A facile iron catalyzed cross-coupling reaction under micro-flow conditions
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A facile iron catalyzed cross-coupling reaction under micro-flow conditions Koji Machitani 1 & Yoshie Tanaka 2 & Yasuhiro Nishiyama 1 & Akira Fujii 1 & Akane Saito 1 & Hajime Mori 1 Received: 25 February 2020 / Accepted: 18 March 2020 # Akadémiai Kiadó 2020
Abstract The iron-catalyzed cross-coupling reaction between an aromatic Grignard reagent and a haloalkane was investigated under micro-flow conditions. The reaction proceeded smoothly by the separate feeding of the substrate solutions to give the desired cross-coupling product. Compared to the previously reported batch reaction, the effect of additives was more predominant in the micro-flow reaction. The flow reactor system appears to contribute to the stabilization and rapid trapping of the unstable catalytic species. Keywords Microreactor . Cross-coupling . Iron . Grignard reagent
Introduction Iron-catalyzed organic reactions have received significant attention in recent years due to the excellent availability of iron deposits, in addition to the low cost and low toxicity of iron compared with rare metal catalysts [1]. In 2004, independent reports into the iron-catalyzed cross-coupling reaction between aromatic Grignard reagents and haloalkanes were published [2–4]. More specifically, they showed that the lowvalent iron complexes [Li(tmeda)]2[Fe(C2H4)] [3], Fe(acac)3 [4], and FeCl3 [2] were effective catalysts in the crosscoupling reactions. Of the reported methods, we focused on that of the Nakamura group [2] due to the low reagent costs, the use of common solvents and the use of only minimal quantities of the Grignard reagent. This reaction enabled the synthesis of aromatic compounds without the requirement for a Pd catalyst. However, although some aromatic compounds were obtained in good yields, it was necessary to strictly control the temperature and ensure that the Grignard reagent was
* Koji Machitani [email protected] * Hajime Mori [email protected] 1
Industrial Technology Center of Wakayama Prefecture 60, Ogura, Wakayama-shi 649-6261, Japan
2
Wakayama Medical University, 580 Mikazura, Wakayama-shi 641-0011, Japan
added slowly, with large amounts of additives also being required to improve the catalyst activity. Thus, to address these issues and improve the substrate scope of the reaction, a number of systems have been reported [5–9]. For example, Bedford described a simple cross-coupling reaction based on the use of catalytic amounts of additives [5]. They also studied various types of ligands and reported that a carbene-based precatalyst and an iron catalyst gave excellent results [6]. In addition, Yamaguchi showed that iron (II) complexes bearing a bipyridine ligand were effective for the cross-coupling reaction [7]. Although these reported methods were useful for the synthesis of aromatic compounds, a number of drawbacks remained, including the requirement for solvent exchange from CH2Cl2 to Et2O, the use of excess Grignard reagent, and the requirement for a particular ligand. It was therefore suggested that specific con
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