Do π-Conjugative Effects Facilitate S_N2 Reactions?

摘要

Rigorous quantum chemical investigations of the S_N2 identity exchange reactions of methyl, ethyl, propyl, allyl, benzyl, propargyl, and acetonitrile halides (X = F~-, Cl~-) refute the traditional view that the acceleration of S_N2 reactions for substrates with a multiple bond at C_β (carbon adjacent to the reacting C_α center) is primarily due to n- conjugation in the S_n2 transition state (TS). Instead, substrate-nucleophile electrostatic interactions dictate S_N2 reaction rate trends. Regardless of the presence or absence of a C_β multiple bond in the S_N2 reactant in a series of analogues, attractive C_β(δ~+)…X(δ~-) interactions in the S_N2 TS lower net activation barriers (E~b) and enhance reaction rates, whereas repulsive C_β(δ~+)…X(δ~-) interactions increase E~b barriers and retard S_N2 rates. Block-localized wave function (BLW) computations confirm that Π-conjugation lowers the net activation barriers of S_N2 allyl (1t, coplanar), benzyl, propargyl, and acetonitrile halide identity exchange reactions, but does so to nearly the same extent. Therefore, such orbital interactions cannot account for the large range of E~b values in these systems.