Theoretical study on the Diels–Alder reaction of cyclopentadiene with methacrolein catalyzed by diethylimidazolium cation

The theoretical studies of how room temperature ionic liquids control desired reactions are very scarce in contrast with their increasing applications in many fields as recyclable solvents, catalysts, and reaction mediums. The present work considers the Diels–Alder (D‐A) reaction of cyclopentadiene with methacrolein in the presence of diethylimidazolium salts as the first prototype of our systemic studies about important organic synthesis reactions catalyzed by room temperature ionic liquids. We show the mechanism details of the D‐A reactions with and without the dialkylimidazolium cation and rationalize the experimental findings based on the results from the quantum chemistry calculations at the AM1, HF/6‐31G(d), and B3PW91/6‐31G(d,p) levels of theory, respectively. It is found that the diethylimidazolium cation acts as a Lewis acid center to catalyze the D‐A reaction, which decreases the barrier and increases the asynchronicity of the D‐A reaction, but does not change the potential energy surface profile of the reaction compared to the noncatalyzed process. The present results rationalize the early experimental findings well and provide the first prototype for theoretically understanding the D‐A reaction in the presence of dialkylimidazolium salts. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007