Olefins metal-catalyzed epoxidation

There are several available terminal oxidants for the transition metal-catalyzed epoxidation of olefins (Table 6.1). Typical oxidants compatible with most metal-based epoxidation systems are various alkyl hydroperoxides, hypochlorite, or iodo-sylbenzene. A problem associated with these oxidants is their low active oxygen content (Table 6.1), while there are further drawbacks with these oxidants from the point of view of the nature of the waste produced. Thus, from an environmental and economical perspective, molecular oxygen should be the preferred oxidant, because of its high active oxygen content and since no waste (or only water) is formed as a byproduct. One of the major limitations of the use of molecular oxygen as terminal oxidant for the formation of epoxides, however, is the poor product selectivity obtained in these processes [6]. Aerobic oxidations are often difficult to control and can sometimes result in combustion or in substrate overoxidation. In... 

Table 6.12 Transition metal-catalyzed epoxidation of olefins with H202 as terminal oxidant.

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). 

Thus, depending on the metal complex used, cyclohexene oxidation can occur via one or more of at least three major pathways, as shown in Reaction 20 path A, radical initiated decomposition of cyclohexenyl hydroperoxide path B, metal catalyzed epoxidation of the olefin and path C, metal catalyzed epoxidation of an allylic alcohol. Ugo found that path B becomes more pronounced when molybdenum complexes are used to modify the oxidation of cyclohexene in the presence of group... 

By-products are formed in subsequent reactions of the terf-alkoxy and tert-alkylperoxy radicals with the hydroperoxide, solvent or olefin. For example, in the metal-catalyzed epoxidation of cyclohexene with ferf-butyl hydroperoxide in benzene, the main by-product was 3-rert-butylperoxy-l-cyclohexene, formed via the sequence433 shown in Eq. (314) [cf. reactions (89)-(94)] ... 

The retarding effect of alcohols on the rate of epoxidation manifests itself in the observed autoretardation by the alcohol coproduct.428,434 446,447 The extent of autoretardation is related to the ratio of the equilibrium constants for the formation of catalyst-hydroperoxide and catalyst-alcohol complexes. This ratio will vary with the metal. In metal-catalyzed epoxidations with fe/T-butyl hydroperoxide, autoretardation by tert-butyl alcohol increased in the order W < Mo < Ti < V the rates of Mo- and W-catalyzed epoxidations were only slightly affected. Severe autoretardation by the alcohol coproduct was also observed in vanadium-catalyzed epoxidations.428 434 446 447 The formation of strong catalyst-alcohol complexes explains the better catalytic properties of vanadium compared to molybdenum for the epoxidation of allylic alcohols.429 430 452 On the other hand, molybdenum-catalyzed epoxidations of simple olefins proceed approximately 102 times faster than those catalyzed by vanadium.434 447 Thus, the facile vanadium-catalyzed epoxidation of allyl alcohol with tert-butyl hydroperoxide may involve transfer of an oxygen from coordinated hydroperoxide to the double bond of allyl alcohol which is coordinated to the same metal atom,430 namely,... 

The rates of metal-catalyzed epoxidations are also influenced by the structure of the olefin and the structure of the hydroperoxide. The relative rates of epoxidation of a series of olefins using a mixture of r-Bu02H and Mo(CO)6 paralleled quite closely those for epoxidations with organic peracids.435... 

Ito, Y. N., Katsuki, T. Oxidation of the C C bond metal catalyzed epoxidation of simple olefins. Asymmetric Oxidation Reactions 2001,19-37. 

R. A. Sheldon, J. A. van Doom, Metal-catalyzed epoxidation of olefins with organic hydroperoxides. 11. The effect of solvent and hydroperoxide structure, J. Catal. 31 (1973) 438. 

K. B. Sharpless, R. C. Michaelson, High stereo- and regioselectivities in the transition metal catalyzed epoxidations of olefinic alcohols by tert-butyl hydroperoxide, J. Am. Chem. Soc. 95 (1973) 6136. 

Sheldon, R.A., and J.A. van Doom, Metal Catalyzed Epoxidation of Olefins with Organic Hydroperoxides, J. Catal. 31 427-435 (1973). 

Epoxides are very versatile intermediates, and asymmetric epoxidation of olefins is an effective approach to the synthesis of enantiomericaUy enriched epoxides [1-3]. Great success has been achieved for the epoxidation of allyhc alcohols [1], the metal-catalyzed epoxidation of unfunctionalized olefins (particularly conjugated cis- and tri-substituted) [2], and the nucleophilic epoxidation of electron-deficient olefins [3]. In recent years, chiral dioxiranes have been shown to be powerful agents for asymmetric epoxidation of olefins. Dioxiranes can be isolated or generated in situ from Oxone (potassium peroxymonosulfate) and ketones (Scheme 3.1) [4,5]. When the di-oxirane is used in situ, the corresponding ketone is regenerated upon epoxidation. Therefore, in principle, a catalytic amount of ketone can be used. When a chiral ketone is used, asymmetric epoxidation should also be possible [6]. Extensive studies have been carried out in this area since the first chiral ketone was reported by Curd in 1984 [7]. This chapter describes some of the recent progress in this area. 

In this multi-authored book selected authors in the field of oxidation provide the reader with an up to date of a number of important fields of modern oxidation methodology. Chapter 1 summarizes recent advances on the use of green oxidants such as H2O2 and O2 in the osmium-catalyzed dihydroxylation of olefins. Immobilization of osmium is also discussed and with these recent achievements industrial applications seem to be near. Another important transformation of olefins is epoxidation. In Chapter 2 transition metal-catalyzed epoxidations are reviewed and in Chapter 3 recent advances in organocatalytic ketone-catalyzed epoxidations are covered. Catalytic oxidations of alcohols with the use of environmentally benign oxidants have developed tremendously during the last decade and in Chapter 4 this area is reviewed. Aerobic oxidations catalyzed by N-hydroxyphtahmides (NHPI) are reviewed in Chapter 5. In particular oxidation of hydrocarbons via C-H activation are treated but also oxidations of aUcenes and alcohols are covered. 

Epoxidation reactions have been widely utilized for over 100 years with peradds, peroxides and, more recently, metal catalysts [7]. However, direct metal-catalyzed aerobic epoxidations are rare and generally require an aldehyde coreductant. In this case, the metal is proposed to catalyze radical formation (A-C, Scheme 5.2) followed by O2 insertion to form acyl peroxide D. Metal-catalyzed aerobic oxidation of aldehydes to peradds has previously been observed [8]. With the formation of species D, either an outer-sphere path similar to a peracid-type oxidation occurs (Path 1) or an inner-sphere metal-catalyzed oxidation in which the metal-based oxidant and substrate interact during oxygen transfer (Path 2 or 3). Mu-kaiyama and coworkers were the first to report an aerobic epoxidation of olefins catalyzed by transition metals using either a primary alcohol or an aldehyde as coreductants [9]. The role of the metal was probed through parallel studies of peracid and metal-catalyzed epoxidations of 2 which yielded different stereochemical outcomes. Therefore, a metal-centered mechanism for olefin epoxidation was proposed which implicates an oxygenase system. Path 2 or 3 (Table 5.1) [10]. 

Sharpless early investigations of epoxidation reactions of secondary allylic alcohols were accompanied by an intriguing observation the enantiomeric alcohols display substantial rate differences in the epoxidation event. This allowed a kinetic resolution process to be considered [78, 79]. As showcased in Equation 12, the racemic substrate 56 features four diastereotopic olefin faces and its epoxidation represents a particularly interesting case [79]. As anticipated, the more nucleophilic, disubstituted olefin displays greater reactivity in the epoxidation to give 58 in > 95 % ee. As is generally the trend for metal-catalyzed epoxidation of allylic alcohols, the anti epoxy alcohol 58 is preferentially formed. Kinetic studies have shown that the use of bulkier tartrate esters, such as diisopropyl tartrate (57), generally further enhances the rate differences between two enantiomeric allylic alcohols [19]. 

In 1990, Jacobsen and subsequently Katsuki independently communicated that chiral Mn(III)salen complexes are effective catalysts for the enantioselective epoxidation of unfunctionalized olefins. For the first time, high enantioselectivities were attainable for the epoxidation of unfunctionalized olefins using a readily available and inexpensive chiral catalyst. In addition, the reaction was one of the first transition metal-catalyzed... 

Metal-Catalyzed Direct Hydroxy-Epoxidation of Olefins," Adam. W. Richter, M.J. Accts. Chem. Res., 1994, 27, 57... 

Zinc compounds have recently been used as pre-catalysts for the polymerization of lactides and the co-polymerization of epoxides and carbon dioxide (see Sections 2.06.8-2.06.12). The active catalysts in these reactions are not organozinc compounds, but their protonolyzed products. A few well-defined organozinc compounds, however, have been used as co-catalysts and chain-transfer reagents in the transition metal-catalyzed polymerization of olefins. 

Optically active epoxides are important building blocks in asymmetric synthesis of natural products and biologically active compounds. Therefore, enantio-selective epoxidation of olefins has been a subject of intensive research in the last years. The Sharpless [56] and Jacobsen [129] epoxidations are, to date, the most efficient metal-catalyzed asymmetric oxidation of olefins with broad synthetic scope. Oxidative enzymes have also been successfully utilized for the synthesis of optically active epoxides. Among the peroxidases, only CPO accepts a broad spectrum of olefinic substrates for enantioselective epoxidation (Eq. 6), as shown in Table 8. 

The reactions of aldehydes at 313 K [69] or 323 K [70] in CoAlPO-5 in the presence of oxygen results in formation of an oxidant capable of converting olefins to epoxides and ketones to lactones (Fig. 23). This reaction is a zeolite-catalyzed variant of metal [71-73] and non-metal-catalyzed oxidations [73,74], which utilize a sacrificial aldehyde. Jarboe and Beak [75] have suggested that these reactions proceed via the intermediacy of an acyl radical that is converted either to an acyl peroxy radical or peroxy acid which acts as the oxygen-transfer agent. Although the detailed intrazeolite mechanism has not been elucidated a similar type IIaRH reaction is likely to be operative in the interior of the redox catalysts. The catalytically active sites have been demonstrated to be framework-substituted Co° or Mn ions [70]. In addition, a sufficient pore size to allow access to these centers by the aldehyde is required for oxidation [70]. 

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