微粒體甲烷單氧化酵素之結構與功能性之模型三核銅金屬簇化物之研究(III)

Abstract

我們首先合成出全新的配位基 7-N3Et ，能與一價銅離子配位形成三核銅離子簇化物 [CuICuICuI(7-N3Et)](X) (X = ClO4, BF4) (2)。此配位基與早先實驗室研發出的 7-Dipy 配位基相似，尾端的兩個銅離子皆以N3進行配位，但本配位基在尾端修飾上八個乙基，模擬 pMMO活性中心的疏水性環境，並藉由 ESI-MS 光譜證實經氧氣或二當量H2O2、TBHP 均可得穩定的三核銅金屬含氧簇化物 [CuIICuII(-O)CuII(7-N3Et)](ClO4)2 (3)。加入過量TBHP氧化劑，此三核銅簇化物會對自身或是鄰近簇化物的乙基進行氮上去烷基反應 (N-dealkylation) 。 以過量TBHP (10~75當量) 作為氧化劑並加入環己烷或甲苯作為外加受質的情況下，可成功氧化環己烷 (CH 鍵能為 99.3 kcal mol-1) 以及甲苯 (CH 鍵能為 90.0 kcal mol-1) 的 CH 鍵。在外加受質存在時，我們發現同時會有分子間 (對受質) 與分子內 (對簇化物氮上乙基) 傳遞氧原子的競爭反應出現，且此現象隨受質的濃度與受質 C-H 鍵能的改變，競爭效果也更加明顯。說明這一系列三核銅簇化物的確具有很高的催化活性，能使靠近簇化物的受質快速地進行羥化反應。而在無外加受質存在時，加入過量TBHP氧化劑 (50當量)，經由GC-MS證實有乙醛產物產生，推測此現象可能是簇化物氮上乙基的 -CH2- 位置先進行羥化反應，再經由氫離子的轉移，脫去乙醛分子。

關鍵字：微粒體甲烷單氧化酵素、三核銅金屬簇化物、催化

In this study, a new ligand 7-N3Et has been synthesized, and it can coordinate with 3 equivalents of CuI ions to form a trinuclear copper complex [CuICuICuI(7-N3Et)](X) (X = ClO4 or BF4) (2). This 7-N3Et ligand is similar to our previous developed 7-Dipy ligand in the scaffold to trap three CuI ions. They all have a pair of symmetric N3 coordination which consists of N1-(2-(diethylamino)ethyl) -N2,N2-diethylethane-1,2-diamine in 7-N3Et instead of 2,2-bis(pyridylmethyl)amine group for 7-Dipy. According to the efficient oxidation of cyclohexane catalyzed by [CuICuICuI(7-Dipy)]+, the [CuICuICuI(7-N3Et)]+ was adopted for comparing the catalytic effects arisen from substitution groups. The ESI-MS spectra of oxygenated [CuICuICuI(7-N3Et)]+ by dioxygen molecules, two equivalents of H2O2, and two equivalents of TBHP present a exactly same cluster signal associated with a [CuIICuII(-O)CuII(7-N3Et)]2+ trinuclear copper complex. However, with more than two equivalents of TBHP, it will encounter N-deethylation to loss a molecular weight in m/z of 28 amu and 56 amu. When one equiv. of [CuICuICuI(7-Dipy)]+ is added in the presence of 10, 25, 50 and 75 equiv. of TBHP , respectively, as oxidant with external substrates such as cyclohexane (C-H BDE : 99.3 kcal/mol) and toluene (C-H BDE : 90 kcal/mol) in the solvent of CH3CN for two hours, substrates can be oxygenated with lower TON because of competing reactions between intra-N-deethylation and inter-oxygenation. In contrast, no external substrate was added in the above condition, and the acetalaldehyde was detected by GC-MS when 50 equiv. of TBHP was used. The production of acetalaldehyde is postulated from first hydroxylation on the -CH2- of the amine group, and then disassociation by proton migration of hydroxyl group to the nitrogen atom of amine.