The chemistry of Boron cation (boronium, borenium, borinium)-promoted transformation reaction

本研究研究了溶剂对手性恶唑硼烷酮促进的不对称醛醇反应的影响，并在恶唑硼烷酮的B-H键变为B-X键时检测了阳离子硼的种类。利用与（R，R）-stien相关的B-Br化合物，详细研究了硼种的存在及其在有机合成中的应用。在不对称反应中，控制假想的手性硼阳离子的两个空轨道是相当困难的。此外，由于硼阳离子具有广泛的高亲电性，因此在亲核试剂的攻击下容易生成中性的三价硼化合物，并且硼阳离子的催化再循环也比较困难。因此，反应体系必须局限于某些协调一致的反应。硼离子的物理有机研究得到了有趣的发展。高氯酸二苯硼被认为是一种常规的标准硼阳离子化合物，通过^<11>B-NMR和计算研究发现它是一种具有共价B-O键的三价硼化合物。硼离子在溶液中的形态为溶剂化的四价形态，而非二价形态。首次利用激光闪光光解技术检测到硼离子的真阳离子形态。

This research was undertaken to study solvent effects on chiral oxazaborolidinone-promoted asymmetric aldol reaction, followed by detection of cationic boron species when the B-H bond of the oxazaborolidinone was changed to the B-X. Using the B-Br compound related to (R,R)-stien, existence of the boron species and its utilization to organic syntheses were investigated in detail. It turned out to be quite difficult to control two vacant orbitals of the hypothetical chiral boron cation in asymmetric reactions. In addition, the complete boron cations lead easily to neutral trivalent boronic compounds by attack of nucleophiles because of the extensively high electrophilicity and it is also difficult to catalytically recycle the boron cation. Thus, the reaction system should be necessarily restricted to some concerted reactions.Physical organic researches on boron cations were interestingly developed. Diphenylboron perchlorate, which was considered as a conventional standard compound of boron cations, was found to be a trivalent boronic compound having a covalent B-O bond by ^<11>B-NMR and calculation studies. Boron cation species in solution were suggested to be solvated tetravalent species, not divalent species. The true boron cationic species were detected for the first time by using laser flash-photolysis.