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 CH(2)CHO(‒) from the reaction of HO(‒) + CH(3)CH(2)OCH(2)CH(2)OCH(3), the unusually large isotope effect (k(D)/k(H) = 5.5) for the reaction of NH(2)(‒) + CH(3)CH(2)OCH(2)CH(3), and the possible kinetic barriers in the reaction of H(‒) + CH(3)CH(2)OCH(2)CH(3). Moreover, we have explored the high site selectivity in the reaction of NH(2)(‒) + CH(3)CH(2)OC(CH(3))(3). In the HO(‒) + CH(3)CH(2)OCH(2)CH(2)OCH(3) 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 E1(cb)-like antiperiplanar conformations. The formation of loosely bonded CH(3)O(‒) and H(2)O moieties was found to be essential for the second E2-type hydrogen transfer, and an intriguing E1(cb)-like gauche transition state (CH(3)OH-Cα-Cβ- OCHCH(2) dihedral = 40.9°) is located, which results in the formation of ionic CH(2)CHO(‒) and neutral CH(3)OH, H(2)O and C(2)H(4) products. The lowest kinetic barrier for the reaction of NH(2)(‒) + CH(3)CH(2)OCH(2)CH(3) is 5.3 kcal mol(‒1) (1 kcal mol(‒1) = 4.2 kJ mol(‒1)), which is 1.5 kcal mol(‒1) higher in energy than the lowest barrier for the reaction HO(‒) + CH(3)CH(2)OCH(2)CH(3). The higher kinetic barrier of the NH(2)(‒) + CH(3)CH(2)OCH(2)CH(3) reaction is consistent with the observation of a larger isotope effect. The lowest kinetic barrier for the reaction of H(‒) + CH(3)CH(2)OCH(2)CH(3) is +5.4 kcal 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 CH(3)CH(2)O(‒) from the reaction of NH(2)(‒) + CH(3)CH(2)OC(CH(3))(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.