Ketone-catalyzed epoxidations, chiral

One highly attractive feature of ketone-catalyzed epoxidation via chiral dioxir-anes is that reliable models can be developed to rationalize the observed enantio-selectivities. For the reaction of a dioxirane with an alkene, two extreme transition states can be envisaged the so-called spiro and planar modes (Fig. 12.3). 

Chen, R., Qian, C. and de Vries, J. G. Asymmetric epoxidation of a,/ -unsaturated ketones catalyzed by chiral ytterbium complexes, Tetrahedron Lett., 2001, 42, 6919-6921. 

Moreover, in the same work, the 0-labeling experiment confirmed chiral dioxiranes to be the intermediates in chiral ketone-catalyzed epoxidation reactions. Murray et al. reported the synthesis and structural characterization of cyclooctatetraene tetraepoxides 90 and 91 through the oxidation of cyclooctatetraene with excess of DMDO lb... 

Shi asymmetric epoxidation Chiral-ketone catalyzed epoxidation of unfunctionalized olefins. 410... 

Abstract Organo-catalyzed asymmetric epoxidation has received much attention in the past 30 years and significant progress has been made for various types of olefins. This review will cover the advancement made in the field of chiral ketone and chiral iminium salt-catalyzed epoxidations. 

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). 

Chiral ketone-catalyzed asymmetric epoxidation has received intensive interest since the first reported by Curci et al. in 1984. The reaction is performed with oxone (potassium peroxomonosulfate) as the primary oxidant which generates the chiral dioxirane catalytic species in situ, which in turn, transfers the oxygen... 

Frohn, M. Shi, Y. Chiral Ketone-Catalyzed Asymmetric Epoxidation of Olefins, Synthesis 2000, 1979-2000. 

Ketone-catalyzed asymmetric and diastereoselective epoxidation of olefins by di-oxiranes generated in situ from chiral ketones and oxone (2KHS05 KH-S04 K2S04) 04ACR497. 

Chiral ketone-catalyzed asymmetric epoxidation of olefins 00S1979. 

Organocatalytic Oxidation. Asymmetric Epoxidation of Olefins Catalyzed by Chiral Ketones and Iminium Salts Wong, O.A. Shi, Y. Chem. Rev. 2008,108,3958. 

Epoxidation of olefins was catalyzed by the ruthenium(II) complex of the above perfluorinated y3-diketone in the presence of 2-methylpropanal (Scheme 50). Unfunctionalized olefins were epoxidized with a cobalt-containing porphyrin complex, and epoxidation of styrene derivatives was catalyzed by chiral salen manganese complexes (248) (Scheme 50). In the latter case, chemical yields were generally high, however, the products showed low enantiomeric excess with the exception of indene (92% ee). [Pd(C7Fi5COCHCOC7Fi5)2] efficiently catalyzed the oxidation of terminal olefins to methyl ketones with f-butylhydroperoxide as oxidant in a benzene-bromoperfluoro-octane solvent system (Scheme 50). In all these reactions, the product isolation and efficient catalyst recycle was achieved by a simple phase separation. 

The asymmetric epoxidation of alkenes constitutes a powerful approach to enantiomerically enriched epoxides, a class of highly versatile intermediates in organic synthesis [1]. Various effective epoxidation systems have been developed, including epoxidation of allylic [2, 3] and homoallylic [4] alcohols, metalunfunctionalized alkenes [5-7], and the nucleophilic epoxidation of electron-deficient alkenes [8]. During the past 10-15 years, much effort has been devoted to chiral ketone-catalyzed asymmetric epoxidation (Scheme 3.1). The subject has been described in great detail in the first edition [9] and other reviews [10]. This chapter provides an update on progress in this area since the first edition [9]. 

For ffie past several decades, several chemocatalysts and biocatalysts have been used for ffie epoxidation of nonfunctionalized aliphatic alkenes. The Katsuki-Jacobsen epoxidation, which is catalyzed by chiral Mn(III)-salen with NaOCl/PhIO as an oxidant, achieves good yields and high stereoselectivities (84-94%ee) for ffie epoxidation of ds-alkenes [21]. The Shi epoxidation, which is catalyzed by ffie fructose-derived ketone and oxone, has been successfully used in ffie epoxidation of frans-alkenes, yielding ffie corresponding oxides with 93-98% ee [74]. However, for nonfunctionalized terminal aliphatic alkenes, chemocatalysts typically display low stereoselectivity [7]. 

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