C(sp2)–C(sp3) Cross-Coupling Enabled by Alkyl Radical Capture at Isolable, Low-Spin (S = 1/2) Cobalt(II)–Monoaryl Catalysts

A cobalt(II) catalyst supported by the ligand 2-(diphenylphosphino)phenol (P,O) was developed for the C(sp2)–C(sp3) Negishi arylation of alkyl(pyridyl)sulfones, which are bench-stable, nonorganohalide C(sp3)–S electrophiles. Employing the catalyst generated in situ from 5 mol % P,O ligand and 5 mol % cobalt(II) bromide, a variety of (hetero)aryl C(sp2)–C(sp3) products were synthesized derived from primary and secondary alkyl sulfones, including difluoromethylation using 2-((difluoromethyl)sulfonyl)pyridine, and cross-coupling of sulfone derived from the thiol-containing ACE inhibitor captopril. Freeze-quench X-band EPR spectroscopy of a catalytic reaction established the catalyst resting state as low-spin (S = 1/2), square-pyramidal (P,O)cobalt(II)–aryl, a rare example of a cobalt(II)–aryl complex detected during a cross-coupling reaction. These data informed the cobalt(II/III/I/0) catalytic cycle involving alkyl radical capture at the (P,O)cobalt(II)–aryl catalyst resting state, enabling selective formation of the C(sp2)–C(sp3) product.