硼陽離子的合成及反應性探討

Abstract

硼陽離子普遍被認為是比一般中性硼烷更強的路易士酸，為了增加硼陽離子的缺電子性，在過去幾年中，數個帶兩個正電荷的硼雙陽離子已經被合成並且完成其結構鑑定，但是卻沒有相關的反應性探討被報導過。先前我們實驗室成功的合成出五甲基環戊二烯基取代的雙取代硼雙陽離子，並發現此硼雙陽離子在與三乙基硼氫化鋰反應後，會對環戊二烯基進行擴環反應，進而生成氮異環碳烯配位的硼雜苯。在此篇論文中，我們深入探討此五甲基環戊二烯基取代的硼雙陽離子與其他試劑的反應，諸如親核試劑、親電子試劑、還原劑等。另一方面，我們也藉由類似的合成路徑，試圖合成胺基取代的雙取代硼雙陽離子。目前我們已成功合成出胺基取代的三配位硼陽離子，然而除去第二個氯離子以生成帶兩個正電荷的硼雙陽離子的反應比想像困難，因此目前尚未達到。

Boron cations are considered as stronger Lewis acid than neutral boranes. To further increase the electron deficiency of boron mono cation, few boron dications have been synthesized and structurally characterized. However, reactivity study of these boron dications has never been reported due to the presence of an over-stabilized boron center. Previously our group had synthesized a Cp*-substituted boron dication ([3][AlCl4]2), and discovered its unusual reaction with superhydride to yield a NHC-stabilized borabenzene (4). This result suggests that the incorporation of an electronically and coordinatively flexible substituent is imperative for realizing reactive boron dications. In this work, further reactivity studies of [3][AlCl4]2 and the synthesis of amino-substituted boron dication, [Cy2NB-IMes]2+, are reported. Reactions of [3]2+ with electrophiles, nucleophiles and reductants show that [3]2+ is more electron deficient than boron monocation and can be transformed into 4 through borylene intermediate. On the other hand, reaction of Cy2N-BCl2 with IMes in chlorobenzene directly gave the chloroborenium, [Cy2NB-IMes]+[Cl] ([6][Cl]). However, the second chloride abstraction of [6]+ was found to be difficult and remains unrealized.