五甲基茂取代基硼陽離子的反應性探討

Abstract

矽氫還原反應是合成有機矽化合物的重要催化反應。在過去的幾十年中，強路易斯酸包括中性硼化合物和三配位硼陽離子已經廣泛地應用於此領域，但是這類型的反應卻不能被雙配位硼陽離子所催化。因此，我們嘗試將五甲基茂取代基置於硼中心，希望能獲得較穩定的雙配位硼陽離子來作為矽氫還原反應的催化劑。我們實驗室在先前已經成功製備出五甲基茂取代基的超配位硼陽離子化合物[Cp*-B-Mes]+ ([1]+)，並且發現它在活化矽烷以進行羰基還原反應方面有非常高的轉換率，能產生所需的矽醚以及過度還原的醚類。但也因為[1]+擁有較高的路易士酸性，造成其不能選擇性地得到較高產率的矽醚產物，為了提高矽氫還原反應的選擇性，我們在苯環取代基上加上推電子取代基團，希望能藉此提高矽醚的產率。因此，本實驗合成了具有推電子基團的硼陽離子，並探討其催化活性。此外，我們還發現這類硼陽離子具有其他的反應，例如Frustrated Lewis Pair的反應，以及與立障較小的路易斯鹼反應。

Hydrosilylation is a key catalytic reaction for the production of organosilicon compounds. In the past decades, strong Lewis acids have also been applied in hydrosilylation studies including neutral boranes and tri-coordinate borenium cations. However, such reaction could not be activated by borinium cations. Therefore, we decided to include Cp* substituent to borinium cation to realize catalytic turn over at the boron center. Thus, the corresponding mesityl-substituted hypercoordinate borinium cation, [Cp*-B-Mes]+ ([1]+) was prepared and examined for hydrosilylation application. Compound [1]+ is highly effective in activating silane for carbonyl reduction yielding the desired silyl ethers and the over reduced ether. The extent of reduction cannot be controlled with [1]+ potentially due to the extreme acidity of [1]+. To improve the selectivity of hydrosilylation, we propose that electron-donating substituent on the phenyl group could improve the selectivity for silyl ether. Thus, the more electron rich boron cations were prepared and their catalytic activities were examined. In addition, we have also investigated the reaction of these boron cations towards Lewis bases to realize Frustrated Lewis Pairs or typical acid-base adducts.