Epoxides from olefin oxidation

The Jacobsen-Katsuki epoxidation reaction is an efficient and highly selective method for the preparation of a wide variety of structurally and electronically diverse chiral epoxides from olefins. The reaction involves the use of a catalytic amount of a chiral Mn(III)salen complex 1 (salen refers to ligands composed of the N,N -ethylenebis(salicylideneaminato) core), a stoichiometric amount of a terminal oxidant, and the substrate olefin 2 in the appropriate solvent (Scheme 1.4.1). The reaction protocol is straightforward and does not require any special handling techniques. 

It should be noted that the related imine-oxaziridine couple E-F finds application in asymmetric sulfoxidation, which is discussed in Section 10.3. Similarly, chiral oxoammonium ions G enable catalytic stereoselective oxidation of alcohols and thus, e.g., kinetic resolution of racemates. Processes of this type are discussed in Section 10.4. Whereas perhydrates, e.g. of fluorinated ketones, have several applications in oxidation catalysis [5], e.g. for the preparation of epoxides from olefins, it seems that no application of chiral perhydrates in asymmetric synthesis has yet been found. Metal-free oxidation catalysis - achiral or chiral - has, nevertheless, become a very potent method in organic synthesis, and the field is developing rapidly [6]. 

This mechanism is consistent with a number of observations. Kinetic studies on prolyl 4-hydroxylase [223] and thymine hydroxylase (EC 1.14.11.6) [224] suggest that cofactor binds first, followed by 02. The bound 02 appears to have superoxide character, as superoxide scavengers are competitive inhibitors of 02 consumption [225,226], It is also clear that the oxidative decarboxylation of the keto acid is a distinct phase of the mechanism from the alkane functionalization step, as these two phases can be uncoupled, particularly when poor substrate analogs are employed [227-229], Evidence for an Fe(IV) = 0 intermediate derives from studies with substrate analogs. Besides the hydroxylation of the 5-methyl group of thymine, thymine hydroxylase can also catalyze ally lie hydrox-ylations, epoxidation of olefins, oxidation of sulfides to sulfoxides, and N-de-... 

G. H. Grosch, U. Muller, M. Schulz, N. Richer, H. Wurz, Oxidation catalyst and process for the production of epoxides from olefines, hydrogen and oxygen using said oxidation catalyst, U.S. Patent No. 6,008,389,1999, Assigned to BASF Corporation. 

It is known that titania/silica can catalyze oxidation reactions [1-3]. Especially, titanium-silicate-1 (TS-1) has been shown to be a very effective catalyst for oxidation reactions. In the TS-1 catalyst, most Ti atoms are isolated from each other by long chains of -O-Si-O-Si-0- and this structure gives high selectivity for the formation of epoxides from olefins [1]. 

The discussion in previous Section 9.3 focused on classic oxidative methods for the synthesis of optically active epoxides from olefins. Oxiranes have long been recognized to be of great utility as chiral building blocks, and indeed their pervasive use attests to their central role in synthesis. Because the developments in epoxidation methods are closely linked to their applications, the section below presents selected transformations of epoxides that have been landmarks in the evolution of the field [30-33]. 

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... 

The epoxidation method developed by Noyori was subsequently applied to the direct formation of dicarboxylic acids from olefins [55], Cyclohexene was oxidized to adipic acid in 93% yield with the tungstate/ammonium bisulfate system and 4 equivalents of hydrogen peroxide. The selectivity problem associated with the Noyori method was circumvented to a certain degree by the improvements introduced by Jacobs and coworkers [56]. Additional amounts of (aminomethyl)phos-phonic acid and Na2W04 were introduced into the standard catalytic mixture, and the pH of the reaction media was adjusted to 4.2-5 with aqueous NaOH. These changes allowed for the formation of epoxides from ot-pinene, 1 -phenyl- 1-cyclohex-ene, and indene, with high levels of conversion and good selectivity (Scheme 6.3). 

High-valent ruthenium oxides (e. g., Ru04) are powerful oxidants and react readily with olefins, mostly resulting in cleavage of the double bond [132]. If reactions are performed with very short reaction times (0.5 min.) at 0 °C it is possible to control the reactivity better and thereby to obtain ds-diols. On the other hand, the use of less reactive, low-valent ruthenium complexes in combination with various terminal oxidants for the preparation of epoxides from simple olefins has been described [133]. In the more successful earlier cases, ruthenium porphyrins were used as catalysts, especially in combination with N-oxides as terminal oxidants [134, 135, 136]. Two examples are shown in Scheme 6.20, terminal olefins being oxidized in the presence of catalytic amounts of Ru-porphyrins 25 and 26 with the sterically hindered 2,6-dichloropyridine N-oxide (2,6-DCPNO) as oxidant. The use... 

Vinylepoxides can be obtained by various strategies, all with their inherent limitations. Racemic epoxidation of olefins is a straightforward route to epoxides, as pure trans- or cis-epoxides can be obtained from ( )- or (Z)-alkenes, respectively. Various oxidants - such as mCPBA and other peracids, H2O2, or VO(acac)2/TBHP - can all be employed in this transformation [1],... 

The asymmetric oxidation of organic compounds, especially the epoxidation, dihydroxylation, aminohydroxylation, aziridination, and related reactions have been extensively studied and found widespread applications in the asymmetric synthesis of many important compounds. Like many other asymmetric reactions discussed in other chapters of this book, oxidation systems have been developed and extended steadily over the years in order to attain high stereoselectivity. This chapter on oxidation is organized into several key topics. The first section covers the formation of epoxides from allylic alcohols or their derivatives and the corresponding ring-opening reactions of the thus formed 2,3-epoxy alcohols. The second part deals with dihydroxylation reactions, which can provide diols from olefins. The third section delineates the recently discovered aminohydroxylation of olefins. The fourth topic involves the oxidation of unfunc-tionalized olefins. The chapter ends with a discussion of the oxidation of eno-lates and asymmetric aziridination reactions. 

The synthesis of the manganese(III) complex of the hexaaza macrocyclic ligand (176), derived from 2,3-butanedione and diethylenetriamine, and its use as a catalyst for the epoxidation of olefins using iodosylbenzene as oxidant has been reported." ... 

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). 

Epoxidation of olefins over Mo containing Y zeolites was studied by Lunsford et al. [86-90]. Molybdenum introduced in ultrastable Y zeolite through reaction with Mo(C0)g or M0CI5, shows a high initial activity for epoxidation of propylene with t-butyl hydroperoxide as oxidant and 1,2-dichloroethane as solvent [88]. The reaction is proposed to proceed via the formation of a Mo +-t-butyl hydroperoxide complex and subsequent oxygen transfer from the complex to propylene. The catalyst suffers however from fast deactivation caused by intrazeolitic polymerization of propylene oxide and resulting blocking of the active sites. 

Polymer-supported persulfonic acid was prepared from potassium persulfate and the cation exchange resin P—SO3H in water. The authors reported various applications of this new oxidizing reagent such as epoxidation of olefins, Baeyer-Villiger reaction and cleavage of disulfide and Af-formylamino acids. 

The search for a new epoxidation method that would be appropriate for organic synthesis should also, preferably, opt for a catalytic process. Industry has shown the way. It resorts to catalysis for epoxidations of olefins into key intermediates, such as ethylene oxide and propylene oxide. The former is prepared from ethylene and dioxygen with silver oxide supported on alumina as the catalyst, at 270°C (15-16). The latter is prepared from propylene and an alkyl hydroperoxide, with homogeneous catalysis by molybdenum comp e ts( 17) or better (with respect both to conversion and to selectivity) with an heterogeneous Ti(IV) catalyst (18), Mixtures of ethylene and propylene can be epoxidized too (19) by ten-butylhydroperoxide (20) (hereafter referred to as TBHP). 

The formation of methylperoxy intermediates—i.e., the product of a formal insertion of O2 into the metal-methyl bond—was substantiated by the observation of epoxidation of allylic alkoxides (Scheme 6), in analogy to the proposed mechanism for the Sharpless epoxidation utilizing tert-butylhydroperoxide (TBHP). A similar oxygen atom transfer from a coordinated alkylperoxide to olefin was also postulated for the epoxidation of olefins with TBHP catalyzed by Cp Mo(0)2Cl [31]. The use of organomolybdenum oxides in olefin epoxidafion cafalysis (albeit not with O2) has recently been reviewed [32]. 

Epoxidation. 1,1,3,3-Tetrachloroacetone can mediate the epoxidation of olefins by hydrogen peroxide in the same way as hcxafluoroacctonc (9, 244-245). It has the advantage that it is probably less toxic, and it is inexpensive and commercially available. The actual oxidant presumably is the hydroperoxide 1, which is converted during the epoxidation into the unstable hydrate (2), from which the tctrachloroacetone can be recovered in >70% yield. Monosubstitutcd alkenes are epoxidized in low yields by this method more highly substituted alkenes are epoxidized in 65 85% yield (VPC).1... 

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