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

The 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(eta(3)-ally1)(4,4'-bis(RfCH2OCH2)-2,2'-bPY)J(OTf), 2a-b where R-f = 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 degrees 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 thennal heating.