具分子內作用力的氮異環碳烯之硼自由基

Abstract

自從Gomberg教授發表第一個自由基三苯甲基自由基後，更多不同的自由基接著被單離及鑑定。第二週期的元素當中，相對於較常見的以碳、氮、氧為中心的自由基，中性硼中心周圍只有五個電子，故在合成上較為困難。因此可以利用路易士鹼或陰離子取代基來配位到硼自由基上以降低硼身上的缺電性。雖然氮異環碳烯是一個穩定硼自由基很好的取代基，但是穩定的hydridoboryl自由基仍沒辦法被成功合成出。因此我們想要在氮異環碳烯旁邊接上hemilabile取代基來讓電子密度分散以使得hydridoboryl自由基更加穩定。藉由改變吡啶的對位的取代基以及硼中心上的取代基，我們可以讓硼自由基更加穩定。利用兩種不同合成自由基的方法可以得到四取代基的中性硼自由基以及三取代基的帶正電硼自由基，且利用他們來進行自由基聚合反應來比較他們差異。

Since the detection of triphenylmethyl radical by Gomberg, a great amount of efforts were devoted to the characterization and isolation of radical species. Compared to other stable radicals of the second-row elements such as carbon, nitrogen, and oxygen, neutral boron-centered radical having only five electrons around the boron center remains a difficult synthetic target. To reduce the electron deficiency of the boron atom, anionic ligand or Lewis base was utilized to occupy the empty orbital of boron. Although N-heterocyclic carbene (NHCs) has been classified as a good ligand to stabilize BR2 neutral radicals, stable hydridoboryl radical (BHR ) still cannot be accomplished. Thus, we propose to tether a hemilabile functional group to NHC ligand to mitigate the reactivity of hydridoboryl radical through coordinating the boron center and delocalizing the unpaired electron density. By changing the para-R group of the pyridyl sidearm and the aryl group on the boron, we were able to further enhance the stability of the radical. With two different radical generation methods, neutral radicals featuring tetra-coordinate boron center and cationic tri-coordinate boron radicals were obtained, and their applications in radical polymerization were also examined.