五甲基茂取代基硼陽離子的催化反應性探討

Abstract

低配位數硼陽離子因硼上的空p軌域與正電荷的存在，而展現出相較中性硼烷化合物有更強的路易士酸性，因此，近期有許多研究都著重在以硼陽離子為中心的路易士酸催化反應。根據硼中心的配位數，硼陽離子可分為三類，包括四配位的boronium ion、三配位的borenium ion、以及雙配位的borinium ion。其中，以雙取代的borinium ion表現出最強的路易士酸性，反應性最高，是高度不穩定的分子，因此borinium ion在催化劑的應用上，至今仍然非常稀少，這也是我們想要在硼化學上突破的地方。 在這項研究中，我們合成了兩個五甲基茂（Cp*）所穩定的超配位硼陽離子(hypercoordinate boron cation)，包含[Cp*BMes]+ 和 [Cp*BCbz]+，並將其應用於路易士酸催化反應中，如：醛、酮、以及醯胺的矽氫化反應，以及吲哚、吡咯的雜環芳烴C(sp2)–H鍵硼化反應。透過催化反應的實驗結果分析，搭配上硼陽離子與Et3PO配位反應之競爭實驗與理論計算，證實了通過將mesityl取代基改為carbazole可以透過將氮上的電子對貢獻電子到缺電的硼中心上，來穩定超配位硼陽離子轉換成雙取代硼陽離子時的過渡態，進而降低超配位硼陽離子催化劑的反應活化能，達到提升催化反應速率的效果。

Boron cations have been well-recognized as strong Lewis acids because of the positive charge on the intrinsically electron deficient boron atom. Therefore, there are substantial works applying boron cations in Lewis acid catalysis. Depending on the coordination number of the boron center, boron cations can be divided into three classes, including tetra-coordinate boronium ion, tri-coordinate borenium ion, and di-coordinate borinium ion. Among these, the di-substituted borinium ion exhibits the highest reactivity towards all sources of electron donor, making it a challenging compound to handle and unsuitable for catalytic transformations. Due to the limited availability of borinium ion-based catalysts, we aimed to explore ways to overcome this challenge. In this study, we synthesized and characterized two hypercoordinate boron cations, [Cp*BMes]+ and [Cp*BCbz]+, which can be viewed as masked borinium ions. We also examined their catalytic performance in hydrosilylation of aldehydes, ketones, and amide; N-silylation of indole; and the C-H borylation of electron-rich heteroarenes. In all cases, the carbazole-functionalized hypercoordinate boron cation, [Cp*BCbz]+, exhibits significantly enhanced reaction kinetics, suggesting that the catalytic performance of hypercoordinate boron cations can be improved by manipulating the interaction between the Cp* and the boron center. Computational and experimental investigations into the base-binding reaction of hypercoordinate boron cations reveal that the coordination of Et3PO to [Cp*BCbz]+ has a lower energy barrier than that of [Cp*BMes]+. The presence of a π-donating carbazole ligand at boron stabilizes the transition state involved in the η5–η1 transformation step, leading to a markedly lowered activation energy of the reactions.