The Chemistry of a Non‐Interacting Vicinal Frustrated Phosphane/Borane Lewis Pair

The dimesitylphosphinocyclopentene/HB(C6F5)2‐derived vicinal trans‐1,2‐P/B frustrated Lewis pair (FLP) 4 shows no direct phosphane–borane interaction. Toward some reagents it behaves similar to an intermolecular FLP; it cleaves dihydrogen, deprotonates terminal alkynes, and adds to organic carbonyl compounds including CO2. It shows typical intramolecular FLP reaction modes (cooperative 1,1‐additions) to mesityl azide, to carbon monoxide, and to NO. The latter reaction yields a persistent P/B FLPNO nitroxide radical, which undergoes H‐atom ion reactions. The FLP 4 serves as a template for the CO reduction by [HB(C6F5)2] to generate a FLP‐η2‐formylborane. The formylborane moiety is removed from the FLP template by reaction with pyridine to yield a genuine pyridine stabilized formylborane that undergoes characteristic borane carbaldehyde reactions (Wittig olefination, imine formation). Most new products were characterized by X‐ray diffraction. Poised to attack: The trans‐1,2‐attachment of the B(C6F5)2 and PMes2 functional groups at a cyclopentane framework leads to a non‐interacting frustrated P/B Lewis pair (see Scheme). The diverse reactivity of this system was studied in detail.