Heterogeneous amination of bromobenzene over titania-supported gold catalysts

Abstract

Amination of bromobenzene with aniline was carried out on heterogeneous Au–TiO₂ catalysts prepared using the deposition–precipitation method. The deposition of gold in loadings in the range 0.1–0.9 wt.% on titania occurred with the formation of gold particles with controlled size and oxidation state as demonstrated from TEM and XPS measurements. In addition, Raman and DR–UV–Vis investigation of these catalysts showed that the interaction of Au particles with the support stabilized the metallic state of gold. Catalytic results demonstrated that the conversion and selectivity depended on the gold-particle size and dispersion, while the mechanism of reaction was in direct relation with the reaction medium. Working in DMC, the solvent was playing the role of base. The advantage of working in DMC was avoiding the formation of KBr which is very difficult to be separated from the reaction products. HBr resulted as a by-product formed adducts with the coupling product, which was separated as crystals at room temperature by a simple filtration. However, in DMC, the reaction occurred with low selectivities. In dioxane, the presence of potassium ethoxide as base was mandatory. The reaction occurred with very high selectivities for moderate conversions. The conversion was limited by KBr released from the reaction. However, the use of the heterogeneous catalyst has the advantage of scavenger, since KBr becomes chemisorbed on the surface and could be simple removed by percolation and the catalyst can be reused.

Introduction

C–N coupling also known as Buchwald–Hartwig amination is already recognized as a reaction with multiple applications in organic syntheses [1], [2], [3], [4]. Versatile organic building blocks and natural products were synthesized using this route [5], and some of these products were implemented at a production scale [6], [7].

This reaction was pioneered by Kosugi et al. [8] using homogeneous Pd-based catalysts and tin amides as N-nucleophiles. Then, an example of palladium(0)-mediated carbon–nitrogen bond formation using stoichiometric quantities of palladium(0) was reported by Boger [9]. Afterward, Buchwald [10], [11] and Hartwig [12], [13] independently improved the catalyst and reaction conditions working under of free amines conditions. Then, the development of the palladium catalysts for this reaction known large achievements, and in this moment, we dispose on a large number of ligands [14]. Improvements in the yield were found by using bidentate phosphine ligands [15], [16], although other discoveries of Wolfe and Buchwald [17] indicated that bidentate binding is unnecessary.

More recently, literature reported non-precious metals as catalysts able to replace noble metals for cross-coupling processes. Among the metals in competition with palladium were mentioned copper [18], [19], [20], [21], [22], [23], nickel [24], and more recently, iron [25], [26], [27], [28]. As an example, iron chloride in the presence of N,N′-dimethylethylenediamine was affording moderate to good yields toward the N1-arylindoles under relatively mild reaction conditions [29]. Recently, the role of iron in cross-coupling reactions was questioned, considering that in fact the reactions were catalyzed by copper impurities of the iron catalyst [30]. However, in spite of the reduced price, palladium catalysts often possess a higher activity than their non-noble metal alternatives, in addition, enabling the conversion of less reactive substrates [31]. Silver was indicated to promote the activity of Pd-catalyzed amination reactions [32].

Working with such homogeneous catalysts is the subject of other severe critics. Even in concentrations of the order of few ppm, they typically led to final compounds containing residual palladium catalysts [33]. Another disadvantage of these procedures is the short lifetime of the catalyst tending to further aggregate and form inactive Pd-black nano-particles [34]. Simple deposition of these catalysts on supports considering adsorption bonds is not improving the behavior. The higher the Pd loading is, the faster the deactivation relative to the catalytic process occurs. Monguchi et al. [35] published the first palladium on charcoal-catalyzed arylamination of aryl bromides and chlorides showing that the phosphane ligand controls the success of the coupling, bidentate ligands being the most effective for this reaction. Other examples were provided latter [36].

Under these conditions, heterogeneization of the active palladium catalysts via stronger bonds appeared as a first approach. In addition, the heterogeneization of the catalysts may allow an easy separation and recycling of expensive palladium catalysts. The reported procedures include heterogeneization of ligands on polymeric supports followed by complexation with palladium [37], [38], [39], [40] or the fixation homogeneous palladium catalysts onto nanoparticles to combine thus the advantages of homogeneous and heterogeneous catalysis [41], [42]. On this basis, it is also possible the development of liquid–liquid biphasic catalysts [43]. The synthesis of cationic imidazolium-based phosphanes is part of the same approaches, exhibiting high stability against air and water [44]. Their complexes with palladium can be immobilized following the supported ionic liquid catalyst concept (SILP). Simple deposition of palladium on typical supports has also been reported [45], [46], [47], [48], [49], [42], [50].

For the conversion of simple, active substrates, ligandless approaches are more attractive for industry by reducing costs dramatically, especially when considering that most of the state of the art ligands are patented [51]. Correa and Bolm [52] reported a ligand-free procedure for the N1-arylation of heterocycles catalyzed by Cu(I) oxide while Taillefer and co-workers [53] indicated an improved behavior for a Cu(II) oxide when using [Fe(acac)₂] as a co-catalyst. Copper and nickel oxide particles supported within charcoal have also been developed for Buchwald–Hartwig aminations. High-loading metal catalysts (5 wt.% of Cu, 5 wt.% of Ni) were active for copper-catalyzed reactions, while relatively inactive insofar as nickel catalysis was concerned [54].

Recent reports indicated that such a reaction can be performed with good results using organocatalysts [55]. In spite of the very good yields, the separation of the catalyst raised typical questions addressed to homogeneous catalysis.

The reaction conditions are very important for Buchwald–Hartwig amination. All the reports about this reaction identified the presence of an alkaline alcoxide as crucial for the success of amination. The effect of solvents on the product distribution and reaction rate was also demonstrated by several authors [56]. Working under microwave activation was not reducing the reaction temperature but shortened the reaction time [57].

The scope of this study was to investigate amination of bromobenzene using another concept. More specifically, we looked for a metallic species able to subtract a proton from the amine followed by its reaction with the aryl-halide, and we selected for this purpose gold.

Gold catalysis has attracted great interest in the last decade [58]. Although Au is not a good catalyst in bulk or in the form of large particles, when prepared as nanoparticles on an oxide support, it becomes one of the most active catalysts [59], [60], [61], [62], [63]. Experimental evidences corroborated with theoretical calculations revealed that the nanoparticles are anchored to oxygen vacancies on the surfaces, which allows the formation of a high areal density of nanoparticles. The smaller the particle, the greater is the fraction of atoms directly in contact with the support and therefore influenced by it, while at the same time, the fraction of coordinately unsaturated surface atoms also increases, and this changes the physical properties of the whole particle [64]. Reciprocally, the presence of gold particles may influence the properties of the support. The reduction of reducible supports (TiO₂, CeO₂, Fe₂O₃) by hydrogen is catalyzed by gold; this could be caused by hydrogen spillover from the metal or by changes induced in the electronic properties of the support [65], [66], [67].

Based on these achievements, Au nanoparticles have been applied as powerful catalysts for various other oxidative transformations, such as in the oxidation of alcohols, aldehydes [68], [69], [70], amines [71], [72], hydrocarbons [73], and in the epoxidation of alkenes [74] utilizing oxygen or air as oxidant. The active species, that is, small metallic clusters of Au⁰, partly oxidized gold particles (Au^δ+), partly reduced gold particles resulted by interacting with electron-rich metal defects of the support (Au^δ−), undercoordinated gold atoms, or cationic Au(I) and/or Au(III) species are in direct dependence with the nature of the reaction [75], [76], [77], [78], [79], [80], [81]. In addition, for oxidative catalytic reactions, it has been shown that gold nanoparticles promote the catalytic activity of TiO₂ [82], [83].

This study reports the results obtained in amination of bromobenzene using titania-supported gold catalysts prepared through the precipitation deposition technique.