A theoretical study on keto-enol tautomerization involving simple carbonyl derivatives

The relative stabilities of the keto and enol forms [Δ E0 (enol-keto)] and the energy barriers to enolization of the keto forms [Δ E≠ (transition state-keto)] for CH3COR (R = CH3, H, F, and CN) and CH3CHY (Y = CH2, NH, and S) are investigated theoretically by Hartree-Fock and Möoller-Plesset second-order calculations with 6–31G** basis sets. Specific and bulk solvent effects are considered by incorporating one water molecule and applying the self-consistent reaction field (SCRF) method to the reaction system, respectively. The Δ E0MP2 values are all positive, in agreement with the lower stability of the enol form in the gas phase as well as in solution. In contrast to a relatively small effect of specific as well as bulk solvation on Δ E0, there is a large lowering of Δ E≠ (by ca. 30 kcal/mol) when solvent effects are accounted for. In general, both Δ E0 and Δ E≠ are depressed in solution and hence enolization is favored thermodynamically as well as kinetically. The keto form is strongly stabilized by a π donor, whereas the enol isomer is stabilized by a π as well as a σ-acceptor substituent, R. As a result, substituent R = F is the most unfavorable whereas R = CN is the most favorable for the enolization. The water catalyzed enolization in the neutral water proceeds concertedly, but carbon deprotonation is more important than carbonyl-oxygen protonation by water in the rate determining step. © 1997 by John Wiley & Sons, Inc.