帶負電荷之雙牙氮異環碳烯

Abstract

我們新合成了一種具有鄰苯二酚的官能基的氮異環碳烯前驅物。這樣的苯咪唑鹽與對苯二硼酸反應之後意外地得到了以硼酸根連結的雙牙苯咪唑鹽，並且藉由單晶X光繞射儀來鑑定它的結構。這樣意外的發現讓我們想要知道它形成的反應機制，所以我們藉由一系列的實驗與理論計算來推論整個反應進行的過程。 我們將此新合成的雙牙苯咪唑鹽加上足夠化學當量的強鹼之後，即可獲得帶負電荷的雙牙氮異環碳烯。將此雙牙氮異環碳烯與硼烷二甲硫醚反應之後，就可以得到雙牙氮異環碳烯的硼烷加合物，此加合物的生成可由異核核磁共振光譜證實，並且藉由單晶X光繞射儀來鑑定它的結構。 最後，我們也合成了此雙牙氮異環碳烯的銠金屬錯合物，並藉由銠金屬上羰基的震動頻率來分析此雙牙氮異環碳烯的電子給予能力。結果證明，此帶負電荷的雙牙氮異環碳烯的推電子能力介於電中性的氮異環碳烯與負電荷的單牙氮異環碳烯之間。

In this work, we synthesized NHC precursors (4a-4c) featuring the catechol functionality. An unexpected spiroborate linked bis-imidazolium molecule (5a) was isolated by treating 4a with 1,4-benzenediboronic acid, and was characterized by single crystal X-ray diffraction. The mechanism of the formation of 4a was clarified by experiments and theoretical calculations. The corresponding free anionic bis-carbene 6a was obtained by treating 5a with stoichiometric amount of base. The corresponding diborane complex 7a and di-Rh(COD)Cl complex 8a were synthesized by treating bis-carbene 6a with BH3•SMe2 and [Rh(COD)Cl]2, respectively. Di-Rh(CO)2Cl complex 9a was obtain by bubbling CO into the solution of 8a. The lower CO stretching frequency of 9a indicates the better donating strength of 6a compared with neutral NHCs.