Noninteracting, Vicinal Frustrated P/B-Lewis Pair at the Norbornane Framework: Synthesis, Characterization, and Reactions

Hydroboration of dimesitylnorbornenylphosphane
with Piers’
borane [HB­(C6F5)2] gave the frustrated
Lewis pair (FLP) 4 in good yield. It has the −PMes2 Lewis base attached at the 2-endo position and the −B­(C6F5)2 group 3-exo oriented at the norbornane
framework. The vicinal FLP 4 was shown by X-ray diffraction
and by spectroscopy to be a rare example of an intramolecular noninteracting
pair of a Lewis acid and Lewis base functionality. The FLP 4 rapidly splits dihydrogen heterolytically at ambient temperature
to yield the phosphonium/hydrido borate zwitterion 5.
It adds to the carbonyl group of benzaldehyde and to carbon dioxide
to yield the adducts 6 and 7, respectively.
Compounds 5–7 were characterized by X-ray diffraction.
Compound 4 adds to the SO function of sulfur
dioxide to give a pair of diastereomeric heterocyclic six-membered
ring products due to the newly formed sulfur chirality center, annulated
with the norbornane skeleton, which were investigated by 31P/11B single and double resonance solid state NMR experiments.
Compound 8 was also characterized by X-ray diffraction.
The FLP 4 undergoes a clean N,N-addition to nitric oxide (NO) to give a norbornane annulated
five-membered heterocyclic persistent FLP-NO aminoxyl radical 12 (characterized, e.g., by X-ray diffraction and EPR spectroscopy).
Additionally, the FLP radical was characterized by 1H solid
state NMR spectroscopy. The radical 12 undergoes a H-atom
abstraction reaction with 1,4-cyclohexadiene to yield the respective
diamagnetic FLP-NOH product 13, which was also characterized
by X-ray diffraction and solid state NMR spectroscopy.