Recoverable cationic Pd-catalyzed Heck reaction under thermomorphic mode

The reaction of allylpalladium chloride dimer and 4,4′-bis(RfCH2OCH2)-2,2′-bpy, 1a–c, in the presence of AgOTf resulted in the synthesis of cationic complex, [Pd(η3-allyl)(4,4′-bis-(RfCH2OCH2)-2,2′-bpy)](OTf), 2a–c where Rf = C9F19 (a), C10F21 (b), C11F23 (c), respectively. The solubility curves of 2a–c which began from virtually zero below −40 °C and increased dramatically at slightly higher temperature were quantitatively measured. Upon changing from −40–90 °C there was several orders of magnitude increase of solubility for 2a–c. The cationic Pd-catalyzed Heck arylation of methyl acrylate was selected to demonstrate the feasibility of recycling usage with 2b,c as the catalyst using DMF as the solvent at 140 °C for 3 h in each run. At the end of each cycle, the product mixtures were cooled to −30 °C, and then catalysts were recovered by decantation. The products were quantified by GC analysis or by the isolated yield. The Heck reaction of C6H5I with methyl acrylate could be catalyzed by 2b with good recycling results for a total of 15 times and also with a 100% selectivity favoring the trans product. To our knowledge, this is the first example of cationic Pd-catalyzed Heck reaction under the thermomorphic mode.

Graphical abstractComplex 2b,c soluble in polar organic solvents at >60 °C but insoluble at −30 °C, were demonstrated as catalysts in the cationic Pd-catalyzed Heck reaction mainly under the thermomorphic mode and recovered for reusage.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Solubility curve of 2a–c was measured and increased greatly with temperature. ► Cationic Pd-catalyzed Heck arylation demonstrates feasibility of recycling usage. ► The first example of cationic Pd-catalyzed Heck reaction under thermomorphic mode.