A DFT Study of the Molecular Mechanisms of the Diels−Alder Reaction between Cyclopentadiene and 3-Phenyl-1-(2-pyridyl)-2-propen-1-one − Role of the Zn2+ Lewis Acid Catalyst and Water Solvent

The molecular mechanism of the Diels−Alder reaction between cyclopentadiene (1) and 3-phenyl-1-(2-pyridyl)-2-propen-1-one (2) in the absence and in the presence of a Zn2+ Lewis acid catalyst has been studied by quantum mechanical calculations at the B3LYP/6-31G* level of theory. A continuum model was selected to represent the effects of the water as solvent. For the uncatalyzed process, two channels, endo and exo, were characterized, and the mechanism corresponded to an asynchronous concerted reaction associated with a [4+2] process. The presence of a Lewis acid catalyst changed the mechanism drastically, the reaction taking place by a polar stepwise mechanism. In the first step, a C−C sigma bond was formed by the nucleophilic attack of 1 on the conjugate position of the Lewis acid coordinated α,β-unsaturated ketone to give a zwitterionic intermediate, while the second step was a ring-closure process by this intermediate to give the final formally [4+2] cycloadduct. The theoretical results have been compared with available experimental data and an understanding of the role of Lewis acids and water solvent emerges from analysis of the results. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)