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

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*CoIII] 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.