Epoxidation alkyl hydroperoxide catalysts

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. 

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 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 chiral ligand (44) was prepared starting from the cyclic a-amino acid (S)-proline80). Recently, similar chiral catalysts and related molybdenum complexes involving optically active N-alkyl-P-aminoalcohols as stable chiral ligands and acetylacetone as a replaceable bidentate ligand, were designed for the epoxidation of allylic alcohols with alkyl hydroperoxides which could be catalyzed by such metal complexes 8,). 

The use of alkyl hydroperoxides in the metal-catalyzed epoxidation of alkenes is their most important synthetic application. High-valence d° metals such as Mo(VI), W(VI), V(V), and Ti(IV) are the most effective catalysts 24 245,278,279,285,286 Used as soluble complexes or as heterogenized supported catalysts, they can give epoxides in near-quantitative yields. 

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... 

Thus, the Milas reagents may be considered to be the progenitors of the metal catalyst/alkyl hydroperoxide reagents5 5 that were later developed cy, inter alia, Halcon, Arco and Shell workers and culminated in the realization of commercial processes for the epoxidation of propylene (reaction II). These reagents involve the very same metal catalysts, e.g. MoVI, WVI, vv and TiIV, as the Milas reagents and they are mechanistically closely related. 

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,... 

There are also several situations where the metal can act as both a homolytic and heterolytic catalyst. For example, vanadium complexes catalyze the epoxidation of allylic alcohols by alkyl hydroperoxides stereoselectively,57 and they involve vanadium(V) alkyl peroxides as reactive intermediates. However, vanadium(V)-alkyl peroxide complexes such as (dipic)VO(OOR)L, having no available coordination site for the complexation of alkenes to occur, react homolyti-cally.46 On the other hand, Group VIII dioxygen complexes generally oxidize alkenes homolytically under forced conditions, while some rhodium-dioxygen complexes oxidize terminal alkenes to methyl ketones at room temperature. 

Since the metal-alkene association preceding the peroxymetalation reaction in mechanism (B) is a pure Lewis acid/Lewis base interaction, it would be expected that compounds having alkylperoxy groups bonded to a Lewis acid center should be active for the epoxidation of alkenes. This is indeed found for boron compounds, which are active as catalysts for the epoxidation of alkenes by alkyl hydroperoxides.246,247 Isolated boron tris(alkyl peroxides), B(OOR)3, have been shown to epoxidize alkenes stoichiometrically, presumably through alkylperoxyboration of the double bond (equation 76).248... 

A material prepared by anchoring titanium(IV) on to the walls of a high-area, crystalline mesoporous silica (MCM41) has been used as an alkene epoxidation catalyst with alkyl hydroperoxides.204 The effect of replacing one of the three O—Si= groups to which the Ti(IV) is bound by an O—Ge= group is reported to lead to an increase in catalytic activity of up to 18% in die epoxidation of cyclohexene, although no explanation is provided and it is notable diat the selectivity towards the formation of cyclohexene oxide (versus cyclohexenol and cyclohexane-1,2-diol) was inferior to that with the non-modified system.205... 

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 general, the metal catalyst-hydrogen peroxide reagent is inferior to the corresponding metal catalyst-alkyl hydroperoxide systems for the epoxidation of olefins (see Section III.B.2). 

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