C−H Activation of Phenyl Imines and 2-Phenylpyridines with Cp*MCl22 (M = Ir, Rh): Regioselectivity, Kinetics, and Mechanism

Sodium acetate promoted C−H activation in a series of para-substituted phenyl imines has been examined using Cp*MCl 2 2 (M = Ir, Rh). The regioselectivity was investigated using a series of meta-substituted phenyl imines (−OMe, −CH 3, −F, −COOMe, −CF 3, and −CN) and 2-phenylpyridines (−OMe, −CH 3, and −CF 3 ). It was found that substrates with electron-donating substituents react significantly faster than substrates with electron-withdrawing substituents, which is consistent with an electrophilic C−H activation mechanism. It was also found that the regioselectivity of the C−H activation was extremely sensitive to steric effects, with a meta methyl group leading to only one regioisomer. Solvent and temperature studies showed that the reaction rate can be increased both by increasing temperature and by using polar solvents such as methanol. The regioselectivity was solvent dependent for the reaction with Cp*IrCl 2 2 but independent of solvent for the reaction with Cp*RhCl 2 2 . The regioselectivity was temperature independent for both metals. With added acid, the aromatic C−H activation was shown to be reversible. Kinetic studies were performed, leading to the conclusion that Cp*M(OAc) + is the key catalytic species responsible for the electrophilic C−H activation.