五甲基茂取代基硼陽離子的合成與反應性探討

Abstract

在過去幾十年，具有強路易斯酸性的中性硼化合物和三配位硼陽離子已經被 廣泛地應用於催化反應上，例如:醛、酮類化合物的矽氫及矽氰加成反應。雙配 位硼陽離子由於反應性過高，所以至今沒有有效的催化應用。為了有效保護雙配 位硼陽離子上的兩個空軌域，我們提出具有五甲基茂取代基 (Cp*) 的超配位硼 陽離子，可藉由 Cp* 與硼中心在 η5 與 η1 之間的切換來達到高反應性及高穩 定性，因此可以視為一種被保護的雙配位硼陽離子 (maskedborinium)。在此基礎 上，我們合成了一系列[Cp*-B-R]+超配位硼陽離子，並將 [Cp*-B-R]+ 應用於路 易士酸催化反應中，包括矽氫及矽氰還原反應。在催化矽氫反應上，[Cp*-B-Mes]+ 是第一個能催化酮類去氧反應的硼陽離子，而藉由調控催化劑的推拉電子性，可 以選擇性的生成矽氫化產物或去氧化產物。[Cp*-B-Mes]+ 在矽氰反應表現也非 常優異，其動力學研究表明，[Cp*-B-Mes]+ 與三甲基氰矽烷 (TMSCN) 作用形 成的三配位硼陽離子是反應中真正的催化劑。透過實驗結果與理論計算得知， Cp* 在矽氰化醛酮反應中，能藉由穩定酸性的三配位硼陽離子，使得催化劑免於 受到親核攻擊而分解。[Cp*-B-Mes]+ 在矽氫還原腈類以及氮,氮-二甲基苯甲醯胺 類表現也非常優異。此研究顯示出藉由引入 Cp* 形成的超配位硼陽離子，可以 穩定硼陽離子中心，並可以被應用在催化反應上。

In the past decades, boron species including neutral boranes and tri-coordinate borenium cations have been widely applied in catalysis, such as hydrosilylation and cyanosilylation. However, two-coordinated boron cation, borinium, has never been utilized for catalytic application due to the high reactivity of the four-electron boron species. We hypothesize that the introduction of electronically and coordinatively flexible ligand, like pentamethylcyclopentadienyl (Cp*), might be the solution to realize catalytically active borinium ion. Hypercoordinated boron cation, stabilized by Cp*, can be viewed as masked borinium with high reactivity and high stability due to the feasible hapticity change of Cp* between η5 and η1. In this study, we have prepared a series of hypercoordinate boron cations, [Cp*-B-R]+, and explored their catalytic applications, including hydrosilylation and cyanosilylation. For hydrosilylation, [Cp*BMes]+ ([1]+) is the first boron cation capable of catalyzing hydrodeoxygenation of ketone. By tuning the electronic property of [Cp*-B-R]+, selective generation of hydrosilylation and hydrodeoxygenation products can be achieved. [1]+ also turned out to be an excellent catalyst in cyanosilylation of ketones. Mechanistic investigation of the process revealed that [1-TMSCN]+ resulted from complexation of [1]+ and TMSCN is the real catalyst. Computational study suggested that the coordination of Cp* on the boron center could prevent the electron deficient B+ center from nucleophilic attack, v resulting in the decomposition of the catalyst. [1]+ also has high catalytic performance in hydrosilylation of nitriles and N,N-dimethyl benzamides. This study showed that the incorporation of a coordinatively flexible substituent at boron is critical in achieving catalytic activity of borinium cations.