Chen Jiawei, Lalancette Roger A, Jäkle Frieder
Department of Chemistry, Rutgers University-Newark, 73 Warren Street, Newark, NJ 07102 (USA), Fax: (+1) 973-353-1264.
Chemistry. 2014 Jul 14;20(29):9120-9. doi: 10.1002/chem.201400057. Epub 2014 Jun 11.
In an effort to develop a new class of redox-active chiral Lewis pairs, pyridine and borane moieties with different steric and electronic properties were introduced onto a planar chiral 1,2-disubstituted ferrocene framework. Metathesis of lithiated, stannylated, or mercuriated pyridylferrocenes with boron halides afforded (pR)-2-[bis(pentafluorophenyl)boryl]-1-(3,5-dimethylpyrid-2-yl)ferrocene (4-Pf), (pR)-2-[dimesitylboryl]-1-(3,5-dimethylpyrid-2-yl)ferrocene (4-Mes), (pS)-2-(bis(pentafluorophenyl)boryl)-1-(2-trimethylsilylpyrid-6-yl)ferrocene (5-Pf), or (pS)-2-[dimesitylboryl]-1-(2-trimethylsilylpyrid-6-yl)ferrocene (5-Mes). The borylated products were analyzed by multinuclear NMR spectroscopy, HRMS, and single-crystal X-ray diffraction. Chiral HPLC and optical-rotation measurements were employed to assess the stereoselectivity of the borylation process and to establish the correct stereochemical assignments. The strength of the B-N interactions were investigated in solution and in the solid state. Compounds 4-Pf and 4-Mes formed robust 'closed' B-N heterocyclic systems that proved to be perfectly stable to air and moisture, whereas 5-Pf established a dynamic equilibrium, in which the B-N heterocycle was observed exclusively at room temperature, but opened up at high temperature according to (19)F NMR exchange spectroscopy data. As a consequence, 5-Pf reacted readily with a molecule of water to generate a ring-opened pyridinium borate. The combination of bulky borane and bulky pyridyl groups in 5-Mes led to a completely 'open' frustrated Lewis pair system with uncomplexed pyridine and borane groups, even at room temperature. Electrochemical studies were performed and the effect of preparative ferrocene oxidation on the structural features was also explored.
为了开发一类新型的氧化还原活性手性路易斯对，将具有不同空间和电子性质的吡啶和硼烷部分引入到平面手性1,2-二取代二茂铁骨架上。锂化、锡化或汞化吡啶基二茂铁与卤化硼的复分解反应得到了（pR）-2-[双（五氟苯基）硼基]-1-（3,5-二甲基吡啶-2-基）二茂铁（4-Pf）、（pR）-2-[二甲基硼基]-1-（3,5-二甲基吡啶-2-基）二茂铁（4-Mes）、（pS）-2-（双（五氟苯基）硼基）-1-（2-三甲基硅基吡啶-6-基）二茂铁（5-Pf）或（pS）-2-[二甲基硼基]-1-（2-三甲基硅基吡啶-6-基）二茂铁（5-Mes）。通过多核NMR光谱、高分辨质谱和单晶X射线衍射对硼化产物进行了分析。采用手性高效液相色谱和旋光测量来评估硼化过程的立体选择性并确定正确的立体化学归属。在溶液和固态中研究了B-N相互作用的强度。化合物4-Pf和4-Mes形成了稳定的“封闭”B-N杂环体系，事实证明它们对空气和水分完全稳定，而5-Pf建立了动态平衡，根据（19）F NMR交换光谱数据，在室温下仅观察到B-N杂环，但在高温下会打开。因此，5-Pf很容易与一分子水反应生成开环的吡啶硼酸酯。即使在室温下，5-Mes中庞大的硼烷和庞大的吡啶基的组合也导致了一个完全“开放”的受挫路易斯对体系，其中吡啶和硼烷基团未络合。进行了电化学研究，并探讨了制备性二茂铁氧化对结构特征的影响。
