以矽烯穩定硼陽離子之合成與反應性研究

Abstract

三配位的硼陽離子已被應用於硼化試劑與催化不飽和分子氫化反應，雖然目前許多以碳烯當作配位基之硼陽離子已被發表出來，但是以碳烯的重類似物當作配位基穩定的硼陽離子仍尚未被發表。在此篇論文中，我們合成出的兩個具有矽烯配位基的硼陽離子，一個是有三氟甲磺酸當作緊密離子對的矽烯配位9-硼二環［3.3.1］壬烷的硼陽離子，另一個是五甲基環戊二烯取代的矽烷配位的硼陽離子。在前者的親核性反應性研究中，微弱的矽硼間作用力均導致矽烯配位基的生成，而在一個電子的還原反應實驗中，我們可以得到一個短暫生成的自由基物種，此自由基會從環境中獲取氫原子成為矽烯與9-硼二環［3.3.1］壬烷；另一方面，由五甲基環戊二烯取代的矽烷配位的硼烷可經除氯反應得到硼陽離子，此硼陽離子可經一個電子的還原反應得到矽自由基，而這些自由基皆可用電子順磁共振儀來證明。

Borenium cations, tri-coordinate boron cations, have been applied as effective borylation reagents and catalysts for hydrogenation of unsaturated molecules. Although several studies on carbene stabilized borenium have been published, the boron cations stabilized by heavier congeners of carbene have never been reported. In an attempt to explore the bonding nature between boron and group 14 metallylenes, we have synthesized, in this work, two boron cations that feature silylene ligand in the coordination sphere. One is a silylene-coordinated 9-BBN borenium that forms a contact ion pair with its counter anion, triflate. The other one is a Cp*-substituted silyl borinium cation resulted from sequential reaction of silylene and boron halide. All of reactivity studies on the contact ion pair of silylene-9-BBN borenium towards various nucleophiles resulted in the dissociation of free silylene, implying the existence of considerably weak Si-B bond. The chemical reduction of the contact ion pair yielded a short-lived radical species, in which the unpaired electron is mainly localized at the boron center. Unfortunately, the radical would undergo further decomposition to yield free silylene and 9-BBN via hydrogen atom abstraction from the environment. On the other hand, the reaction between NHSi and Cp*BCl2 led to the formation of silylborane, that could be further transformed in to boron cation by chloride abstraction. The chemical reduction of the cation resulted in the formation of a boron cation tethered silicon radical, which has been investigated with EPR spectroscopy.