Cross-Electrophile Coupling to Form Sterically Hindered C(sp²)–C(sp³) Bonds: Ni and Co Afford Complementary Reactivity

The formation of sterically hindered C(sp²)-C(sp³) bonds could be a useful synthetic tool but has been understudied in cross-electrophile coupling. Here, we report two methods that couple secondary alkyl bromides with aryl halides that contain sterically hindered C-X bonds: 1) <i>ortho</i>-substituted aryl bromides with nickel catalysts and 2) di-<i>ortho</i>-substituted aryl iodides with cobalt catalysts. Stoichiometric experiments and deuterium labeling studies show that 1) [Co] is better than [Ni] for oxidative addition of di-<i>ortho</i>-substituted Ar-I and 2) [Co] is better than [Ni] for radical capture/reductive elimination steps with di-<i>ortho</i>-substituted arenes. For both metals, Ar-H side products observed in reactions with low-yielding di-<i>ortho</i>-substituted aryl iodides appear to arise from Ar• formation and hydrogen-atom transfer from the solvent. While the origins of the differences in scope are not yet understood, these studies demonstrate a previously unknown complementarity between nickel and cobalt in cross-electrophile coupling.