Oxygen epoxidations using

Oxidizing enzymes use molecular oxygen as the oxidant, but epoxidation with synthetic metalloporphyrins needs a chemical oxidant, except for one example Groves and Quinn have reported that dioxo-ruthenium porphyrin (19) catalyzes epoxidation using molecular oxygen.69 An asymmetric version of this aerobic epoxidation has been achieved by using complex (7) as the catalyst, albeit with moderate enantioselectivity (Scheme 9).53... 

The complexation of the base with the oxirane in nonpolar solvents drives the fi-deprotonation to occur from the syn face (see above), but also influences its regio- and stereochemistry. During the aggregation step, two conformational structures can be formed depending on which lone pair of the oxygen is used as the donor site. The relative stabilities of these complexes are largely influenced by nonbonded interactions between the base and the syn substituents on the epoxide (Scheme 16). 

Several methods for the asymmetric epoxidation of electron-poor alkenes rely on the use of metal peroxides associated with chiral ligands . Enders and coworkers reported that ( )-a,/ -unsaturated ketones may be epoxidized using stoichiometric quantities of diethylzinc and a chiral alcohol, in the presence of molecular oxygen (equation 33). The best enantioselectivities were found using (/ ,/ )-Af-methylpseudoephedrine 54 as R OH... 

Epoxidation Catalyzed by Metalloporphyrins. Metalloporphyrins, which have thoroughly studied as catalysts in alkane oxygenations, have also been tested as epoxidation catalysts.119,122,244,245,307 Iodosylbenzene (PhIO), sodium hypochlorite, alkyl hydroperoxides, potassium hydrogen persulfate, and molecular oxygen are the oxygen sources used most frequently in these oxidations.119... 

There are many other reactions devised for oxygen transfer such as various oxidants combined with transition metals with or without macrocyclic hosts and industrial specific epoxidations using oxygen, to name just a few. 

As discussed in Section 10.1, asymmetric epoxidation of C=C double bonds usually requires electrophilic oxygen donors such as dioxiranes or oxaziridinium ions. The oxidants typically used for enone epoxidation are, on the other hand, nucleophilic in nature. A prominent example is the well-known Weitz-Scheffer epoxidation using alkaline hydrogen peroxide or hydroperoxides in the presence of base. Asymmetric epoxidation of enones and enoates has been achieved both with metal-containing catalysts and with metal-free systems [52-55]. In the (metal-based) approaches of Enders [56, 57], Jackson [58, 59], and Shibasaki [60, 61] enantiomeric excesses > 90% have been achieved for a variety of substrate classes. In this field, however, the same is also true for metal-free catalysts. Chiral dioxiranes will be discussed in Section 10.2.1, peptide catalysts in Section 10.2.2, and phase-transfer catalysts in Section 10.2.3. 

Chlordane was introduced as an insecticide in 1945 and was the first cyclodiene insecticide that was used in agriculture (Eisler, 1990). It was the second most important organochlorine pesticide after toxaphene from 1976 to 1977 (Stansley Roscoe, 1999). It has been used on agricultural crops and extensively in the control of termites (Smith, 1991). Chlordane and heptachlor can be metabolized into two persistent (oxygenated) epoxides—oxychlordane and heptachlor epoxide—in mammals (Nomeir Hajjar, 1987) such that the two compounds are always measured together with chlordane and heptachlor. 

Fig. 11 Structures of chiral ruthenium complexes used as catalysts in epoxidations using a variety of oxygen donors...

Carbon-oxygen bonds can also be formed by the intramolecular reaction between an alkyne and an epoxide using 5 mol% AUCI3 in acetonitrile 20,21... 

The work of Davis was, however, unable to distinguish which oxygen was attacked on the titanium peroxo complex when the alkene co-ordinates. Therefore, Neurock and Manzer conducted a theoretical study of the mechanism of alkene epoxidation using TS-1 with aqueous hydrogen peroxide.32 The workers concluded that their calculations to predict both the structure and relative bands in the IR spectra for TS-1 were in good agreement with experimental data. The calculations indicated that the oxygen closest to the titanium centre was the active site for alkene attack. The result was the direct formation of... 

Recently, the first asymmetric cell-free application of styrene monooxygenase (StyAB) from Pseudomonas sp. VLB 120 was reported [294]. StyAB catalyses the enantiospecific epoxidation of styrene-type substrates and requires the presence of flavin and NADH as cofactor. This two-component system enzyme consists of the actual oxygenase subunit (StyA) and a reductase (StyB). In this case, the reaction could be made catalytic with respect to NADH when formate together with oxygen were used as the actual oxidant and sacrificial reductant respectively. The whole sequence is shown in Fig. 4.106. The total turnover number on StyA enzyme was around 2000, whereas the turnover number relative to NADH ranged from 66 to 87. Results for individual substrates are also given in Fig. 4.106. Excellent enantioselectivities are obtained for a- and -styrene derivatives. 

Epoxidation using m-chloroperoxybenzoic acid (RCO3H) is a syn addition of oxygen to a double bond. The original bond stereochemistry is retained. 

Some noteworthy intramolecular nucleophilic ring openings have been reported in the recent literature, which can be used to prepare functionalized heterocycles of synthetic interest. For example, the highly oxygenated epoxide 100 undergoes rearrangement induced by boron trifluoride etherate, whereby anchimeric assistance from the pendant phenylthio substituent leads to an intermediate episulfonium ion 101 which subsequently suffers 5-e o-tet cyclization to form the tetrahydrofiiran derivative 102 <03TL5547>. 

Whereas so far oxidation reactions with O2 have been discussed, an important heterogeneous catalytic oxidation reaction is the epoxidation of propylene to propylene epoxide. Using hydroperoxide the reaction can be done over Ti dispersed on silica [133]. Ti becomes 4 coordinated as Si in Si02. With hydrogen-peroxide the reaction has been found to occur for Ti incorporated in the lattice of MFI zeolite [134]. The unique property of these systems is that the electrophilic nature of oxygen intermediates does not cause reaction with the allylic CH bond. It is proposed that the adsorbed intermediate is ... 

Ozone has also been employed in alkene epoxidations using metalloporphyrins <9UOC3725>. Elemental fluorine in aqueous acetonitrile epoxidizes alkenes in good to excellent yields (Equation (42)). It has been suggested that the actual oxygen transfer agent is HOF <90JOC5155>. 

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