A general synthetic route to [ Cu ( X ) (

In transitionmetal-catalysed reactions the accessibility of the catalyst is one of the most important factors in dictating the usefulness of any catalyticmethod. The development of catalytic systems based on inexpensive metals such as iron and copper have gained increased attention during the last few years. In this context, N-heterocyclic carbene (NHC) copper species of the type [Cu(X)(NHC)] (X = Cl, Br, I) have shown to be efficient catalysts for several transformations, such as the reduction of carbonyl compounds, hydrosilylation and [3+2] cycloaddition of alkynes and azides. They can also be used as carbene transfer reagents to other transition metals such as gold or palladium. In addition, these systems have exhibited interesting biological activity as antitumour agents. The most common synthetic strategy to prepare [Cu(X)(NHC)] (X = Cl, Br, I) complexes is the reaction of a free carbene with a copper source. The free carbene can be isolated in the first instance or generated in situ (Scheme 1). The main drawbacks of this procedure are the need for an inert atmosphere and strictly anhydrous conditions as well as the use of strong and expensive bases. In 2010 an improved procedure was developed by our group: the synthesis of several [Cu(Cl)(NHC)] complexes using Cu2O as a copper source in different solvents was reported (Scheme 1). This methodology allows the use of air-stable and economical starting materials and generates water as the only side-product. McQuade and co-workers have employed this system to validate their hypothesis that such syntheses could be performed in a continuous flow apparatus. In 2012, Jiang and co-workers reported the synthesis of some [Cu(Cl)(NHC)] by treatment of imidazolium salts with weak bases in the presence of a copper source. However, this protocol requires high temperature and environmentally unfriendly solvents such as 3-chloropyridine. Very recently, Cisnetti and co-workers showed that aqueous ammonia can promote the formation of Cu–NHC complexes from the corresponding imidazol(idin)ium salts. Despite these recent improvements, access to a general synthetic route leading to [Cu(X)(NHC)] complexes under milder and environmentally friendly conditions remains highly desirable. Recently, one of us has reported a one-step methodology to prepare [Au(X)(NHC)] (X = Cl, Br, I) complexes, using imidazol(idin)ium salts, a gold source and a weak base. This protocol proceeds under mild conditions and has proven to be robust and general, as it is applicable to the synthesis of a wide range of NHC–Au complexes. Due to the great versatility of this protocol and the closely related chemistry of gold and copper, we were interested in extending this straightforward methodology to the preparation of [Cu(X)(NHC)] complexes (Fig. 1). Therefore we carried out the synthesis of [Cu(Cl)(IPr)] (2a) following the optimised conditions for gold, e.g., treating, in air, IPr HCl (1a) with CuCl in the presence of 1 equivalent of K2CO3, using technical grade acetone at 60 1C. After 24 h an encouraging 80% conversion to the desired complex 2a was observed. By doubling the amount of base, 2a was isolated in 92% yield (Scheme 2). Noteworthily, the reaction time can be reduced to 1 h by using 10 equivalents of base. After a brief optimisation of the reaction conditions (see ESI†), we found the best conditions for the synthesis of [Cu(Cl)(IPr)] are the ones used for the synthesis of [Au(Cl)(IPr)]. In order to Scheme 1 Synthetic routes to [Cu(X)(NHC)] complexes.