- Vicinal Elimination from 2-Fluoroisopropyl Cation in the Gas Phase
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An electron bombardment flow (EBFlow) reactor has been used to assay C3H5+ isomer distribution from different sources.Quenching the ions by fluoride abstraction followed by 19F NMR of recovered neutral C3H5F permits analysis of the ratio of CH3C=CH2+ to allyl cation.Electron impact on 1-bromopropene yields predominantly allyl cation, in agreement with a previously reported mass spectrometric measurement.Ionization of isobutene yields an 80:20 mixture of CH3C=CH2+ and allyl cation.Electron impact on tert-butyl fluoride yields only CH3C=CH2+.The difference between the two latter results means that ionized isobutene is not an intermediate in production of C3H5+ from tert-butyl fluoride.Therefore, the major fragment of tert-butyl fluoride, (CH3)2CF+ (1), must be undergoing vicinal elimination of hydrogen fluoride to yield C3H5+.Theoretical calculations provide evidence that this does not take place via an isolated, electronically excited state of 1.Ab initio calculations on the ground-state C3H6F+ potential energy surface reveal six stable isomers of 1,3-8, all of which have stable, isoelectronic C3H6O analogues.Two of these, 7 and 8 (isoelectronic to allyl alcohol and acetone enol, respectively), correspond to ion-molecule complexes of C3H5+ and hydrogen fluoride.More extensive calculations on 8 illustrate the difference between an ion-molecule complex and a hydrogen-bonded complex.Other isomers of 1, CH3CHFCH2+ (2) and CH3CH+CH2F (11), do not have stable, isoelectonic C3H6O analogues.Neither are they stable C3H6F+ isomers.Ab initio calculations show that 2 undergoes barrier-free rearrangement to CH3CH2CHF+ (3), while 11 represents one of the resonance structures of 2-methylfluoriranium, 4.Ab initio results show good agreement with empirical estimates for ΔHf(stand.) of 3, of methylvinylfuoronium (5), and of methyl-2-propenylfluoronium (9).These lead to a calculated heat of formation of 8 that is 19 kcal/mol higher than that of 1.Ion 1 corresponds to the global energy minimum, and its two lowest energy isomers are calculated to be 3 and fluoretanium (6).Together the experimental and theoretical results show that 1 undergoes a 1,3-hydrogen shift to 8 enroute to elimination in preference to rearrangement to 6.
- Stams, Dion A.,Johri, Kamalesh K.,Morton, Thomas Hellman
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- Structures of Organic Cations in the Gas Phase. Neutral Products from Fluoride Abstraction Examined by NMR
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Structures of 2-fluoroisopropyl cation (1) and 2-propenyl cation (2) in the gas phase have been examined by quenching via ion-molecule reactions and examination of the neutral products by NMR.Both ions are produced by 70-eV electron bombardment of tert-butyl fluoride, and both react with the parent neutral to yield tert-butyl cation.The ion-molecule reaction of 1 yields 2,2-difluoropropane as >90percent of the neutral product.The ion-molecule reaction of 2 yields 2-fluoropropene, as confirmed by preparing 2 from an other source and examining the product from its reaction with tert-butyl-d6 fluoride (3).Allyl fluoride is not detected among the the reaction products.Since this product is observed when allyl bromide is bombarded with 70-eV electrons in the presence of tert-butyl fluoride, its absence in electron bombardment of tert-butyl fluoride alone implies that 2 is the only C3H5+ ion produced and that it does not rearrange to the allyl cation under the reaction conditions.The deuterated neutral products from electron impact on 3 alone or in the presence of diethyl ether indicate that 1 does not scramle its hydrogens, even when it contains enough internal energy to expel HF to form 2.The use of deuterium isotope shifts in 19F NMR to study the position and extent of label in neutral products is exemplified as a new tool for probing structures of gaseous cations.
- Redman, Eric W.,Johri, Kamalesh K.,Morton, Thomas Hellman
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- Reduction of fluorinated cyclopropene by nitrogenase
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Reduction of the first known halogen-containing substrate by nitrogenase (N2ase), 3,3-difluorocyclopropene (DFCP), was investigated. Reduction requires both N2ase proteins (MoFe and Fe protein), ATP, and an exogenous reductant (dithionite, DT), as with N2 and known alternative substrates of the enzyme. Two major products providing evidence for reductive C-F bond cleavage were confirmed, propene (P1, requiring 6e -/6H+) and 2-fluoropropene (P2, 4e-/4H +). Both were identified by GC-MS and NMR spectroscopy, and had the same Km constants (0.022 atm, 5.4 mM). Reduction of 1,2-dideuterated DFCP (d2-DFCP) further revealed that (i) in both P1 and P2, two deuterium atoms are retained, one on carbon-1 and one on carbon-3, indicating that C=C bond cleavage rather than C-C bond cleavage is involved during DFCP reduction at least to P2 (assuming no F migration); (ii) no selectivity was observed in formation of cis and trans isomers of 1,3-d2-2- fluoropropene, whereas cis-1,3-d2-propene is the predominant 1,3-d2-propene product, indicating that one of the bound reduction intermediates on the pathway to propene is constrained geometrically. A reduction mechanism, consistent with hydride transfer as a key step, is discussed. Reductive C-F bond cleavage is an ability of N2ase that further demonstrates the unique and remarkable scope of its catalytic prowess.
- Ni, Feng,Lee, Chi Chung,Hwang, Candy S.,Hu, Yilin,Ribbe, Markus W.,Mckenna, Charles E.
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p. 10346 - 10352
(2013/08/23)
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- Preparation of monofluoroalkenes
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There is described a process for the preparation of compounds of formula I in whichR is hydrogen, optionally substituted alkyl, optionally substituted alkenyl optionally substituted cycloalkyl, optionally substituted phenyl or optionally substituted heterocyclyl, andR1 and R2 are hydrogen, an organic radical or together with the ring to which they are attached, form an optionally substituted ring, which may contain one or more hetero atoms, which comprises reacting a compound of formula II with a compound of formula III in which the ring Q is an optionally substituted heteroaromatic ring, which optionally comprises one or more further hetero atoms and to which is optionally fused an optionally substituted ring.
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- Isotopomer Distributions of Neutral Products from a Doubly Labeled Cation in the Gas Phase. Interconversion of 1-Fluoro-1-propyl Cation and 1-Fluoroisopropyl Cation on the C3H6F+ Potential Energy Surface
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The title cations CH3CH2CHF+ (3) and CH3CHCH2F+ (4) are formed as transient intermediates in the gas phase.These are labile on the millisecond timescale as free ions but can be intercepted in ion-neutral complexes.When 3 is generated as free cation by reaction of CF3+ with propionaldehyde, it rearranges to (CH3)2CF+ (1), as shown by recovery of 2-fluoropropene as a neutral product from its deprotonation in an EBFlow experiment.The same neutral product is recovered when 1 is produced directly by reaction of acetone with CF3+ in the EBFlow.Neutral productsindicative of 3 and 4 (allyl fluoride and 1-fluoropropene) are recovered when these cations are formed in + PhO.> ion-neutral complexes by electron bombardment of CH3CDFCH2OPh (6).Analysis of the isotopic distribution in the recovered neutrals from EBFlow radiolysis of CH3CDF*CH2OPh (where the asterisked carbon is 13C-labeled) allows an assessment of the primary rearrangement pathways.The distribution of label is assayed by using 19F NMR.Rearrangement of the R+ moiety to form deuterated 1 occurs in about half of the complexes formed.In the remainder, methyl transfer (to form deuterated 3) is 2-3 times faster than fluoride transfer (to form deuterated 4).Scrambling of deuterium in the neutral products provides evidence that 3 and 4 interconvert within the ion-neutral complexes.
- Shaler, Thomas A.,Morton, Thomas Hellman
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p. 6771 - 6779
(2007/10/02)
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- Bestimmung der Geschwindigkeitskonstanten der Reaktion von 1CH2 mit Fluor- und Chlorethen
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The rate constants for the addition of 1CH2 to fluoroethene and chloroethene have been determined using the known rate constant for the reaction of 1CH2 with ketene.The experiments yielded the results k4a(296 K) = (8.2 -/+ 0.6)E13 cm3 mol-1 s-1 k4b(296 K) = (2.9 -/+ 0.4)E13 cm3 mol-1 s-1. - Keywords: Chemical Kinetics / Gases / Photochemistry / Radicals
- Simon, F. G.,Heydtmann, H.
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p. 543 - 545
(2007/10/02)
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- Process for the preparation of fluorinated nitroalkanes
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In the conjugated nitrofluorination of an olefin by reacting an olefin of the formula STR1 in which R1, R2, R3 and R4 each independently is hydrogen, fluorine, chlorine, bromine, alkyl, halogenoalkyl or cycloalkyl, or R2 and R4 together are an alkylene radical of 3 to 6 carbon atoms, with hydrogen fluoride and nitric acid to produce an α-fluorinated nitroalkane of the formula STR2 the improvement which comprises effecting the reaction in a vessel protected against corrosion employing about 1 to 1.1 mols of hydrogen fluoride and about 1 to 2 mols of nitric acid per mol of olefin. The process uses much less HF than heretofore and the products are useful intermediates in making herbicides. Some of the products are new.
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- SYNTHESE D'ETHYLENIQUES MONOFLUORES
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Monofluorinated ethylenic compounds have been prepared by action of potassium tertiobutylate either on 2-fluorotosylates or 2-fluorochlorides.In the case of 2-fluorochlorides the action, at higher temperature, of basic salt such as KF in ethylene glycol, may be used to prepare monofluorinated ethylenics compounds.
- Baklouti, A.,Chaabouni, M. M.
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p. 181 - 190
(2007/10/02)
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