No abstract

a Catalysts which have low antimicrobial toxicity and are aprotic, yet which can act as Brønsted acidic catalysts in the presence of protic additives have been developed. The catalysts are recyclable, considerably more active (i.e. can be used at 10-50 times lower loadings) and of broader scope than their antecedent generation.Since Forbes and Davis reported the design of a class of phosphonium-and 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.1,2 These materials afford the practitioner the flexibility of a system which combines strong acidity with the traditional advantages 3 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 3o,4 (i.e. 2) and traditional imidazolium-ion based ILs which incorporate acidic counteranions (i.e. 3). 5 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. 6,7 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). 8a Along the same lines, we recently demonstrated that triazolium ion-based species could act in a similar fashion at loadings of 1-2 mol%. 8b It was proposed that the low resonance stabilisation energy of the imidazolium ion would allow

Imidazolium derived ionic liquid catalysts have been developed which are aprotic and of low antimicrobial and antifungal toxicity, yet which can act as efficient Brønsted acidic catalysts in the presence of protic additives. The catalysts can be utilised at low loadings and can be recycled 15 times without any discernible loss of activity.Over the last decade Ionic Liquids (ILs) have been extensively investigated as potential replacements for volatile organic compounds for use as (inter alia) both tunable reaction media and catalytic solvents.

The ability of triazolium salts to serve as a precatalyst for both an acid and a powerful base/nucleophile (controlled by additives) has been exploited in a process characterised by a unique in situ catalyst modification strategy.

The asymmetric carbonyl-ene reaction of trifluoropyruvate with five alkenes catalysed by [Pd{(R)-BINAP}]-(SbF 6 ) 2 were carried out in good yields and enantioselectivities (up to 96% yield and 96% ee) in low antimicrobial toxicity C2-substituted imidazolium ionic liquids (ILs). Toxicity data was included in the selection criteria for reaction optimisation after a preliminary IL screen. The Pd(II) catalyst immobilised in an IL was recycled and reused up to 7 times without decrease of either yield or ee. One IL prepared, which was determined to be of high antimicrobial toxicity was assigned a low priority for future applications. † Electronic supplementary information (ESI) available. See

A new, organocatalytic approach to the hydrolytic deprotection of dithianes has been developed involving a low‐toxicity imidazolium‐ion‐based catalyst in an aqueous medium. A complimentary solvent‐free method without added water and involving a sacrificial aldehyde is also reported. The catalyst does not appear to operate by a specific‐acid catalysis mechanism.

Aprotic pyridinium ions incorporating electron-withdrawing substituents on the aromatic ring are powerful catalysts for the acetalization of aldehydes and the formation of dithianes, dithiolanes, dioxanes, and dioxolanes. Under optimum conditions the best catalyst can be used at a loading as low as 0.1 mol% and can outperform a Brønsted acid catalyst with a pK a of 2.2.

Acid Catalysis. -Imidazolium derived ionic liquid catalysts are developed which are aprotic, but can act as Broensted acidic catalysts in the presence of protic additives. The catalysts can be recycled 15 times without loss of activity. -(MYLES, L.; GORE, R.; SPULAK, M.; GATHERGOOD, N.; CONNON*, S. J.; Green Chem. 12 (2010) 7, 1157-1162, DOI:10.1039/c003301d ; Cent. Synth. Chem. Biol., Trinity Coll., Univ. Dublin, Dublin 2, Ire.; Eng.) -M. Paetzel 48-039