A new generation of aprotic yet Brønsted acidic imidazolium salts : low toxicity , high recyclability and greatly improved activity †

Since Forbes and Davis reported the design of a class of phosphoniumand imidazolium ion-based ionic liquids (e.g. 1, Fig. 1A) equipped with a pendant acidic sulfonic acid moiety, interest in Brønsted acidic ionic liquids (BAILs) has gathered considerable pace. These materials afford the practitioner the flexibility of a system which combines strong acidity with the traditional advantages associated with the use of nonvolatile ionic liquids. Subsequently two other strategies for the design of acidic imidazolium-ion based ionic liquids (ILs) were reported: the protonated imidazole conjugate acids (i.e. 2) and traditional imidazolium-ion based ILs which incorporate acidic counteranions (i.e. 3). While these systems have found application in a wide-range of acid-catalysed reactions, the potential uncertainties from an environmental standpoint (to the best of our knowledge the toxicity and biodegradation profiles of these materials have yet to be established) and potential storage difficulties associated with the fact that these materials are strongly Brønsted acidic, remain. We therefore became interested in the design of aprotic salts which could serve as acidic catalysts only when used in conjunction with an additive. These materials hold promise as catalysts which can be designed to be readily storable and of minimal toxicity/environmental impact, the catalytically useful acidity of which could be controlled in an ‘on–off’ fashion. In short, these catalysts would be acidic only when required. Our inspiration for this work came from the serendipitous discovery that N-alkyl pyridinium ions could catalyse the acetalisation of benzaldehyde in the absence of any discernible acidic species in solution. It was later demonstrated that this phenomenon also occurred in the case of N-alkyl imidazolium ions, which allowed the design of a suite of demonstrably low antimicrobial toxicity salts (of which 4, Fig. 1A, proved the most active) capable of promoting the acetalisation of aldehydes (inter alia) at low catalyst loadings (e.g. 5–10 mol%, Fig. 1B). Along the same lines, we recently demonstrated that triazolium ion-based species could act in a similar fashion at loadings of 1–2 mol%. It was proposed that the low resonance stabilisation energy of the imidazolium ion would allow