Recyclable Palladium Catalysts for the Heck/Sonogashira Reaction under Microwave‐assisted Thermomorphic Conditions

Abstract

AbstractThe reaction of allyl palladium(II) chloride dimer and 4,4′‐bis(RfCH2OCH2)‐2,2′‐bpy, 1a–b, in the presence of AgOTf resulted in the synthesis of cationic palladium complex, [Pd(η3‐allyl)(4,4′‐bis‐(RfCH2OCH2)‐2,2′‐bpy)](OTf), 2a–b where Rf = C9F19 (a), C10F21 (b), respectively. The reaction of [PdCl2(CH3CN)] or K2PdCl4 with 1b, gave rise to the synthesis of [PdCl2(4,4′‐bis‐(C10F21CH2OCH2)‐2,2′‐bpy)], 3b. The quantitatively determined solubility curves of 2a–b and 3b in DMF indicated dramatic increase of solubility for 2a–b and 3b from −40 to 90 °C. The catalyst‐recoverable Pd‐catalyzed Heck/Sonogashira reactions were successfully achieved in DMF with microwave‐assistance. The cationic Pd‐catalyzed Heck arylation of methyl acrylate was selected to demonstrate the feasibility of recycling 2a–b using DMF as a solvent under microwave‐assisted thermomorphic conditions. At the end of each cycle, the product mixtures were cooled, and then the catalysts were recovered by decantation. The Heck arylation catalyzed by 2b under microwave‐assisted mode exhibited good recycling results favoring the trans product. To our knowledge, this is the first example of cationic Pd‐catalyzed Heck arylation under microwave‐assisted thermomorphic conditions. Additionally, recoverable 3b‐catalyzed Sonogashira reactions were also achieved successfully in DMF. The reactions under microwave‐assistance gave much better results in yield and in efficiency than that under conventional thermal heating.