Molecular Organization of Reagents in the Kinetics and Catalysis of Liquid-Phase Reactions: XIII. Cyclic Transition States Involving Solvent Molecules in the Mechanism of Aminolysis of Cyclocarbonates in an Alcohol Medium

AbstractTrends in the influence of a proton-donor solvent on the reaction of aminolysis of ethylene carbonate, which proceeds through concerted and stepwise mechanisms, are studied. The study is based on kinetic data of model reactions in alcohol solutions and results of DFT quantum chemical calculations of reaction paths that involve hydrogen-bonded transition states and intermediates containing different numbers of solvent molecules in the proton-transfer and stabilization cycles. The calculated data on the activation energies of the reaction, which includes cycles of different sizes, indicate that the stepwise mechanism is preferable compared to the concerted one when the reaction proceeds in methanol. The activation barrier of the stepwise path of the reaction is lower by 2.9 kcal/mol than that for the concerted path. Only the proton-transfer cycle is essential for the progress of the reaction, and the presence of a stabilization cycle does not lead to a decrease in the activation barrier.