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A theoretical study on keto‐enol tautomerization involving simple carbonyl derivatives
Author(s) -
Lee Doyoung,
Kim Chang Kon,
Lee BonSu,
Lee Ikchoon,
Lee Byung Choon
Publication year - 1997
Publication title -
journal of computational chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.907
H-Index - 188
eISSN - 1096-987X
pISSN - 0192-8651
DOI - 10.1002/(sici)1096-987x(19970115)18:1<56::aid-jcc6>3.0.co;2-z
Subject(s) - keto–enol tautomerism , chemistry , tautomer , enol , deprotonation , protonation , substituent , solvation , computational chemistry , medicinal chemistry , solvent , photochemistry , catalysis , organic chemistry , ion
The relative stabilities of the keto and enol forms [Δ E 0 (enol‐keto)] and the energy barriers to enolization of the keto forms [Δ E ≠ (transition state‐keto)] for CH 3 COR (R = CH 3 , H, F, and CN) and CH 3 CHY (Y = CH 2 , 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 E 0 MP2 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 E 0 , there is a large lowering of Δ E ≠ (by ca. 30 kcal/mol) when solvent effects are accounted for. In general, both Δ E 0 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.