鍺烯與錫烯對不同硼化合物之反應性探討

Abstract

硼烷是一個含有三個取代基以及一個空p軌域的中性分子，基於硼原子中心的路易士酸性，可以在有機化學中有許多應用。為了增加此分子的缺電子性已提高其反應性，增加硼原子的正電荷是一個有效的方法，在硼陽離子的其中一種分類為三配位硼陽離子，代表在單一分子中具有兩個取代基及一個中性配基，相較於反應性極高的二配位硼陽離子與過於被穩定的四配位硼陽離子，平面的三配位硼陽離子對有機上的轉變反應同時擁有不錯的穩定性與反應性。另一方面，其中一種名為單芽氮異環碳烯之重元素相似物的性質及配位特性已經被研究了數十年，碳烯之重元素相似物擁有兩個活性位置：一對孤電子對(路易士鹼)與一個空p軌域(路易士酸)，對於重元素相似物來說，受到金屬中心的s與p軌域混成不佳的影響，路易士酸性因而提高且路易士鹼性下降。 受到碳烯穩定的三配位硼陽離子的啟發，我們決定致力於將碳取代為其重元素相似物，包含鍺與錫，來穩定三配位硼陽離子。我們認為必須要先合成一系列的碳烯之重元素相似物與硼烷的加成物，接著再加入不同的拔氫或拔鹵素試劑來產生三配位硼陽離子。然而，初步的結果顯示碳烯之重元素相似物與硼烷的反應對個別試劑的立體障礙程度有高度的關聯性。目前為止的結果為，其一是碳烯之重元素相似物上的取代基會轉移到硼中心上，其二是兩者完全沒有發生反應。

Borane is a neutral molecule with three substituents and one empty p orbital. There are many applications of boranes in organic chemistry based on the Lewis acidity of the boron center. In order to enhance the electron deficiency of the molecule to improve reactivity, accumulation of the positive charges on boron is an effective way. One of the classes of boron mono-cation is borenium, which has two substituents and one neutral ligand. Compared to the extremely reactive borinium and over stabilized boronium, planar tri-coordinate borenium possesses moderate stability and reactivity for application in organic transformations. On the other hand, being the heavier analogues of N-heterocyclic carbene (NHC), N-heterocyclic metallylenes (NHEs, E = Si, Ge, Sn, Pb) have been investigated for decades. Metallylenes possess two active sites: a lone pair (Lewis base) and a vacant p orbital (Lewis acid). For heavier analogues, the poor s-p orbital mixing leads to increased Lewis acidity and decreased Lewis basicity at the metal center. Inspired by the reactivity of carbene-stabilized borenium cations, we decided to dedicate our efforts to replace carbon with other group fourteen elements, including germanium and tin. We propose that the metallylene-borane adducts could be synthesized, and followed by treating with particular hydride or halide abstracters would form the borenium moiety. However, preliminary results show that the reaction between metallylene and borane is highly dependent on the steric hindrance of both reagents. Thus far, metallylene either transferred its substituents to boron center or does not react with boron derivatives at all.