酚硼烷與酚硼烷自由基之合成、磁性質、與穩定性探討

Abstract

苯酚與酚鹽為天然抗氧化劑，而其氧化反應之產物一般被認為是苯氧自由 基，然而，對於苯氧自由基之分解反應的研究尚屬少見。在這篇論文中，我們製備一系列的酚硼烷自由基分子，並且經由電子自旋共振實驗可以發現，不成對電子與中心硼原子核(11B、10B)的超精細耦合常數分別為4.8G 及1.6G。有趣的是，相對於另一個穩定的二均三甲苯基硼取代的苯氧自由基分子，我們製備的酚硼烷自由基分子在室溫下無法穩定存在，其分解產物為碳-碳偶聯產物，實驗結果證實其反應中間體為自由基二聚體，且此二具體的穩定性與溶劑的配位能力有高度關聯性。另一方面，我們也發現酚鹽的雙電子氧化反應會形成不穩定的單重態雙自由基分子，而後迅速分解成複雜的混合物。

Phenols and phenolates have been known as natural antioxidants and their oxidation products are generally the phenoxyl radicals; however, the studies of the decomposition pathway of phenoxyl radicals are relatively rare. In this work, a series of bisphenoxyboryl monoradicals have been prepared and their ESR measurements confirm the delocalization of the unpaired electron to the boron center with hyperfine-coupling constants of 4.8 G and 1.6 G to the 11B (I = 3/2) and 10B (I = 3) nuclei, respectively. Interestingly, compared to the persistent dimesitylboryl phenoxyl radical, the bisphenoxyboryl mono-radicals are unstable at room temperature and decompose to yield the carbon-carbon coupling product.The decomposition process of the reported mono-radicals is postulated to proceed through an ESR-silent radical dimer, which features a longer lifetime in non-coordinating solvent. On the other hand, two-electron oxidation of the borylated bisphenoxides results in a complex mixture, which is proposed to arise from the formation of the unstable singlet borylated bisphenoxydiradicals.