Journal
CHEMISTRY-A EUROPEAN JOURNAL
Volume 16, Issue 4, Pages 1221-1231Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/chem.200901519
Keywords
acid-base interactions; density functional calculations; ionic liquids; Knoevenagel condensation; organocatalysis
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Funding
- CICYT [MAT 2006-14274-C02-01]
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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 oil 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.
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