Short CommunicationAmphiphilic ionic palladium complexes for aqueous–organic biphasic Sonogashira reactions under aerobic and CuI-free conditions
Graphical abstract
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.
Section snippets
Synthesis and characterization of 1, 2 and 3
At room temperature, 2 and 3 were synthesized through exchanging the cation of NH4+ in 1 [30] by the imidazolium-based cations, which were fully characterized by 1H NMR, 31P NMR, CHN-elemental analysis, and single-crystal X-ray analysis. The molecular structures of 2 and 3 were depicted in Fig. 1, which were both composed of the imidazolium-based cations and the Pd-complex anions. The Pd-complex anion possesses the square-planar geometry which is similar to that in PdCl2(PPh3)2 [31]. The Pd
Conclusions
The ionic Pd(II)-complexes of 2 and 3 were synthesized through exchanging the cation of NH4+ in 1 by the imidazolium-based organic moieties. The difference of the counter-cations could dramatically change the properties of the corresponding Pd-complexes of 1–3, in terms of the aqueous solubilities and the catalytic behaviors in water, respectively. 2 and 3 were featured with the amphiphilic character due to the dually presence of the lipophilic imidazolium-based cation and the hydrophilic anion
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (Nos. 21273077 and 21076083).
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