Asymmetric epoxidation of alkenes

Asymmetric epoxidations of alkenes have been intensively studied since Sharpless initial report on asymmetric epoxidation of allylic alcohols in 1980. This reaction, discussed in Section 9.1.3, has become one of the most widely employed reactions in asymmetric synthesis, due to its reliability and high enantioselectivity [2],... 

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... 

Figure 6.14. Chiral salen complexes used in the asymmetric epoxidation of alkenes.[7]...

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). 

Ru(CO)(porph 0 (porpff ""=chiral poprphyrin from reaction of pyrrole with l,2,3,4,5,6,7,8-octahydro-l 4,5 8-dimethanothracene-9-carboxaldehyde) is made from (porph 0 and RUjCCO). As Ru(CO)(porpff )/(Cl2PyNO)/CgHg it catalysed the asymmetric epoxidation of alkenes with e.e. values up to 88% at room temperatures (better yields were obtained in sealed tubes up to 125°C) [878],... 

Oxidation of chiral sulfonimines (R"S02N=CHAr)and chiral sulfamyl-imines (R RNS02N=CHAr)affords optically active 2-sulfonyloxaziridines and 2-sulfamyloxaziridines, respectively. These chiral, oxidizing reagents have been used in the asymmetric oxidation of sulfides to sulfoxides (15-68% ee), 11-13 selenides to selenoxides (8-9% ee] enolates to a-hydroxycarbonyl compounds (8-37% ee) and in the asymmetric epoxidation of alkenes (15-40% ee)... 

The photocatalyzed asymmetric epoxidation of alkenes using a binap-equipped Ru-salen complex was also reported <99SL1157>. 

Chiral porphyrins are also effective in the asymmetric epoxidation of alkenes. For example, a Cj-symmetiic iron porphyrin (29) <99JA460> catalyzes the efficient epoxidation of terminal alkenes, such as 30, with very good ee s and up to 550io turnovers. Similarly, trons-disubstituted... 

Table 6.4 Asymmetric epoxidation of alkenes using ulose (3) as catalyst at 0 C.

Alternatively, chiral epoxides can be prepared efficiently by other routes, in particular asymmetric epoxidation of alkenes. For related reviews in... 

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... 

Asymmetric epoxidations of alkenes catalysed by chiral monomeric organorhenium (VII) and organomolybdenum(VI) compounds,160 ketones,161 and salen-metal complexes162 have been reviewed. The advances in the catalysed Baeyer-Villiger oxidation... 

Asymmetric epoxidation of alkenes. Two groups have prepared chiral (salen)-manganese complexes such as 2 from ( + )- or (-)-l and salicylaldehyde derivatives... 

In practice in the literature of the past 20 years the important results with ruthenium in epoxidation are those where ruthenium was demonstrated to afford epoxides with molecular oxygen as the terminal oxidant. Some examples are presented (see later). Also ruthenium complexes, because of their rich chemistry, are promising candidates for the asymmetric epoxidation of alkenes. The state of the art in the epoxidation of nonfunctionalized alkenes is namely still governed by the Jacobsen-Katsuki Mn-based system, which requires oxidants such as NaOCl and PhIO [43,44]. Most examples in ruthenium-catalysed asymmetric epoxidation known until now still require the use of expensive oxidants, such as bulky amine oxides (see later). 

Some examples of asymmetric epoxidations of alkenes using chiral ruthenium porphyrins have been reported for example, the previously reported D4-sym-metrical chiral ruthenium porphyrin complex Run(D4-Por )(CO)(MeOH) [58], which produced (R)-styrene oxide in 57% ee with Cl2PyNO as a donor, was readily converted into the dichloro derivative A [59] (Fig. 11). This di-chlororuthenium porphyrin gave (R)-styrene oxide in 69% ee using Cl2PyNO and was highly active (875 TON in 1.5 h). The use of unsubstituted pyridine N-oxide or NMO as oxidants resulted in low substrate conversions as well as... 

The importance of this reaction also lies in the fact that asymmetric epoxidation of alkenes other than allylic alcohols is possible with this catalytic system (see Section 9.3.4). The third reaction relates to catalysts developed by Unilever for improved detergent action in the presence of hydrogen peroxide. The important point to note is that catalytic intermediates with metal-oxo groups play a pivotal role in all these reactions. 

Asymmetric Epoxidation of Alkenes Other Than Allyl Alcohols... 

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). 

This class of catalysts covers chemocatalysts that do not contain a transition metal. The class has been known for many years, but it is relatively recently that the term organocatalyst has been used (209). A wide variety of transformations can be performed, which is currently an area of intense research (209-218). Table 5 (220-252) summarizes some key transformations in which organocatalysis can be useful. Reactions range from the asymmetric epoxidation of alkenes, which need not be conjugated to another functional group, to aldol reactions and... 

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... 

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