Department of Chemistry, Queen's University, 90 Bader Lane, Kingston, Ontario.
Chemistry. 2010 Apr 26;16(16):4895-902. doi: 10.1002/chem.200903005.
We report herein that the reaction between a series of Hantzsch's ester analogues 1 a-d with the Lewis acidic species B(C(6)F(5))(3) results in facile transfer of hydride to boron. The main products of this reaction are pyridinium borohydride salts 2 a-d, which are obtained in high to moderate yields. The N-substituted substrates (N-Me, N-Ph) reacted in high yield 90-98 % and the connectivity of the products were confirmed by an X-ray crystallographic analysis of the N-Me borohydride salt 2 a. Unsubstituted Hanztsch's ester 1 a reacted less effectively generating only 60 % of the corresponding borohydride salt, with the balance of the material sequestered as the ester-bound Lewis acid-base adduct 3 a. Formation of the Lewis acid-base adduct could be minimized by increasing the steric bulk about the ester groups as in 1 d. The connectivity of the carbonyl-bound adduct was confirmed by an X-ray crystallographic analysis of 3 e the product of the reaction of methyl ketone 1 e with B(C(6)F(5))(3). We also explored the generation of these pyridinium salts by employing frustrated Lewis pair methodology. However, the reaction of mixtures of the corresponding pyridine and B(C(6)F(5))(3) with hydrogen gas only resulted in formation of trace amounts of the pyridinium borohydride, along with the Lewis acid-base adduct of the starting material and B(C(6)F(5))(3). The 1,2-dihydropyridine adduct was the final product of this reaction. This was ascribed to the low basicity of the pyridine nitrogen and the complicating formation of an ester bound Lewis acid-base adduct.
我们在此报告，一系列 Hantzsch 酯类似物 1a-d 与路易斯酸物种 B(C(6)F(5))(3)之间的反应导致氢容易转移到硼上。该反应的主要产物是吡啶硼氢化盐 2a-d，以高产率至中等产率获得。N-取代的底物（N-Me、N-Ph）以高产率 90-98%反应，产物的连接性通过 N-Me 硼氢化盐 2a 的 X 射线晶体学分析得到证实。未取代的 Hantzsch 酯 1a 反应效果较差，仅生成 60%相应的硼氢化盐，其余物质被酯基路易斯酸碱加合物 3a 捕获。通过增加酯基周围的空间位阻，如在 1d 中，可以最小化路易斯酸碱加合物的形成。通过 X 射线晶体学分析羰基键合加合物 3e 的产物，即甲基酮 1e 与 B(C(6)F(5))(3)的反应产物，证实了羰基键合加合物的连接性。我们还探索了通过使用受阻路易斯对方法生成这些吡啶鎓盐。然而，相应的吡啶和 B(C(6)F(5))(3)与氢气混合物的反应仅导致痕量的吡啶硼氢化盐的形成，以及起始材料和 B(C(6)F(5))(3)的路易斯酸碱加合物。1,2-二氢吡啶加合物是该反应的最终产物。这归因于吡啶氮的低碱性和酯基路易斯酸碱加合物的复杂形成。
