Trigonal Phosphine Umpolung: Electrophilicity Driven by a Redox-Active Boron Cluster

Trivalent phosphines are classically defined by their nucleophilic character. The reversal of polarity, or umpolung, at phosphorus is a conceptually significant transformation leading to new reactivity. While geometrically constrained phosphines have been demonstrated to exhibit electrophilic character, similar behavior for trigonal phosphines is rare. In this study, we report direct experimental evidence of electrophilic behavior in a non-constrained trigonal phosphine enabled by the attachment of a redox-active boron cluster. The diphosphine 1-P^tBu₂-2-PⁱPr₂-closo-C₂B₁₀H₁₀ undergoes selective addition of anionic nucleophiles, including ⁿBu^- and CN^-, at its P^tBu₂ group. These reactions are accompanied by a two-electron reduction of the carborane cage and its conversion from a neutral closo- to a dianionic nido- structure, demonstrating that cluster-centered redox activity drives phosphorus-centered electrophilicity. The resulting nido-carboranyl phosphines exhibit enhanced nucleophilicity at the remaining phosphine arm, enabling subsequent trapping with alkyl halides, carbon disulfide, and electron-deficient fluoroarenes. These findings establish redox-active boron clusters as platforms for inducing umpolung and provide a new strategy for accessing ambiphilic reactivity at phosphorus.