Theoretical study of the gas-phase decomposition of neutral ?-amino acid ethyl esters. Part 1?The elimination ofN,N-dimethylglycine ethyl ester and ethyl 1-piperidineacetate (original) (raw)
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Journal of Physical Organic Chemistry, 2003
Theoretical studies of the thermolysis in the gas phase of two α-amino acid ethyl esters, ethyl picolinate and ethyl 1-methylpipecolinate, were carried out using ab initio theoretical methods, at the MP2/6–31G(d) and MP2/6–311+G(2d,p) levels of theory. The reactions studied have two steps: the first corresponds to the formation of a neutral amino acid intermediate via a six-membered cyclic transition state, and the second is the rapid decarboxylation of this intermediate via a five-membered cyclic transition state. The progress of the first step of the reactions was followed by means of the Wiberg bond indices. The results indicate that the transition states have an intermediate character between reactants and products, and the calculated synchronicities show that the reactions are concerted and slightly asynchronous. The bond-breaking processes are more advanced than the bond-forming processes, indicating a bond deficiency in the transition states. The kinetic parameters calculated for both reactions agree very well with the available experimental results. Copyright © 2003 John Wiley & Sons, Ltd.
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 Thermolysis Reaction of Ethyl β-Hydroxycarboxylates in the Gas Phase
The Journal of Physical Chemistry A, 2002
Theoretical studies on the thermolysis of three ethyl -hydroxycarboxylates in the gas phase were carried out using ab initio theoretical methods, at the MP2/6-31G(d) and MP2/6-311++G(2d,p)//MP2/6-31G(d) levels of theory. Two competitive reaction pathways for the decomposition process have been studied. The first pathway describes a two-step mechanism, with the formation in a first step of an aldehyde, or a ketone, and an enol intermediate, followed by the tautomerization of this intermediate to ethyl acetate. The second pathway is a one-step mechanism with formation of ethylene and a carboxylic acid. Both processes occur via sixmembered cyclic transition states. The thermolysis is favorable along the first pathway with the first step as the rate-limiting step for the global process. The progress of the principal reactions was followed by means of the Wiberg bond indices. The results indicate that the transition states are late, and the calculated synchronicities show that the reactions are concerted and highly synchronous. The bond-breaking processes are more advanced than the bond-forming ones indicating a bond deficiency in the transition states. The kinetic parameters calculated for the studied reactions agree very well with the available experimental results. A theoretical study on the kinetic deuterium isotope primary, and Rand -secondary, effects has also been carried out.
Theoretical study of the thermolysis reaction of ethyl beta-hydroxycarboxylates in the gas phase
Journal of Physical Chemistry A, 2002
Theoretical studies on the thermolysis of three ethyl -hydroxycarboxylates in the gas phase were carried out using ab initio theoretical methods, at the MP2/6-31G(d) and MP2/6-311++G(2d,p)//MP2/6-31G(d) levels of theory. Two competitive reaction pathways for the decomposition process have been studied. The first pathway describes a two-step mechanism, with the formation in a first step of an aldehyde, or a ketone, and an enol intermediate, followed by the tautomerization of this intermediate to ethyl acetate. The second pathway is a one-step mechanism with formation of ethylene and a carboxylic acid. Both processes occur via sixmembered cyclic transition states. The thermolysis is favorable along the first pathway with the first step as the rate-limiting step for the global process. The progress of the principal reactions was followed by means of the Wiberg bond indices. The results indicate that the transition states are late, and the calculated synchronicities show that the reactions are concerted and highly synchronous. The bond-breaking processes are more advanced than the bond-forming ones indicating a bond deficiency in the transition states. The kinetic parameters calculated for the studied reactions agree very well with the available experimental results. A theoretical study on the kinetic deuterium isotope primary, and Rand -secondary, effects has also been carried out.
Kinetics and mechanism of elimination of ethyl acetate in the gas phase: A theoretical study
Using the PM3 semi-empirical quantum mechanical molecular orbital method, a procedure was devised to study the gas phase pyrolytic reaction of ethyl acetate in order to gain a deeper insight into both its kinetics and mechanism. By considering the involvement of formal charges and geometrical changes in the activation, a mechanism was proposed in which a pre-equilibrium of acidic proton transfer is followed by the rate limiting bond polarization of C-O bond in a cyclic transition state. The reactions involve a non-synchronous break in the β β β β-carbon-hydrogen and the α α α α-ether oxygen bonds through a six-centred transition state. The results obtained showed that the rate constant and the computed Arrhenius parameters compare well with the experimental values in the literature.
Theoretical study of the decomposition of ethyl and ethyl 3-phenyl glycidate
Journal of Molecular Modeling, 2013
The mechanism of the decomposition of ethyl and ethyl 3-phenyl glycidate in gas phase was studied by density functional theory (DFT) and MP2 methods. A proposed mechanism for the reaction indicates that the ethyl side of the ester is eliminated as ethylene through a concerted six-membered cyclic transition state, and the unstable intermediate glycidic acid decarboxylates rapidly to give the corresponding aldehyde. Two possible pathways for glycidic acid decarboxylation were studied: one via a fivemembered cyclic transition state, and the other via a fourmembered cyclic transition state. The results of the calculations indicate that the decarboxylation reaction occurs via a mechanism with five-membered cyclic transition state.
A DFT study on the kinetics and mechanism of cyclodiglycine thermal decomposition in the gas phase
Progress in Reaction Kinetics and Mechanism, 2016
Theoretical investigations using density functional theory have been performed on the kinetics and mechanism of cyclodiglycine (piperazine-2,5-dione) thermal decomposition in the gas phase. Five major possible paths have been proposed and analysed. α-Lactam, β-lactam, 4-imidazolidinone and smaller species such as HNCO, H 2 CNH, H 2 CCO, CO, H 2 and HCN can be produced through these paths. The route yielding aziridine and HNCO is the optimum channel for this reaction from the energy point of view. Finally, according to the quantum theory of atoms in molecules, electron localisation function and localised orbital locator analyses, it was confirmed that a concerted mechanism operates for the reaction and all critical bonds of the transition states have covalent character.
Computational Study of the Aminolysis of Esters. The Reaction of Methylformate with Ammonia
The Journal of Organic Chemistry, 2003
The aminolysis of esters is a basic organic reaction considered as a model for the interaction of carbonyl group with nucleophiles. In the present computational study the different possible mechanistic pathways of the reaction are reinvestigated by applying higher level electronic structure theory, examining the general base catalysis by the nucleophile, and a more comprehensive study the solvent effect. Both the ab initio QCISD/6-31(d,p) method and density functional theory at the B3LYP/6-31G(d) level were employed to calculate the reaction pathways for the simplest model aminolysis reaction between methylformate and ammonia. Solvent effects were assessed by the PCM method. The results show that in the case of noncatalyzed aminolysis the addition/elimination stepwise mechanism involving two transition states and the concerted mechanism have very similar activation energies. However, in the case of catalyzed aminolysis by a second ammonia molecule the stepwise mechanism has a distinctly lower activation energy. All transition states in the catalyzed aminolysis are 10-17 kcal/mol lower than those for the uncatalyzed process.
FUDMA JOURNAL OF SCIENCES
Kinetics and thermodynamics of gas-phase thermal decomposition of 1-phenylethyl acetate to vinyl benzene and acetic acid (ethanoic) were carried out using the density functional theory (DFT) method at B3LYP/6-31++G**. Geometric parameters obtained include atomic charge distribution, dihedral angles, bond lengths, and bond angle for the ground state reactant (GS), transition state (TS), and the product (PRD) while the thermodynamic parameters such as a change in entropy change (∆S), change in enthalpy (∆Hreaction) and free Gibbs energy were calculated at 623K with an interval of 25K. Kinetic parameters determined include activation energy (Ea), Pre-exponential Arrhenius factor (log A) and rate (k). Geometric results revealed that the decomposition reaction is through asynchronous cleavage of α-ether oxygen bonds and β-carbon-hydrogen in the six-membered cyclic transition state: C2-H1 and C5-O7 bond breaking occurred first while the C9-H1 bond formation process is lagging behind in a single step. The ∆S (5.867 J/mol/K); ∆Hreaction (38.45 kcal/mol), ∆G (39.69 kJ/mol), Ea (43.7 kcal/mol), log A (12.70) and k (6.1 x 10-2 S-1) compared well with the experimental available results in literature at 623K. The intrinsic reaction coordinate (IRC) on the TS structures shows that the reactant connects to the respective minima while the Wilberg bond index shows that the TS possesses 'an early' character closer to the reactant than the products. The theoretical calculation method can be used to study the thermodynamics, mechanism and kinetics of the thermal decomposition of acetates thus reducing the cost, laboratory experiments time and exposure to hazardous chemicals.