Probing the Oxidative Addition Transition State in Nickel Metallaphotoredox Catalysis Using Intramolecular ¹³C...

The merger of nickel and photoredox catalysis has enabled a wide range of C-C and C-X bond formations under mild conditions. Central to the mechanistic understanding of these catalytic reactions is the identification of the nickel species involved in the key oxidative addition step. Intramolecular ¹³C kinetic isotope effects (KIEs) in conjunction with DFT analysis are a reliable, quantitative approach to identify the oxidation state and ligand environment at the metal center during oxidative addition. This methodology is applied to evaluate the mechanism of two distinct nickel metallaphotoredox-catalyzed reactions. Specifically, we probe the oxidative addition step of nickel metallaphotoredox catalyzed C(sp²)-C(sp³) cross-coupling reactions that utilize either neutral bipyridyl ligands (NiCl₂(dtbbpy)(H₂O)₄) or anionic diketonate ligands (Ni(THMD)₂) and utilize this information to elucidate the operational catalytic cycles in these reactions. In the N-methyl selective arylation of trialkyl amines catalyzed by NiCl₂(dtbbpy)(H₂O)₄, a Ni(I)(dtbbpy)(α-aminoalkyl) complex is invoked as the most likely species involved in the oxidative addition step. An alternative mechanism is also identified that is consistent with the experimental KIEs, involving oxidative addition to a Ni(0)(dtbbpy)(methyleneiminium) bromide complex. In the deaminative arylation of sterically hindered primary amines catalyzed by Ni(THMD)₂, our results indicate that an anionic Ni(I)(THMD)(Cl) complex is likely involved in the oxidative addition step, a finding that accounts for the rate acceleration observed in the presence of tetrabutyl ammonium chloride as an additive. These results illustrate that intramolecular ¹³C KIEs are an effective experimental probe to address challenging mechanistic questions in nickel metallaphotoredox catalysis.