Epoxide process

Thallium-Catalyzed Epoxidation Process. The use of Tl(III) for olefin oxidation to yield glycols, carbonyls, or epoxides is weU known... 

The emergence of the powerful Sharpless asymmetric epoxida-tion (SAE) reaction in the 1980s has stimulated major advances in both academic and industrial organic synthesis.14 Through the action of an enantiomerically pure titanium/tartrate complex, a myriad of achiral and chiral allylic alcohols can be epoxidized with exceptional stereoselectivities (see Chapter 19 for a more detailed discussion). Interest in the SAE as a tool for industrial organic synthesis grew substantially after Sharpless et al. discovered that the asymmetric epoxidation process can be conducted with catalytic amounts of the enantiomerically pure titanium/tartrate complex simply by adding molecular sieves to the epoxidation reaction mix-... 

The Payne epoxidation with benzonitrile/ hydrogen peroxide is also an efficient epoxidation process. It is often the method of choice for industrial batch-type applications, but on a small scale the need for continuous pH control is inconvenient. 

In the epoxidation process (Figure 4.4), the oxygen of the enone s carbonyl function first coordinates with the zinc atom. The ethylperoxy anion then attacks the (3-position, which constitutes a Michael-type addition. The subsequent cyclization gives the epoxy ketone and the zinc alkoxide. 

J. D. Jewson, C. A. Jones, and R. M. Dessau, Direct epoxidation process of olefins using palladium-titanosilicate catalyst containing gold promoter, PCT Int. Appl. WO 2001062380 A1 30 Aug 2001. 

With this system we converted 135 mM styrene (relative to the total liquid volume) to styrene oxide in 10 h at a cell dry weight of around lOg/L aqueous phase, with an average activity of 152 U/L total liquid volume. This corresponds to a space-time yield of 1.1 g (5)-styrene oxide per liter and hour. These are the highest specific activities reported thus far for a microbial epoxidation process. ... 

LIKELY COSTS OF LARGE SCALE HYDROXYLATION OR EPOXIDATION PROCESSES WITH WHOLE CELL BIOCATALYSTS... 

Z.H. Ge, R. Thompson, S. Cooper, D. Elhson and P. Tway, Quantitative monitoring of an epoxidation process hy Eourier-Transform infrared spectroscopy. Process Contr. Qual., 7(1), 3-12 (1995). 

The resulting polyol resembles the product that is hypothesized for the oligomerization of triglycerides via air oxidation, with the exception that there is a large increase in the hydroxyl content of the polyol product, and there is very little, if any, of the starting epoxide left unreacted. In addition, the epoxidation process does not produce low molecular weight chain scission products, which are a by-product of the blown oil process. The hydroxylation of epoxidized triglycerides is illustrated in Fig. 17. 

Mimoun and coworkersdescribed the first well-defined example of a d° metal aUtylperoxidic species 49 which epoxidized simple olefins with high selectivity. Several features of the epoxidation performed by 49 resemble those of the Halcon catalytic epoxidation process " . Novel tungsten complexes containing 2 -pyridyl alcoholate ligands like 50 have been synthesized and tested as catalysts in the epoxidation of cw-cyclooctene with TBHP in the absence of solvent . The system displayed modest catalytic activity (100% conversion in 60 h) but excellent product selectivity. 

Title Direct Epoxidation Process Using a Mixed Catalyst System... 

The epoxidation of propene is analogous to that of ethylene catalyzed by silver. However, the selectivity is much lower. Due to the pronounced oxidation sensitivity of the allyl CH3-group, excessive combustion occurs as a side reaction. The heterogeneous process has no practical significance, therefore, as it has to compete with a highly selective liquid phase epoxidation process. 

Catalytic systems containing Te02, HBr and AcOH have been used industrially by Oxirane to convert ethylene to ethylene glycol via the formation of mono- and di-acetate (equations 131 and 132).359-361 The overall yield from ethylene to ethylene glycol is more than 90%, making this reaction competitive with respect to the older silver-catalyzed ethylene epoxidation process. 

In the Halcon epoxidation process, the reaction of the zirconium(IV) methyl trialkoxide (201) with 02 yields the epoxy alkoxide (203), via intramolecular epoxidation of the coordinated allyl alcohol by the incipient methyl peroxide complex (202).630... 

An important improvement in the asymmetric epoxidation process is the finding, reported in 1986, that by adding molecular sieves to the reaction medium virtually all reactions can be performed with a catalytic amount (5-10 mol %) of the Ti-tartrate complex [3]. Previously, only a few structural classes of allylic alcohols were efficiently epoxidized by less than stoichiometric amounts of the complex, and most reactions were routinely performed with stoichiometric quantities of the reagent. 

For the asymmetric epoxidation reaction, dry alcohol-free dichloromethane (the use of dichloromethane stabilized with methanol must be avoided) is usually the solvent of choice It is inert to the reagents, has good solvent power for the components of the reaction, and supports good epoxidation rates. A fortunate consequence of the asymmetric epoxidation process is that ligation of the allylic alcohol to the Ti center aids in solubilization of the substrate. Substrates that normally may be only modestly soluble in the above-mentioned solvents will be brought into solution as they complex with the Ti-tartrate catalyst. 

Optically active tartrate esters are the source of chirality for the asymmetric epoxidation process. With a few subtle exceptions, the esters used conventionally—dimethyl (DMT), diethyl (DET), and diisopropyl tartrate (DIPT)—are equally effective at inducing asymmetry during the crucial epoxidation event. The minor exceptions that have been noted include (a) a slight improvement... 

The nonconventional tartrate esters 1-3 have been used to probe the mechanism of the asymmetric epoxidation process [20a]. These chain-linked bistartrates when complexed with 2 equiv. of Ti(0-f-Bu)4 catalyze asymmetric epoxidation with good enantiofacial selectivity. 

Ti(IV) isopropoxide [Chemical Abstracts nomenclature 2-propanol, Ti(4+) salt], is the Ti species of choice for preparation of the Ti-tartrate complex in the asymmetric epoxidation process. The use of Ti(IV) f-butoxide has been recommended for reactions in which the epoxy alcohol product is particularly sensitive to ring opening by the alkoxide [18]. The 2-substituted epoxy alcohols are one such class of compounds. Ring opening by f-butoxide is much slower than by i-propoxide. With the reduced amount of catalyst that now is sufficient for all asymmetric epoxidations, the use of Ti(0-f-Bu)4 appears to be unnecessary in most cases, but the concept is worth noting. 

The addition of activated molecular sieves (zeolites) to the asymmetric epoxidation milieu has the beneficial effect of permitting virtually all reactions to be earned out with only 5-10 mol % of the Ti-tartrate catalyst [3,4]. Without molecular sieves, only a few of the more reactive allylic alcohols are epoxidized efficiently with less than an equivalent of the catalyst. The role of the molecular sieves is thought to be protection of the catalyst from (a) adventitious water and (b) water that may be generated in small amounts by side reactions during the epoxidation process. 

Before commencing, the attention of the reader is drawn to the terms enantiofacial selectivity and diastereoselectivity. The usage in this chapter does not conform to the strictest possible definitions of these terms. In particular, enantiofacial selectivity is used with reference to the selection and delivery of oxygen by the epoxidadon catalyst to one face of the olefin in preference to the other. This usage extends to chiral allylic alcohols (primarily the 1-substituted allylic alcohols) when the focus of the discussion is on face selection in the epoxidation process. Diastereoselectivity is used in the discussion of kinetic resolution when the generation of diastereomeric compounds is emphasized. 

The presence of a stereogenic center at Cj of an allylic alcohol introduces an additional factor into the asymmetric epoxidation process in that now both enantiofacial selectivity and dias-tereoselectivity must be considered. It is helpful in these cases to examine epoxidation of each enantiomer of the allylic alcohol separately. Epoxidation of one enantiomer proceeds normally and produces an erythro epoxy alcohol in accord with the rules shown in Figure 6A.1. 

The variable enantioselectivities seen in these results likely stem from conformational restraints imposed by the cyclic structures, which prevent the allylic alcohols from attaining an ideal conformation for the epoxidation process (see Fig. 6A.9, below, for the proposed ideal conformation). 

An alternate mechanism invoking an ion-pair transition-state assembly has been proposed to account for the enantioselectivity of the asymmetric epoxidation process [137]. In this proposal, two additional alcohol species are required in the transition-state complex. This... 

---