Heterogeneous Suzuki cross-coupling reactions over palladium/hydrotalcite catalysts

doi:10.1016/j.jcis.2006.06.058

Journal of Colloid and Interface Science

Volume 302, Issue 2, 15 October 2006, Pages 568-575

https://doi.org/10.1016/j.jcis.2006.06.058 Get rights and content

Abstract

The efficiency of various heterogeneous solids consisting of palladium supported on hydrotalcite as catalysts in the Suzuki cross-coupling reaction between bromobenzene and phenylboronic acid was studied. Based on the catalytic activity results, the reaction develops to an acceptable extent with 100% selectivity at moderate temperatures in the presence of some of the catalysts. The best results were provided by a catalyst consisting of an acetate–pyridine complex of Pd supported on hydrotalcite that gave high conversion values even after three reuses. The reactions conditions were very mild (a temperature of 55 °C and atmospheric pressure). In fact, the catalyst provided conversion and selectivity results surpassing those of existing heterogeneous phase catalysts and most homogeneous phase catalysts for the same purpose.

Graphical abstract

Introduction

The palladium-catalyzed Suzuki cross-coupling reaction has for some years been one of the most powerful available tools for building carbon–carbon bonds in organic syntheses [1], [2], [3]. Most applications of the Suzuki reaction involve homogeneous catalysts consisting of Pd complexes with a variety of ligands (particularly phosphines) [2], [4], [5], [6]. Some recent cross-coupling processes, however, use palladium immobilized on various supports including sepiolites [7], [8], silica [9], zeolite and zeolitic materials [10], [11], [12], carbon [13], [14] and organic complexes bound to inorganic solids [15], [16], [17].

These heterogeneous catalysts are used mainly to overcome some problems of homogeneous catalytic processes such as the need to recycle the catalyst or potential environmental pollution issues—which is leading to an increasing use of water as solvent. Also, palladium ligands and precursors are expensive, which restricts their industrial use.

Hydrotalcite is a naturally occurring mineral of the layered double hydroxide (LDH) family, the members of which constitute a major class of anionic clays. Hydrotalcite is structurally related to brucite [Mg(OH₂)]: magnesium cations are at the centers of octahedra the vertices of which are occupied by hydroxyl groups to form stacks. In hydrotalcite, some Mg²⁺ cations are replaced by aluminum cations, which introduces a charge deficiency in its layers. In order to ensure electroneutrality in the overall structure, the positive charge is countered by carbonate anions present in a disorderly manner in the interlayer region, which contains crystallization water as well [18]. This type of hydrotalcite has been used as such and as a catalyst precursor by our group in organic processes including the epoxidation of limonene [19], [20] or the Meerwein–Ponndorf–Verley reaction [21], [22], [23], [24] and, recently, as a support for palladium in the Suzuki cross-coupling reaction [25], [26].

In this work, we prepared various heterogeneous Pd²⁺ catalysts on hydrotalcite and used them in the Suzuki cross-coupling reaction between bromobenzene and phenylboronic acid (Scheme 1) in the presence of inorganic bases. The objective of this study was to establish which palladium catalyst was the more active (a supported palladium complex, a intercalated palladium complex, a supported palladium salt, or a catalyst with palladium included in the hydrotalcite structure). The catalysts and the hydrotalcite precursor were characterized for structure and surface properties using various instrumental techniques. The effects of the solvent and temperature on the catalytic activity were also studied. Finally, the catalyst was subjected to reuse tests with various aryl halides and phenylboronic acid to check whether the results were universal.

Section snippets

Hydrotalcite

The Mg/Al hydrotalcite used as catalyst precursor was obtained using a coprecipitation method described elsewhere [27]. In a typical synthetic run, a solution containing 0.3 mol of Mg(NO₃)₂⋅6H₂O and 0.15 mol of Al(NO₃)₃⋅9H₂O (Mg/Al = 3) in 250 ml of deionized water was used. The solution was slowly dropped over 500 ml of an Na₂CO₃ solution at pH 10 at 60 °C under vigorous stirring, the pH being kept constant by adding appropriate volumes of 1 M NaOH during precipitation. The suspension thus

Elemental analysis

Table 2 shows the results of the elemental analysis of each catalyst.

As can be seen, the HT solid possesses an Mg/Al ratio identical to the theoretical value. This metal ratio was preserved in the catalysts obtained from this hydrotalcite (i.e., neither immobilization of the PdAc₂Py₂ complex nor exchange of carbonate ions with PdCl²⁻₄ ions or impregnation of the surface with PdCl₂ altered the chemical composition of the brucite layers). The HT-Pd catalyst also exhibits an [Mg(II) +

Conclusions

The results obtained in this work show that various hydrotalcites containing palladium are active catalysts in the Suzuki cross-coupling reaction—particularly those obtained by supporting an acetate–pyridine complex of palladium on hydrotalcite. The biphenyl conversion in the reaction between bromobenzene and phenylboronic acid was found to depend on the particular base used, of which K₂CO₃ proved the best among those tested. The influence of the nature of the solvent was examined, using the

Acknowledgements

The authors gratefully acknowledge funding by Spain's Ministerio de Educación y Ciencia, Fondos Feder and by the Consejería de Educación y Ciencia de la Junta de Andalucía.

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Heterogeneous Suzuki cross-coupling reactions over palladium/hydrotalcite catalysts

Abstract

Graphical abstract

Introduction

Section snippets

Hydrotalcite

Elemental analysis

Conclusions

Acknowledgements

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