Mechanistic Insights into Cobalt-Catalyzed Kumada C(sp2)–C(sp2) Cross-Coupling Enabled by High-Denticity Cobalt(I) Intermediates

Understanding the role of low-valent cobalt species in cross-coupling catalysis remains a central challenge, particularly in distinguishing two-electron organometallic pathways from radical-based mechanisms. Herein, we report the isolation and comprehensive characterization of cobalt(II) and cobalt(I) intermediates supported by a tetradentate diazapyridinophane (tBuN4) ligand and elucidate their roles in Kumada cross-coupling reactions. Structural, spectroscopic, and computational studies reveal partial ligand-centered reduction in the “Co(I)” species, which function as single-electron initiators for halogen atom transfer from aryl bromides. Mechanistic investigations suggest that the resulting aryl radicals follow two distinct pathways: solvent-mediated hydrogen atom transfer leading to protodehalogenation, or interception by Grignard reagents to form C(sp2)–C(sp2) bonds through an SRN1-type mechanism (radical-nucleophilic aromatic substitution). Overall, these findings establish a radical-mediated framework for Co-catalyzed Kumada coupling and highlight ligand denticity as a key factor in governing reaction pathways.