Spectroscopic Characterization of the 1-Boratricyclo-[4.1.0.0^2,7]-heptane Radical with a Delocalized Four-Center-One-Electron Bond

The boron atom is a highly electrophilic reagent due to the presence of its empty p orbital, making it prone to undergo electrophilic addition reactions with the carbon-carbon double bonds of olefins. In this study, the classical C=C reaction pathway occurs when a boron atom attacks the C=C bond of cyclohexene, resulting in the formation of the η² (1,2)-BC₆H₁₀ complex (<b>A</b>) that contains a borirane radical subunit. This complex can further undergo photoisomerization, leading to the formation of a 3,4,5,6-tetrahydroborepine radical (<b>C</b>) through the cleavage of C-C bonds. In addition, two 1-boratricyclo[4.1.0.0^2,7]heptane radicals with chair (<b>B</b>) and boat (<b>B'</b>) conformations were observed through α C-H cleavage reactions. Bonding analysis indicates that these radicals involve a four-center-one-electron (4c-1e) bond. Under UV light irradiation, these two radicals undergo ring-opening and rearrangement reactions, resulting in the formation of a 1-cyclohexen-1-yl-borane radical (<b>D</b>), which is a sp² C-H activation product. These findings delineate a potential pathway for the synthesis of organoboron radicals through boron-mediated C-H and C-C bond cleavage reactions in cycloolefins.