Alkyl hydroperoxides epoxidation

We emphasize that the above mechanism is strictly valid only for H202 and alkyl hydroperoxide epoxidations of alkenes catalyzed by TS-1 and Ti-MCM-41. In view of the observation of similar titanium oxo species when H2 + 02 are brought in contact with TS-1 or Ti-MCM-41 (54), similar conclusions may be drawn for that system as well. A radical mechanism involving the Ti=0 groups had been proposed earlier by Khouw et al. (221) for the hydroxylation of alkanes. No spectroscopic investigation of the TS-l/H202/alkane has yet been reported. 

The addition of an oxygen atom to an olefin to generate an epoxide is often catalyzed by soluble molybdenum complexes. The use of alkyl hydroperoxides such as tert-huty hydroperoxide leads to the efficient production of propylene oxide (qv) from propylene in the so-called Oxirane (Halcon or ARCO) process (79). 

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). 

As electrophilic substitutes for peracids, the use of borate ester induced decomposition of alkyl hydroperoxides and molybdenum VI peroxy-complexes have been reported in the recent literature. Although these reagents have led to the epoxidation of olefins in greater than 90% yield there are no reports yet of their application to steroid olefins. 

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 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]. 

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... 

The molybdenum-catalyzed conversion of alkenes into epoxides by alkyl hydroperoxides is an important commercial process.17,18 The synthetic potential of such reactions in regard to more complex organic molecules has been evaluated19 alkyl hydroperoxides are used as oxidants in the... 

Olefin epoxidation by alkyl hydroperoxides catalyzed by transition metal compounds occupies an important place among modern catalytic oxidation reactions. This process occurs according to the following stoichiometric equation ... 

TABLE 8. Results of the synthesis of dialkyl peroxides through reaction of alkyl hydroperoxides with epoxides... 

L Ti, Zr, Hf. Characteristic for group IV transition metal catalysts for epoxidation reactions is the intermediate formation of a mono- or bidentate coordinated alkyl hydroperoxide, hydrogen peroxide or a bidentate coordinated peroxo group in the catalytically active species. 

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 direct epoxidation of simple aikenes by hydrogen peroxide or alkyl hydroperoxides is a longstanding goal in oxidation chemistry. The reaction is usually catalyzed by suitable high-valent metals, mainly belonging to group 5 and 6, through formation of metal peroxo species. ... 

---