Unfunctionalized olefins metal-catalyzed epoxidation

Asymmetric epoxidation of olefins is an effective approach for the synthesis of enan-tiomerically enriched epoxides. A variety of efficient methods have been developed [1, 2], including Sharpless epoxidation of allylic alcohols [3, 4], metal-catalyzed epoxidation of unfunctionalized olefins [5-10], and nucleophilic epoxidation of electron-deficient olefins [11-14], Dioxiranes and oxazirdinium salts have been proven to be effective oxidation reagents [15-21], Chiral dioxiranes [22-28] and oxaziridinium salts [19] generated in situ with Oxone from ketones and iminium salts, respectively, have been extensively investigated in numerous laboratories and have been shown to be useful toward the asymmetric epoxidation of alkenes. In these epoxidation reactions, only a catalytic amount of ketone or iminium salt is required since they are regenerated upon epoxidation of alkenes (Scheme 1). 

Epoxides are very versatile intermediates, and asymmetric epoxidation of olefins is an effective approach to the synthesis of enantiomericaUy enriched epoxides [1-3]. Great success has been achieved for the epoxidation of allyhc alcohols [1], the metal-catalyzed epoxidation of unfunctionalized olefins (particularly conjugated cis- and tri-substituted) [2], and the nucleophilic epoxidation of electron-deficient olefins [3]. In recent years, chiral dioxiranes have been shown to be powerful agents for asymmetric epoxidation of olefins. Dioxiranes can be isolated or generated in situ from Oxone (potassium peroxymonosulfate) and ketones (Scheme 3.1) [4,5]. When the di-oxirane is used in situ, the corresponding ketone is regenerated upon epoxidation. Therefore, in principle, a catalytic amount of ketone can be used. When a chiral ketone is used, asymmetric epoxidation should also be possible [6]. Extensive studies have been carried out in this area since the first chiral ketone was reported by Curd in 1984 [7]. This chapter describes some of the recent progress in this area. 

In 1990, Jacobsen and subsequently Katsuki independently communicated that chiral Mn(III)salen complexes are effective catalysts for the enantioselective epoxidation of unfunctionalized olefins. For the first time, high enantioselectivities were attainable for the epoxidation of unfunctionalized olefins using a readily available and inexpensive chiral catalyst. In addition, the reaction was one of the first transition metal-catalyzed... 

This chapter presents an overview of existing strategies for asymmetric catalytic epoxidation of unfunctionalized olefins with synthetic catalysts. The significant progress in metal-catalyzed oxo transfer and dioxirane epoxidation has increased the accessibility of several classes of optically active epoxides and concurrently highlighted important problems yet to be solved. 

In short, the experimental results presented above collectively form a more coherent understanding of the [Mn(salen)] -catalyzed epoxidation of unfunctionalized olefins. Side-on approach of the substrate at the metal-oxo species leading to stepwise C-0 bond formation offers a straightforward explanation for product selectivity and additive effects. The degree of C-0 bond formation reflects the position of the transition state along the reaction coordinate, and it is this position that is critical to the level of asymmetric induction in the [Mn(salen)]-catalyzed epoxidation. 

Asymmetric manganese-salen-catalyzed epoxidation of unfunctionalized olefins was reported by Jacobsen et al. [74] in 1990, which allowed the enantioselective epoxidation of unfunctionalized olefins. In particular, the high enantioselectivities obtained for Jacobsen epoxidation on cis-olefins, nicely complement the Sharpless epoxidation and dihydroxylation protocols, which give reduced enantioselectivities for these substrates. The Sharpless and Jacobsen procedures are frequently used asymmetric oxidative reactions in API synthesis. More recently, organocatalytic procedures such as Shi epoxidations [75] were also employed to avoid toxic transition metal catalysts. 

Eor recent reviews on metal catalyzed highly enantioselective epoxidation of unfunctionalized olefins, see ... 

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