When Weaker Can Be Tougher: On the Role of Oxidation State (I) in P- vs. N-Ligand Derived Ni-Catalyzed Trifluoromethylthiolation of Aryl halides
Kalvet, Indrek; Guo, Qianqian; Tizzard, Graham J.; Schoenebeck, Franziska (2017). When Weaker Can Be Tougher: On the Role of Oxidation State (I) in P- vs. N-Ligand Derived Ni-Catalyzed Trifluoromethylthiolation of Aryl halides. ACS Catalysis, 7 (3), 2126−2132. DOI: 10.1021/acscatal.6b03344.
Kalvet, Indrek; Guo, Qianqian; Tizzard, Graham J.; Schoenebeck, Franziska
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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)NiI, 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)NiI–Br and (dppf)NiI–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 NiI complexes fully support productive NiI/NiIII catalysis, as the competing β-F elimination is disfavored. Moreover, N-derived NiI complexes are predicted to be more reactive than their Ni0 counterparts in catalysis. These data showcase fundamentally different roles of NiI in carbon–heteroatom bond formation depending on the ligand sphere.