A Simple Model to Predict Preferable Aldol Products from Unsymmetrical Ketones Using Local Hard−Soft Acid−Base Concept

Abstract

Unsymmetrical ketones produce two types of enolate intermediates upon deprotonation, which are known as “less substituted” and “more substituted” ones. The major final aldol product will depend on which one of these two enolate intermediates is more reactive toward the incoming reactant (i.e., aldehyde in the case of aldol condensation reaction) in the next stage of the reaction. As here the active sites belong to two different chemical systems, “local hardness” values are reported to be more reliable (Langenaeker et al. J. Phys. Chem. 1995, 99, 6424, and Roy et al. J. Phys. Chem. 1998, 102, 3746) to predict the intermolecular reactivity order of these two intermediates. But in this article we have proposed a very simple model which simultaneously represents both kinds of enolate intermediates. So the present study can be considered as a first one in which an intermolecular problem has been recasted as an intramolecular one, and thus “local softness” and related reactivity descriptors have been used, instead of the “local hardness”, to predict the intermolecular reactivity orders. The generated results show that the model works at satisfactory level.