Effects of Additives on Catalytic Arene C–H Activation: Study of Rh Catalysts Supported by Bis-phosphine Pincer Ligands

Hydrogen–deuterium exchange (H/D exchange) is a method commonly used for studying catalytic activation of C–H­(D) bonds by transition metal complexes. In this study, a series of additives were studied for H/D exchange of toluene-d 8 with acetic acid (HOAc) using (RPNP)­Rh­(X) complexes (R = phosphine substituents including cyclohexyl, isopropyl, and tert-butyl; X = trifluoroacetate or acetate) as the precatalysts. Cu­(OAc)2 and AgOAc additives were found to benefit Rh-mediated C–H­(D) activation of toluene with meta–para selectivity by facilitating the conversion to active (RPNP)Rh species and stabilizing the Rh catalysts from decomposition to inactive Rh(s). In contrast, nonoxidizing Lewis acid additives, such as B­(OMe)3 or NaOAc, were not effective at facilitating Rh-catalyzed toluene C–H activation. The complexes (RPNP)­RhIII(H)­(X)2 and [(RPNP)­RhI(CO)]­[X] (X = TFA or OAc) were found to be intermediates of the catalytic the H/D exchange.