硼烷基取代氮異環碳烯：合成與反應機構探討

Abstract

氮異環碳烯對於過渡金屬甚至是主族元素一直是相當重要的配基，為了要更加廣泛研究氮異環碳烯及其錯合物的應用性，許多種不同推電子基團修飾的碳烯分子已經被報導，並應用在催化反應上。然而，具有拉電子基團的氮異環碳烯研究，卻比較少被探討。為了要了解拉電子的硼取代基對於氮異環碳烯的影響，我們合成一系列、在1號位有不同硼烷基取代的咪唑鹽，並嘗試其碳烯的合成。有趣的是，當我們嘗試要進行去質子化的過程當中，我們觀察到硼烷基的1,2-位移的現象，進而產生2-硼烷取代咪唑。為了研究此1,2-位移反應的反應機構，我們設計了一系列的實驗。在交錯實驗中，我們證明此1,2-位移反應是一個分子間的重組過程；在自由基捕捉實驗中顯示，在去質子化的過程中會有硼-氮斷鍵的發生，並生成兩個自由基。搭配上交錯實驗的結果，推測這兩個自由基分子應有一定的穩定度，可以脫離溶劑作用進行分子間重組。最後，我們也根據一系列的研究結果搭配理論計算，推測出一個較為完善的、可能的反應機構。

N-Heterocyclic Carbenes (NHCs) have been regarded as promising supporting ligands for their great bonding ability toward transition metal and main group element. To broaden chemistry of NHCs and NHC complexes, several electron-donating functional groups have been incorporated into the imidazole ring. Electron-withdrawing group, however, received considerable less attention. In an attempt to realize the N-borylated N-heterocyclic carbene, a series of N-borylated imidazolium salts have been prepared and characterized. Interestingly, the deprotonation of the N-boryl imidazolium salt initiated a 1,2-boryl shift from nitrogen atom to the C2 carbon of imidazole to yield the corresponding 2-boryl imidazole. In order to understand the migration process, we have carried out the crossover experiment, which confirmed the inter-molecular migration processes upon deprotonation. Also, radical trapping experiments revealed the presence of two unusual radical species, a Li+ coordinated NHC radical and a diarylboryl radical, originated from homolytic B-N bond breaking. Therefore, the 1,2-boryl migration of NHC can be explained. Upon generation of the putative N-boryl NHC, a B-N bond cleavage occurred to generate two stable radicals, which could escape the solvent cage and recombine randomly to accomplish the intermolecular migration process.