Send Orders of Reprints at reprints@benthamscience.net 238 Current Organic Chemistry, 2013, 17, 238-256 Ionic Liquids and the Heck Coupling Reaction: An Update Patrícia Prediger, a,b Yves Génisson a* and Carlos Roque Duarte Correia b* a UMR5068, CNRS-Université Paul Sabatier, Toulouse, France b Instituto de Química, Universidade Estadual de Campinas, UNICAMP, C.P. 6154, CEP. 13084-971, Campinas, São Paulo, Brazil Abstract: In this review update we provide a critical analysis of recent results concerning the use of ionic liquids (ILs) as viable, effec- tive, and environmental friendly media for the Heck reactions (HR). The examples selected illustrate the multifaceted action of ILs as nanoparticles stabilizers, promoters of ligand-assisted HR, promoters of phosphine-free HR, solvent for the Heck-Matsuda reactions em- ploying arenediazonium salts, in the dehydrative Heck olefination, and as charged-tagged pre-catalysts. These selected reports aim at demonstrating the scope and the impact of Ils on new discoveries, as well as to illustrate the new opportunities and challenges they offer for those interested in catalysis and organic synthesis in general. Keywords: Ionic liquids, Heck reaction, Palladium-nanoparticles , N-heterocyclic carbenes. 1. INTRODUCTION After a decade of exponential development, the field of ionic liquids (ILs) technology has reached a substantial degree of matur- ity in several aspects, including preparation, purification and phys- icochemical characterization [1]. Over the same period, other do- mains of application of ILs have progressively grown such as ana- lytical chemistry [2] or biomass valorization [3]. The area of cataly- sis and organometallic chemistry, with the early identification of ILs as unique media [4], has also strongly contributed to their cur- rent wide recognition [5]. Yet, as demonstrated by the profusion of recent works reported here, many opportunities and challenges are still open in this domain. ILs have been proposed as an eco-friendly alternative to com- mon volatile molecular solvents. This arises from several key fea- tures such as their thermal stability, negligible vapor pressure and they are virtually non-inflammable [6]. The tunable physicochemi- cal profile of ILs, such as water/solvent relative miscibility or liquid state temperature range, has also called the attention of the chemical community, offering prospects for innovative process developments [7]. Their potential for recyclability in particular underlies all the above-mentioned physicochemical characteristics of ILs. Among the Pd(0)-catalyzed cross couplings, the reaction be- tween an aryl halide and an olefin, known as the Heck-Mizoroki arylation holds a prominent place [8]. It has become a versatile and ubiquitous synthetic tool for the creation of strategic Csp 2 -Csp 2 bonds in complex molecules. A myriad of reaction cocktails have been proposed for the Heck coupling, varying the source of Pd(0), the metal ligand, the solvent and the base. However, such a sophis- tication of the reaction systems normally goes along with more constraining reaction conditions and costly reagents [9]. In this context, ILs have offered unexpected opportunities for a striking simplification of the Heck coupling procedure, potentially acting as a recoverable reaction medium and reaction promoter [10]. This has further led to specifically functionalized ionic structures, called Task Specific ILs (TSILs), designed to accelerate a reaction step (as * Address correspondence to this author at the Instituto de Química, Universidade Estadual de Campinas, UNICAMP, C.P. 6154, CEP. 13084-971, Campinas, São Paulo, Brazil; Tel: +55 19 35213086; Fax: +55 19 3521 3023: E-mail: roque@iqm.unicamp.br a base) or to complex the metallic species (as a ligand) [11]. Other approaches, such as the catalyst’s immobilization, were also pur- sued with the development of supported ILs [12]. A comprehensive overview of the Heck reaction in ILs has been published by Bellina and Chiappe in March 2010 [13]. The present review intends to give an update of the work published in this highly active area from mid 2010. 2. THE DIFFERENT TYPES OF EFFECT OF ILS ON THE HECK REACTION One of the most important roles of the ionic medium on palla- dium-catalyzed transformations may be the stabilization of the ac- tive catalytic species. This has notably led to the development of practical aerobic and ligand-free coupling procedures. The general promoting effects of ILs on palladium-catalyzed cross couplings, including the Heck reaction, are basically of two main types: nanoparticles stabilization or diaminocarbene ligands generation. 2.1. ILs as Nanoparticles Stabilizer The first effect of ILs on palladium catalysis deals with nanoparticles stabilization [14]. Indeed, the efficiency of the Heck coupling largely depends on the generation of stable Pd(0) colloidal solutions from a precatalyst. In situ Pd nanoparticles generation in standard ammonium or imidazolium ILs have been examined in depth [15]. It may be easily accomplished, for example by reduc- tion (either upon hydrogenation or strong heating) of a Pd(II) salt or decomposition of an organometallic complex [16]. The formation of a dispersed metallic suspension, generally characterized in size and shape by Transmission Electron Microscopy (TEM) analysis [17] is commonly opposed to the eventual precipitation of inactive crystalline Pd black. Combined electrostatic and steric stabilizing effects of the IL on the nanoparticles have been invoked. 2.2. Diaminocarbene Precursor The formation of diaminocarbene transition metal ligands through the somewhat favorable abstraction of the hydrogen in the 2-position of an imidazolium nucleus is well documented [18]. The participation of imidazolium based ILs as carbenic species precur- 18 5- 8/13 $58.00+.00 © 2013 Bentham Science Publishers