Solvent-free Catalytic Reaction : Ideal Green Reaction Using Silicon Catalysts with Weakly Coordinating Anion

用弱配位阴离子(如B[3,5-（CF_3)_2CoH_3]_4- (TFPB)和carabadodecaborate）的钠盐或铯盐从五配位氯硅烷中提取氯，得到具有分子内配位酰胺基团的四配位硅基阳离子。这些盐的x射线结构分析表明，硅原子的近平面几何形状反映sp^2平面度和明显的长Si-O键距离。位于硅原子中的正电荷会被分子内弱的O□Si配位所稳定。^<29>Si核磁共振谱也支持硅原子的硅离子特征。这些盐作为路易斯酸表现出有效的催化活性，促进硅烯醇醚与反应性较差的羰基化合物（如酮和脂肪醛）之间的交叉醛醇反应，从而提供相应的高收率醛醇加合物。此外，在无溶剂条件下，用[H(OEt_2)_2]+[B(C_6F_5)_4]-原位脱硅制备的阳离子硅在酮与烯醇化硅醛醇反应中表现出较高的催化活性。因此，在阳离子硅催化剂的存在下，位阻较低的反应性酮与硅烯醇酯顺利反应，产生高收率的醛醇产物。由5配位氯硅烷制备的硅基阳离子对促进硅烯醇酸酯与α、β-不饱和羰基化合物或腈的Michael加成反应具有较高的催化活性。

Chloride abstraction from pentacoordinate chlorosilanes by sodium or cesium salts of weakly coordinating anions such as B[3,5-(CF_3)_2CoH_3]_4- (TFPB) and carabadodecaborate gave tetracoordinate silyl cations having intramolecularly coordinating amide groups. X-ray structure analysis of these salts indicates the near-planar geometries at the silicon atoms reflecting sp^2 planarity and markedly long Si-O bond distances. The positive charge located in the silicon atoms would be stabilized by weak intramolecular O□Si coordination. The ^<29>Si NMR spectra also support silylium ion character of the silicon atoms. These salts exhibit effective catalytic activity as Lewis acid to promote the cross-aldol reaction between silyl enol ethers and less reactive carbonyl compounds such as ketones and aliphatic aldehydes, affording high yields of the corresponding aldol adducts. Furthermore, cationic silicon species, which were generated in situ by protodesilylation of silicon enolates with [H(OEt_2)_2]+[B(C_6F_5)_4]-, exhibited high catalytic activity as a Lewis acid in the aldol reaction of ketones with silicon enolates under the solvent-free conditions. Thus, sterically hindered less reactive ketones smoothly reacted with silicon enolates in the presence of cationic silicon catalyst, giving aldol products in high yields. Silyl cations, which were prepared from 5-coordinate chlorosilanes, showed high catalytic activity in promoting the Michael addition of silicon enolates to α,β-unsaturated carbonyl compounds or nitriles.