THEORETICAL INVESTIGATION ON BASE-INDUCED 1,2-ELIMINATIONS IN THE MODEL SYSTEM F-+CH3CH2F - THE ROLE OF THE BASE AS A CATALYST

A theoretical investigation has been performed on the ps-phase reactions of the F- + C2H5F model system using a high-level density-functional method. The purpose is a better understanding of the nature of the base-induced elimination reactions, in particular the role of the base as a catalyst, the prevalence of anti-E2 over syn-E2 elimination, the prevalence of E2 elimination over S(N)2 substitution, and the reaction mechanism. The base has been found to play a key role as a catalyst. The uncatalyzed transition-state (TS) energies are very high. The uncatalyzed syn TS is lowest, in contrast to the prevalent view that the anti TS would be more strongly stabilized by favorable interaction of the developing carbanionic lone pair at C(beta) with the backside lobe of the sigma*(C(alpha)-F). Upon catalysis by the base, the transition state of the anti mode is selectively stabilized, leading to the prevalence of anti-E2 over syn-E2 elimination. One reason for the selective stabilization is the favorable electrostatic interaction of the F- base with the C(alpha)-F dipole of the anti TS. A second factor is the very low energy and, thus, the good acceptor capability of the C2H5F 8a' LUMO in the strongly rearranging, loose anti-E2 transition state. The anti-E2 elimination prevails over the S(N)2 substitution. This is ascribed to the lower energy and entropy barrier for the anti-E2 elimination as well as to the preferential formation of a reactant complex which is predestined to react further via the anti-E2 pathway. The anti-E2 elimination (and not only the syn-E2) is found to preferentially produce FHF- and C2H4. The prevalence of anti-E2 over S(N)2 is therefore in excellent agreement with the experimental result that reaction of F- and C2H5F exclusively yields FHF- and C2H4. However, we reinterpret this observation as being the result of anti-E2 and not syn-E2 elimination. A qualitative MO theoretical analysis is given, which enables one to understand the coplanarity of the reaction and to predict which reaction, E2 or S(N)2, dominates for a given general substrate C2H5L (Scheme III). On the basis of a simple MO theoretical concept, an ''E2/S(N)2 spectrum'' is proposed, which comprises the Bunnet-Cram E2H and the Winstein-Parker E2H/E2C as well as the S(N)2/S(N)1 mechanistic spectra. The base-induced eliminations studied are of the E2H category. No E2C-like interactions are present in the transition state. An intermediate anion is never formed. The syn-E2 reaction is only slightly E1cb-like, whereas the anti-E2 elimination is virtually ideal E2. Interestingly, there is no distinct channel on the anti-E2 reaction energy surface leading from the reactant complex to the transition state. Instead, the system shows a very weak tendency to proceed via an E1cb-like route (initial C(beta)-H bond elongation) or via an E1-like route (initial C(alpha)-F bond elongation). An important characteristic of the anti-E2 elimination, which is not contained in the E2H formalism, is the pronounced shift of the abstracted proton from the C(beta) to the C(alpha) position in the transition state.