An efficient and recyclable water-soluble cyclopalladated complex for aqueous Suzuki reactions under aerial conditions

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Abstract

Several water-soluble cyclopalladated complexes with five- or six-membered rings have been prepared as air-stable solids from Schiff base ligands bearing an N-phenyl sulfonate groups. Cyclopalladated complexes with six-membered rings show high catalytic efficiency for the Suzuki reactions of aryl bromides with phenylboronic acid in aqueous solvents under mild conditions. Palladium complex 1 can be used for five reaction cycles in high conversions for the Suzuki reactions in neat water without additives. The catalytic process for the Suzuki couplings is proved by TEM analysis to proceed on Pd(0) nanoparticles. Surfactant-protected palladium nanoparticles present lower activities and poorer recyclability for the coupling reactions than those generated in situ without additives.

Graphical abstract

A new water-soluble cyclopalladated complex was proved to be an efficiently recyclable catalyst precursor for Suzuki reactions in neat water under aerial conditions without additives.

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Introduction

In the past two decades, the palladium-catalyzed Suzuki reaction of aryl halides with arylboronic acids has evolved into one of the most important and powerful methods to form biaryls [1], [2], [3], [4], [5], [6], [7], [8]. Under increasing environmental awareness, water is an attractive alternative to traditional organic solvents because it is inexpensive, nonflammable, nontoxic, and environmentally sustainable. Since Casalnuovo’s initial report of TPPMS (triphenylphosphine monosulfonate)/Pd(OAc)2 catalyst system for cross coupling reactions in aqueous solvents [9], Suzuki reaction in aqueous phase including neat water and water–organic mixed solvents has been received much more attention [10], [11], [12], [13], [14], [15].

Recently, cyclopalladated complexes have attracted much attention as exciting catalyst precursors to cross coupling reactions [16], [17], [18], [19], [20]. The first water-soluble cyclopalladated imine complex reported by Ryabov was applied as catalyst for ester hydrolysis [21]. The use of the hydrophilic cyclopalladated complexes in water is expected to perform as a recyclable and highly active catalyst system for the Suzuki reaction in aqueous solvents. There have been several examples of hydrophilic cyclopalladated complexes or Pd pincer complexes (these so-called pincer complexes are usually denoted as Pd-ECE, such as Pd-NCN, Pd-PCN or Pd-SCS-pincer complex, where E is a neutral two-electron donor atom, and C represents the anionic carbon atom) reported as precatalysts for aqueous Suzuki reactions [22], [23], [24], [25], [26], [27], [28], [29], [30]. The simple Pd-oximed complexes with a hydroxy substituent on the aromatic rings reported by Nájera are highly active catalysts for Suzuki couplings of aryl bromides, activated aryl chlorides or heteroaromatic chlorides [22], [23]. Pd-NCN and Pd-PCN pincer complexes reported by SanMartin and his co-workers were employed as good catalyst for Suzuki couplings in neat water, although these two precatalysts were not exactly water soluble [24], [25]. A hydrophilic Pd-CNC pincer complex was proved to be a highly efficient recyclable homogeneous catalyst for Suzuki couplings, and the reaction could be repeated up to five times [26]. Additionally, the polymer-supported SCS-pincer complexes [27], [28] and the oximed cyclopalladated complex attached silica [29] were also used as recyclable heterogeneous catalysts for Suzuki couplings, and it was suggested that the nanopalladium generated gradually in situ was the active catalytic species for the reactions. Combined with a water-soluble phosphine ligand (t-Bu-Amphos), cyclopalladated imine complex with sulfonate groups could be efficiently used for coupling reaction of 4-bromotoluene with phenylboronic acid in eleven reaction cycles, but those Pd complexes were not efficient precatalysts on their own [30]. As one of the most popular classes of cyclopalladated derivatives, a cyclopalladated imine complex has proved to be efficient for Suzuki reactions in aqueous solvent with TBAB (tetrabutylammonium bromide) additive [31]. However, no cyclopalladated imine complexes have been reported as efficient and recyclable catalytic precursors for Suzuki reaction in neat water without supports or additives.

Herein, we report several water-soluble cyclopalladated imine complexes and its catalytic performance toward Suzuki reactions in aqueous phase. It was found that the chelated structure of Pd complexes were crucial for the catalytic efficiency. The new complex 1 with six-membered rings is proved to be a highly efficient and recyclable catalyst precursor for the Suzuki reactions of aryl bromides in neat water without additives.

Section snippets

Preparation of ligands and complexes

Condensation of the sodium salt of 3,5-dialkyl sulfanilic acid (or 2,6-dialkyl aniline) with anthracene-9-carbaldehyde in anhydrous methanol yielded the desired Schiff base ligands. Then the hydrophilic cyclopalladated complexes (1 and 2) were prepared by a traditional C–H activation reaction of the sulfonated Schiff base ligands with Li2PdCl4 in the presence of sodium acetate in anhydrous methanol (Scheme 1). It has been proved that the weakly coordinating sulfonate anion does not strongly

Conclusions

The water-soluble cyclopalladated complex 1 with six-membered rings has been proved to be a highly efficient and recyclable catalyst precursor for Suzuki reactions of aryl bromides in neat water. TEM analysis suggests an active species of Pd(0) nanoparticles for the Suzuki reactions. Surfactants or excess of phenylboronate can occupy active free sites of palladium nanoparticles and result in lower catalytic activity.

Materials

Methanol and dichloromethane were purified by standard methods. The sodium salts of 3,5-dialkyl sulfanilic acid [38], [39] and the benzaldehyde imine ligands [30] were prepared by modifications of literature procedures. All other reagents were used as received from commercial sources.

Apparatus

GC yields were determined by comparison to an internal standard. 1H NMR and 13C NMR were recorded on a Bruker 300 MHz. TEM analysis was carried on by a JEM 100-CX II.

Preparation of ligand 1

Anthracene-9-carbaldehyde (0.22 g, 1.07 mmol) and

Acknowledgements

This work was supported by NSFC No. 20372042, Shandong Scientific Plan 032090105 and the Science Foundation of Shandong Y2006B18.

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