Asymmetric epoxidation (AE) is undoubtedly one of the most significant reactions since one or even two stereogenic centers are constructed, and the resulting epoxides can be further transformed in the synthesis of chiral molecules. Over the past decades, numerous efforts have been dedicated to the catalytic AE to achieve epoxides with high enantioselectivities and yields. For example, the Ti-catalyzed epoxidation by Sharpless, the salen-Mn(III) complexes by Jacobsen and Katsuki independently, and the chiral ketones by Shi are among the greatest breakthroughs in AE. Despite the tremendous progress of these recognized protocols, currently, the pursuit of efficient catalytic systems is still highly desirable from both the atom economic and ecological point of view. Notably, the natural non-heme enzymes can activate oxygen to carry out oxidation reactions under mild conditions, which may offer an alternative way to the sustainable goal of AE. Inspired by these structure motifs and powerful oxidative properties of the active sites of such metalloenzymes, many biomimetic catalysts have been synthesized and applied to the AE. Indeed, the bioinspired AE is an appealing method because low catalyst loadings (as low as 0.01 mol%) and environmentally benign oxidants like hydrogen peroxide (H2O2) are often employed in this catalytic system. More importantly, probing into the concomitant metal-oxygen intermediates in bioinspired models will also benefit the illumination of mechanisms in non-heme enzymes in return. The landmark studies of AE catalyzed by bioinspired aminopyridine manganese complexes began at 2003 by Stack and co-workers. They described that a manganese complex [Mn(R,R-MCP)(OTf)2 could rapidly catalyze the epoxidation of electron-deficient olefins using peracetic acid (AcOOH) as the oxidant with high turnover numbers (TONs). After the pioneer work of Stack, versatile manganese and iron complexes containing aminopyridine N4 ligands were reported and proved to be active in the catalytic AE with H2O2 as the oxidant. Herein, we will present the preparation, reactivity and substrate scope of bioinspired manganese complexes supported by various novel N4 ligands.

Highly enantioselective epoxidation of olefins with H2O2 catalyzed by bioinspired N4 manganese complexes

254th National Meeting (2017)

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