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

The effect of substituents on carbamoylmethyl-cyclopentadienyl (Cp-A) ligands on the neutral rhodium(III)-catalyzed oxidative [4+21 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(lll) complex with a dimethyl-substituted Cp-A ligand (CPA3) 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(lll) complex with a diphenyl-substituted Cp-A ligand Cp m or Cp A2 ) showed high catalytic activity, which is comparable to the activity of the Cp*Rh w complex. Interestingly, a rhodium(lll) 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(lll)-catalyzed C-H bond functionalization reactions.