A study on the effect of Lewis acid catalysis on the molecular mechanism of the cycloaddition between (E)-methyl cinnamate and cyclopentadiene

The molecular mechanism of the Diels-Alder reaction between (E)-methyl cinn amate and cyclopentadiene has been analyzed by means of AMI semiempirical m ethod and compared to the experimental results. Stationary points for two r eactive channels, endo-cis and exo-cis, on potential energy surfaces have b een characterized. Three Lewis acids, boron trifluoride (BF3), aluminum tri chloride (AlCl3) and catechol boron bromide (CBB) have been used as catalys ts taking into account the formation of a complex between the boron or alum inum atom and the carbonyl oxygen of (E)-methyl cinnamate. The molecular me chanism of the uncatalyzed reaction corresponds to a concerted process. In the presence of BF3 and AlCl3, enhancement of both the asynchronicity and c harge transfer, with small decreased energy barriers, were obtained. With C BB, the molecular mechanism changes and the reaction takes place along a st epwise mechanism. The initial step corresponds to the nucleophilic attack o f cyclopentadiene on the dienophile double bond to give an intermediate, fo llowed by its closure in the second step, yielding the corresponding final cycloadduct. The inclusion of the CBB catalyst drastically decreases the en ergy barrier associated with the carbon-carbon bond formation of the first step, relative to the concerted process. The role of the Lewis acid catalys t emerges from the analysis of the results. (C) 2001 Elsevier Science Ltd. All rights reserved.