Sonogashira C–C coupling reaction in water using tubular reactors with catalytic metal inner surface

A new reaction setup equipped with micro tubular reactor with thin catalytic metal inner surface (Pd or Pd–Cu alloy) was developed for the reactions in high-pressure and high-temperature water (HPHT-H2O) flow. To evaluate the catalytic performance of this micro tubular reactor, Sonogashira C–C coupling of iodobenzene and ethynylbenzene was investigated by using this system under alkaline aqueous medium. Under the optimized reaction conditions of 250 °C and 16 MPa of pressure, diphenylethyne was obtained with good yield and 100% selectivity within a very short residence time of ∼1.6 s. The reaction proceeds without any additional promoters, additives or organic solvents. Leaching of catalytic metal from the reactor tube was not found under the studied reaction conditions. Alloying of Pd with Cu remarkably improved the efficiency of the reaction due to co-catalytic effect that is related to the easy electron transfer between two metals.