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

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posted on 19.02.2016, 05:42 by Muhammad Sajid, Gerald Kehr, Thomas Wiegand, Hellmut Eckert, Christian Schwickert, Rainer Pöttgen, Allan Jay P. Cardenas, Timothy H. Warren, Roland Fröhlich, Constantin G. Daniliuc, Gerhard Erker
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.