Abstract
Organocatalyzed Michael, Mannich, and aldol reactions of aldehydes or ketones, as nucleophiles, have triggered several discussions regarding their reaction mechanism. H2 (18) O has been utilized to determine if the reaction proceeds through an enamine or enol mechanism by monitoring the ratio of (18) O incorporated into the final product. In this communication, we describe the risk of H2 (18) O as an evaluation tool for this mechanistic investigation. We have demonstrated that exchange of (16) O/(18) O occurs in the aldehyde or ketone starting material, caused by the presence of H2 (18) O and amine catalysts, before the Michael, Mannich, and aldol reactions proceed. Because the newly generated (18) O starting aldehydes or ketones and (16) O water affect the incorporation ratio of (18) O in the final product, the use of H2 (18) O would not be appropriate to distinguish the mechanism of these organocatalyzed reactions.
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