Evaluation of the olefinic double bond influence in the unimolecular homogeneous gas phase elimination of alkenyl acetates (original) (raw)
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Journal of Molecular Modeling, 2011
The mechanism for the decarbonylation of (E)-2butenal and (E)-2-methyl-3-pheny-2-propenal was studied with different levels of ab initio and DFT methods. Reactants, products and transition structures were optimized for two kinds of reaction channel: a one-step reaction which involves a three-membered cyclic transition state, and a two-step reaction which involves an initial fourmembered cyclic transition state. According to our calculations, these two possible mechanisms entail similar energetic costs, and there are only small differences depending on the reactant. The elimination of (E)-2methyl-3-pheny-2-propenal yields different products depending on the channel followed. Only one of the three possible one-step mechanisms leads directly to (E)-βmethylstyrene (the main product according to experiment). This fact is reasonably well reproduced by our results, since the corresponding transition state gave rise to the lowest activation Gibbs free energy.
Journal of Physical Organic Chemistry, 2010
The gas-phase elimination of 1,1-dimethoxycyclohexane yielded 1-methoxy-1-cyclohexene and methanol. The kinetics were determined in a static system, with the vessels deactivated with allyl bromide, and in the presence of the free radical inhibitor cyclohexene. The working temperature was 310-360 -C and the pressure was 25-85 Torr. The reaction was found to be homogeneous, unimolecular, and follows a first-order rate law. The temperature dependence of the rate coefficients is given by the following Arrhenius equation: log k(s S1 ) ¼ [(13.82 W 0.07) -(193.9 W 1.0)(kJ mol S1 )](2.303RT) S1 ; r ¼ 0.9995. Theoretical calculations were carried out using density functional theory (DFT) functionals B3LYP, MPW1PW91, and PBE with the basis set 6-31G(d,p) and 6-31GRR(d,p). The calculated values for the energy of activation and enthalpy of activation are in reasonably good agreement with the experimental values using the PBE/6-31G (d,p) level of theory. Both experimental results and theoretical calculations suggest a molecular mechanism involving a concerted polar four-membered cyclic transition state. The transition state structure of methanol elimination from 1,1-dimethoxycyclohexane is characterized by a significantly elongated C-O bond, while the C b -H bond is stretched to a smaller extent, as compared to the reactant. The process can be described as moderately asynchronic with some charge separation in the TS.
MP2 study of the gas phase elimination mechanism of some neutral amino acids and their ethyl esters
Journal of Physical Organic Chemistry, 2008
The mechanisms of the gas phase elimination of N,N-dimethylglycine, picolinic acid, and N-phenylglycine and their ethyl esters have been examined at Mö ller-Plesset MP2/6-31G (d, p) level of theory. The ethyl esters of these 2-amino carboxylic acids produce the corresponding amino carboxylic acid and ethylene in a rate-determining step. However, the unstable intermediate amino carboxylic acid rapidly decarboxylate to give the corresponding amino compound. These calculations imply a concerted, semi-polar six-membered cyclic transition state type of mechanism for the ethyl esters, and a non-synchronous five-membered cyclic transition state for the amino acids decarboxylation. The present results support previous mechanistic consideration of the elimination of the above-mentioned compounds in the gas phase.
Theoretical study of the oxidation mechanism of aromatic amines
Journal of the Chemical Society, Perkin Transactions 2, 1991
The overall mechanism of the chemical step involved in the electrochemical oxidation of tertiary aromatic amines has been investigated by means of the quantum-chemical A M 1 method. By using the parent compound aniline as a model it is found that, in good accord with previous electrochemical data, dimerization of t w o radical cations takes place before deprotonation, the dimerization step involving a higher energy barrier.
ACS Omega
We report the influence of substituents and physical conditions on activation energies for the noncatalyzed amination (C−N cross-coupling reactions) of aryl halides. We uncover a significant correlation between the barrier heights of the C−N bond formation and Hammett σ parametersa formal measure of the electron-withdrawing or-donating ability of substituents on the aryl halides. Our results indicate that such correlations are useful predictive tools for the amination of aryl halides over a wide range of substituent types. From 54 cases studied (six substituents occupying specific positions relative to halogen atoms), the 2-COOHPhI + NH 2 n Pr amination reaction is predicted to possess the lowest noncatalyzed activation free energy (135.6 kJ mol −1) using the B3LYP method. The lower barriers for the 2-COOHPhX (for X = Cl, Br, and I) compounds are shown to originate from collusion between steric and electronic effectsspecifically, the momentary formation of a hydrogen bond between an oxygen site on the ortho-COOH and the lone pair of the entering amine. Internal reaction coordinate (IRC) path calculations afforded us these and other key insights into the nature of the reactions. The control exerted by substituents on the arrangement of the transition state structure, as well as the sensitivity of the reaction barriers to temperature and solvent polarity, are discussed. These results offer new perspectives from which to assess the nature of the C−N bond formation and suggest new avenues for future exploration, especially in progress toward the metal-free amination of aryl compounds.
Oxidations of Amines. III. Duality of Mechanism in the Reaction of Amines with Chlorine Dioxide
Journal of the American Chemical Society, 1967
to the induction period gave straight-line plots when submitted to the pseudo-first-order treatment (chlorite was employed in excess). The results are summarized in Table V. Estimation of Activation Energy for Chlorine Dioxide Reaction with p-Nitrobenzyl-N,N-dimethylamine. The reaction of this amine was studied at 26.95 f 0.2", at 14.3 =k 0.2", and at 40.7 f 0.2". Although the Arrhenius plot showed some scatter, the Arrhenius activation energy, E,, was estimated at 12.9 Z!Z 2 kcal/mole, and AF* was estimated to be 13.3 =k 2 kcal/mole.
Theoretical study on the elimination kinetics in the gas phase of allyl methyl compounds
Monatshefte für Chemie - Chemical Monthly, 2018
The thermal decomposition kinetics of allyl methyl amine, allyl methyl ether, and allyl methyl sulfide in the gas phase has been studied theoretically using the M06-2x/aug-cc-pVTZ quantum chemical approach. The observed activation parameters are consistent with a concerted unimolecular mechanism involving a non-planar cyclic six-membered transition state. Based on the optimized ground state geometries, a natural bond orbital analysis of donor-acceptor interactions reveals that the stabilization energies corresponding to the electronic delocalization from the lone-pair (LP) non-bonding orbitals on the heteroatom to the neighboring r à C2ÀC3 antibonding orbitals decrease from allyl methyl amine to allyl methyl sulfide. This delocalization fairly explains the increase of occupancies of LP orbitals on the heteroatom from allyl methyl sulfide to allyl methyl amine. The results also suggest that the kinetics of the thermolysis of the studied compounds are dominated by LP ! r à electronic delocalization effects. Analysis of bond order, bond indices, and synchronicity parameters demonstrates that these reactions proceed through a concerted and slightly asynchronous mechanism.
Journal of Physical Organic Chemistry, 2015
The gas-phase elimination of 2-methyl-2-propenal catalyzed by HCl yields propene and CO gas, while E-2-pentenal with the same catalyst gives butene and CO gas. The kinetics determinations were carried out in a static system with the reaction vessels deactivated with allyl bromide and the presence of the free radical inhibitor toluene. Temperature and pressure ranges were 350.0-410.0°C and 34-76 Torr. The elimination reactions are homogeneous and unimolecular, and follow a first-order rate law. The rate coefficients for the reactions are expressible by the following Arrhenius equations: Product formation from 2-methyl-2-propenal propene : log k 1 s À1 lmol À1 À Á ¼ 12:58 ± 0:47 ð Þ À165:0 ± 5:86 ð Þ kJmol À1 2:303RT ð Þ À1 Product formation from E-2-pentenal butene : log k 1 s À1 lmol À1 À Á ¼ 12:82 ± 0:44 ð Þ À171:3 ± 5:52 ð Þ kJmol À1 2:303RT ð Þ À1 Data from the kinetic and thermodynamic parameters of these catalyzed elimination reactions implies a mechanism of a concerted five-membered cyclic transition state structure for the formation of the corresponding olefin and carbon monoxide.