Effects of the leaving group and of ion pairing on the stereochemistry of E2 reactions. An ab initio study

Computational studies of the stereochemistry of E2 reactions of ethyl derivatives, EtX, at MP2/6-31+G*//MP2/6-31G* give the following results (X, base, Delta Delta H-double dagger(syn - anti in kcal mol(-1))): F, OH-, 6.3; F, LiOH, -21.4; F, NaOH, -11.6; Cl, OH-,8.0; Cl, LiOH, -18.7; Cl, NaOH, -12.4; NMe3+, OH-, -11.1; NMe3+, LiOH, 3.7. The activation barriers are much higher for the ion-paired bases than for hydroxide, reflecting their much weaker basicity. As deduced from results in solution, ion pairing of the base promotes syn elimination with negative leaving groups (F-, Cl-) but the reverse is true for neutral leaving groups (NMe3). The effects are strikingly larger in the gas phase than the typically modest ones found in solution. The effects of the leaving group and the base on the nature of the transition structure are discussed.