On the Solvent and Temperature Driven Stereoselectivity of the Diels-Alder Cycloaddition Reactions of Furan with Maleic Anhydride and Maleimide

In quantum chemical calculations, the Diels-Alder cycloaddition reaction of maleic anhydride with furan favors the endo-isomer kinetically in the gas phase. This preference, however, changes in favor of the exo-isomer on inclusion of solvent effects. For example, in acetonitrile, the exo-isomer was calculated to form twice as fast as the endo-isomer at room temperature, an observation that corroborates well with the experiments. The free energy change of the reaction is such that it allows faster reversal of the endo-isomer to the reactants in an equilibrium process and, thus, the predominance of the more stable exo-isomer eventually. The reaction of maleimide with furan is similar to that of maleic anhydride. However, it favors predominantly endo selectivity at temperatures below 320 K which is in excellent agreement with the experimental studies of temperature dependence of this DA reaction. Further, a gedanken calculation of the DA reaction of cyclopropene with furan has been investigated to delineate the effect of the retro-pathway on the stereoselectivity.