具活性高自旋三價鐵中心介穩定鐵-氧中間物探討
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2017
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各種鐵–氧中間物長期以來被認為參與酵素催化氧合及氧化反應過程中；其中，iron–superoxo是路徑中重要的過程。實驗室先前的研究發現Fe(BDPP)在−80 °C下將氧氣活化形成iron–superoxo中間物，並且可以進行氫原子轉移反應。另外，我們亦嘗試不同的反應，發現有別於文獻先前關於aldehyde deformylation報導，經由動力學實驗帶入Hammett equation得到負的ρ值，代表所經過的反應途徑有別於以往，非傳統的親核反應。另一方面，我們利用Fe(BDPP)與DMPO反應，展示文獻上第一個直接藉由iron–superoxo中間物形成iron–alkylperoxo中間物的模型分子。至今，有許多實驗及光譜證據顯示iron–alkylperoxo中間物可以經由O–O斷鍵形成高價Fe=O物種。使用isopropyl 2-iodoxybenzoate (IBX-ester) 與Fe(BDPP)反應嘗試合成Fe=O物種，但從UV–vis滴定實驗顯示鐵:氧比為2:1。稍後我們發現此介穩定物種對於受質並無反應發生，但若先預混入受質可鑑定出25%的產物，由於無法單離出此物種，故藉由Evans method搭配密度泛函理論了解其特性，推測此介穩定物質非Fe=O物種，而是相對穩定的FeIII2(μ–O)中間物。
Several iron–oxygen intermediates have long been known participating in enzymatic oxygenation and oxidation processes. The initial processes of the reactions is the formation of iron–superoxide. In our earlier work about activation of dioxygen, an end-on iron–superoxo intermediate was formed from the reaction of Fe(BDPP) compound with dioxygen at −80 °C. The iron–superoxo intermediate can proceed hydrogen atom abstraction. To extend the reaction diversity, the iron–superoxo intermediate was reacted with aldehyde deformaylation bearing unique negative ρ value analyzed by Hammett equation. The negative ρ value suggests the reaction does not fit the nucleophilic trend. On the other hand, we demonstrated the first iron–alkylperoxo intermediate directly via iron–superoxo intermediate reacting with DMPO. There are strong evidence in literature that conversion of iron(III)–alkylperoxo complex to its corresponding iron–oxo complex occurs via O–O bond cleavage. Reacting Fe(BDPP) with isopropyl 2-iodoxybenzoate (IBX-ester) at −80 °C gave a metastable intermediate. The UV–vis titration experiments revealed that the intermediate has iron to oxygen atom of 2:1 ratio. From reactivity tests the resulting species unexpectedly shows no reactivity towards neither hydrogen atom abstraction nor oxygen atom transfer reaction. Only when the hydrogen donating substrate was premixed before IBX-ester added, about 25% product is analyzed. These results rule out the assignment of iron–oxo species but imply an inert intermediate postulated to be a dimeric Fe–O–Fe species. Since the intermediate could not be isolated due to its instability, Evans method of the resulting solution and DFT calculations were utilized to identify the proposed intermediate.
Several iron–oxygen intermediates have long been known participating in enzymatic oxygenation and oxidation processes. The initial processes of the reactions is the formation of iron–superoxide. In our earlier work about activation of dioxygen, an end-on iron–superoxo intermediate was formed from the reaction of Fe(BDPP) compound with dioxygen at −80 °C. The iron–superoxo intermediate can proceed hydrogen atom abstraction. To extend the reaction diversity, the iron–superoxo intermediate was reacted with aldehyde deformaylation bearing unique negative ρ value analyzed by Hammett equation. The negative ρ value suggests the reaction does not fit the nucleophilic trend. On the other hand, we demonstrated the first iron–alkylperoxo intermediate directly via iron–superoxo intermediate reacting with DMPO. There are strong evidence in literature that conversion of iron(III)–alkylperoxo complex to its corresponding iron–oxo complex occurs via O–O bond cleavage. Reacting Fe(BDPP) with isopropyl 2-iodoxybenzoate (IBX-ester) at −80 °C gave a metastable intermediate. The UV–vis titration experiments revealed that the intermediate has iron to oxygen atom of 2:1 ratio. From reactivity tests the resulting species unexpectedly shows no reactivity towards neither hydrogen atom abstraction nor oxygen atom transfer reaction. Only when the hydrogen donating substrate was premixed before IBX-ester added, about 25% product is analyzed. These results rule out the assignment of iron–oxo species but imply an inert intermediate postulated to be a dimeric Fe–O–Fe species. Since the intermediate could not be isolated due to its instability, Evans method of the resulting solution and DFT calculations were utilized to identify the proposed intermediate.
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超氧化鐵錯合物, 介穩定中間物, iron–superoxo, aldehyde deformaylation, metastable intermediates, iron–μ–oxo