Rhodium Catalysts Based on Phenyl Substituted Cp Ligands for Indole Synthesis via Oxidative Coupling of Acetanilides and Alkynes

Rhodium-catalyzed oxidative coupling of acetanilides and alkynes via C-H activation is the most powerful synthetic tool for producing the indole motif from commercially available precursors. However, this reaction usually requires large catalyst loadings (5 mol% of rhodium). In this study, a 1,2-diphenylcyclopentadienyl ligand-based catalyst was developed that works well at 1 mol% loading of rhodium. DFT calculations of the C-H activation step provided insight into its high catalytic activity. The catalyst efficiency was also demonstrated in the synthesis of naturally occurring isocoumarins, such as polygonolide, tubakialactone B and penicimarine F. The developed catalytic protocols tolerate a wide range of functional groups, for example, halide, nitro, hydroxy, and alkoxy.