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

The formation of sterically hindered C(sp 2 )–C(sp 3 ) 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) ortho -substituted aryl bromides with nickel catalysts and 2) di- ortho -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- ortho -substituted Ar–I and 2) [Co] is better than [Ni] for radical capture/reductive elimination steps with di- ortho -substituted arenes. For both metals, Ar–H side products observed in reactions with low-yielding di- ortho -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.