Synthesis, Structure and Reactivities of Pentacoordinated Phosphorus–Boron Bonded Compounds

The isolation and reactivities of two pentacoordinated phosphorus–tetracoordinated boron bonded compounds were
explored. A strong Lewis acidic boron reagent and electron-withdrawing ligand system were required to form the
pentacoordinated phosphorus state of the P–B bond. The first compound, a phosphoranyl-trihydroborate, gave a THF
stabilised phosphoranyl-borane intermediate upon a single hydride abstraction in THF. This compound could undergo a
unique rearrangement reaction, that involved a two-fold ring expansion, to give an unusual fused bicyclic compound or it
could act as a mono-hydroboration reagent. The hydroboration reactivity of the intermediate was found to be more reactive
towards alkynes over alkenes with good to moderate regioselectivity towards the terminal carbon. The second compound,
a phosphoranyl-triarylborate, was found to have a vastly different reactivity to the trihydroborate as it was highly stable
towards acids and bases. This is thought to be due to the large bulk around the P–B bond as shown in the crystal structure