Triethylsulfonium Bistriflimide as the Reaction Medium in Catalyzed and Uncatalyzed Cycloaddition [4 + 2]

Theoretical Investigation of the Kinetic Effect of the 1-Ethylpyridinium Trifluoroacetate Ionic Liquid in the Acceleration of Diels–Alder Reactions of Isoprene with Acrylic Acid and Acrylonitrile

The mechanism, the regioselectivity, the stereoselectivity and the kinetics of Diels–Alder reactions of isoprene with acrylic acid and acrylonitrile have been studied, at the B3PW91/6-31G(d,p) level of theory, both in the gas phase and in the presence of organic [dichloromethane (DCM)] and ionic liquid [1-ethylpyridinium trifluoroacetate (EPTFA)] solvents. Intrinsic reaction coordinate calculations show that these reactions take place through an asynchronous concerted mechanism leading to the endo para cycloadducts as the major products in the gas phase and to the exo para cycloadducts as the major products both in organic and in ionic liquid solvents. The explicit solvation model involving the coordination of one molecule of the solvent with the dienophiles shows a considerable decrease of the activation energy when passing from DCM to EPTFA. The enhancement of these cycloaddition reactions can be explained by the strong hydrogen bonding created between the ion pair of the ionic liquid and the oxygen atom of the dienophile reagents. Moreover, density functional theory-based reactivity indices also show an increase of the reaction polarity and consequently of the reaction rate, when replacing DCM solvent by EPTFA solvent. The results obtained give evidence that the ionic liquid EPTFA is an excellent solvent for Diels–Alder reactions in comparison with conventional organic solvents.

N-hexylpyridinium bis(trifluoromethylsulfonyl)imide and Lewis acids — catalytic systems for Diels-Alder reaction

A comparative study of the Diels-Alder reactions between cyclopentadiene (1) and dienophiles (2 a–c) in N-hexylpyridinium bis(trifluoromethylsulfonyl)imide in a temperature range of 20–45°C is reported. The reaction rate constants and activation energies were calculated. Moreover, the catalytic systems based on N-hexylpyridinium bis(trifluoromethylsulfonyl)imide and Lewis acids were tested as a reaction media to perform the Diels-Alder reaction. Yb, Y, Mg, Zn triflates and chlorides (0.005 to 0.1 mmol) were used as catalysts. The recycling of catalytic system consisted of YCl3 and ionic liquid was performed.