Cage-shaped borate esters with tris(2-oxyphenyl)methane or -silane system frameworks bearing multiple tuning factors: geometric and substituent effects on their Lewis acid properties.
Cage-shaped borate esters with tris(2-oxyphenyl)methane or -silane system frameworks bearing multiple tuning factors: geometric and substituent effects on their Lewis acid properties.
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DOI：
10.1002/chem.201002789
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发表时间：
2011-03
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通讯作者：
M. Yasuda;Hideto Nakajima;R. Takeda;S. Yoshioka;S. Yamasaki;K. Chiba;A. Baba
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文献类型：
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作者：
M. Yasuda;Hideto Nakajima;R. Takeda;S. Yoshioka;S. Yamasaki;K. Chiba;A. Baba
硼配合物，含有新的三齿配体，三（邻氧基芳基）甲烷和硅烷，制备。这些配合物具有围绕硼中心的笼状结构，并显示出比开放型硼化合物更高的刘易斯酸性和催化活性。笼形配体通过改变几何形状来确定硼酸盐的性质，并且通过螯合作用始终与金属中心结合。合成的化合物为L·B（OC（6）H（4））（3）CH、L·B（OC（6）H（4））（3）SiMe及其衍生物（L=四氢呋喃或吡啶作为外部配体）。理论计算表明，笼状硼酸盐具有较大的二面角（C（ipso）-O-B-O）。由于二面角的几何效应，笼形硼酸盐比开放形硼酸盐具有更大的刘易斯酸性。在笼形骨架的芳基上引入吸电子基团增加了刘易斯酸性。桥头硅取代桥头碳降低了硼配合物的刘易斯酸性，这是因为大的硅原子减小了C（ipso）-O-B-O的二面角。对位氟取代的化合物B（OC（6）H（3）F）（3）CH和邻苯基取代的化合物B（OC（6）H（3）Ph）（3）CH的配体交换速率小于未取代的硼酸酯B（OC（6）H（4））（3）CH的配体交换速率。B（OC（6）H（4））（3）SiMe的配体交换速率远快于B（OC（6）H（4））（3）CH。笼状硼酸盐对桤木反应和Mukaiyama型Aldol反应的催化效果优于开放型硼酸盐B（OPh）（3）和强刘易斯酸BF（3）·OEt（2）。笼形硼酸酯与庞大的取代基在邻位选择性地催化反应与空间位阻较小的底物，而未取代的硼酸酯表现出没有选择性。
Boron complexes that contain new tridentate ligands, tris(o-oxyaryl)methanes and -silanes, were prepared. These complexes had a cage-shaped structure around a boron center and showed higher Lewis acidity and catalytic activity than open-shaped boron compounds. The cage-shaped ligands determined the properties of the borates by altering the geometry and were consistently bound to the metal center by chelation. The synthesized compounds were L⋅B(OC(6)H(4))(3)CH, L⋅B(OC(6)H(4))(3)SiMe, and its derivatives (L=THF or pyridine as an external ligand). Theoretical calculations suggested that the cage-shaped borates had a large dihedral angle (C(ipso)-O-B-O) compared with open-shaped borates. The geometric effect due to the dihedral angle means that compared with open-shaped, the cage-shaped borates have a greater Lewis acidity. The introduction of electron-withdrawing groups on the aryl moieties in the cage-shaped framework increased the Lewis acidity. Substitution of a bridgehead Si for a bridgehead C decreased the Lewis acidity of the boron complexes because the large silicon atom reduces the dihedral angle of C(ipso)-O-B-O. The ligand-exchange rates of the para-fluoro-substituted compound B(OC(6)H(3)F)(3)CH and the ortho-phenyl-substituted compound B(OC(6)H(3)Ph)(3)CH were less than that of the unsubstituted borate B(OC(6)H(4))(3)CH. The ligand-exchange rate of B(OC(6)H(4))(3)SiMe was much faster than that of B(OC(6)H(4))(3)CH. A hetero Diels-Alder reaction and Mukaiyama-type aldol reactions were more effectively catalyzed by cage-shaped borates than by the open-shaped borate B(OPh)(3) or by the strong Lewis acid BF(3) ⋅OEt(2). The cage-shaped borates with the bulky substituents at the ortho-positions selectively catalyzed the reaction with less sterically hindered substrates, while the unsubstituted borate showed no selectivity.