1 research outputs found
Mechanism of the Piperidine-Catalyzed Knoevenagel Condensation Reaction in Methanol: The Role of Iminium and Enolate Ions
The free energy profile
of the piperidine catalyzed Knoevenagel
condensation reaction of acetylacetone with benzaldehyde has been
obtained by theoretical calculations. The carbinolamine formation
step involves catalysis by methanol solvent, and its decomposition
takes place via hydroxide ion elimination without a classical transition
state, leading to the iminium ion. Hydroxide ion deprotonates the
acetylacetone, forming an enolate that attacks the iminium ion and
leads to an addition intermediate. The final step involves
elimination of piperidine catalyst. Our analysis suggests the iminium
ion formation has the highest barrier and the catalytic effect of
piperidine is facilitating the elimination step rather than activation
of the benzaldehyde electrophile. Experimental measures of the kinetics
lead to an observed free energy barrier of 20.0 kcal mol<sup>–1</sup>, in good agreement with the theoretical value of 21.8 kcal mol<sup>–1</sup> based on the free energy profile