When Weaker Can Be Tougher: The Role of Oxidation State (I) in P-vs N-Ligand-Derived Ni-Catalyzed Trifluoromethylthiolation of Aryl Halides

The direct introduction of the valuable SCF3 moiety into organic molecules has received considerable attention. While it can be achieved successfully for aryl chlorides under catalysis with Ni degrees(cod)(2) and dppf, this report investigates the Ni-catalyzed functionalization of the seemingly more reactive aryl halides Arl and ArBr. Counterintuitively, the observed conversion triggered by dppf/Ni degrees is ArCI > ArBr > Arl, at odds with bond strength preferences. By a combined computational and experimental approach, the origin of this was identified to be due to the formation of (dppf)Ni-1, which favors beta-F elimination as a competing pathway over the productive cross-coupling, ultimately generating the inactive complex (dppf)Ni(SCF2) as a catalysis dead end. The complexes (dppf)Ni-1-Br and (dppf)Ni-1-I were isolated and resolved by X-ray crystallography. Their formation was found to be consistent with a ligand-exchange-induced comproportionation mechanism. In stark contrast to these phosphine-derived Ni complexes, the corresponding nitrogen-ligand-derived species were found to be likely competent catalysts in oxidation state I. Our computational studies of N-ligand derived Ni-1 complexes fully support productive Ni-1/Ni-111 catalysis, as the competing beta-F elimination is disfavored. Moreover, N-derived Ni-1 complexes are predicted to be more reactive than their Ni counterparts in catalysis. These data showcase fundamentally different roles of Ni-1 in carbon-heteroatom bond formation depending on the ligand sphere.