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

Abstract

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 Ni⁰(cod)₂ and dppf, this report investigates the Ni-catalyzed functionalization of the seemingly more reactive aryl halides ArI and ArBr. Counterintuitively, the observed conversion triggered by dppf/Ni⁰ is ArCl > ArBr > ArI, 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^I, which favors β-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^I-Br and (dppf)Ni^I-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^I complexes fully support productive Ni^I/Ni^III catalysis, as the competing β-F elimination is disfavored. Moreover, N-derived Ni^I complexes are predicted to be more reactive than their Ni⁰ counterparts in catalysis. These data showcase fundamentally different roles of Ni^I in carbon-heteroatom bond formation depending on the ligand sphere.