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

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AbstractThe effect of substituents on carbamoylmethyl‐cyclopentadienyl (CpA) 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 CpA ligand (CpA3) showed high catalytic activity and improved the regioselectivity as compared to a commercially available Cp*RhIII complex. On the other hand, in reactions with internal alkynes, a rhodium(III) complex with a diphenyl‐substituted CpA ligand (CpA1 or CpA2) showed high catalytic activity, which is comparable to the activity of the Cp*RhIII complex. Interestingly, a rhodium(III) complex with an electron‐deficient di(ethoxycarbonyl)‐substituted Cp ligand (CpE) 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 CpA and CpE ligands to the rhodium(III)‐catalyzed C−H bond functionalization reactions.