Hydrogenation in Ionic Liquids

Abstract

One of the principal present-day challenges facing the field of transition metal catalysis is the efficient recycling and reuse of catalysts and ligands. The use of ILs is rapidly advancing in this discipline. Due to their tunable physico-chemical properties, which differ markedly from those of conventional organic and aqueous media, ILs can provide a means of catalyst immobilization. The non-nucleophilic and weakly co-ordinating nature of many classes of ionic liquid provides an inert reaction medium that can extend the lifetime of a catalyst. Recyclability of the catalyst system is a key attribute of IL media and it is this enhancement of catalyst performance that is driving research in this field. Low-polarity compounds, for example diethyl ether and n-hexane, are poorly soluble in common ILs, providing a suitable accompanying solvent for biphasic catalysis. The positive aspects of homogeneous and heterogeneous catalysis are combined using a biphasic system, in which the catalyst resides in the IL, but the substrates/products are retained in the alternate phase. Thus, the biphasic system provides a cost-effective way to successfully separate the desired product by simple decantation, leaving the catalyst immobilised in the IL and ready for reuse. Product isolation is often simplified even in monophasic catalysis if the substrate is soluble in the IL medium, but the product can be separated by simple extraction or distillation, due to the low vapour pressure of the IL. The reduced polarity of the hydrogenated products in comparison with the substrate can also be exploited for separation from the IL/catalyst phase. Increasing the difference in polarity between the IL and the hydrogenated product can also render the product insoluble in the IL, thus allowing facile decantation of the product from the IL, leaving the IL/catalyst phase ready for the next reaction. Research into catalytic hydrogenations in IL s began in 1995 with the almost simultaneous work of Chauvin[1] and Dupont.[2] Since then this field has been extended from conventional hydrogenation using transition metal catalysts to transfer hydrogenation and the effect of nanoparticles on hydrogenation reactions in ILs. Supported ionic liquid catalysis (SILC) is also a relatively new field. Using this method, the added benefit of selectivity provided by the homogeneous catalyst can be combined with the attributes of heterogeneous biphasic catalysis. The homogeneous catalyst is, in effect, immobilised on a heterogeneous support. One of the first investigations in this area was carried out by Mehnert et al.[3] in 2002, with a flurry of papers ensuing from 2007-2009. It is intended that this chapter should cover recent progress in hydrogenation reactions carried out in ILs. Wasserscheid and Schulz[4] contributed a chapter in ‘The Handbook of Homogeneous Hydrogenation’ covering homogeneous hydrogenation in ILs which covers