Iron-Catalyzed Enantioselective Epoxidations with Various Oxidants: Evidence for Different Active Species and Epoxidation Mechanisms

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• 作者：Alexandra M. Zima ; Oleg Y. Lyakin ; Roman V. Ottenbacher ; Konstantin P. Bryliakov ; Evgenii P. Talsi
• 刊名：ACS Catalysis
• 出版年：2017
• 出版时间：January 6, 2017
• 年：2017
• 卷：7
• 期：1
• 页码：60-69
• 全文大小：604K
• ISSN：2155-5435

文摘

Iron complexes with chiral bipyrrolidine-derived aminopyridine (PDP) ligands are among the most efficient Fe-based bioinspired catalysts for regio- and stereoselective oxidation of C–H and C═C moieties with hydrogen peroxide. Besides hydrogen peroxide, other oxidants (peroxycarboxylic acids and organic hydroperoxides) can be effectively used. In this work, we have examined the mechanistic landscape of the Fe(PDP) catalyst family with various oxidants: H₂O₂, organic hydroperoxides, and peracids. The combined EPR spectroscopic, enantioselectivity, Hammett, Z-stilbene epoxidation stereoselectivity, and ¹⁸O-labeling data witness that the same oxoiron complexes [(L)Fe^V═O(OC(O)R)]²⁺ are the actual epoxidizing species in both the catalyst systems (L)Fe/H₂O₂/carboxylic acid and (L)Fe/AlkylOOH/carboxylic acid. On the contrary, in the systems (L)Fe/R²C(O)OOH (R² = CH₃ or 3–Cl-C₆H₄), in the presence or in the absence of carboxylic acid, the epoxidation is predominantly conducted by the acylperoxo-iron(III) intermediates [(L)Fe^III(OOC(O)R²)]²⁺, in a concerted fashion.