Functionalized Cyclopentadienyl Ligands and Their Substituent Effects on a Rhodium(III)‐Catalyzed Oxidative [4+2] Annulation of Indole‐ and Pyrrole‐1‐Carboxamides with Alkynes

Abstract The effect of substituents on carbamoylmethyl‐cyclopentadienyl (Cp A ) ligands on the neutral rhodium(III)‐catalyzed oxidative [4+2] annulation of indole‐ and pyrrole‐1‐carboxamides with alkynes, in which the C−H bond cleavage is the partially rate‐limiting step, was investigated. As a result, in the reactions with terminal alkynes, a rhodium(III) complex with a dimethyl‐substituted Cp A ligand (Cp A3 ) showed high catalytic activity and improved the regioselectivity as compared to a commercially available Cp*Rh III complex. On the other hand, in reactions with internal alkynes, a rhodium(III) complex with a diphenyl‐substituted Cp A ligand (Cp A1 or Cp A2 ) showed high catalytic activity, which is comparable to the activity of the Cp*Rh III complex. Interestingly, a rhodium(III) complex with an electron‐deficient di(ethoxycarbonyl)‐substituted Cp ligand (Cp E ) that shows high catalytic activity toward the annulation of benzamides with internal alkynes, in which the C−H cleavage is rate‐limiting, showed low catalytic activity toward the reactions with both terminal and internal alkynes. The ligand effects above provide important guidelines for the application of our Cp A and Cp E ligands to the rhodium(III)‐catalyzed C−H bond functionalization reactions.