Catalytic Activity of Pd(II) Complexes with Triphenylphosphito Ligands in the Sonogashira Reaction in Ionic Liquid Media
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Catalytic Activity of Pd(II) Complexes with Triphenylphosphito Ligands in the Sonogashira Reaction in Ionic Liquid Media I. Błaszczyk • A. M. Trzeciak • J. J. Zio´łkowski
Published online: 14 October 2009 Ó Springer Science+Business Media, LLC 2009
Abstract The reactivity of palladacycle dimeric complexes with substituted triarylphosphito ligands P(OR)3 (R = Ph, m-MeC6H4, o-MeC6H4, C6H3-2,4-tBu2) as well as their non-orthometallated analogues PdCl2[P(OR)3]2 was tested in a copper-free Sonogashira reaction with iodobenzene and phenylacetylene as substrates and imidazolium ionic liquids as the reaction medium. The ionic liquids [bmim][PF6], [bmim][BF4] and [emim][SO4Et] were chosen. The palladium complexes studied showed high activity, and the yield of diphenylacetylene ranged from 31 to 98% in 1 h. The best results were obtained in [bmim][PF6] for PdCl2[P(O-m-MeC6H4)3]2 (84%) and for orthopalladated dimer with the same phosphite (98%). Keywords Palladium Orthometallation Sonogashira Ionic liquids
1 Introduction Recently the Sonogashira reaction has attracted a lot of attention as an efficient way to obtain alkenyl- and arylacetylenes, substituted alkynes, [1] symmetrical diynes, [2] conjugated oligomers and polymers, [3] materials for nonlinear optical and molecular electronics [4]. The Sonogashira coupling is a palladium-catalyzed C(sp2)–C(sp) coupling between aryl or alkenyl halides and terminal alkynes (Fig. 1) [5]. Typical substrates of the Sonogashira reaction are iodoor bromoaryl compounds, also functionalized, that react I. Błaszczyk A. M. Trzeciak (&) J. J. Zio´łkowski University of Wrocław, 14 F. Joliot-Curie Str., 50-383 Wrocław, Poland e-mail: [email protected]; [email protected]
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with phenylacetylene or other alkynes [1, 5–8]. Because of the air sensitivity of most catalysts, coupling reactions are performed under an inert atmosphere, often in polar organic solvents like THF, DMF, [9, 10] DMA, [3] or in water [11, 12]. Recently it has been shown that ionic liquids can also serve as good solvents in Sonogashira reactions [9, 13–18]. When 2 mol% of the palladium catalyst [(bisimidazole)2PdClMe] was used in the most frequently applied ionic liquid, [bmim][PF6], with NEt3 as a base, a yield of 85% was obtained after 2 h [6]. In Sonogashira reaction performed in a microflow system using PdCl2(PPh3)2 in [bmim][PF6] the coupling product was obtained with the yield 96, 80 and 78% in three subsequent runs [13]. The superior performance of [bmim][PF6] among the four ionic liquids screened in reaction of iodobenzene with phenylacetylene catalyzed by Pd-carbene complex was also recognized [14]. Another example is provided by the catalyst precursor PdCl2[P(OPh)3]2, which facilitates the preparation of diphenylacetylene already in room temperature with 35% yield in 1 h. Increasing the temperature to 80 °C, optimal for this reaction, made it possible to obtain 83% of the product in 4 h [15]. However, the ionic liquid should be carefully selected because ionic liquids can act as inhibitors. Such
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