent dependent.In dimethylformamide (DMF), an adduct of DMF and sodium borohydride is formed which reduces simple aryl halides by a non-radical, likely nucleophilic route.Dimethylamino- substituted products are formed, as are simple dechlorinated species.In dimethylacetamide or in ethers, a radical-based reaction involving activated titanocene borohydride takes place, and only dechlorinated products result.
- Liu, Yumin,Schwartz, Jeffrey
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p. 4471 - 4482
(2007/10/02)
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- The Mechanism of Titanium Complex-Catalyzed Reduction of Aryl Halides by Sodium Borohydride Is Strongly Solvent Dependent
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The titanium complex-catalyzed reduction of aryl halides by sodium borohydride in dimethylacetamide (DMA) or ethers proceeds by electron transfer from a reduced titanium species, yielding an intermediate aryl radical.
- Liu, Yumin,Schwartz, Jeffrey
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p. 940 - 942
(2007/10/02)
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- Picosecond radical kinetics. Rate constants for reaction of benzeneselenol with primary alkyl radicals and calibration of the 6-cyano-5-hexenyl radical cyclization
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The cyclopropylcarbinyl radical ring opening was used as a radical clock to determine rate constants for benzeneselenol trapping in THF and in toluene. Hydrogen atom transfer trapping from PhSeH appeared to be partially diffusion controlled. An operational Arrhenius function for trapping in THF is log (kT·M s) = 11.03 - 2.21/2.3RT. The recommended function for PhSeH trapping in other low-viscosity organic solvents is log (kT·M s) = 10.87 - 2.10/2.3RT. The rate constant for trapping at 25°C is 2.1 × 109 M-1 s-1. The kinetic values are expected to apply for PhSeH trapping of simple primary alkyl radicals. As a check on this assumption, cyclization of the 6-cyano-5-hexenyl radical (9), produced from the corresponding PTOC ester radical precursor, was calibrated with PhSH and PhSeH trapping. The two trapping agents gave essentially equivalent results. The cyclizations of both (E)- and (Z)-9 are described by log (kr·s) = 11.0 - 3.8/2.3RT. This fast rearrangement (kr = 1.6 × 108 s-1 at 25°C) could prove to be useful as a radical clock for timing fast second-order processes.
- Newcomb, Martin,Varick, Thomas R.,Ha, Chau,Manek, M. Beata,Yue, Xu
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p. 8158 - 8163
(2007/10/02)
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- Synthesis, Structure, and Reactions of Stable Titanacyclopentanes
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Titanacyclic compounds of the formula Cp*2Ti(CH2CH2C(CH2CHR)CH2) (5a; R=H and 5b; R=C6H5, Cp*=pentamethylcyclopentadienyl), the first stable titanacyclopentanes, have been prepared by the reaction of bis(pentamethylcyclopentadienyl)titanium-ethylene complex (3) with methylenecyclopropanes (4), and their structures were determined based on both spectroscopic data and X-ray crystallography.Complex 5b crystallized in space group P21/a (Z=4) with cell constants, a=21.832(3), b=8.580(1), c=14.759(2) Angstroem, β=96.81(1) deg, U=2744.9(6) Angstroem3 (4261 reflections, R=0.053).The reaction of 5 with carbon monoxide afforded spiroheptan-5-ones in 98percent yield.The thermal decomposition of 5 has been investigated, and possible mechanisms of the reactions have been proposed based on deuterium-labeled experiments.A novel formal reductive elimination of organic ligands giving 1-phenylspirohexane has been observed in the thermolysis of 5b.A structure-reactivity relationship has been discussed.
- Mashima, Kazushi,Sakai, Nozomu,Takaya, Hidemasa
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p. 2475 - 2483
(2007/10/02)
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- Homocoupling of Alkyl Halides and Cyclization of α,ω-Dihaloalkanes via Activated Copper
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The homocoupling of alkyl halides utilizing a highly activated form of zerovalent copper has been studied.Allyl and benzyl halides produce high yields of their respective homocoupled dimers 1,5-hexadiene and 1,2-diphenylethane.An 83percent yield of tetradecane was produced from n-heptyl iodide.The yield drops substantially for the corresponding bromo and chloro compounds.The yield is also strongly solvent and temperature dependent.Secondary and tertiary alkyl iodides and bromides produce moderate to low yields of homocoupling accompanied by substantial amounts of the corresponding alkane and alkenes.The copper-mediated cyclization of α,ω-dihaloalkanes has also been examined.The yield of cycloalkanes is moderate to high for the smaller rings with the yield decreasing substantially as the ring size increases.The compound meso-1,2-dibromo-1,2-diphenylethane was found to produce exclusively trans-stilbene in high yield.The activated copper was produced by reducing CuI*PR3 with a preformed solution of lithium naphthalenide.Complexes using triethylphosphine and tri-n-butylphosphine were both studied.The product yields were found to be similar in both cases.
- Ginah, Francis O.,Donovan, Thomas A.,Suchan, Scott D.,Pfennig, Deborah R.,Ebert, Greg W.
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p. 584 - 589
(2007/10/02)
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- The Gas-Phase Acidities of the Alkanes
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The gas-phase acidities of 15 simply alkanes have been determined in a flowing afterglow-selected ion flow tube (FA-SIFT) by a kinetic method in which alkyltrimethylsilanes are allowed to react with hydroxide ions to produce a mixture of trimethylsiloxide ions by loss of alkane and alkyldimethylsiloxide ions by loss of methane.The reaction is proposed to proceed by addition of hydroxide ion to the silane to form a pentacoordinate siliconate ion intermediate which decomposes through two transition states, one in which negative charge is placed on a methyl group and the other in which negative charge is placed on the alkyl group.The ratio of siloxide ions produced is proposed to be correlated to the relative basicity of the methyl and alkyl anions.The method is calibrated by making use of the known acidities of methane (ΔH0acid=416.6 kcal/mol) and benzene (ΔH0acid= 400.7 kcal/mol).In general, methyl substitution is found to stabilize alkyl anions in the gas phase except that the ethyl anion is found to be more basic that the methyl anion.By combining the gas-phase acidities with the bond dissociation energies, the electron affinities (EA) of the corresponding alkyl radicals can be calculated.Many simple alkyl radicals are found to have negative EA's.The results for the alkyl groups studied are as follows, where the first number is the ΔH 0acid (kcal/mol) of the corresponding alkane and the second number (in parentheses) is the EA (kcal/mol) of the alkyl radical: ethyl 420.1 (-6.4), isopropyl 419.4 (-9.5), cyclobutyl 417.4 (-7.5), cyclopentyl 416.1 (-7.0), sec-butyl 415.7 (-5.8), n-propyl 415.6 (-1.9), tert-butyl 413.1 (-5.9), isobutyl 412.9 (0.8), 3-butenyl 412.0 (1.7), cyclopropyl 411.5 (8.4), cyclopropylmethyl 410.5 (3.2), 1-methylcyclopropyl 409.2 (8.0), neopentyl 408.9 (4.8), vinyl 407.5 (16.1), 2-propenyl 405.8 (15.8).
- DePuy, Charles H.,Gronert, Scott,Barlow, Stephan E.,Bierbaum, Veronica M.,Damrauer, Robert
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p. 1968 - 1973
(2007/10/02)
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- A Convenient Method for Kinetic Studies of Fast Radical Rearrangements. Rate Constants and Arrhenius Function for the Cyclopropylcarbinyl Radical Ring Opening
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An indirect method for studying the kinetics of radical rearrangements employing N-hydroxypyridine-2-thione esters as radical precursors and hydrogen atom transfer from thiophenol as the basis trapping reaction is demonstrated in a study of an archetypal fast radical reaction, the cyclopropylcarbinyl radical ring opening.Rate constants measured over the temperature range -37 to 50 deg C gave a temperature-dependent function for the ring opening of log(k/s-1)=(13.0+/-0.14)-(6.8+/-0.2)/θ and a rate constant for ring opening at 25 deg C of 1.0E8 s-1.The kinetic values from this work were combined with rate data from low-temperature ESR studies and higher temperature nitroxyl radical trapping studies to give an Arrhenius function of log(k/s-1)=13.15-7.05/θ that is recommended for calculations of rate constants for the title ring opening.
- Newcomb, Martin,Glenn, Anne G.
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p. 275 - 277
(2007/10/02)
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- Isomeric Characterization of Gaseous Ions. Minimizing C4H8.+ Rearrangment by Dissociating the Corresponding Neutrals
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With use of neutralization-reionization (NR) mass spectrometry, five gaseous C4H8.+ isomers can be structurally characterized by neutralization with sodium to produce excited C4H8 molecules whose dissociation products are reionized to both negative and positive ions.Such NR mass spectra are less characteristic when produced by Cs neutralization, which forms more highly exicted C4H8, or by Hg neutralization followed by dissocation of C4H8 using multiple collisions; on average each collision adds ca. 1.7 eV to the molecule, so that isomerization may occur between collisions.The Na NR spectra show that C4H8.+ ions from all butyl acetate isomers have the expected structure except that 2-C4H8.+ is formed from n-butyl acetate. 2-C4H8.+ and methylcyclopropane.+ appear to be the major products from methylcyclopentane.+. 1-Butanol.+ and cyclohexane.+ predominatly form cyclobutane.+, while β- and γ-valerolactone.+ give 1-C4H8.+ and methylocyclopropane.+, respectively, as major products.
- Feng, Rong,Wesdemiotis, Chrysostomos,Zhang, Mei-Yi,Marchetti, Mauro,McLafferty, Fred W.
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p. 1986 - 1991
(2007/10/02)
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- Rate Constants and Arrhenius Functions for Hydrogen Atom Transfer from tert-Butyl Thiol to Primary Alkyl Radicals
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The radical clock method was used to determine rate constants for hydrogen atom transfer from t-BuSH to primary alkyl radicals over the temperature range -45 to 50 deg C.Three clocks, cyclization of the 5-hexenyl radical (3a), ring opening of the cyclopropylcarbinyl radical (3b), and rearrangement of the 2,2-dimethyl-3-butenyl radical (3c), were used in THF, and 3a was used in toluene.Arrhenius functions for the two studies with 3a and the one with 3b were quite similar, and the Arrhenius function for 3c, when adjusted for direct comparison to reactions with simple primary radicals, was also similar.Combining the results from 3a and 3b in THF gave an Arrhenius function for hydrogen atom transfer from t-BuSH to primary radicals of log(k/M-1 s-1) = (8.37 +/- 0.08) - (2.00 +/- 0.09)/2.3RT; the calculated rate constant at 25 deg C is 8.0 * 106 M-1 s-1.This study ultimately relates the t-BuSH rate constants to those for hydrogen atom transfer to primary radicals from both Bu3SnH and PhSH and serves to compare the latter two sets of kinetic data; it also demonstrates that the fast radical clock reactions of 3a and 3b are calibrated against equivalent standards.
- Newcomb, Martin,Glenn, Anne G.,Manek, M. Beata
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p. 4603 - 4606
(2007/10/02)
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- Cyclopropane formation from the reaction of (γ-haloalkyl)iron compounds with silver(1+)
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Reaction of C5H5(CO)2Fe(CH2)3Br with AgBF4 in benzene leads to formation of cyclopropane in 73% yield.
- Casey, Charles P.,Smith, Laura J.
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p. 2419 - 2421
(2008/10/08)
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- NEW REDUCTION OF SULFOXIDES AND SULFILIMINES TO SULFIDES WITH ZINC/1,4-DIBROMOBUTANE. A PROPOSAL FOR A RADICAL-FORMING ELECTRON TRANSFER PROCESS
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Based upon (13)C NMR spectroscopic observations of the deuteriorated methylcyclopropane and 1-butene, a bromoalkyl radical process has been considered in the reduction of sulfoxides and sulfilimines to sulfides by zinc/1,4-dibromobutane.KEYWORDS --- sulfide; reduction; sulfoxide; sulfilimine; zinc; 1,4-dibromobutane; single electron transfer; (13)C NMR; deuterio-methylcyclopropane; deuterio-butene-1
- Nagasawa, Kazuo,Yoneta, Akemi,Sugimoto, Hideki,Ito, Keiichi
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p. 4351 - 4354
(2007/10/02)
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- Radical-Based Dephosphorylation and Organophosphonate Biodegradation
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Products resulting from the degradation of organophosphonates by Escherichia coli are identified and used as a basis for evaluating mechanisms which may be the chemical basis of the biodegradation.One mechanistic hypothesis which is consistent with the biodegradation products involves radical-based dephosphorylation.Chemical modeling of this process is achieved by the reaction of alkylphosphonic acids with lead(IV) tetraacetate and electrochemical oxidation at a platinum anode.
- Frost, J. W.,Cordeiro, M. L.,Li, D.
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p. 2166 - 2171
(2007/10/02)
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- γ-Silicon Stabilization of Carbonium Ions in Solvolysis. 2. Solvolysis of 4-(Trimethylsilyl)-2-butyl p-Bromobenzenesulfonates
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The solvolysis of 4-(trimethylsilyl)-2-butyl p-bromobenzenesulfonate produces racemic substitution products and a minor amount of methylcyclopropane at a rate which is several-fold faster than the solvolysis of its carbon analogue; polarimetric and conductometric rates are not significantly different.Added LiClO4, KOH, KOBs, and LiOBs produced normal salt effects on the polarimetric rate; b values varied from 0.2 to 130, depending on the nature of the salt and solvent.The deuterium isotope rate effects vary somewhat with solvent as follows: α-d, 1.12-1.15; β-d3, 1.115-1.16; β-d2, 0.975-1.07.It is concluded that the γ-silyl group promotes the solvolysis and stabilizes the intermediate carbonium ion.The "W" and "sickle" conformations of the transition states for the formation and/or reaction of the carbonium ion intermediate have approximately equal energies.
- Shiner, V. J.,Ensinger, Mark W.,Rutkowske, Randy D.
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p. 804 - 809
(2007/10/02)
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- STEREOSPECIFIC PHOTODIMERIZATION OF UNSATURATED COMPOUNDS INDUCED BY NICKEL COMPLEXES
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Irradiation of (ethylene)bis(triphenylphosphine)nickel (1) or bis(triphenylphosphine)nickelacyclopentane induced stereospecific cyclodimerization of substituted olefins such as acrylonitrile and methylacrylate affording trans-1,2-disubstituted cyclobutanes, while 1,7-octadiene was converted to trans-bicyclooctane.Ethylene was photochemically dimerized to cyclo- and linear dimers induced by 1.
- Miyashita, Akira,Ikezu, Shohgo,Nohira, Hiroyuki
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p. 1235 - 1238
(2007/10/02)
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- Reactions of Cyclopropylcarbinyl Halides with (Trimethylstannyl)alkalis.Evidence That Kinetically Free Intermediates Need Not Be Involved in Cyclopropylcarbinyl to 3-Butenyl Rearrangements
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Studies on the reactions of cyclopropylcarbinyl bromide and iodide with trimethylstannyl anionoids have been made with emphasis on counterion, solvent, and addend (tert-butylamine and dicyclohexylphosphine) effects.Both halides yield cyclopropylcarbinyl- and 3-butenyltrimethylstannanes as major products.Depending upon reaction parameters much or all of the latter is shown to be formed by a mechanism or mechanisms which do not involve kinetically free radical or ionic intermediates.With cyclopropylcarbinyl bromide no electron transfer (ET) (free radical) component isobserved in THF or in THF containing 18-crown-6.However, in 20 percent THF- 80 percent benzene a small extent of the ET mechanism is observed in the presence of DCPH.The iodide reacts in THF via SN2 (or equivalent) and halogen-metal exchange (HME) mechanisms leading to the formation of both methylcyclopropane and 1-butene in the presence of traps, indicating that the cyclopropylcarbinyl anion can undergo rearrangement to the 3-butenyl anion in competition with other processes.In the presence of 18-crown-6 and DCPH a small ET component is observed with the iodide.
- Alnajjar, Mikhail S.,Smith, Gary F.,Kuivila, Henry G.
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p. 1271 - 1276
(2007/10/02)
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- The Reactions of Cyclopropyl(methyl)bromonium Ion with Benzene in the Gas Phase
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The thermal gaseous cyclo-C3H5BrCH3(+) ion, formed by the reaction of CH3ClCH3(+) ion, but not of CH3FCH3(+) ion, with cyclopropyl bromide, reacts with benzene as a methylating and as an allylating agent with equal facility.The loss of the cyclic structure results either from a concentrated ring cleavage during the attack or from rearrangement of cyclopropylarenium ion after the attack.The methylating ability of cyclopropyl(methyl)bromonium ion toward benzene falls between those of dimethylfluoronium and dimehtylchloronium ions, i.e.CH3FCH3(+) > cyclo-C3H5-BrCH3(+) > CH3ClCH3(+).
- Colosimo, Marcello,Bucci, Roberto
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p. 461 - 464
(2007/10/02)
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- Reactions and Properties of Some Trimethyleneplatinum(IV) Complexes. Part 11. Platinacyclobutanes derived from Methylcyclopropane
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It is shown that platinacyclobutanes derived from methylcyclopropane exist as a mixture of isomers with partial structures PtCH2CHMeCH2 and PtCH2CH2CHMe, and that these isomers interconvert easily.In some cases decomposition of these platinacyclobutanes gives ylide complexes L>, e.g.L = 2-methylpyridine, while in other cases but-1-ene complexes , e.g.L = CD3CN, are formed.In both cases the products are formed selectively from the least stable isomer of the platinacyclobutane.At higher temperatures or on photolysis, platinacyclobutane complexes such as decompose to give but-1-ene, 2-methylpropene, propene, and ethylene and the relative yields depend on the experimental conditions.Possible mechanisms are discussed.
- Al-Essa, Razak J.,Puddephatt, Richard J.,Perkins, Duncan C. L.,Rendle, Melvyn C.,Tipper, Charles F. H.
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p. 1738 - 1745
(2007/10/02)
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- An Experimental and Theoretical Investigation of the Mechanism of Deoxygenation of Carbonyl Compounds by Atomic Carbon
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The reactions of atomic carbon with butanal and butanone have been studied by both MNDO calculations and experiment.The calculations indicate the preferred path to involve addition of carbon across the C=O bond to form an oxiranylidene which rearranges to ketene.Since the ketene is formed with enough excess energy to undergo dissociation to carbon monoxide and a carbene, it should be detectable only under conditions where the excess energy is rapidly dissipated.While no ketene was observed when the reactions were carried out in the gas phase, a ketene intermediate was trapped by addition of water following the reaction of arc-generated carbon atoms with frozen butanal.
- Dewar, Michael J. S.,Nelson, Donna J.,Shevlin, Philip B.,Biesiada, Keith A.
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p. 2802 - 2807
(2007/10/02)
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- The 147-nm Photolyses of Tetramethylene Sulfide, Tetramethylene Sulfoxide, and Tetramethylene Sulfone
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Tetramethylene sulfide, sulfoxide, and sulfone have been photolyzed with 147-nm radiation in the gas and condensed phases.Mechanisms which assume diradical intermediates are presented for the photodecomposition of each compound.The major reaction in all three cases is the elimination of ethylene by a β-cleavage of the initially formed diradical: *XCH2CH2CH2CH2*->*XCH2CH2*+C2H4.The competing β-cleavage produces X=CH2 and C3H6.This second β-cleavage is less important in the sulfoxide and the sulfone than it is in the sulfide.The direct elimination of the sulfur atom, and any attached oxygen atoms, is of no importance in the sulfide and of increasing importance as the oxidation state of the sulfur atom increases.The similarity between the mass spectral fragmentation and the vacuum ultraviolet photochemistry of these compounds is noted and the possible involvement of triplet states is mentioned.
- Scala, A. A.,Colon, Ismael,Rourke, W.
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p. 3603 - 3607
(2007/10/02)
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- Ion-Molecule Complexes in Unimolecular Fragmentations of Gaseous Cations. Alkyl Phenyl Ether Molecular Ions
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A decomposition pathway that bears a formal resemblance to first-order elimination in solution is demonstrated for parent ion fragmentations of alkyl phenyl ethers under electron impact (EI) and chemical ionization (CI).The sequence of steps in the gas phase, parent ion -> ion-molecule complex -> fragments, is analogous to first-order elimination in solution (which goes through ion pairs).Such a mechanism for expulsion of PhOH+. from molecular ions has been tested by examing neutral products from 70-eV electron bombardment of neopentyl phenyl ether in a specially constructed electron bombardment flow (EBFlow) reactor.The C5H10 isomers 2-methyl-1-butene (2) and 2-methyl-2-butene (3) are recovered in the same ratio (2/3=1.14) as is produced by gas-phase deprotonation of tert-amyl cation.This result is validated by a mass spectrometric study of deuterated analogues, for which the ratio of γ-transfer (corresponding to product 2) to α-transfer (corresponding to product 3) is calculated to be 1.46.Intermediacy of an ion-molecule complex, PhOCH2C(CH3)3+. -> .CH3CH2(CH3)2C+> -> PhOH+. + 2 or 3, predicts this outcome, where the species in brackets represents an electrostatically bound comlex of the neutral phenoxyl radical and an alkyl cation.This mechanism explains the published mass spectrometric (EI an CI) and EBFlow results for n-propyl and n-butyl phenyl ethers.Several confirmatory experiments support the intermediacy of an ion-molecule complex and rule out other interpretations of the experiment data.
- Morton, Thomas Hellman
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p. 1596 - 1602
(2007/10/02)
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- The photolysis of tetrahydrofuran and of some of its methyl derivatives at 185 nm
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The uv photolysis of tetrahydrofuran, 1, 2-methyltetrahydrofuran, 2, cis-2,5-dimethylterahydrofuran, 3, trans-2,5-dimethyltetrahydrofuran, 4, and 2,2,5,5-tetramethyltetrahydrofuran, 5, has been investigated by product analysis in the liquid phase, and quantum yields have been determined.The photolysis of tetrahydrofuran itself was also studied in the gas phase at pressures ranging from 1 to 120 atm (pressurizing gas N2); and very little difference was found between the photolytic behaviour of the vapour at 120 atm and that of the liquid.The major products are in all cases the cyclopropanes and the corresponding carbonyl compounds, as well as the olefinic alcohols and the carbonyl compounds that are isomeric with the starting material.These products are considered to be formed by the two primary processes and . The cyclopropanes formed in reaction retain some excess energy (apparently more then is needed to realize the trimethylene form), and in the photolysis of tetrahydrofuran vaapour the hot cyclopropane rearranges to a considerable extent into propene.The propene to cyclopropane yield ratio falls strongly with increasing pressure, to a value of 0.065 at 120 atm.A similar value is observed in the liquid phase photolysis.The five-membered oxyl alkyl diradical from reaction is the likely intermediate in the cis-trans photoisomerization that is observable with the 2,5-dimethyltetrahydrofurans (Φ(cis -> trans) ca. Φ(trans -> cis) ca. 0.2).The photolysis of these compounds also demonstrates that steric factors have a strong bearing on the course of the reaction, e. g. the quantum yield of methylcyclopropane from the cis compound is 0.22, vs. 0.08 from the trans compound.Molecular hydrogen is produced if the tetrahydrofurans carry hydrogen in α-position.Its production is enhanced if the α-position is shared with a methyl group (1 gives a hydrogen quantum yield of 0.07, 2 of 0.17, 3 of 0.27, 4 of 0.29, and 5 of zero).
- Kizilkilic, Nuray,Schuchmann, Heinz-Peter,Sonntag, Clemens Von
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p. 2819 - 2826
(2007/10/02)
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- (Cycloalkylmethyl)bis(η5-cyclopentadienyl)titanium(III) Complexes
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Reaction of bis(cyclopentadienyl)titanium dichloride (1) and isopropylmagnesium bromide (2) with the methylenecycloalkanes 3-8 (cyclopropane), 35 (cyclobutane), and 37 (cyclopentane) leads to the (cycloalkylmethyl)bis(cyclopentadienyl)titanium(III) compounds 9-14, 36, and 38.The cyclopropylmethyl complexes 12-14, which have et least two geminal methyl substituents, are stable.Compounds 9-11, 36, and 38, in which appropriate substituents are lacking, rearrange by ring opening followed by C=C bond isomerization to give the (η3-allyl)bis(η5-cyclopentadienyl)titanium complexes 23-26 and 40.
- Lehmkuhl, Herbert,Fustero, Santos
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p. 1361 - 1370
(2007/10/02)
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- Ring-opening of Some Radicals containing the Cyclopropylmethyl System
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Mono- and bi-cyclic radicals containing the cyclopropylmethyl system are readily generated by interaction of the appropriate halides with triphenyl- or tributyl-stannane.Each radical studied underwent ring-opening by fission of the more substituted βγ-bond.In the case of the secondary radical (12b) the new double bond was formed preferentially in the trans-configuration.Rate constants, which cannot be determined with high accuracy by this method, lie in the range 1E7-3E8 s-1 at 25 deg C.When generated by the flow method in the e.s.r. cavity α-hydroxycyclopropylmethyl radicals undergo β-fission followed by 1,5-hydrogen atom t ransfer to afford enoxyl radicals.The latter reaction occurs more slowly in water than in non-polar solvents.The rigid hydroxynortricyclyl (43) undergoes preferential fission of the less substituted βγ-bond, possibly because of the dipolar nature of the transition state.
- Beckwith, Athelstan L. J.,Moad, Graeme
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p. 1473 - 1482
(2007/10/02)
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- Competitive reactions of nucleophiles. Solvolyses of cyclopropylcarbinyl and cyclobutyl methanesulfonates in the presence of NaBH4
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Cyclopropylcarbinyl (I) and cyclobutyl (II) mesylates were solvolysed in aqueous diglyme with or without added NaBH4 under a variety of conditions. The reaction products were isolated and analyzed by VPC. A mixture containing cyclopropylcarbinyl, cyclobutyl and minor quantities of homoallyl products, was obtained in all cases. The proportion of the unsaturated compound was greater in hydrocarbon than in carbinol products, the composition of the mixture depending upon reaction conditions and the starting mesylate. Thus, solvolysis of both I and II yielded cyclopropylcarbinol and cyclobutanol in a ratio of nearly 1:1, while in the hydrocarbon products methylcyclopropane predominated over cyclobutane by a factor of 3 in the reactions of I and by a factor of 2 in the reactions of II. It was shown that a direct displacement reaction of the BH4 ion on the primary mesylate I is not competitive with the solvolysis and cannot therefore be responsible for the above results. It was found that a better yield in hydrocarbons was obtained with I and II than with (1-methylcyclopropyl)carbinyl (III) and 1-methylcyclobutyl (IV) mesylates. The unexpected fact that the BH4- ion competes more efficiently with water for intermediates formed in solvolyses of I and II than for those of III and IV was confirmed by determination of competition factors of N3- ion vs water. In all cases, intermediates formed from I and II showed more discrimination in their reactions with reagents differing in nucleophylicity than did those formed from III and IV. The results indicate that the attack of the nucleophile occurs mostly on the ion pair formed in the rate determining step. The latter reaction is apparently subject to reactivity relationships typical for a direct displacement reaction and not characteristic for free carbonium ions. These facts should be borne in mind when conclusions concerning the structure of carbonium ions are drawn from the composition and structure of solvolysis products. It is concluded that the ion pairs trapped by NaBH4 have different structures, depending upon the starting isomeric mesylate. It is shown that the formation of an equilibrating mixture of classical ions or ion pairs as intermediates in these reactions cannot account for experimental results.
- Majerski,Borcic,Sunko
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p. 301 - 313
(2007/10/05)
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