Theoretical Studies of Structural Effects on the Mechanism of Acyl-Transfer Reactions1

Potential energy profiles have been determined for the two series of reactions:  (i) X- + HCOY, where X = Y = H, F, or Cl, and (ii) X- + RCOX, where X = F or Cl and R = SiH3, CH3, H, CN, or NO2. Energies of all stationary points, including reactants, ion−dipole complexes, stable adducts, transition states, and products, were evaluated at the Hartree−Fock (HF) and the second-order Moller−Plesset (MP2) correlation levels with the 6-311++G** for reaction series i and with the 6-31+G* basis set for reaction series ii. The results predict that acyl-transfer reactions can proceed through single-well, double-well, and triple-well energy profiles in the gas phase depending on the nucleophile, X-, nucleofuge Y-, and acyl group R. Factors that favor the single-well or triple-well profile with a stable tetrahedral adduct are (a) stronger bond formation of the C−X or C−Y bond, (b) stronger nucleophilicity of X- and poorer leaving ability of the nucleofuge, Y-, (c) wide energy gap between the two antibonding MOs, π*CO...