Lewis acidic ionic liquids for the synthesis of electrophilic alkenes via the Knoevenagel condensation

1-Butyl-3-methylimidazolium chloroaluminate, [bmim]Cl·AlCl3, N=0.67 and 1-butylpyridinium chloroaluminate, [bpy]Cl·AlCl3, N=0.67 ionic liquids were found to work well as the Lewis acid catalyst and solvent in the Knoevenagel condensations of benzaldehyde and substituted benzaldehydes with diethyl malonate to give benzylidene malonates. The benzylidene malonates subsequently underwent Michael additions with diethyl malonate. The extent of Michael product formed during the reaction was found to vary with the Lewis acidity and the molar proportion of ionic liquid. The influence of Lewis acidity of the ionic liquid on the Knoevenagel and Michael products is demonstrated. In the case of 2-hydroxyarylaldehydes, the reactions led to the formation of 3-ethoxycarbonyl coumarins under ambient conditions. © 2002 Elsevier Science Ltd. All rights reserved. Ever since the ionic liquids came into the spotlight during the last decade, rigorous and consistent efforts in this area have paved the way for alternative and new synthetic strategies in the realm of synthetic organic chemistry. The pioneering work of Seddon and others have unravelled several remarkable properties of ionic liquids that can be ably exploited in tackling problems encountered in synthetically useful reactions. Their unprecedented ability to solvate a broad spectrum of substrates of organic and inorganic nature has widened the horizon of their applicability. Infinitesimally small vapour pressure, recyclability without any diminution in the desirable properties (except in the case of chloroaluminate ionic liquids), high thermal stability and ease of handling are the features possessed by these liquids. The hallmark of such liquids is our ability to alter their properties as desired, by manipulating their structure with respect to the choice of organic cation, anion and side chain attached to the organic cation. All these properties have bestowed entirety to these liquids for their application in developing environmentally benign chemical technologies. The Knoevenagel reaction is well known for its immense potential in the synthesis of electrophilic olefins from active methylene and carbonyl compounds. More than a century old now, a wide array of catalysts have been employed to accomplish this reaction, each affording variable yields of olefins. The reaction is catalysed by weak bases under homogenous conditions. However, the past decade has witnessed a rapid emergence of heterogeneous catalysts comprising of mainly inorganic materials such as clays, zeolites, etc. owing to their environmentally benign properties, which have been aptly employed in this reaction. Apart from these properties, the feature which encouraged their application in Knoevenagel reaction is that most of them are ditopic in nature, containing both acidic and basic sites, while others are solely acidic or basic. Some Lewis acid catalysts have also been Scheme 1. Reaction of benzaldehyde and substituted benzaldehydes with diethyl malonate in the Lewis acidic [bmim] Cl·xAlCl3 and [bpy]Cl·xAlCl3 ionic liquids. * Corresponding author. Tel.: 91-22-2816750; fax: 91-22-2816750; e-mail: [email protected] 0040-4039/02/$ see front matter © 2002 Elsevier Science Ltd. All rights reserved. PII: S0040 -4039 (01 )02341 -3 J. R. Harjani et al. / Tetrahedron Letters 43 (2002) 1127–113