C–N cross-coupling on supported copper catalysts: The effect of the support, oxidation state, base and solvent Alina Tirsoaga a , Bogdan Cojocaru a , Cristian Teodorescu a , Florin Vasiliu a , Maria Nicoleta Grecu a , Daniela Ghica a , Vasile I. Parvulescu a,⇑ , Hermenegildo Garcia b,⇑ a Department of Organic Chemistry, Biochemistry and Catalysis, Faculty of Chemistry, University of Bucharest, Bdul Regina Elisabeta 4-12, Bucharest 030016, Romania b Instituto de Tecnología Química CSIC-UPV, Universitat Politécnica de Valencia, Av. De los Naranjos s/n, 46022 Valencia, Spain article info Article history: Received 5 April 2016 Revised 10 June 2016 Accepted 19 June 2016 Keywords: Heterogeneous catalysis C–N cross coupling Copper catalysts Precipitation-deposition technique Amination of bromobenzene abstract A series of supported copper catalysts at two different loadings (1 and 2 wt%) have been prepared by deposition precipitation on various supports including TiO 2 , ZnO, Al 2 O 3 and active carbon and submitted or not to reductive treatments to favor the increase in population of Cu(I). The samples have been char- acterized by textural measurements, electron microscopy and spectroscopic techniques including EPR and XPS, concluding the presence of dispersed copper oxides on the support with small particle size and contrasting prevalence of Cu(II) or Cu(I). The catalytic activity of all these catalysts for the C–N cou- pling of aniline and bromobenzene has been evaluated. A strong influence of the support, copper oxida- tion state, solvent, nature of the base was observed, the optimal conditions being the use of ZnO or TiO 2 as supports and toluene/dioxane as solvent and EtOK as base. t-C 5 H 11 OK as base in either THF or toluene give rise to the formation of t-C 5 H 11 phenyl ether in some extent. The catalyst undergoes deactivation during the reaction, but about 88% of the activity of the fresh sample could be regained by dioxane wash- ings before reuse. XPS indicates that the most likely origin of catalyst deactivation is adsorption on the copper catalyst surface of KBr and inorganic salts formed as byproducts during the reaction. Ó 2016 Elsevier Inc. All rights reserved. 1. Introduction Cross-coupling is a very powerful strategy to generate aryl- carbon and aryl-heteroatom covalent bonds and may occur follow- ing different routes [1–15]. Among these, the formation of the C–N bonds elicited a large interest in the synthesis of compounds with pharmaceutical, cosmetic, agrochemical and optical devices [16– 23] applications. Because of this it received a great industrial and academic importance in the past two decades [24]. Initiated by Migita et al. [25] using aminostannanes, the amination of aryl halides is mostly known as a Buchwald-Hartwig reaction [26–31]. Classical Buchwald-Hartwig reaction represents a palladium- mediated cross-coupling procedure [32,33] under homogeneous catalytic conditions, using strong bases, aprotic and/or polar and non-polar solvents and different ligands [34–39]. Along with the choice of the ligand, an important component of the C–N cross-coupling reaction is the nature of the base (e.g. car- bonates, phosphates, alkaline hydroxides, alkoxides, inorganic flu- oride or silylamide) that plays an important role on the evolution of the reaction, being involved in the deprotonation of the N- substrate. A suitable base affords considerable benefits in the func- tional group tolerance of Pd-catalyzed amination reactions. Classi- cal solvents usually employed in the Buchwald-Hartwig amination reaction [40] are nonpolar and aprotic being thus able to dissolve all reactants with the exception of NaO-t-Bu (the traditional base of this reaction). Although palladium [32,33] is the most widely used active metal of the Buchwald-Hartwig cross-coupling reactions, copper, nickel and iron have been also considered as alternatives because they are less expensive, and require cheaper bidentate ligands such as phenanthrolines, 1,3-diketones, imines, a-amino acids, salicy- lamides, diamines, lowering thus the overall synthesis procedure costs [41]. The literature already reported a broad variety of nucleophiles that were successfully used in homogeneous copper-mediated cross-coupling reactions to form C(aryl)–N bond: amines, anilines, amides, imides, ureas, carbamates, sulfonamides as well as aro- matic heterocycles (imidazoles, pyrazoles, thiazoles, tetrazoles, benzimidazoles, indazoles) [42,43]. http://dx.doi.org/10.1016/j.jcat.2016.06.011 0021-9517/Ó 2016 Elsevier Inc. All rights reserved. ⇑ Corresponding authors. E-mail addresses: vasile.parvulescu@chimie.unibuc.ro (V.I. Parvulescu), hgar- cia@qim.upv.es (H. Garcia). Journal of Catalysis 341 (2016) 205–220 Contents lists available at ScienceDirect Journal of Catalysis journal homepage: www.elsevier.com/locate/jcat