Trihydroborates and Dihydroboranes Bearing a Pentacoordinated Phosphorus Atom: Double Ring Expansion To Balance the Coordination States

Abstract The first trihydroborate bearing a pentacoordinated phosphorus atom was synthesized as a new P−B bonded compound. Hydride abstraction of the trihydroborate gave an intermediary dihydroborane, which showed hydroboration reactivity and was trapped with pyridine whilst maintaining the P−B bond. The dihydroborane underwent a rearrangement, which involved a double ring expansion to compensate for the unbalanced coordination states of the phosphorus and boron atoms, to give a new fused bicyclic phosphine‐boronate.