Density functional study of the proline-catalyzed direct aldol reaction

The proline-catalyzed direct aldol reaction between acetone and acetaldehyde has been investigated using density functional theory. Proline catalyzes the reaction according to the enamine mechanism characteristic of the natural class I aldolase enzymes. Although it has been postulated that the rate-limiting step in the proposed mechanism is enamine and/or C-C bond formation, the initial reaction between proline and acetone is accompanied by a very large barrier which may inhibit further progression of the reaction. However, an alternative lower energy reaction pathway is utilized when the ionizing solvent DMSO is present to assist in the formation and stabilization of separated charges. The direct aldol reaction between acetone and acetaldehyde illustrates the catalytic potential of simple organic molecules in asymmetric synthesis.