An Investigation of the endo Product Selectivity in the Diels-Alder Reaction

The Diels-Alder reactions of cyclopentadienes with a variety of substituted ethylenes were carried out, and their reaction rates and the endo : exo isomer ratios of the products were determined. It was shown that the faster reaction is characterized by a higher endo isomer distribution and the existence of a linear correlation between the reaction rate and the endo isomer distribution. The results were interpreted in terms of the concept of secondary orbital interaction in the transition state.