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

Abstract

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.