Proton shuffling in acid/base-catalyzed enolizations: a computational study

Abstract

Enolization of acetaldehyde catalyzed by the combined action of a general base (ammonia) and a general acid (formic acid) was examined by density functional theory at the B3LYP/6-311 + G(3df,2p) level while manipulating distance relationships among the reactants. Computations were carried out in the gas phase, in the presence of four water molecules, and with a dielectric constant of 78.4. Enolization involves an early transition state where general-base catalysis is more developed than general-acid catalysis. Although formic acid does not promote enolization by itself, it does facilitate α-proton transfer from acetaldehyde to the general base by several orders of magnitude. Formic acid accomplishes this feat via a hydrogen bond at a van der Waals distance to the carbonyl oxygen as opposed to forming a low-barrier hydrogen bond. A low-barrier hydrogen bond would indeed be capable of accelerating the enolization were it not for the energy cost of generating it. Formic acid may also facilitate enolization by internal solvation of the ammonium ion that is partially formed in the transition state via carbon-to-nitrogen proton transfer. General-base catalysis by trimethylamine, which is out of position to coordinate with the formic acid carboxyl, actually has lower activation energy than that of ammonia catalysis, possibly owing to basicity/shielding effects. Computations also demonstrate that the proton removed by the ammonia nitrogen remains on the nitrogen throughout rather than being transferred via low-energy rotation processes and secondary proton transfers to an oxygen atom of formic acid or the enol itself. Finally, stepwise and concerted mechanisms for enolizations have been proposed in the literature, with experimental evidence being provided for both. The concerted/non-concerted disagreement seems to stem from the continuum of organic mechanisms that Nature bestows onto organic chemistry. Thus, acid/base catalysis varies from stepwise at one extreme to synchronous at the other extreme with an infinite number of concerted mechanisms in between. Since the degree of concertedness undoubtedly depends upon the particular acid, base, substrate, and solvent, disparate enolization models are to be expected. Copyright © 2012 John Wiley & Sons, Ltd.