CAAC-Coordinated Boron Dication: The Effect of π-Acidic Ligand on the Reactivity of Boron Electrophile

在本論文中，我們合成並鑑定了環烷基胺基碳烯配位之二氯硼烷加成物、三配位硼陽離子以及硼正二價陽離子。經由單晶X光繞射分析可確認，環烷基胺基碳烯配位之硼正二價陽離子之結構近似於已被報導的氮異環碳烯配位之硼正二價陽離子，為一「直線型」結構。實驗觀察以及理論計算結果都顯示，比起氮異環碳烯配位之硼正二價陽離子，環烷基胺基碳烯配位之硼正二價陽離子有更強之路易士酸性。經由反應性研究發現，環烷基胺基碳烯配位之硼正二價陽離子具有三個親電子位置。根據親核基的性質，親核反應分別會發生在硼二價陽離子中心、五甲基環戊二烯基團之配位碳以及環烷基胺基碳烯之碳烯中心上。這種在單一分子上具有三個親電子位置的小分子非常少見。如果將環烷基胺基碳烯配位之硼正二價陽離子以及三配位硼陽離子以還原劑還原，分別可以得到硼自由基陽離子以及硼自由基分子。由於硼中心具有較高的缺電子性質，硼自由基陽離子的電子自旋共振光譜圖具有極高之硼超精細耦合常數值。CAAC-coordinated dichloroborane adduct 1a, chloroborenium [2a]+ and boron dication [3a]2+ have been synthesized and characterized. The structure of [3a]2+ is confirmed by X-ray diffraction analysis and is similar to the previously reported NHC-coordinated boron dication. The enhanced Lewis acidity of [3a]2+ compared to its NHC-congener was demonstrated by experimental observation and theoretical calculation. Reactivity study revealed that [3a]2+ possesses three electrophilic sites (the boron center, the Cp* ring and the CAAC carbene center) according to the nature of the nucleophile. This is a rare case that three electrophilic sites reside on a single small molecule. Chemical reduction of [3a]2+ and [2a]+ yielded the corresponding radical cation [6]+• and neutral radical [7]•, respectively. The exceptionally large boron hyperfine coupling constant of [6]+• is attributed to the enhanced electron-deficiency of the boron center