摘要/Abstract

近年路易斯酸B（C₆F₅）₃催化的醛酮还原及胺化反应研究表明，缺电子的路易斯酸B（C₆F₅）₃也可以作为一种"耐水"的催化剂在"有水"条件下进行催化反应.这些研究成果对进一步扩展受限路易斯酸碱对（FLPs）化学的研究领域和应用前景提供了更多可能.本文以硅烷作为还原剂，在路易斯酸B（C₆F₅）₃催化下可在温和条件下实现醛与烷基羟胺类化合物的直接还原胺化反应，并且在还原过程中N-O键不会发生断裂，可中等至高产率地制备各种烷氧基取代的羟胺衍生物.对反应机理研究发现，在中性条件下苯甲醛与苄氧基羟胺的反应仅得到缩合中间产物肟醚，而在HCl或过量H₂O的参与下苯甲醛与苄氧基羟胺的直接还原胺化均可顺利进行；对反应机理的研究表明苄氧基羟胺会与路易斯酸硼烷在过量H₂O的参与下发生质子化，在硅烷的作用下转化成具有一定还原性的"硼氢化胺盐"活性中间体并进而促使中间产物肟醚的还原.对醛与羟胺的直接还原胺化反应研究表明，在"有水"条件下路易斯酸B（C₆F₅）₃不仅仅是一种"耐水"的催化剂，在某些反应中水可能直接影响着催化反应，尤其是对醛酮的直接还原胺化反应.因此，继续深入研究有"水"条件下路易斯酸硼烷参与的催化反应机理不仅对FLPs化学的发展至关重要，对其他相应催化体系的研究也具有重要的参考价值.
关键词: 受限路易斯酸碱对, 三(五氟苯基)硼, 氢化硅烷, 还原胺化反应, 水
Recently the research work concerning B(C₆F₅)₃ catalyzed reductive and amination of aldehydes and ketones revealed that this extremely electron-deficient borane is, actually, a rather water-tolerant catalyst. This fact considerably broadens the scope of the water/base tolerant FLP chemistry. In this project, an efficient one pot reductive amination method has been developed by reaction of aldehydes and alkoxyamines with hydrosilanes as the hydride sources and B(C₆F₅)₃ as catalyst without cleavage of the N-O bond. This protocol can be used to prepare the secondary and tertiary alkoxyamines by starting from the primary and secondary ones, respectively. A special attention has been paid to elucidate the role of water in the reductive amination. When benzaldehyde reacts with benzoxylamine, only the condensation product oxime ether could be observed. Whereas surprisingly when excess amount of water is added, the reductive amination goes successfully like the alkoxyamine hydrochloride works. The detailed NMR data has shown that a transformation of the intermediate oximes ArCH=NOR to the "ammonium borates"[ArCH=NHOR]⁺[X-B(C₆F₅)₃]^-(X=Cl, OH) can take place in the reaction system, while the latter can be converted into the well-known active intermediate "ammonium hydroborates"[ArCH=NHOR]⁺[H-B(C₆F₅)₃]^- to reduce the C=N bond under mild condition in the presence of hydrosilanes. That means the deprotonation reaction of the Lewis acid water adduct H₂O-B(C₆F₅)₃ could be a key step for the B(C₆F₅)₃ catalyzed reaction under moist condition. In this case the adduct H₂O-B(C₆F₅)₃ acts as a Brønsted acid as HCl does. Meanwhile a simulative experiment under different ratio of water has been fulfilled to prove this speculation. The C=N bond of Benzalaniline (PhCH=NPh) and Benzyloxy oxime ether (PhCH=NOCH₂Ph) could be reduced only in presence of 2 equiv. H₂O rather than equivalent. Based on this study it has shown that in the frustrated Lewis pair (FLP) chemistry, the Lewis acid B(C₆F₅)₃ is not only a highly effective and water tolerant catalyst, the "disfavored" deprotonation of H₂O-B(C₆F₅)₃ adductis possibly playing an important role in reductive amination reaction. To clarify in detail the actual role of water in the reductive amination reaction under the "moist" conditions would enable the further development of FLP and related catalyzed reactions.
Key words: frustrated Lewis pairs, B(C₆F₅)₃, hydrosilanes, reductive amination, water


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