Couzijn Erik P A, Ehlers Andreas W, Schakel Marius, Lammertsma Koop
Department of Organic Chemistry, Faculty of Sciences, Vrije Universiteit, De Boelelaan 1083, NL-1081 HV Amsterdam, The Netherlands.
J Am Chem Soc. 2006 Oct 18;128(41):13634-9. doi: 10.1021/ja0645887.
The exceptional stability of recently reported pentaorganosilicates is investigated by bond energy analyses. Experimental coupling constants are used to probe their electronic structure, entailing bonds with mixed ionic-covalent character. Our analyses reconfirm that the axial bonds are more prone to heterolytic cleavage than are the equatorial bonds. Aryl substituents provide substantial electronic stabilization by charge delocalization, but cause steric crowding due to ortho-hydrogen repulsion. In contrast, silicates with two ax,eq biaryl groups are not congested. The remaining substituent is confined to an equatorial site, where it is insensitive to elimination. These concepts adequately explain the experimentally observed stability trends and are valuable for designing other stable pentaorganosilicates.
通过键能分析研究了最近报道的五有机硅酸盐的异常稳定性。实验耦合常数用于探测它们的电子结构，涉及具有混合离子-共价特征的键。我们的分析再次证实，轴向键比赤道键更容易发生异裂裂解。芳基取代基通过电荷离域提供显著的电子稳定性，但由于邻位氢排斥会导致空间拥挤。相比之下，含有两个轴向-赤道联芳基的硅酸盐则不存在拥挤现象。其余取代基局限于赤道位置，在那里它对消除反应不敏感。这些概念充分解释了实验观察到的稳定性趋势，并且对于设计其他稳定的五有机硅酸盐具有重要价值。
