Computational Studies of the Gas Phase Reactions of Ethers with Anions: Kinetic Barriers, Isotope Effects, Consecutive Eliminations and Site Selectivity

Abstract

Bimolecular elimination reactions (E2) are fundamentally important processes in organic chemistry. Our current work focuses on a computational investigation of several interesting and unexpected experimental results previously obtained in our laboratory. In particular, we have examined the detailed mechanisms for generating CH2CHO− from the reaction of HO− + CH3CH2OCH2CH2OCH3, the unusually large isotope effect ( kH/ kD = 5.5) for the reaction of NH2− + CH3CH2OCH2CH3, and the possible kinetic barriers in the reaction of H− + CH3CH2OCH2CH3. Moreover, we have explored the high site selectivity in the reaction of NH2− + CH3CH2OC(CH3)3. In the HO− + CH3CH2OCH2CH2OCH3 reaction, three ion-neutral encounter complexes were located and fully optimized. The corresponding transition states were confirmed during the first E2 hydrogen-transfer process and they all possess E1cb-like antiperiplanar conformations. The formation of loosely bonded CH3O− and H2O moieties was found to be essential for the second E2-type hydrogen transfer, and an intriguing E1cb-like gauche transition state (CH3OH–C α–Cβ–OCHCH2 dihedral = 40.9°) was located, which results in the formation of ionic CH2CHO− and neutral CH3OH, H2O, and C2H4 products. The lowest kinetic barrier for the reaction of NH2− + CH3CH2OCH2CH3 is −5.3 kcal mol−1 (–22.2 kJ mol−1), which is 1.5 kcal mol−1 (6.3 kJ mol−1) higher in energy than the lowest barrier for the reaction HO− + CH3CH2OCH2CH3. The higher kinetic barrier of the NH2− + CH3CH2OCH2CH3 reaction is consistent with the observation of a larger isotope effect. The lowest kinetic barrier for the reaction of H− + CH3CH2OCH2CH3 is +5.4 kcal mol−1 (22.6 kJ mol−1), indicating that, although H− is a strong base, this reaction cannot occur at room temperature, which agrees well with the experimental results. The high selectivity in the formation of CH3CH2O− from the reaction of NH2− + CH3CH2OC(CH3)3 is explained by an electrostatic potential analysis of the ether molecule. Thus, this computational study provides important insight into the detailed mechanisms of elimination reactions.