Electronic Directing Group Modification for Improved Ni(II)-Mediated C(sp3)–H Activation: A Hammett Investigation of 8-Aminoquinoline

The 8-aminoquinoline (AQ) directing group has received considerable attention for facilitating superior C–H bond activation in unactivated substrates compared to other N,N-bidentate directing groups. However, for nickel, the role of AQ in C–H activation reactions has not been fully realized as of yet. Kinetic experiments and a Hammett analysis of 5-substituted AQs in C(sp3)–H activation have uncovered a disconnected trend between electron-rich and electron-poor substituted AQs that yield a nonlinear Hammett plot in the Ni(II)-mediated C(sp3)–H activation of tertiary ureas. We have identified and synthesized paramagnetic tris-AQ ligated anionic Ni(II) species, such as Na[fac-(1d–)3Ni] (complex 5). We propose that paramagnetic complexes participate in a pre-equilibrium with the active species that undergoes C–H activation. We hypothesize that these coordinatively saturated paramagnetic tris-ureate complexes are favored for electron-withdrawing directing groups, subsequently influencing the initial rates of C–H activation. These observations also point toward the importance of catalyst speciation and the role of ligand electronic properties, not just steric features that result in speciation changes and subsequent changes in reaction rates in Ni catalysis.