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 BB 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 (E-1/2=-1.05/-1.55V) were not sufficiently negative to reduce MesBC(4)Ph(4) (E-1/2=-1.69V), a suitable diborene, IiPr(iPr)BB(iPr)IiPr, was tailor-made to comply with these requirements. With a halfwave potential of E-1/2=-1.95V, this diborene ranks amongst the most powerful neutral organic reductants known and readily reacted with MesBC(4)Ph(4) by SET to afford a stable boron-centered radical-anion radical-cation pair.