Catalytically Relevant Intermediates in the Ni-Catalyzed C(sp)-H and C(sp)-H Functionalization of Aminoquinoline Substrates.

This Article describes the synthesis and characterization of cyclometalated aminoquinoline Ni σ-aryl and σ-alkyl complexes that have been proposed as key intermediates in Ni-catalyzed C-H functionalization reactions. These Ni complexes serve as competent catalysts for the C-H functionalization of aminoquinoline derivatives with I. They also react stoichiometrically with I to form either aryl iodides or β-lactams within minutes at room temperature. Furthermore, they react with Ag salts at -30 °C to afford isolable five-coordinate Ni species. The Ni σ-aryl complexes proved inert toward C(sp)-I bond-forming reductive elimination under all conditions examined (up to 140 °C in DMF). In contrast, a Ni σ-alkyl analogue underwent C(sp)-N bond-forming reductive elimination at 140 °C in DMF to afford a β-lactam product. However, despite the ability of this latter Ni species to participate in stoichiometric product formation, the complex was not a competent catalyst for β-lactam formation. Overall, these results suggest against the intermediacy of Ni species in these C-H functionalization reactions.