Palladium Supported on Silk Fibroin as Efficient Catalyst for Sonogashira Coupling and Sonogashira‐Cacchi Type Annulation

A palladium catalyst supported on silk fibroin (Pd/SF) has been tested in the Sonogashira coupling reactions of a wide range of aryl iodides and terminal alkynes. The best catalytic performance was obtained at 90 °C using only 0.25 mol % metal loading under copper-free conditions, in the presence of a H2O/EtOH (3 : 2 v/v) solvent mixture and triethylamine as the base, affording the coupling products in good yields. The Pd/SF-based protocol was then successfully extended to the synthesis of benzofurans through Sonogashira-Cacchi type annulation of 2-iodophenol with terminal alkynes. Preliminary investigations on the recyclability and the heterogeneous behaviour of the catalyst were performed: hot filtration test and leaching test evidenced the presence of a small amount of soluble active palladium species into the reaction mixture. Nevertheless, Pd/SF could be reused in recharging test and it can be recycled in four consecutive runs with no significant deactivation.