Development of Titanium Dioxide-Supported Pd Catalysts for Ligand-Free Suzuki–Miyaura Coupling of Aryl Chlorides

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.

The Direct Mechanocatalytic Suzuki–Miyaura Reaction of Small Organic Molecules

“…These observations are in accordance with the results of Sajiki et al, who investigated the mechanism of the heterogeneous catalyzed Suzuki reactions on Pdclusters. [26] We further monitored the reaction kinetics of our reaction by synchrotron in situ X-ray diffraction (XRD) (cf. Figure 2, Supporting Information Chapter 4.2) and followed the reaction kinetics.…”

Enhancing the Catalytic Activity of Palladium Nanoparticles via Sandwich-Like Confinement by Thin Titanate Nanosheets

“…The Pd 3d 5/2 signal of L-titanate@Pd@L-titanate was found at a binding energy (BE) of 335.6 eV, which was significantly shifted from the 335.0 eV reported for bulk Pd . BEs of Pd nanoparticles deposited on TiO 2 range from 334.6 to 335.5 eV. − These reported shifts in comparison to the value of bulk Pd might be ascribed to electronic metal–support interactions. Along this line, the shift to higher BE observed for L-titanate@Pd@L-titanate would indicate a slightly positively charged species of Pd δ+ (δ < 1). , A shift to higher BE of small metal nanoparticles can, however, also originate from final state effects or an ill-defined reference level.…”

Development of Facile and Simple Processes for the Heterogeneous Pd-Catalyzed Ligand-Free Continuous-Flow Suzuki–Miyaura Coupling

“…It is utilized for the production of (hetero)biaryl structures, which are fundamental components of active pharmaceutical ingredients (APIs) [4,[8][9][10][11][12][13][14], functional materials [15], and natural products [16,17] in a variety of synthetic chemistry fields. The development of a highly active and durable heterogeneous Pd catalyst is important for minimizing toxic metal contamination in the products and reducing environmental pollution caused by metal leaching during the catalytic cycle in transition-metal-catalyzed coupling reactions and treatment after the reactions [18][19][20][21][22].…”