Assessment of the suitability of imidazolium ionic liquids as reaction medium for base-catalysed reactions -: Case of Knoevenagel and Claisen-Schmidt reactions

For the Knoevenagel condensation of benzaldehyde and malononitrile in 1-butyl-3-methylimidazolium hexafluorophosphate [(bmim)PF6] to which alkali metal hydroxide in ethanol had been previously added, the product is formed with high selectivity and the ionic liquid could be reused for at least five runs without the need of additional base. Low to moderate mass balances were observed in the first runs until the ionic liquid becomes saturated and the mass balances become almost complete. For the Claisen-Schmidt condensation of acetophenone with benzaldehyde, besides the expected conjugated ketone formed with high selectivity, minor amounts of ethyl benzoate are also formed. Reuse of the ionic liquid require additional amounts of base. Acid-base titration shows that most of the base (up to 80%) may disappear by reaction with the ionic liquid in the absence of any reagent. Therefore, addition of extra amounts of base is necessary in order to achieve high substrate conversion in subsequent runs and the reactions become not catalytic. This raises the point that imidazolium ionic liquids are only suitable for a small range of base catalysed reactions, and that imidazolium liquids substituted at the 2-position will be more suited as medium for based-catalysed reactions. (C) 2004 Elsevier B.V. All rights reserved.