Evidence for the Participation of Two Distinct Reactive Intermediates in Iron(III) Porphyrin Complex-Catalyzed Epoxidation Reactions

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• 作者：Wonwoo Nam ; Mi Hee Lim ; Ha Jin Lee ; and Cheal Kim
• 刊名：Journal of the American Chemical Society
• 出版年：2000
• 出版时间：July 19, 2000
• 年：2000
• 卷：122
• 期：28
• 页码：6641 - 6647
• 全文大小：118K
• 年卷期：v.122,no.28(July 19, 2000)
• ISSN：1520-5126

文摘

We have studied the competitive epoxidations of olefins with cis- and trans-stilbenes and withcyclooctene and trans-stilbene in iron porphyrin complex-catalyzed epoxidation reactions by H₂O₂, tert-butylhydroperoxide (t-BuOOH), and m-chloroperoxybenzoic acid (m-CPBA) in protic solvent (i.e., a solvent mixtureof CH₃OH and CH₂Cl₂) and aprotic solvent (i.e., a solvent mixture of CH₃CN and CH₂Cl₂) at room temperatureunder catalytic reaction conditions. The competitive epoxidations were also carried out with in situ generatedhigh-valent iron(IV) oxo porphyrin cation radical complexes in aprotic solvent under stoichiometric reactionconditions. By determining the ratios of epoxide products formed in the competitive epoxidations, we wereable to conclude unambiguously that the reactive species generated in protic solvent are high-valent iron(IV)oxo porphyrin cation radical complexes 3 and the intermediates formed in aprotic solvent are oxidant-ironporphyrin intermediates 2. A protic solvent such as methanol is proposed to function as a general-acid catalyst,thereby increasing the rate of O-O bond cleavage of 2 to form 3. In the absence of general-acid catalysis suchas in aprotic solvent, the rate of O-O bond cleavage of 2 is relatively slow and 2 transfers its oxygen toolefins prior to the formation of 3. To further examine the effect of the general-acid catalysis on the nature ofepoxidizing intermediates, we carried out competitive epoxidations in the solvent mixtures of alcohol/CH₂Cl₂using alcohols of varying pK_a values and in the presence of an acid (i.e., HClO₄) in aprotic solvent. Theproduct ratios were found to vary depending on the strength of the solvent acidity, demonstrating that thereaction of 2 with olefin competes with the O-O bond cleavage of 2 that leads to the formation of 3. We alsoreported for the first time that a high-valent iron(IV) oxo porphyrin cation radical intermediate containingelectron-deficient porphyrin ligand shows an unexpected preference for trans-stilbene over cis-stilbene in thecompetitive epoxidations of cis- and trans-stilbenes.