Speciation and Reactivity of Mono- and Binuclear Ni Intermediates in Aminoquinoline-Directed C–H Arylation and Benzylation

Abstract

This paper describes detailed organometallic studies of the aminoquinoline-directed Ni-catalyzed C−H functionalization of 2,3,4,5-tetrafluoro-N-(quinolin-8-yl)benzamide with diaryliodonium reagents. A combination of 19 F NMR spectroscopy and X-ray crystallography is used to track and characterize diamagnetic and paramagnetic intermediates throughout this transformation. These provide key insights into both the cyclometalation and oxidative functionalization steps of the catalytic cycle. The reaction conditions (solvent, ligands, base, and stoichiometry) play a central role in the observation of a Ni II precyclometalation intermediate as well as in the speciation of the Ni II products of C−H activation. Both mono-and binuclear cyclometalated Ni II species are observed and interconvert, depending on the reaction conditions. Cyclic voltammetry reveals that the Ni II/III redox potentials for the cyclometalated intermediates vary by more than 700 mV depending on their coordination environments, and these differences are reflected in their relative reactivity with diaryliodonium oxidants. The oxidative functionalization reaction affords a mixture of arylated and solvent functionalization organic products, depending on the conditions and solvent. For example, conducting oxidation in toluene leads to the preferential formation of the benzylated product. A series of experiments implicate a Ni II/III/IV pathway for this transformation.