Olefins epoxidation with alkyl hydroperoxides

Metal oxide-based catalysts were also studied57 in the early days of development of olefin epoxidation with alkyl hydroperoxides. M0O3 was an excellent catalyst, WO3 showed moderate activity and other oxides (V2O5, Se02,... 

G. Yin, A. M. Danby, D. Kitko, J. D. Carter, W. M. Scheper, D. H. Busch, Olefin epoxidation by alkyl hydroperoxide with a novel cross-bridged cyclam manganese complex Demonstration of oxygenation by two distinct reactive intermediates. [Erratum to document cited in CA146 316400], Inorg. Chem. 46 (2007) 2173. 

The phenomenon that early transition metals in combination with alkyl hydroperoxides could participate in olefin epoxidation was discovered in the early 1970s [30, 31]. While m-CPBA was known to oxidize more reactive isolated olefins, it was discovered that allylic alcohols were oxidized to the corresponding epoxides at the same rate or even faster than a simple double bond when Vv or MoVI catalysts were employed in the reaction [Eq. (2)] [30]. 

Molybdenum complexes are the most effective catalysts known for the selective epoxidation of olefins with alkyl hydroperoxides (210-212). Commonly known is the Arco or Halcon process for the large-scale manufacture of propylene oxide from propylene. This process uses t-BuOOH or ethyl benzene hydroperoxide (EBHP) as an oxidant and Mo(CO)6, for example, as a source of Mo. The Mo(CO)6 acts as a catalyst precursor, which is converted into a soluble active form by complexation with diols (3). Chemists have designed several supported versions of the catalysts for this epoxidation chemistry. A clear classification can be made on the basis of the nature of the support. 

All the polymers of Table III have been applied for the epoxidation of olefins with alkyl hydroperoxides. For example, the polymers with iminodiacetic acid or diethylene triamine groups were used for the regioselective epoxidation of (E)-geraniol with t-BuOOH to the 2,3-epoxide (225), whereas the Mo anchored to the diphenylphosphinopolystyrene catalyst is used in the epoxidation of cyclohexene with t-BuOOH (228). The polymer-supported molybdenyl thioglycolate has also been used for the catalytic oxidation of thiols and phosphines with air or pyridine N-oxide as the oxidant (234). 

In this context it is worth noting that neither the titanium(IV) tartrate catalyst nor other metal catalyst-alkyl hydroperoxide reagents are effective for the asymmetric epoxidation of unfunctionalized olefins. The only system that affords high enantioselectivities with unfunctionalized olefins is the manganese(III) chiral Schiff s base complex/NaOCl combination developed by Jacobsen [42]. There is still a definite need, therefore, for the development of an efficient chiral catalyst for asymmetric epoxidation of unfunctionalized olefins with alkyl hydroperoxides or hydrogen peroxide. 

R. A. Sheldon, Molybdenum-catalyzed epoxidation of olefins with alkyl hydroperoxides. 

The epoxidation of two cycloalkenones, a- and P-isophorone, with alkyl hydroperoxides demonstrates that active and selective titania-silica aerogels can be prepared by the sol-gel method combined with extraction of the solvent with supercritical COg at low temperature. The key factors for obtaining high activity in the epoxidation of bulky cyclic olefins are the high Ti-distribution in the silica matrix, the mesoporous structure and high surface area. 

The most well-known example is the catalytic epoxidation of olefins with alkyl hydroperoxides that is used for the commercial production of propylene oxide (see earlier). The reaction is catalyzed by high-valent compounds of early transition metals, e.g. Movl, WVI, Vv and TiIV, and involves a peroxometal type mechanism [6,7] as shown (reaction 12). Mo compounds are particularly effective homo-... 

In this review we shall focus on the use of heterogeneous catalysts for the liquid phase epoxidation of olefins with alkyl hydroperoxides or hydrogen peroxide. The latter is generally the oxidant of choice for fine-chemicals production owing to a better availability and lower price. Emphasis is placed on methods with a broad scope in organic synthesis. 

These mesoporous mixed titania-silica oxides are hydrophilic materials and are excellent catalysts for epoxidations of olefins, allylic alcohols and a,jff-unsaturated ketones with alkyl hydroperoxides in non-aqueous media [37]. Their performance can be improved even further by adding organic or inorganic bases to neutralize acid sites present on the surface [38,39], The latter cause side-reactions, especially with acid sensitive epoxides. Amine addition was particularly effective and led to the development of a mesoporous Ti-Si mixed oxide containing surface-tethered tertiary amino groups as an active, selective, and recyclable catalyst for the epoxidation of allylic alcohols [38]. 

Several framework titanium-substituted mesoporous silicates, including Ti-MCM-41 (42,43), Ti-HMS (198), Ti-MCM-36 (180), Ti-MCM-48 (199), and Ti-SBA-15 (200), have shown promising activity for the epoxidation of bulky olefins with alkyl hydroperoxides as oxidants. Unfortunately, compared with the microporous MFI-type titanium silicates, the mesoporous materials exhibit low activity for epoxidation reactions. The hydrophilic nature of mesoporous silica catalysts with isomorphous titanium substitution is considered to be one of the major reasons for the low activity (179). Various attempts have been made to improve the activity. Using a different synthetic procedure, titanium species have been grafted onto... 

Reaction conditions depend on the reactants and usually involve acid or base catalysis. Examples of X include sulfate, acid sulfate, alkane- or arenesulfonate, chloride, bromide, hydroxyl, alkoxide, perchlorate, etc. RX can also be an alkyl orthoformate or alkyl carboxylate. The reaction of cycHc alkylating agents, eg, epoxides and a2iridines, with sodium or potassium salts of alkyl hydroperoxides also promotes formation of dialkyl peroxides (44,66). Olefinic alkylating agents include acycHc and cycHc olefinic hydrocarbons, vinyl and isopropenyl ethers, enamines, A[-vinylamides, vinyl sulfonates, divinyl sulfone, and a, P-unsaturated compounds, eg, methyl acrylate, mesityl oxide, acrylamide, and acrylonitrile (44,66). 

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... 

Metal alkoxides undergo alkoxide exchange with alcoholic compounds such as alcohols, hydro-xamic acids, and alkyl hydroperoxides. Alkyl hydroperoxides themselves do not epoxidize olefins. However, hydroperoxides coordinated to a metal ion are activated by coordination of the distal oxygen (O2) and undergo epoxidation (Scheme 1). When the olefin is an allylic alcohol, both hydroperoxide and olefin are coordinated to the metal ion and the epoxidation occurs swiftly in an intramolecular manner.22 Thus, the epoxidation of an allylic alcohol proceeds selectively in the presence of an isolated olefin.23,24 In this metal-mediated epoxidation of allylic alcohols, some alkoxide(s) (—OR) do not participate in the epoxidation. Therefore, if such bystander alkoxide(s) are replaced with optically active ones, the epoxidation is expected to be enantioselective. Indeed, Yamada et al.25 and Sharp less et al.26 independently reported the epoxidation of allylic alcohols using Mo02(acac)2 modified with V-methyl-ephedrine and VO (acac)2 modified with an optically active hydroxamic acid as the catalyst, respectively, albeit with modest enantioselectivity. 

The epoxidation of electon-defident olefins using a nucleophilic oxidant such as an alkyl hydroperoxide is generally nonstereospecific epoxidation of both cis- and /nmv- ,/3-unsatii rated ketones gives the trans-epoxide preferentially. However, the epoxidation of cis-ofi-unsaturated ketones catalyzed by Yb-(40) gives civ-epoxides preferentially, with high enantioselectivity, because the oxidation occurs in the coordination sphere of the ytterbium ion (Scheme 26).132... 

These complexes are effective catalysts in epoxidation reactions with H2O2 and alkyl hydroperoxides. Several detailed mechanistic studies have been carried out in particular, it has been shown that, when the alkyl chain contains a double bond, no autoepoxidation is observed both in the solid state and in solution. Nevertheless, if f-BuOOH is added, the epoxidation of the olefinic moiety immediately takes place. Therefore, it has been suggested that these complexes are not the active species in the oxygen transfer step to the substrate, but they behave as catalysts for the primary peroxidic oxidant. On the basis of kinetic, spectroscopic and theoretical studies, the authors provided a mechanism, whose key steps are sketched in Scheme 12. In this context a major role appears to be played by the fluxionality of the particular ligands used . ... 

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). 

Effect of Olefin Structure. The reaction rate of the epoxidation depends on olefin structure. In general, the more alkyl substituents bonded to the carbon atoms of the double bond, the faster the reaction rate. This was shown by a reaction of 2-methyl-2-pentene, cyclohexene, and 2-octene with cumene hydroperoxide under the same conditions (Table V). The yield of epoxide was quantitative. The results indicate that 2-methyl-2-pentene reacts faster than cyclohexene and 2-octene. 

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