Computational Chemistry Studies Of [1,7] Sigmatropic Hydrogen Shift In Z,Z-1,3,5-heptatriene Systems And Esterification Reactions

Density functional theory has been used to study mechanisms of the reactions involving [1,7]-sigmatropic hydrogen shift in Z,Z-1,3,5-heptatriene systems, esterification of acetyl chloride with methanol and acid-catalyzed esterification of carboxylic acids with methanol.  The calculations employed the B3LYP functional, with 6-31+G(d) basis sets. Mechanism routes were computed, with complete optimization of all intermediates and transition states.  The effect of electron-donating and electron-withdrawing substituents in Z,Z-1,3,5heptatriene systems was investigated.  Analysis of the geometries, energies and electronic characteristics of the sigmatropic transposition compared to those of the unsubstituted cases provided insights into substituent effects of the reactions.  The study revealed that the inductive and mesomeric effects of heteroatoms or heterosubstituents are of great importance in the energetics of the transformation.  Steric effects also play an important role due to the geometrical constraints of the reaction.  Generally increasing the electron density of the  system, decreases the electron density on the protropic proton resulting in the decrease of the activation energy.  For the esterification of acetyl halides with methanol, the calculations suggest that the reaction proceeds through a loose transition state with a concerted SN2 mechanism.  The nucleophilic methanol attacks the carbonyl carbon at Bürgi-Dunitz trajectory and the interaction is subject to general base catalysis, either by a second molecule of methanol or a solvent molecule.  This explains the mixed reaction kinetics observed from experimental data.