Alkenes, enantioselective epoxidation, catalysts

Ordinary alkenes (without an allylic OH group) have been enantioselectively epoxidized with sodium hypochlorite (commercial bleach) and an optically active manganese-complex catalyst. Variations of this oxidation use a manganese-salen complex with various oxidizing agents, in what is called the Jacobsen-Katsuki... 

Several catalysts that can effect enantioselective epoxidation of unfunctionalized alkenes have been developed, most notably manganese complexes of diimines derived from salicylaldehyde and chiral diamines (salens).62... 

A number of chiral ketones have been developed that are capable of enantiose-lective epoxidation via dioxirane intermediates.104 Scheme 12.13 shows the structures of some chiral ketones that have been used as catalysts for enantioselective epoxidation. The BINAP-derived ketone shown in Entry 1, as well as its halogenated derivatives, have shown good enantioselectivity toward di- and trisubstituted alkenes. 

The binaphthyl azepinium salt 59 (TT= tris(tetrachlorobenzenediolato)phosphate(V)) and corresponding azepine 60 were developed as effective catalysts for the enantioselective epoxidation of unfunctionalised alkenes, with enantiomeric excesses up to 87% <06TA2334>. 

Among many other methods for epoxidation of disubstituted E-alkenes, chiral dioxiranes generated in situ from potassium peroxomonosulfate and chiral ketones have appeared to be one of the most efficient. Recently, Wang et /. 2J reported a highly enantioselective epoxidation for disubstituted E-alkenes and trisubstituted alkenes using a d- or L-fructose derived ketone as catalyst and oxone as oxidant (Figure 6.3). 

Although the Sharpless catalyst was extremely useful and efficient for allylic alcohols, the results with ordinary alkenes were very poor. Therefore the search for catalysts that would be enantioselective for non-alcoholic substrates continued. In 1990, the groups of Jacobsen and Katsuki reported on the enantioselective epoxidation of simple alkenes both using catalysts based on chiral manganese salen complexes [8,9], Since then the use of chiral salen complexes has been explored in a large number of reactions, which all utilise the Lewis acid character or the capacity of oxene, nitrene, or carbene transfer of the salen complexes (for a review see [10]). 

Highly enantioselective epoxidation of unfunctionalized alkenes was developed by using chiral metalloporphyrin catalysts.1214-1218 Remarkable anion axial ligand effects were observed with [Fe(TPFPP)X] complexes (X = triflate, perchlorate, nitrate).1219 Hexafluoroacetone was found to be an efficient cocatalyst with H202,1220 and alkenes could be epoxidized by ozone at ambient temperature.1221... 

Homologous biphenyl and binaphthyl tertiary azepines (4) and quaternary iminium salts, prepared from (+)-(5,5 )-L-acetonamine, behave as effective catalysts for the enantioselective epoxidation of unfunctionalized alkenes with Oxone (ee up to 83%).113... 

Another strategy for positioning a catalytic center across the entrance of a conical cavity is to employ a cavitand functionalized at one entrance by a pendent chelate arm (Scheme 13.16). Enantioselective epoxidations of aromatic alkenes was realized with catalysts 62, 63, and 64, 65, although the enantioselectivity remained modest [46]. (For experimental details see Chapter 14.13.11). The reaction requires the slow addition (over 1 h) of a solution of alkene 66 and Oxone to a solution of the catalyst. Both the size of the cavity and the structure of the bridged ketone influenced the reactivity. Hence, whilst the formation of the diol 68 was observed when 62 and 63 were used, the presence of 64 and 65 resulted only in the formation of epoxide 67. 

Catalyst Ih proved to be the best for the enantioselective epoxidation of terminal alkenes. Table 2.4 reports some representative data. Excellent enantioselectivities can be observed in many cases. Dienes were also investigated (Table 2.5), In this case, the epoxidation occurred exclusively at the terminal double bond with complete regioselectivity and ee up to 98%. To the best of our knowledge, any other chiral metal catalyst reported in the literature would lead to the electrophilic asymmetric epoxidation of the more electron-rich double bond [50,51],... 

Several examples are known of the enantioselective conversion of alkenes into epoxides with the use of polymer-supported oxidation catalysts. This can be traced to the pioneering work by Julia and Colonna in 1980. They demonstrated that highly enantioselective epoxidations of chalcones and related a, 3-unsaturated ketones can be achieved with the use of insoluble poly(a-amino acids) (116, Scheme 10.20) as catalysts [298-301]. The so-called Julia-Colonna epoxidation has been the object of several excellent reviews [302-306]. The terminal oxidant is H202 in aq. NaOH. With lipophilic amino acids as the components, such as (SJ-valine or (SJ-leucine, enantioselectivities as high as 96-97% ee were obtained. The enan-tioselectivity depends of several factors, including the side-chain of the amino acid, the nature of the end groups and the degree of polymerization. Thus, for instance,... 

As for the diols, the symmetric compounds have found most uses for nonsymmetric diols, a versatile synthesis via silyl ketones using the SAMP/RAMP methodology has been developedl5. Both enantiomers of the simplest symmetric diol, 2,3-butanediol (11), are often used in asymmetric synthesis, mostly for the formation of acetals and ketals with carbonyl compounds and subsequent reactions with acidic catalysts (Section D. 1.1.2.2.), Grignard reagents (Section D. 1.3.1.4.) and other carbanions (Sections D. 1.5.1., D. 1.5.2.4.), and diastereoselective reductions (Section D.2.3.3.). Precursors of chiral alkenes for cycloprotonations (Section D.1.6.1.5.) and for chiral allenes (Section B.I.), and chiral haloboronic acids (Section D. 1.1.2.1.) are other applications. The free diol has been employed as a chiral ligand in molybdenum peroxo complexes used for enantioselective epoxidation of alkenes (Section D.4.5.2.2.). 

The majority of [M(ra/en)]-based complexes immobilized onto carbon materials with catalytic properties refer to Mn(lll) complexes that have been shown to behave as highly efficient homogeneous catalysts in the chemoselective [98,99] and enantioselective epoxidation of a large variety of alkenes [99,100]. To date, as noted in Section 8.3, their heterogenization onto carbon materials was carried out with ACs and CXs. 

Because of this catalyst degradation, organometallic catalysts are currently the best synthetic reagents for enantioselective epoxidation of olefins. Chiral Mn(III)-salen complexes yield up to 99% ee for cw-disubstituted, tri- and tetra-substituted alkenes [62], but the best results require less desirable oxidants - iodosyl benzene or hypochlorite. Other catalysts accept a more limited substrate range the Sharpless-Katsuki titanium-tartrate ester [65] for allylic alcohols and the JuUa-Colonna epoxidation for a,P-unsaturated ketones [66]. 

KHSO5) is used most commonly, although hydrogen peroxide in acetonitrile is an alternative. Formation of the dioxirane of the ketone (see Scheme 5.50) is followed by enantioselective epoxidation of the alkene. Two examples of the asymmetric epoxidation using the catalyst 64 are shown in Schemes 5.70 and 5.71. 

Other metal-based epoxidation catalysts have been explored to overcome some of the hmitations of the Sharpless procedure. One drawback with the Sharpless asymmetric epoxidation is the slightly lower ees often obtained when using cis-olefin substrates. The group of Yamamoto have achieved highly enantioselective epoxidations of ds-alkenes using vanadium(V) oxytriisopropoxide in the presence of C2-symmetric bishydroxamic acid ligands such as (4.23). In contrast to the Sharpless procedure this process is not hampered by the presence of air or... 

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