Use of “Homeopathic” Ligand‐Free Palladium as Catalyst for Aryl‐Aryl Coupling Reactions

Abstract

Abstract:We have previously shown that the use of ligand-free palladium employing Pd(OAc) 2 as catalyst precursor in the Heck reaction of aryl bromides is possible if low catalyst loadings, typically between 0.01 -0.1 mol % are used. We have now tested this phenomenon, which we have dubbed "homeopathic" palladium, in biaryl formation using the Suzuki, the Negishi and the Kumada cross-coupling reactions. The Suzuki reaction of aryl bromides, both activated and deactivated, is possible using 0.02 -0.05 mol % of Pd(OAc) 2 . In this reaction turnover frequencies up to 30,000 have been reached with activated substrates. Even aryl chlorides could be reacted if strongly electron-withdrawing substituents were present. The Negishi coupling with a variety of arylzinc halides was possible on aryl bromides containing electronwithdrawing substituents. The Kumada reaction only gave low yields of products under "homeopathic conditions.

Palladium-Catalyzed Suzuki Reaction in Aqueous Media

“…de Vries 等 [93] 报道了一个在 N-甲基吡咯烷酮(NMP)/ 水混合溶液中 Pd(OAc) 2 催化的 Suzuki 反应体系. 催化 剂用量为 0.02 mol%, 碱为 K 2 CO 3 , 在 90 ℃条件下反应 4 h, 可催化芳基溴代物.…”

Synthesis of 4-Methoxybiphenyl Using Pd-Containing Catalysts Based on Polymeric Matrix of Functionalized Hypercrosslinked Polystyrene

“…Pd/C, which is simple in preparation and can be easily separated from the reaction mixture, is the most well-known ligandless catalyst [2][3][4][5]. Palladium salts [1,6] can be also attributed to the ligandless catalysts of the Suzuki reaction. The main issue with ligandless catalysts, however, is irreversible leaching of palladium and a change of particle size distribution due to an aggregation process.…”

Catalytic Applications of Metal Nanoparticles in Imidazolium Ionic Liquids

“…[88] Well-defined and in-situ-prepared Pd 0 nanoparticles in ionic liquids have been reported to promote CÀC coupling reactions such as Heck and Suzuki processes (Table 1, entries 6-9). In most cases, however, the Pd 0 nanoparticles are simple reservoirs of homogeneous Pd 0 catalytically active species akin to those observed with phosphane-free palladi- [89][90][91] such as simple Pd II compounds [92] or palladacycles. [93,94] It is highly probable that in these cases the Heck coupling reaction proceeds through the oxidative addition of the aryl halide on the nanoparticles surface, and the oxidized Pd species thus formed are detached from the surface and enter into the main catalytic cycle.…”