Photolytic activation of Ni(II)X2L explains how Ni-mediated cross coupling begins
Max Kudisch, Reagan X. Hooper, Lakshmy Kannadi Valloli, Justin Earley, Anna Zieleniewska, Yu Jin, Stephen DiLuzio, Rebecca W. Smaha, Hannah J. Sayre, Xiaoyi Zhang, Matthew J. Bird, Amy A. Cordones, Garry Rumbles, Obadiah G. Reid
Abstract
Nickel photocatalysis has recently become vital to organic synthesis, but how the Ni(II)X2L pre-catalyst (X = Cl, Br; L = bidentate ligand) becomes activated to Ni(I)XL has remained puzzling and is typically addressed on a case-by-case basis. Here, we reveal a general mechanism where light induces photolysis of the Ni(II)-X bond, either via direct excitation or triplet energy transfer. Photolysis produces Ni(I)XL and a halogen radical, X•. Subsequent hydrogen atom abstraction, often from the solvent, produces a C(sp3) radical, R•, that recombines with Ni(I) to form organonickel(II) complexes, Ni(II)XRL. Rather than acting as a loss pathway, Ni(II)XRL behaves as a light-activated reservoir of Ni(I) via photolysis of the Ni(II)-C bond. These results explain the role of the solvent in protecting the catalyst from off-cycle dimerization, demonstrate that two photons are often required to drive the reaction, and show how tuning the ligand can control the concentration of active Ni(I) species. Nickel(II) dihalide precatalysts with bidentate nitrogen ligands are widely used in cross-coupling reactions, notably in combination with photosensitizers, forming catalytic systems that currently drive major conceptual and synthetic thrusts within organic chemistry. Here the authors show a general mechanism by which these precatalysts are converted to the reduced, catalytically active species, using a range of characterization and spectroscopic techniques.