Roles of Lewis Acid Catalysts in Diels-Alder Reactions between Cyclopentadiene and Methyl Acrylate

The Diels-Alder reaction of cyclopentadiene with methyl acrylate catalyzed by AlCl₃ has been theoretically investigated. M06-2X level DFT calculations have shown that the formation of two C−C bonds is asynchronous in the cycloaddition both in the endo path and in the exo path, thus making a good contrast to the well-known concept of [4+2] reactions based on the orbital symmetry arguments. It was found that the catalyst facilitates the cycloaddition and brings a higher endo selectivity in the highly asynchronous process, as compared with the reaction of the diene and the dienophile without the catalyst.