An AM1 theoretical study on the effect of Zn2+ Lewis acid catalysis on the mechanism of the cycloaddition between 3-phenyl-1-(2-pyridyl)-2-propen-1-one and cyclopentadiene

The mechanism of the Diels–Alder reaction between 3-phenyl-1-(2-pyridyl)-2-propen-1-one and cyclopentadiene has been investigated with the AM1 semiempirical method. Stationary points for two reactive channels, endo-cis and exo-cis, have been characterized. The role of the Lewis acid catalyst has been modeled taking into account the formation of a complex between Zn2+ and the carbonyl oxygen atom and the pyridyl nitrogen atom of the 3-phenyl-1-(2-pyridyl)-2-propen-1-one system with and without the presence of two molecules of water around the cation. The mechanism of the uncatalyzed reaction corresponds to a concerted process, but in the presence of Lewis acid catalyst the mechanism changes and the reaction takes place through a stepwise mechanism. A first step involves the nucleophilic attack of the cyclopentadiene in the double bond of the dienophile which produces an intermediate. A second step involves the closure of the intermediate yielding the corresponding final cycloadduct. The inclusion of the Zn2+ catalyst drastically decreases the energy barrier associated with the carbon–carbon bond formation of the first step in comparison to the concerted process.