雙取代鋁自由基之合成

Abstract

近年來含有硼(B)、鎵(Ga)、銦(In)或鉈(Tl)的雙取代13族自由基已成功被合成和發表。然而，由於鋁的缺電子特性與高路易士酸性，目前尚未成功分離出雙取代鋁自由基。這裡我們提出三種分子設計來嘗試合成以鋁為中心的自由基。借鑑於先前關於硼陽離子自由基 ([cAACCyBTMP]•+)的研究，我們採用了正交推拉效應(orthogonal push-pull effect)。我們成功分離並利用單晶X射線繞射鑑定了三取代的中性鋁自由基[cAACCyAlHMDS(Cl)]•，並且這是目前第二個被發表的單體形式的中性三取代鋁自由基。然而，我們嘗試進一步拔氯生成[cAACCyAlHMDS]•+卻未能成功。為此，我們將HMDS替換為Cp*取代基，目的是為了使π電子能夠更好的給到鋁中心。我們合成了起始物cAACCyAlCp*(Cl2)。雖然未能成功分離經過還原後的[cAACCyAlCp*(Cl)]•，但EPR訊號證實了這個自由基的存在。然而，由於該化合物的穩定性不足，無法進一步進行拔氯的過程，因此未能成功合成雙取代鋁自由基陽離子。我們推測是因為鋁的高路易士酸性是鹵素難以拔除的原因，因此，我們將不帶電的cAAC改為帶負電的NHB來提昇鋁原子上的電子密度，並成功合成了[NHBAlCp*]+，並通過單晶結構確認其結構，也對其進行了單電子還原反應。雖然目前仍無法分離[NHBAlCp*]•，但自由基捕捉實驗成功證實了[NHBAlCp*]•的存在，為鋁自由基分子的研究前進一大步。

Di-substituted group 13 radical species, such as those with B, Ga, In, or Tl, have recently been reported. However, the isolation of di-substituted aluminum radical has not been achieved, primarily due to its electron deficiency and high Lewis acidity. Here, we propose three synthetic strategies to achieve aluminum centered radicals. Drawing inspiration from our previous work on the boron radical cation ([cAACCyBTMP]•+), the orthogonal push-pull effect and captodative effect were utilized as the design principle for the di-substituted aluminum radical. To this end, a tri-substituted neutral aluminum radical [cAACCyAlHMDS(Cl)]•, the precursor of the anticipated radical cation, was successfully isolated and characterized by SC-XRD. However, all attempts at the following chloride abstraction to generate [cAACCyAlHMDS]•+ were unsuccessful. Considering that one Al-N π-interaction might be insufficient in stabilizing low-coordinate Al atom, HMDS group was replaced with Cp* ligand for a better π-electron donating ability. Chemical reduction of cAACCyAlCp*(Cl2) was performed. Although not successfully isolated, the observed EPR signals suggested the formation of [cAACCyAlCp*(Cl)]• in the solution. Unfortunately, its instability prevented the subsequent chloride abstraction, so the desired di-substituted aluminum radical cation was not accomplished. To mitigate the electron deficiency of low-coordinate Al radical, neutral cAAC ligand is replaced by the isoelectronic boryl anion. Consequently, di-substituted aluminum radical [NHBAlCp*]• was successfully generated from one-electron reduction of [NHBAlCp*]+. While the isolation of [NHBAlCp*]• remains challenging, its existence could be verified via radical trapping experiment.