Oxone, asymmetric olefin epoxidation

Subsequently, high chemoselectivity and enantioselectivity have been observed in the asymmetric epoxidation of a variety of conjugated enynes using fructose-derived chiral ketone as the catalyst and Oxone as the oxidant. Reported enantioselectivities range from 89% to 97%, and epoxidation occurs chemoselectively at the olefins. In contrast to certain isolated trisubstituted olefins, high enantioselectivity for trisubstituted enynes is noticeable. This may indicate that the alkyne group is beneficial for these substrates due to both electronic and steric effects. 

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). 

Previously, some fluorocyclohexanones were used in a catalytic amount with Oxone for asymmetric epoxidation reaction, but they gave a poor ee . It was found later that chiral ketones derived from fructose work well as asymmetric epoxidation catalysts and show high enantioselectivity in reactions of /rani-disubstituted and trisubsti-tuted olefins ". Cis and terminal olefins show low ee under these reaction conditions. Interestingly, the catalytic efficiency was enhanced dramatically upon raising the pH. Another asymmetric epoxidation was also reported using Oxone with keto bile acids. ... 

Chiral dioxirane that was also generated in situ from the corresponding ketone and Oxone was first used for catalytic asymmetric epoxidation by Curd et al., although enantioselectivity was low [7], Later, Yang et al. disclosed that this approach had a bright prospect if used with a combination of Oxone and chiral ketone 3 [8]. Ketone 3 is converted into the corresponding dioxirane in situ, which epoxidizes olefins (Scheme 6B.5). 

Yang et al. have applied C2-symmetric chiral dioxiranes, generated in situ from corresponding chiral ketones 75 and Oxone, for asymmetric epoxidation of trans-olefins and trisubstituted olefins (33-87% ee) <1996JA491, 1996JA11311>. 

Yang s chiral ketones 75 have also been used as catalysts in the kinetic resolution of acyclic secondary allyl silyl ethers <2001JOC4619>. Dioxiranes generated in situ from dehydrocholic acid derivatives 122 and Oxone have been used in the asymmetric epoxidations of cinnamic acid derivatives with product ee s up to 95% <2001TA1113, 2002JOC5802> and unfunctionalized olefins (up to 98% ee) <2006T4482>. 

Dioxiranes are remarkably versatile oxidizing agents which show encouraging potential for asymmetric synthesis, particularly asymmetric epoxidation. Dioxiranes can be generated in situ from Oxone (KHSO5) and ketones (Scheme 1). In principle, only a catalytic amount of ketone is required, so with a chiral ketone there exists the opportunity for catalytic asymmetric epoxidation." Since the first asymmetric epoxidation of olefins with a chiral dioxirane reported by Curci in 1984," this area has received intensive interest and significant progress has been made." - ... 

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

Shi and co-workers <97JOC2328, 97JA11224> have optimized their chiral dioxirane protocol for the asymmetric epoxidation of non-functionalized rra/i.v-olefins (e.g., 44), such that the chiral ketone 42 can be used in catalytic quantities with potassium peroxomonosulfate (Oxone) as the stoichiometric oxidant. The key to preserving the lifetime of the chiral auxiliary is pH control during the reaction the optimum range was found to be 10.5 or above, which is conveniently maintained with potassium carbonate. 

In 1996, Shi made a huge development in this area, reporting the asymmetric epoxidation of alkenes using chiral dioxiranes generated in situ. The epoxidation works well for disubstituted tra s-olefins, and trisubstituted olefins using a fructose-derived ketone as a catalyst and oxone as an oxidant (Scheme 1.9) [26]. 

Recently, considerable efforts have been made to discover new organocatalytic systems for asymmehic epoxidation. In 2003, A arwal and coworkers reported that the asymmetric epoxidation of olefins proceeded in good yields and with moderate enantioselectivities using Oxone (Wako Chemicals, Osaka, Japan) as an oxidant in the presence of a 48-type catalyst (Scheme 1.22) [261]. According to their proposal, the protonated ammonium salt species can act not only as a phase-transfer catalyst to carry the real oxidant species to the organic phase but also as a promoter to activate the chiral oxidant via hydrogen-bonding stabilization, as depicted in 63. 

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. 

Dioxirane-based systems for asymmetric epoxidation have received much attention over the past decade, and they have emerged as one of the most effective methods for producing enantiomerically enriched oxiranes. Their reactivity stems from (he ring strain associated with the three membered ring and the relatively weak 0-0 bond. They are readily attacked even by poor nucleophiles such as olefins. The general method to produce a dioxirane is through the use of a ketone and a stoichiometric oxidant (usually Oxone) in either acetonitrile or dimethoxymethane (Scheme 1.8). 

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