Highly Active Pd-PEPPSI Complexes for Suzuki-Miyaura Cross-coupling of Aryl Chlorides: an Investigation on the Effect of
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doi: 10.1007/s40242-019-9222-2
Highly Active Pd-PEPPSI Complexes for Suzuki-Miyaura Cross-coupling of Aryl Chlorides: an Investigation on the Effect of Electronic Properties ZHANG Yingying, HAN Fangwai, ZHANG Mengyao, ZHANG Huixin, LI Ying, WANG Ru, ZENG Yongfei* and LIU Guiyan* Tianjin Key Laboratory of Structure and Performance for Functional Molecules, Key Laboratory of Inorganic-organic Hybrid Functional Material Chemistry, Ministry of Education, College of Chemistry, Tianjin Normal University, Tianjin 300387, P. R. China Abstract Three new Pd-pyridine enhanced precatalyst preparation stabilization and initiation(PEPPSI) complexes with halogen groups on the N-heterocyclic carbene and pyridine were prepared. Their structures have been clearly characterized by nuclear magnetic resonance spectroscopy and X-ray single-crystal diffraction. The effects of the electronic properties of halogen groups on the catalytic activity in the Suzuki-Miyaura cross-coupling of aryl chlorides were investigated. These Pd-PEPPSI complexes could catalyze the cross-coupling reaction efficiently with a low catalyst loading(0.05%, molar ratio) at room temperature and the products were obtained in high yields. Keywords Pd-pyridine enhanced precatalyst preparation stabilization and initiation(PEPPSI) complex; Suzuki-Miyaura cross-coupling reaction; Aryl chloride
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Introduction
for the Suzuki-Miyaura reaction of aryl chlorides were reported by Organ et al. in 2006[23]. On this basis, other three PdPEPPSI complexes(1d―1f) were also been synthesized by introducing sterically groups(i-Pent, c-Pent, or i-Bu) onto the ortho-positions of the N-aryl moieties of the NHCs, which were efficient to catalyze the tetra-ortho-substituted
Palladium-catalyzed Suzuki-Miyaura cross-coupling(SMC) reaction as a versatile method for forming carbon-carbon bonds are widely utilized in scientific research and industrial production[1,2]. Typically, the introduction of ligands in SMC reaction usually improves catalytic efficiency. So, the development of highly active ligands for SMC reactions becomes important. In 1991, Ardunego et al.[3] isolated the free N-heterocyclic carbene(NHC) for the first time. From then on, NHCs have attracted widespread attention in terms of their better σ-donating properties. For example, in the design and synthesis of catalysts, NHCs are usually preferred because they can stabilize the catalysts easily[4―7]. So, palladium catalytic system is no exception and NHCs have been successfully applied in Pd-catalyzed Suzuki-Miyaura reactions in the past decades[8―12]. However, NHC-Pd-catalyzed SMC reaction of aryl chlorides still faces enormous challenges, such as with low palladium loadings under mild conditions[13]. In recent years, various efforts have been made and various pre-catalysts have been reported[14―22]. Among them, pyridine enhanced precatalyst preparation stabilization and initiation(PEPPSI) palladium complexes showed good catalytic performance[23]. The main Pd-PEPPSI precatalysts reported are shown in Scheme 1. Initially, t
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