Partially and fully ????-brominated Mn-porphyrins in P450 biomimetic systems: Effects of the degree of bromination on electrochemical and catalytic properties

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• 作者：Nascimento ; Eliane do ; Silva ; Gilson de F. ; Caetano ; Fabiana A. ; Fernandes ; Marcela A.M. ; et. al.
• 关键词：Manganese porphyrins ; β ; -Halogenated porphyrins ; Iodosylbenzene ; Biomimetic catalysis
• 刊名：Journal of Inorganic Biochemistry
• 出版年：2005
• 出版时间：May, 2005
• 年：2005
• 卷：99
• 期：5
• 页码：1193-1204
• 全文大小：263 K

文摘

β-Hexabromo-5,10,15,20-tetrakis(4-carbomethoxyphenyl)porphyrinatomanganese(III) chloride (Mn(III)(Br₆TCMPP)Cl) was prepared by selective Br₂-hexabromation of its parent non-brominated manganese complex (Mn(III)(TCMPP)Cl), whereas the octabrominated analogue β-octabromo-5,10,15,20-tetrakis(4-carbomethoxyphenyl)porphyrinatomanganese(III) chloride (Mn(III)(Br₈TCMPP)Cl) was synthesized via metallation of the corresponding free-base. β-Octabromo-5,10,15,20-tetrakis(4-carbomethoxyphenyl)porphyrin was obtained by demetallation of its brominated Cu(II) derivative, which, in its turn, was prepared by either a Br₂ or an N-bromosuccinimide protocol. Relative to Mn(III)(TCMPP)Cl (E_1/2 = −0.16 V vs. normal hydrogen electrode, CH₂Cl₂), the Mn(III)/Mn(II) reduction potential of Mn(III)(Br₈TCMPP)Cl and Mn(III)(Br₆TCMPP)Cl showed anodic shifts of 0.43 and 0.33 V, respectively, which corresponded to a linear shift of 0.05 V per bromine added. These manganese complexes were evaluated as cytochrome P450 mimics in catalytic iodosylbenzene (PhIO)-oxidations of cyclohexane and cyclohexene. In aerobic PhIO-oxidation of cyclohexene, epoxidation and allylic autoxidation reactions were inversely related, competitive processes; the most efficient P450-mimics were the least effective autoxidation catalysts. Mn(III)(Br₆TCMPP)Cl was more efficient as epoxidation or hydroxylation catalyst than both its fully and non-β-brominated counterparts were. There was no linear relationship between the catalytic efficiency and both the number of bromine substituents and the Mn(III)/Mn(II) potential; these observations were compared to Lyons system literature data and discussed. Analogously to enzymatic optimum pH effects, an optimum redox potential effect is suggested as relevant in designing and understanding cytochrome P450 biomimetic catalysts.