Abstract
The catalyst activities of various heterogeneous palladium catalysts supported by anatase-, rutile- and brookite-type titanium oxide for ligand-free Suzuki–Miyaura cross-coupling reactions of aryl chlorides were evaluated. Palladium acetate [Pd(OAc)2], supported on anatase-type titanium oxide (TiO2) via acetonitrile solution impregnation process without reduction [Pd/TiO2 (anatase-type)], demonstrated the highest catalyst activity in comparison to those of other titanium oxide (rutile- or brookite-type) supported Pd(OAc)2 without reduction and reduced Pd/TiO2 (anatase-type) [Pd(red)/TiO2 (anatase-type)]. Various aryl chloride and bromide derivatives were smoothly coupled with arylboronic acids including heteroarylboronic acids in the presence of 5–10 mol% Pd/TiO2 (anatase-type) without the addition of any ligands. Although the fresh Pd/TiO2 (anatase-type) catalyst was surprisingly comprised of ca. 1:2 mixture of palladium(II) and palladium(0) species according to X-ray photoelectron spectroscopy (XPS), in spite of no reduction process, significant further increment of palladium(0) species was observed during the Suzuki–Miyaura coupling reaction, and Pd/TiO2 (anatase-type) was converted into a catalyst, which contained palladium(0) species as the main component [ca. 1:5 mixture of palladium(II) and palladium(0) species]. Therefore, the reduction via the electron donation process to the palladium(II) species may have occurred during the reaction on anatase-type titanium oxide.
Highlights
We developed heterogeneous Pd catalysts immobilized on a ligand-like tertiary amine polystyrene-divinylbenzene-based polymer for the ligand-free Suzuki–Miyaura coupling reaction of aryl chlorides [39,40]
We prepared and investigated the physical properties of the anatase, rutile- and brookite-type TiO2 -supported Pd catalysts by using X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM), and comprehensively investigated the effect on the catalyst activities depending on the crystal form of TiO2 to the TiO2 -supported Pd-catalyzed ligand-free
Suzuki–Miyaura coupling reaction of aryl hard to reuse, while the fresh catalyst was suitable for the ligand-free Suzuki–Miyaurachlorides
Summary
Palladium (Pd)-catalyzed Suzuki–Miyaura coupling reaction [1,2,3,4,5,6,7,8,9,10,11,12] of aryl halides and arylbolonic acid derivatives is one of the most reliable and useful synthetic methods for constructing biaryl derivatives as fundamental skeletons of various biologically active compounds [2,4,6], pharmaceuticals [2,4] and functional materials [5,6]. Heterogeneous Pd-catalyzed ligand-free Suzuki–Miyaura coupling reactions have attracted attention as environmentally friendly, metal-contamination-free (or low) and cost-effective methods over the past couple of decades [7,8,9,10,11,12,17,18,19]. We developed heterogeneous Pd catalysts immobilized on a ligand-like tertiary amine polystyrene-divinylbenzene-based polymer for the ligand-free Suzuki–Miyaura coupling reaction of aryl chlorides [39,40]. We prepared and investigated the physical properties of the anatase-, rutile- and brookite-type TiO2 -supported Pd catalysts by using X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM), and comprehensively investigated the effect on the catalyst activities depending on the crystal form of TiO2 to the TiO2 -supported Pd-catalyzed ligand-free. The anatase-type TiO2 -supported Pd catalyst was found to have quite an efficient catalyst activity toward the Suzuki–Miyaura coupling reaction, including the coupling of aromatic chlorides due to the quite highly distributed small Pd nanoparticles (1–2 nm) on the anatase-type TiO2
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