Electron deficient olefins, epoxidation with

Scheme 61 Epoxidation of electron deficient olefins with silver complexes.

Iodosylbenzene is sufficiendy reactive on its own to epoxidize electron-deficient olefins such as tetracyanoethylene (43). It is possible that coordinated monomeric iodosylbenzene is substantially more reactive than polymeric iodosylbenzene and that complexation of a monomeric form is sufficient to provide the requisite reactivity with normal olefins. 

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). 

A high catalyst loading (typically 20-30 mol%) is usually required for the epoxidation with ketone 26 because Baeyer-Vilhger oxidation presumably decomposes the catalyst during the epoxidation. The fused ketal moiety in ketone 26 was replaced by a more electron-withdrawing oxazohdinone (32) and acetates (33) with the anticipation that these replacements would decrease the amount of decomposition via Baeyer-Villiger oxidation (Fig. 8) [71, 72]. Only 5 mol% (1 mol% in some cases) of ketone 32 was needed to get comparable reactivity and enantioselectivity with 20-30 mol% of ketone 26 [71]. Since dioxiranes are electrophilic reagents, they show low reactivity toward electron-deficient olefins, such as a, 3-unsaturated esters. Ketone 33, readily available from ketone 26, was found to be an effective catalyst towards the epoxidation of a, 3-unsaturated esters [72]. 

Next to the base-catalyzed asymmetric epoxidations of electron-deficient olefins with chiral hydroperoxides described above, a few examples of uncatalyzed epoxidations with... 

The Ni(II) complexes of salen analogues with bipyridine units 36 showed higher catalytic activity than Ni(IIXsalen) in the epoxidation of an electron-deficient olefin, allyl chloride. Their enhanced reactivity was ascribed partially to their higher stability relative to the salen complex (98). 

Selective epoxidation of olefins by vanadium(V) alkyl peroxo complexes has also been reported (76). These complexes are very effective stereo-selective reagents for the transformation of olefins into epoxides. The mechanism consists of binding of the olefin to the metal to displace one of the peroxo-oxygen atoms, nucleophilic attack of the bound oxygen atom on the coordinated electron-deficient olefin, dissociation of the epoxide, and reaction of the remaining vanadium intermediate with... 

Ti(0-/-Pr)4-catalyzed epoxidation works for allyl alcohols with an electron-deficient olefin. The epoxidation of different allyl alcohols bearing an electron-withdrawing group has been attested [596-599] and Eq. (257) compares the stereochemical outcome of a few methods of epoxidation [596]. The titanium-based method generally results in considerable improvement of syn selectivity. The stereoselectivity of the reaction depicted by Eq. (258) is the reverse of that afforded by alkaline peroxide epoxidation [599]. 

In the presence of cinchona derivatives as catalysts, peroxides or hypochlorites as Michael donors react with electron-deficient olefins to give epoxides via conjugate addition-intramolecular cyclization sequence reactions. Two complementary methodologies have been developed for the asymmetric epoxidation of electron-poor olefins, in which either cinchona-based phase-transfer catalysts or 9-amino-9(deoxy)-epi-dnchona alkaloids are used as organocatalysts. Mechanistically, in these two... 

A. Murphy, G. Dubois, T. D. P. Stack, Efficient epoxidation of electron-deficient olefins with a cationic manganese complex, J. Am. Chem. Soc. 125 (2003) 5250. 

D. E. De Vos, B. F. Sels, M. Reynaers, Y. V. Subba Rao, P. A. Jacobs, Epoxidation of terminal or electron-deficient olefins with H2O2, catalysed by Mn-trimethyltriazacyclonane complexes in the presence of an oxalate buffer. Tetrahedron Lett. 39 (1998) 3221. 

The double bonds in diethyl maleate and fumarate are very electron deficient olefins. These were indeed the most difficult compounds to epoxidize. The usual treatment with slight excess of oxidant for short periods of time did not result in any reaction. With a large excess, however (5-10 molequivalent) progress could be monitored at room temperature and after 12 hours about 90% conversion was achieved. Thus cis and traits diethyl epoxysuccinate were obtained in 65% and 50% yield respectively. These epoxides are important intermediates in many syntheses, immuno-pharmacological studies and polymerization processes and are always made stepwise by indirect methods. A full retention of configuration was observed and neither epoxide was contaminated with the other (figure 12). 

Additionally, some non-cinchona catalysts have also been applied for the epoxidation of chalcones and electron-deficient olefins. Maruoka s group [156] demonstrated that 3 mol% of ammonium salt 96 with NaOCl as oxidant allows the epoxidation of chalcones with excellent yields and enantioselectivities (Scheme 12.24). 

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