Construction of a Neutral Antiaromatic Dibenzo[a,e]diborapentalene via Reductive Intramolecular Cyclization: Oxidation to a Highly Stable Radical Cation.

The incorporation of heteroatoms into π-conjugated frameworks has proven to be an efficient strategy for modulating electronic properties, enabling the discovery of unprecedented structures, reactivities, and physical properties. Herein, we report the facile synthesis of neutral antiaromatic dibenzo[a,e]diborapentalene 3a and its π-extended derivative 3b, which have been characterized by NMR spectroscopy and X-ray crystallography. The strong antiaromaticity of the central pentalene cores is well established by both experimental and computational analyses. Moreover, their unique oxidation reactivity has been thoroughly investigated. Treatment of 3a with different equivalents of AgSbF6 led to the formation of the corresponding radical cations 7a or 8a. While 7a is generated via direct one-electron oxidation of 3a, the formation of 8a is proposed to proceed through initial generation of a dicationic intermediate, followed by fluoride abstraction and further one-electron oxidation. Notably, 8a exhibits high stability in air. The reactivity of 3a toward Ph2S2 or gray selenium afforded the oxidized products 9 and 10, respectively, demonstrating the boron center as a versatile reaction platform. These results emphasize the unique electronic structures and reactivity arising from the substitution of carbon with boron in comparison to carbonaceous analogues.