Direct Evidence for Competitive C-H Activation by a Well-Defined Silver XPhos Complex in Palladium-Catalyzed C-H Functionalization

ABSTRACT: Increasing evidence indicates that silver salts can play a role in the C-H activation step of palladium-catalyzed C-H functionalization. Here we isolate a silver(I) complex by C-H bond activation and demonstrate its catalytic competence for C-H functionalization. We demonstrate how silver carbonate, a common but highly insoluble additive, reacts with pentafluorobenzene in the presence of a bulky phosphine, XPhos, to form the C-H bond activation product Ag(C6F5)(XPhos). By isolating and fully characterizing this complex and the related carbonate and iodide complexes, [Ag(XPhos)]2(mu-kappa 2,kappa 2-CO3) and [AgI(XPhos)]2, we show how well-defined Ag(I) complexes can operate in conjunction with palladium complexes to achieve C-H functionalization even at ambient temperature. Reactions are tested against the standard cross-coupling of C6F5H with 4-iodotoluene, catalyzed by palladium acetate at 60 degrees C in the presence of silver carbonate and Xphos. Key observations are that (a) PdI(C6H5)(XPhos) reacts stoichiometrically with Ag(C6F5)(XPhos) to form Ph-C6F5 instantly at room temperature; (b) catalytic cross coupling can be achieved using 5% Ag(C6F5)(XPhos) as the sole silver source; and (c) palladium acetate (typical precatalyst) can be replaced for catalytic cross coupling by the expected oxidative addition compound PdI(C6H5)(XPhos). These investigations lead to a catalytic cycle in which Ag(I) plays the C-H bond activation role and palladium plays the coupling role. Moreover, we show how the phosphine can be exchanged between silver complexes, ensuring that it is recycled even though silver carbonate is consumed during catalytic cross-coupling.