Comparative Investigations of Cp*-Based Group 9 Metal-Catalyzed Direct C-H Amination of Benzamides

Abstract

Key mechanistic features of the [Cp*MCl2](2) (M = Ir, Rh, Co; all are in group 9) catalyzed C-H amination of benzamides with organic azides were investigated with a strong emphasis on the metal effects on the reaction mechanism, revealing that the Rh- and Ir-catalyzed reactions follow a similar reaction profile, albeit with different individual kinetic and thermodynamic parameters. The observation that the Ir-based system was much superior in terms of the rates and efficiency in comparison to Rh was attributed to the intrinsically strong relativistic effects in iridium. While a cobalt system [Cp*Co-III] showed little catalytic activity for most azides examined, plausible [(BA)(Cp*)CoNR](+) intermediates of these reactions were characterized as a "Co(III)-nitrenoid radical" species with a weak ("one electron-two center type") Co-NPh bond. Its Rh and Ir analogues are characterized as diamagnetic metal nitrenoids with a strong M=NR double bond. The provided experimental and computational investigations indicate that the rate-limiting step of the reaction resides in the final stage (protodemetalation) that takes place via a concerted metalation-deprotonation (CMD) mechanism. While experimental measurements of thermodynamic parameters were in good agreement with DFT calculations, theoretical predictions on the electronic nature of key intermediates and energy barriers were successfully used to rationalize the experimentally observed reactivity pattern.