Boron as a powerful reductant: synthesis of a stable boron-centered radical-anion radical-cation pair

Despite the fundamental importance of radical-anion radical-cation pairs in single-electron transfer (SET) reactions, such species are still very rare and transient in nature. Since diborenes have highly electron-rich BB double bonds, which makes them strong neutral reductants, we envisaged a possible realization of a boron-centered radical-anion radical-cation pair by SET from a diborene to a borole species, which are known to form stable radical anions upon one-electron reduction. However, since the reduction potentials of all know diborenes (E1/2 =-1.05/-1.55 V) were not sufficiently negative to reduce MesBC4 Ph4 (E1/2 =-1.69 V), a suitable diborene, IiPr⋅(iPr)BB(iPr)⋅IiPr, was tailor-made to comply with these requirements. With a halfwave potential of E1/2 =-1.95 V, this diborene ranks amongst the most powerful neutral organic reductants known and readily reacted with MesBC4 Ph4 by SET to afford a stable boron-centered radical-anion radical-cation pair.