Cobalt-Catalyzed [2σ+2π] Cycloaddition via Site-Selective Capture of Bicyclobutane Diradical.

Strained bicyclo[1.1.0]butanes (BCBs) have emerged as versatile precursors for accessing highly reactive intermediates, enabling unconventional bond constructions through strain-release pathways. Herein, we report a cobalt(II)-catalyzed strategy that combines thermal activation of BCBs with selective radical capture by Co(II) to afford head-to-head fused bicyclo[2.1.1]hexanes (BCHex). We propose that thermal activation of the BCB generates a reactive diradical intermediate, which is intercepted by a tailored bidentate phosphine Co(II) cationic complex via oxidative radical coupling, followed by a [2σ+2π] cycloaddition with indoles. Mechanistic investigations, including 31P NMR spectroscopy, cyclic voltammetry, electron paramagnetic resonance (EPR), and density functional theory (DFT) studies, provide insights into the plausible reaction pathway. The involvement of paramagnetic Co(II) species is consistent with the broad features observed in both EPR and nuclear magnetic resonance (NMR) spectra of the catalyst. In addition, four structurally distinct Co(II) complexes were synthesized, fully characterized by UV-Vis and Fourier-transform infrared (FTIR) spectroscopy, and evaluated as catalysts for the transformation. The cationic nature of the Co(II) species was further confirmed by phosphine oxide trapping experiments and single-crystal X-ray crystallography.