Relative energies, ∆E, were calculated as the difference between the energy of the combined structure and the sum of the energies of the isolated reactants. Harmonic vibrational frequencies were calculated numerically to confirm the nature of stationary points along the PES and to calculate Gibbs free energies. Relative Gibbs free energies of enolate intermediates and transition states, ∆G min and ∆G ‡ , respectively, were calculated as the difference between the Gibbs free energy of the combined structure and the sum of the Gibbs free energies of the isolated reactants.