Bifunctional Acid–Base Ionic Liquid Organocatalysts with a Controlled Distance Between Acid and Base Sites

Bifunctional acid-base ionic liquid organocatalysts with different distances between the two sites have been synthesised, and their activity for the Knoevenagel condensation has been tested. As has been found to be the case with enzymes, the distance between the acidic and basic sites determines the activity of the bifunctional organocatalyst, and at the optimal distance the reaction rate increases by two orders of magnitude with respect to the purely acidic or basic counterpart organocatalysts. The experimental results have been rationalised through the study of the reaction mechanism of the Knoevenagel condensation between malononitrile and benzaldehyde by means of DFT calculations. It has been found that it consists of two consecutive steps. First, deprotonation of malononitrile on the basic site to obtain a methylene carbanion intermediate takes place, and second, co-adsorption and activation of benzaldehyde on the acid centre of this intermediate followed by the C-C bond-formation reaction. The calculations and the kinetic study indicate that there is an inversion of the rate-controlling step when the distance between the acidic and the basic sites is modified, with a direct implication on the reaction rate.