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.

以 Piers 的硼烷 [HB-(C6F5)2] 对二甲基硼杂环戊烷进行水合硼化反应，成功得到了具有良好产率的受阻路易斯对（FLP）4。该受阻路易斯对在2-endo位上附着有-PMes2路易斯碱，而在诺伯烷骨架上3-exo位取向的-B-(C6F5)2基团。通过X射线衍射和光谱学研究表明，邻近的FLP 4 是一个罕见的分子内非相互作用的路易斯酸和路易斯碱功能的例子。FLP 4 在室温下迅速异裂二氢，生成磷铵/氢化硼酸的双性离子5。该双性离子与苯甲醛的羰基以及二氧化碳反应，分别生成加成物6和7。化合物5-7通过X射线衍射进行了表征。化合物4与二氧化硫的S-氧官能团反应，由于新形成的具有诺伯烷骨架环化的手性硫中心，产生了两个对映异构的杂环六元环产物，这些产物通过31P/11B单共振和双共振固态核磁共振实验进行了研究。化合物8也通过X射线衍射进行了表征。FLP 4 对一氧化氮（NO）进行干净的N,N-加成，生成环化的五元杂环持久FLP-NO氨基自由基12（例如，通过X射线衍射和EPR光谱学进行了表征）。此外，FLP自由基通过1H固态核磁共振光谱学进行了表征。自由基12与1,4-环己二烯发生氢原子提取反应，生成相应的抗磁性FLP-NOH产物13，该产物也通过X射线衍射和固态核磁共振光谱学进行了表征。