Elsevier

Coordination Chemistry Reviews

Volume 311, 15 March 2016, Pages 1-23
Coordination Chemistry Reviews

Review
Schiff base-derived homogeneous and heterogeneous palladium catalysts for the Suzuki–Miyaura reaction

https://doi.org/10.1016/j.ccr.2015.11.010Get rights and content

Highlights

  • Use of Schiff bases as ligands for Pd catalyzed Suzuki reactions is described.

  • An insight into state of the art applications of has been presented.

  • Applications of Schiff base-derived heterogeneous systems have also been presented.

Abstract

The ligand-assisted palladium (Pd)-catalyzed Suzuki–Miyaura cross-coupling reaction is one of the most attractive protocols in organic chemistry and phosphines have been established as the best ligand system for this transformation. However, these phosphines have significant limitations, such as high toxicity, sensitivity to air and moisture, handling problems, and high costs. Recently, Schiff bases have been recognized as excellent alternatives to phosphines in Suzuki–Miyaura reactions. Similar to phosphines, the steric and electronic characteristics of Schiff bases can be manipulated by selecting suitable condensing aldehydes and amines. Many Schiff base-derived homogeneous and heterogeneous Pd catalysts have been reported for Suzuki–Miyaura reactions and this review provides insights into the state-of-the-art in applications of these Schiff base-derived Pd catalysts in the Suzuki–Miyaura reaction.

Introduction

Palladium-mediated cross-coupling reactions, such as those reported by Suzuki–Miyaura [1], [2], Heck [3], [4], Stille [5], [6], Kumada [7], Negishi [8], [9], and Sonogashira [10], [11], and others [12], have revolutionized the chemical industry related to the synthesis of various natural products, pharmaceuticals, agrochemicals, and other materials. Among the various cross-coupling methods that are available, the reactions between aryl halides and arylboronic acids, the so-called Suzuki–Miyaura reaction (Scheme 1), is the most extensively studied in organic chemistry due to its operational simplicity, excellent functional group tolerance, easy accessibility, and the environmental friendly properties of the starting reagents, which are some of the key factors responsible for increased research into this particular reaction [13], [14], [15]. Since the first report of this reaction in 1979 [1], [2], there has been an exponential increase in the number of related studies. Thus, thousands of Pd catalysts (both homogeneous and heterogeneous) with various ligand systems can be used to perform this coupling reaction [13], [16], [17], [18], [19], [20], [21], [22], [23]. In some cases, particularly when using aryl bromides or iodides as substrates, the reaction can be performed under extremely mild conditions with a short reaction time and a very low Pd loading. It should be noted that the usual trend in the activation of aryl halides in the Suzuki–Miyaura or other Pd-catalyzed cross-coupling reactions follows the order of: R  I > R  Br  R  Cl [22], [24], [25]. The low reactivity of aryl chlorides is due to the high Csingle bondCl bond strength [24], [25]. In general, the activation of aryl chlorides requires severe reaction conditions with a relatively high Pd loading (up to 10 mol%) [27], [28]. However, using appropriate ligand systems, aryl chloride activation can even occur at room temperature with a low loading of Pd [28], [29], [30], [31], [32], [33].

The Pd-based systems are the most extensively studied catalytic systems for Suzuki–Miyaura reactions, but in the past two decades, there has also been significant progress in cross-coupling reactivity using other metals, such as Au [34], [35], Ni [36], [37], [38], Cu [39], [40], and Fe [41], [42]. From the perspectives of cost and accessibility, the first row transition metals are more attractive than Au- or Pd-based systems. However, this issue is outside the scope of the current review, so it is not discussed further.

Section snippets

Effects of ligands in the Suzuki–Miyaura reaction

It is known that in transition metal-based homogeneous catalysis, the activity and selectivity of a catalytic system is controlled by the characteristics of the ligand attached to the metal, and thus selecting an appropriate ligand is crucial for the catalyst's success. Some reports [43], [44], [45] have described ligand-free Suzuki–Miyaura reactions using simple Pd salts such as [Pd(OAc)2], [Pd(dba)2], and PdCl2 as catalysts, but these metal salts often act as precursors for Pd nanoparticles

Schiff bases and their complexes with palladium for the Suzuki–Miyaura reaction

Schiff bases were first discovered by Schiff [109]. Subsequently, Schiff bases have been explored intensively as ligands and they have played important roles in the development of coordination chemistry and catalysis. Schiff bases are typically prepared via condensation of a primary amine with an aldehyde or ketone. The presence of a dehydrating agent such as MgSO4 usually favors Schiff base formation [86]. The common structural feature of these compounds is an imine group, which is generally

Schiff base-derived heterogeneous catalysts

The first part of this review focused on Schiff base-derived homogeneous catalysts for the Suzuki reaction and some excellent catalytic systems were discussed. However, the main problem associated with these homogeneous catalysts is their separation and subsequent reuse. Thus, many efforts have been made to graft homogeneous Pd compounds into/onto various supports via different immobilization techniques. Among these strategies, the most widely used is the Schiff base condensation approach,

Conclusions

In recent decades, the number of reports of ligand-mediated Pd-catalyzed Suzuki–Miyaura cross-coupling reactions has grown in an exponential manner. Among the various ligands described, phosphines have been established as the best system for this transformation. The main advantage of phosphines is that their steric and electronic properties can be manipulated easily by changing the functional groups attached to the phosphorus atoms. However, despite their success, phosphines are not the most

Acknowledgments

DST, New Delhi (Project No: EMR/2015/000021), and Vienna University of Technology (for a visiting fellowship). We also thank Esther Knittl for language editing.

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