Amphiphilic ionic palladium complexes for aqueous–organic biphasic Sonogashira reactions under aerobic and CuI-free conditions

Highlights

• Syntheses of the water-soluble ionic complexes.

• The amphiphilic palladium complexes for aqueous-organic biphasic Sonogashira reactions.

• Sonogashira reactions under aerobic and CuI-free conditions.

Abstract

The ionic Pd(II)-complexes of 2 (ammonium-[1-(2-hydroxyethyl)-3-methylimidazolium] bis[3-(diphenylphosphino)benzenesulfonate]-dichloropalladium(II) ([(NH₄)(Hemim)][PdCl₂(TPPMS)₂])) and 3 (bis[1-n-butyl-3-methylimidazolium] bis[3-(diphenylphosphino)benzenesulfonate]-dichloropalladium(II) ([Bmim]₂[PdCl₂(TPPMS)₂])) were synthesized and fully characterized. The single crystal X-ray diffraction analyses show that 2 and 3 are composed of the imidazolium-based cations and [PdCl₂(TPPMS)₂]^2 − anions. The properties of such imidazolium-based Pd-complexes of 2 and 3, in terms of the aqueous solubilities and the catalytic behaviors in water, could be dramatically varied. When 2 and 3 were applied as the precatalysts for the Sonogashira coupling of iodobenzene with phenylacetylene under aerobic and CuI-free conditions, the much higher yields of 1,2-diphenylethyne were obtained due to their amphiphilicity. The wide generality of 2 was available for aqueous–organic biphasic Sonogashira reactions.

Introduction

Palladium-catalyzed Sonogashira reaction is a powerful tool for the formation of Csp²Csp bond, and the most important method for the synthesis of internal acetylenes [1]. In a typical Sonogashira procedure, the reaction is conducted using a phosphine-ligated palladium complex with a catalytic amount of copper(I) iodide as a co-catalyst in the presence of base under inert atmosphere [1], [2]. However, the in situ formed copper(I) acetylides with moisture- and air-sensitivity induce unwanted homocoupling products of terminal alkynes through Glaser reaction [2], [3], [4]. In the last few decades, a variety of efficient palladium catalysts containing hindered phosphines [5], [6], [7], [8], [9], palladacycles [10], [11] and N-heterocylic carbene (NHC) [12], [13], [14], [15] have been developed for the copper-free Sonogashira reactions, which allow the manipulation of the Sonogashira reaction with low catalyst loadings under copper-free conditions and without exclusion of air and moisture. The absence of copper also makes the Sonogashira reactions possibly be performed in water medium [14], [15], [16].

Aqueous organometallic catalysis (AOC) by water-soluble phosphine complexes has been developed into a most successful way for product isolation and/or catalyst recycling [17], [18], [19]. The solubility of the organometallic catalysts in water is determined by the solubility of the ligands, and the sulfonated phosphines are generally the most important ones in AOC [20], [21], [22]. Typically, under aqueous–organic biphasic condition in AOC, the addition of a phase-transfer catalyst, such as surfactant [23], [24] or cyclodextrin [25], [26], is required in order to eliminate the serious mass transfer limitation observed with hydrophobic substrates. The use of amphiphilic phosphines has recently been found to be a promising alternative to suppress the mass transfer factor because the amphiphilic phosphines incorporate the surface-active property and the coordinating ability to the metal into the same compound [27], [28], [29]. Unfortunately, the syntheses of amphiphilic phosphines are often time-consuming, laborious and expensive. The activities in the functionalization of ionic liquids in our group highlight us to replace the typical counter-cation of Na⁺ or NH₄⁺ in TPPMS by the imidazolium-based cation, which can provide amphiphilicity via incorporating lipophilic organic moieties. On the basis of this conception, herein, the amphiphilic TPPMS-ligated Pd complexes of 2 and 3 were synthesized (see Supplementary Information) and fully characterized (Scheme 1), which were further studied as the precatalysts for aqueous–organic biphasic Sonogashira reactions under aerobic and CuI-free conditions.