Alkenes catalytic asymmetric epoxidation

As shown in cycle (b) in Scheme 10.1, the iminium-oxaziridinium pair can also effect catalytic asymmetric epoxidation of alkenes. Early work in this field by Bohe et al. included investigation of the norephedrine-derived oxaziridinium salt 34 (33% ee in the catalytic epoxidation of traws-stilbene [41] ee up to 61% was achieved when 34 was employed stoichiometrically [42]), or the L-proline-derived material 35 (39% ee in the epoxidation of trans-3-phenyl-2-propenol [43]). Rapid... 

The catalytic asymmetric epoxidation of alkenes offers a powerful strategy for the synthesis of enantiomerically enriched epoxides and enantioselective oxidation reactions in ionic liquids have been summarised previously.[39] Complexes based on chiral salen ligands - usually with manganese(III) as the coordinated metal - often afford excellent yields and enantioselectivities and the catalytic cycle for the reaction is depicted in Scheme 5.5 J40 ... 

A review of nonenzymatic asymmetric epoxidations covering the literature through 1983 has been published elsewhere. Improved enantioselectivity (to as high as 64% ee) for epoxidations of schiral oxaziridines has been described and results are included in a review of synthetic tq>pli-cations of oxaziridines. A summary of catalytic asymmetric epoxidations of alkenes is present in Table 12, together with brief comments on each method. 

Asymmetric epoxidation The catalytic asymmetric epoxidation of alkenes has been the focus of many research efforts over the past two decades. The non-racemic epoxides are prepared either by enantioselective oxidation of a prochiral carbon-carbon double bond or by enantioselective alkylidenation of a prochiral C=0 bond (e.g. via a ylide, carbene or the Darzen reaction). The Sharpless asymmetric epoxidation (SAE) requires allylic alcohols. The Jacobsen epoxidation (using manganese-salen complex and NaOCl) works well with ds-alkenes and dioxirane method is good for some trans-alkenes (see Chapter 1, section 1.5.3). 

Catalytic asymmetric epoxidation of alkenes has been achieved by means of dioxiranes formed in situ from OXONE and dioxolane-containing cyclic ketones such as 34, 35 <1999JOC6443>, and 260 <1999TA2749> derived from... 

TABLE 2.4 Catalytic asymmetric epoxidation of terminal alkenes with hydrogen peroxide mediated by Ih... 

This iminium salt was tested in the catalytic asymmetric epoxidation of several alkenes at 0 °C, and a comparison of the results with those obtained using catalyst (17) is presented in Table 5.2 [24]. These results indicated that catalyst (24) in general induces much higher enantioselectivity in asymmetric epoxidation than others that were screened, providing in some cases dramatic improvements in ee over catalyst (17). 

TABLE 5.10 Catalytic asymmetric epoxidation of several alkenes mediated by catalyst (31) ... 

TABLE 5.14 Catalytic asymmetric epoxidation of various alkenes using TPPP and catalyst 36 Ph... 

Metal complexes of enantiomericaUy pure N,N -ethylenebis(salicylideneaminato) (salen) complexes in combination with stoichiometric oxidants currently provide the most selective method for the catalytic asymmetric epoxidation of unfunctionalised alkenes. The use of C2-symmetric salen complexes of manganese(lll) were reported independently in 1990 by Jacobsen and coworkers and Katsuki and coworkers. The first generation catalysts are represented by the general structure (4.33). The complex with R = Bu is known as Jacobsen s catalyst. All of the first generation catalysts are composed of a enantiopure diamine core and possess large substituents at the 3/3 and 5/5 positions. Subsequently Katsuki and coworkers developed second generation catalysts such as (4.34) with axially chiral groups at the 3/3 positions. 

A more recent alternative approach, developed by Jacobsen and co-workers, concerns the catalytic asymmetric epoxidation of unfunctionalized olefins using cheap NaOCl as oxidant in the presence of Mn complexes of chiral Schiff bases as catalysts, the so-called salene (Fig. 3-4). Values of 97% e.e. have been achieved using cis-disubstituted or trisubstituted alkenes. Equation 3-15 describes the Jacobsen epoxidation of olefins schematically. 

Halterman RL, Jan S-T (1991) Catalytic asymmetric epoxidation of unfunctionalized alkenes using the first D. -symmetric metallotetraphenylporphyrin. J Org Chem 56 5253-5254... 

The synthesis, structure, and catalytic properties of a Pd11 complex with a partially hydrogenated ligand, shown in Figure 31, are described.393 This study provides the first asymmetric epoxidation of alkenes catalyzed by a palladium complex.393... 

A more versatile method to use organic polymers in enantioselective catalysis is to employ these as catalytic supports for chiral ligands. This approach has been primarily applied in reactions as asymmetric hydrogenation of prochiral alkenes, asymmetric reduction of ketone and 1,2-additions to carbonyl groups. Later work has included additional studies dealing with Lewis acid-catalyzed Diels-Alder reactions, asymmetric epoxidation, and asymmetric dihydroxylation reactions. Enantioselective catalysis using polymer-supported catalysts is covered rather recently in a review by Bergbreiter [257],... 

After the "asymmetric epoxidation" of allylic alcohols at the very beginning of the 80 s, at the end of the same decade (1988) Sharpless again surprised the chemical community with a new procedure for the "asymmetric dihydroxylation" of alkenes [30]. The procedure involves the dihydroxylation of simple alkenes with N-methylmorpholine A -oxide and catalytic amounts of osmium tetroxide in acetone-water as solvent at 0 to 4 °C, in the presence of either dihydroquinine or dihydroquinidine p-chlorobenzoate (DHQ-pClBz or DHQD-pClBz) as the chiral ligands (Scheme 10.3). 

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). 

Of the numerous catalytic asymmetric methods developed for the functionalisation of alkenes, epoxidation has emerged as one of the most versatile and reliable methods... 

The breakthrough came already in 1996, one year after Curd s prediction, when Yang and coworkers reported the C2-symmetric binaphthalene-derived ketone catalyst 6, with which ee values of up to 87% were achieved. A few months later, Shi and coworkers reported the fructose-derived ketone 7, which is to date still one of the best and most widely employed chiral ketone catalysts for the asymmetric epoxidation of nonactivated alkenes. Routinely, epoxide products with ee values of over 90% may be obtained for trans- and trisubstituted alkenes. Later on, a catalytic version of this oxygen-transfer reaction was developed by increasing the pH value of the buffer. The shortcoming of such fructose-based dioxirane precursors is that they are prone to undergo oxidative decomposition, which curtails their catalytic activity. 

The development of asymmetric cyclopropanation protocols has been actively studied and in recent years remarkable progress has been made. The extent of chiral induction that can now be obtained in these reactions approaches the level of other classic catalytic asymmetric reactions on alkenes, such as catalytic hydrogenation and the Sharpless epoxidation.37... 

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