Pd-PEPPSI-IPentCl: a new highly efficient ligand-free and recyclable catalyst system for the synthesis of 2-substituted indoles via domino copper-free Sonogashira coupling/cyclization

Abstract

A pyridine-containing decidedly resourceful Pd-N-heterocyclic carbene complex, Pd-PEPPSI-IPent^Cl (PEPPSI = pyridine enhanced precatalyst preparation, stabilization, and initiation), is used as a first class recyclable catalytic system for the synthesis of 2-substituted indoles via domino copper-free Sonogashira coupling/cyclization. The catalyst showed a greater performance in the cascade reaction of various 2-bromo anilines with different terminal acetylenes under mild (60 °C) and green conditions (ethanol : water) even in the absence of a copper catalyst and an inert atmosphere. The catalyst is used for the first time in these reactions. The findings suggest that 0.1 mol% of the catalyst is sufficient, and that the catalyst is recyclable and can be reused for up to six cycles.