Unsaturated Vicinal Frustrated Lewis Pair Formation by Electrocyclic Ring Closure and Their Reaction with Nitric Oxide

The Lewis acidic β-styryl-B­(C6F5)2 reagent 6a undergoes a clean 1,1-carboboration reaction with 1-(PMes2)-2-cyclohexenyl acetylene 9 at 60 °C to give the vicinal P/B-substituted conjugated triene product 10a. At 80 °C this undergoes a stereoselective thermally induced disrotatory electrocyclic ring closure to give the cyclohexadiene-derived P/B system 11. Subsequent TEMPO oxidation gave the substituted phenylene-bridged P/B product 12. Both 11 and 12 are active phosphane/borane frustrated Lewis pairs (FLPs). The FLP 11 reacts in a typical way with phenylacetylene to give the phosphonium/alkynylborate product 13. Compound 12 cleaves dihydrogen at near ambient conditions to give the respective phosphonium/hydridoborate zwitterion 14. Both the FLPs 11 and 12 cooperatively add P/B to the nitrogen atom of nitric oxide (NO) within minutes at room temperature to give the persistent P/B FLPNO• radicals 19 and 21, respectively (both characterized by X-ray diffraction and by EPR spectroscopy). The FLPs 11 and 12 are thermally robust. At elevated temperatures (11: 75 °C, 12: 100 °C) they undergo a coupling reaction with dimethyl acetylenedicarboxylate with carbon–carbon bond activation at a P-mesityl substituent.

路易斯酸性β-苯乙烯基-二(五氟苯基)硼试剂6a在60 ℃下与1-二(均三甲苯基)膦基-2-环己烯基乙炔9发生洁净的1,1-碳硼化反应，得到邻位P/B取代的共轭三烯产物10a。在80 ℃条件下，该产物会经历立体选择性热诱导对旋电环化环合过程，生成环己二烯衍生的P/B体系11。后续经TEMPO氧化，得到取代亚苯基桥联的P/B产物12。11与12均为具有反应活性的膦/硼受阻路易斯酸碱对（frustrated Lewis pairs, FLPs）。FLP 11可与苯乙炔按典型反应模式反应，得到鏻/炔基硼酸盐产物13。化合物12可在接近室温的环境中裂解二氢，生成对应的鏻/氢合硼酸盐两性离子14。FLPs 11和12均可在室温下数分钟内协同将P/B单元加成至一氧化氮（NO）的氮原子上，分别得到稳定的P/B型FLPNO•自由基19和21（二者均通过X射线衍射与电子顺磁共振光谱完成表征）。FLPs 11和12具备优异的热稳定性，在升温条件下（11：75 ℃，12：100 ℃），它们可与乙酰二羧酸二甲酯发生偶联反应，在P-均三甲苯基取代基位点实现碳-碳键活化。