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 ((PNP)-P-R)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 ((PNP)-P-R)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 ((PNP)-P-R)Rh-III(H)(X)(2) and [((PNP)-P-R)Rh-I(CO)] [X] (X = TFA or OAc) were found to be intermediates of the catalytic the H/D exchange.