Theoretical study of the gas-phase elimination kinetics of several heterocyclic carbamates

The homogeneous, unimolecular, gas-phase elimination kinetics of several heterocyclic carbamates was studied at the MP2/6-31G level of theory under the average experimental conditions for each substrate. The elimination products for these carbamates are the corresponding carbamic acid and isobutylene. The intermediate acids are unstable and decarboxylate at the reaction temperature. Calculated thermodynamic and kinetic parameters are in good agreement with the experimental values, displaying the same reactivity order observed experimentally. An increase in the electron-withdrawing effect of the substituent on the carbamate nitrogen, increases the elimination rate. Transition state structures are described as six-membered rings with some departure from planarity. NBO charge analysis reveals strong polarization at the O(ester) δ−Cδ+ bond. Bond indexes and synchronicity parameters are consistent with a concerted polar type of mechanism with O(ester) δ−Cδ+ bond breaking as the most important process. Copyright © 2007 John Wiley & Sons, Ltd.