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-（TFP B）和carabadocaborate的钠盐或铯盐从五配位氯硅烷中夺取氯，得到分子内具有配位酰胺基的四配位硅阳离子。对这些盐的X射线结构分析表明，硅原子的几何结构接近平面，反映了sp^2平面性和明显长的Si-O键距。位于硅原子中的正电荷将通过弱的分子内O□Si配位而稳定。1H <29>Si NMR谱也支持硅原子的硅离子特征。这些盐表现出有效的催化活性，如刘易斯酸，以促进甲硅烷基烯醇醚和反应性较低的羰基化合物，如酮和脂肪族醛之间的交叉羟醛缩合反应，提供高产率的相应的羟醛加成物。此外，在无溶剂条件下，烯醇化硅与[H（OEt_2）_2]+[B（C_6F_5）_4]-原位脱硅生成的阳离子硅物种作为刘易斯酸对烯醇化硅与酮的羟醛缩合反应具有很高的催化活性。因此，在阳离子硅催化剂存在下，空间位阻较小的反应活性酮顺利地与硅烯醇化物反应，以高产率得到羟醛产物。以5-配位氯硅烷为原料合成的硅烷基阳离子对烯醇化硅与α，β-不饱和羰基化合物或腈的Michael加成反应具有很高的催化活性。

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.