Asymmetric epoxidation of terminal alkenes

M. Collardon, A. Scarso, P. Sgarbossa, R. A. Michelin, G. Strukul, Asymmetric epoxidation of terminal alkenes with chiral Pt complexes, /. Am2. Chem. Soc. 128 (2006) 14006. 

FIGURE 2.5 Asymmetric epoxidation of terminal alkenes with hydrogen peroxide catalyzed by Pt(ll) chiral complexes Ih-k. 

M. Colladon, A. Scarso, G. Strukul, Towards a greener epoxidation method. Use of water-surfactant media and catalyst recycling in the platinum-catalyzed asymmetric epoxidation of terminal alkenes with hydrogen peroxide, Adv. Synth. Catal. 349 (2007) 797. 

The introduction of chiral bidentate diphosphines allowed for asymmetric epoxidations of terminal alkenes. Screening a number of different chiral diphosphines in the epoxidation of 4-methylpentene using 2 mol% of catalyst and 1 equivalent of aqueous hydrogen peroxide as the terminal oxidant revealed that the catalyst containing S,S-Chiraphos (34) allowed for an efficient chirality transfer from the catalyst to the substrate [141]. With this particular catalyst combination, the U-isomer of 4-methylpentene oxide was obtained in 60% yield and 75% enantiomeric excess. The... 

Asymmetric epoxidation of terminal alkenes with hydrogen peroxide was optimized with electron-poor chiral Pt(II) complexes bearing a pentafluorophenyl residue, as described in Section 23.3.1.6. The same catal3rtic system was made more sustainable by the employment of water as the solvent under micellar conditions. Surfactant optimization revealed the preferential use of neutral species like Triton-XIOO to solubihze both the catalyst and substrates. In several cases an increase of the asymmetric induction was observed (Scheme 23.43). The use of an aqueous phase and the strong affinity of the catalyst for the micelle allowed the recycling of the catalytic system by means of phase separation and extraction of the reaction products using an apolar solvent (hexane). The aqueous phase containing the catalyst was reused for up to three cycles with no loss of activity or selectivity. 

Colladon, M., Scarso, A., Sgarbossa P, et al. (2006). Asymmetric Epoxidation of Terminal Alkenes with Hydrogen Peroxide Catalyzed by Pentafluorophenyl Pt(II) Complexes, J. Am. Chem. Soc., 128, pp. 14006-14007. 

The asymmetric epoxidation of /i-alkenes and terminal alkenes proved to be more difficult, though a recent finding, describing the use of a modified salen complex to epoxidize ( )-0-methylstyrene to form the corresponding epoxide in 83% ee, represents another important step forward. Alternatively, chiral (D2-symmetric) porphyrins have been used, in conjunction with ruthenium or iron, for efficient asymmetric oxidation of trans- and terminal alkenes[92]. 

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... 

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],... 

CPO-catalyzed asymmetric epoxidation of terminal olefins, (a) 1,1-disubstituted terminal alkenes and (b) o)-bromo-2-methylalkenes. 

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). 

NR = nonreactive toward hydrocarbons PO — oxidation of phosphines to phosphine oxides MF — peroxometallacyclic adduct formation with cyanoalkenes NSE=nonstereoselective epoxidation SE-stereoselective epoxidation AE = asymmetric epoxidation HA-hydroxylation of alkanes HB=hydroxylation of arenes OA=oxidation of alcohols to carbonyl compounds K = ketonization of terminal alkenes SO — oxidation of SO to coordinated SO4 MO — metallaozonide formation with catbonyl compounds I — oxidation of isocyanides to isocyanates. 

The methodology described above allows the asymmetric epoxidation of allylic alcohols or cis-substituted conjugated alkenes and the resolution of terminal epoxides. The asymmetric synthesis of trans-di- and trisubstituted epoxides can be achieved with the dioxirane formed from the fructose-derived ketone 64, developed by Shi and co-workers. The oxidizing agent potassium peroxomonosulfate... 

Chiral epoxides are extensively employed high-value intermediates in the synthesis of chiral compounds due to their ability to react with a broad variety of nucleophiles. In recent years a lot of research has been devoted to the development of catalytic methods for their production [551, 1141], The Katsuki-Sharpless method for the asymmetric epoxidation of allylic alcohols [1142,1143] and the asynunetric dihydroxylation of alkenes are now widely applied and reliable procedures. Catalysts for the epoxidation of nonfunctionalized olefins have been developed more recently [555, 1144]. Although high selectivities have been achieved for the epoxidation of cA-alkenes, the selectivities achieved with trans- and terminal olefins were less satisfactory using the latter methods. 

Three different chiral biphenyl iminium salts have been prepared and tested as asymmetric catalysts for epoxidation of prochiral alkenes with oxone in MeCN-H20 in the presence of a base. The complexes (74) and (75) have electron-withdrawing 3,3 -substitutents on the terminal phenyl units but salt (75) lacks the t-butyl and methoxy groups at the central biphenyl unit. The salt (76) bears no t-butyl or methoxy groups or electron-withdrawing substitutents at a biphenyl unit. The catalytic reactivity of these complexes in terms of the yield and ee of the epoxidized product is in the following order (76) > (75) > (74). This suggests that substitution on the biphenyl unit introduces steric bulk which is damaging for catalytic activity and enantioselectivity. ... 

This catalytic system provides high enantioselectivities for a range of epoxides (Figure 19.2), including those derived from trisubstituted and traus-1,2-disubstituted alkenes, with complete stereospecificity (retention of the alkene geometry in the epoxide product) [17]. The reaction has been shown to be chem-oselective for the alkene of enynes [18], provided monoepoxides upon reaction with conjugated trans-dienes [19] and afforded up to 93% ee for the asymmetric epoxidation of fluoro-olefins [20]. However, decreased enantioselectivity was observed for both cis- and terminal alkenes. The catalytic system has also been applied to the resolution and desymmetrization of cyclic trisubstituted alkenes [21]. 

Figure 19.6 Lactam catalyst for the asymmetric epoxidation of 1,1-disubstituted terminal alkenes.

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