Propene, epoxidation

The first evidence of the capacity of supported gold catalysts to epoxidate propene was described by Haruta et al. using dioxygen in the presence of H2 as reductant, which allows 02 activation at low temperatures [237]. Although initial selectivities by Au/Ti02 were low, promising improvements were achieved with different supports [230-232, 238-246]. One of them, TS-1, is known to be suitable for the selective epoxidation of propene with H202 [246]. For that reason, many early studies focused on its use. 

Commercially available 30% hydrogen peroxide solution can oxidize alkenes readily in the presence of a carbo-diimide promoter <1996SL649, 1998JOC2564, 1998JOC1730> (Equation 67). A method to epoxidize propene using aqueous hydrogen peroxide and a reaction controlled phase-transfer catalysts was developed <20040PD131>. 

Up till now it has not been possible to carry out the analogous reaction with propene. Numerous researchers have attempted to develop a process for the direct oxidation of propene into propene epoxide (PO). Only indirect routes have, up to now, been applied in successful selective processes (see Section 5.5.4). Those indirect processes involve the use of hydrogen peroxide, organic peroxides and peracids, hypochlorides, etc. (see e.g. SMPO, Chapter 2). The reason that it is difficult to epoxidize propene is the facile formation of an allylic intermediate because the C-H groups in the methyl group become activated. 

Since the early 1980s, EniChem has been a pioneer in the development of the process, holding a portfolio of patents [10c,dj. The integration of HP synthesis by means of alkylanthraquinone/alkylanthrahydroquinone (RAQ/RAHQ) cycle technology, with PO production, by means of propene epoxidation with H P, is possible because of the peculiar properties of the TS-1 catalyst (Scheme 6.5). TS-1 can selectively epoxidize propene using diluted HP [12]. A water- methanol mixture is the solvent for the epoxidation the alcohol is necessary to obtain a sufficient reaction rate. Therefore, a cost-saving feature in this process is the fact that the crude H P produced can be used directly in the epoxidation of propene. Moreover, integration of the two processes is also allowed by the easily accomplished separation of propene and PO from the water-methanol mixture. Methanol, after separation and purification, can be recycled to the epoxidation step. 

Alternative Names/Abbreviations 1,2-Epoxypropane, propylene epoxide, propene oxide, PO... 

Many of the reactions we ve already encountered can yield a chiral product from an achi ral starting material Epoxidation of propene for example creates a chirality center by adding oxygen to the double bond... 

Figure 7 7 shows why equal amounts of (R) and (5) 1 2 epoxypropane are formed m the epoxidation of propene There is no difference between the top face of the dou ble bond and the bottom face Peroxyacetic acid can transfer oxygen to either face with equal facility the rates of formation of the R and S enantiomers of the product are the same and the product is racemic... 

Recently (79MI50500) Sharpless and coworkers have shown that r-butyl hydroperoxide (TBHP) epoxidations, catalyzed by molybdenum or vanadium compounds, offer advantages over peroxy acids with regard to safety, cost and, sometimes, selectivity, e.g. Scheme 73, although this is not always the case (Scheme 74). The oxidation of propene by 1-phenylethyl hydroperoxide is an important industrial route to methyloxirane (propylene oxide) (79MI5501). 

The manufacture and uses of oxiranes are reviewed in (B-80MI50500, B-80MI50501). The industrially most important oxiranes are oxirane itself (ethylene oxide), which is made by catalyzed air-oxidation of ethylene (cf. Section 5.05.4.2.2(f)), and methyloxirane (propylene oxide), which is made by /3-elimination of hydrogen chloride from propene-derived 1-chloro-2-propanol (cf. Section 5.05.4.2.1) and by epoxidation of propene with 1-phenylethyl hydroperoxide cf. Section 5.05.4.2.2(f)) (79MI50501). 

The nonfluonnated double bond m 3-perfluoroalkyl-l-propene is epoxidized with difficulty by ni-chloroperoxybenzoic acid [20] (equation 12)... 

A more efficient agent than peroxy compounds for the epoxidation of fluoro-olefins with nonfluonnated double bond is the hypofluorous acid-acetomtrile complex [22] Perfluoroalkylethenes react with this agent at room temperature within 2-3 h with moderate yields (equation 13), whereas olefins with strongly electron-deficient double bond or electron-poor, sterically hindered olefins, for example l,2-bis(perfluorobutyl)ethene and perfluoro-(l-alkylethyl)ethenes, are practically inert [22] Epoxidation of a mixture of 3 perfluoroalkyl-1-propenes at 0 C IS finished after 10 mm in 80% yield [22] The trifluorovinyl group in partially fluorinated dienes is not affected by this agent [22] (equation 13)... 

Table 1.6.1. Epoxidation of 2-methyl-2-propene-l-ol benefieial effeets of eatalytie reaetions and derivatization.

In general, 2-substituted allylic alcohols are epoxidized in good enantioselectivity. Like glycidol, however, the product epoxides are susceptible to ring opening via nucleophilic attack at the C-3 position. Results of the AE reaction on 2-methyl-2-propene-l-ol followed by derivatization of the resulting epoxy alcohol are shown in Table 1.6.1. Other examples are shown below. 

The one general exception to the rule that ethers don t typically undergo Sn2 reactions occurs with epoxides, the three-membered cyclic ethers that we saw in Section 7.8. Epoxides, because of the angle strain in the three-membered ring, are much more reactive than other ethers. They react with aqueous acid to give 1,2-diols, as we saw in Section 7.8, and they react readily with many other nucleophiles as well. Propene oxide, for instance, reacts with HC1 to give l-chloro-2-propanol by Snj2 backside attack on the less hindered primary carbon atom. We ll look at the process in more detail in Section 18.6. 

Posner recently reported a very simple and fast way to activate epoxides towards nucleophilic opening by ketone lithium enolate anions by use of BF3 Et20 (1 equiv.) [73]. The application of this procedure to the nucleophilic opening of propene oxide with the lithium enolate of 2-cycloheptanone, obtained by the conjugate addition of trimethylstannyllithium to 2-cycloheptenone, afforded the stan-... 

The commonly held view of the uniqueness of Ag for ethylene epoxidation may soon change in view both of the propene epoxidation work of Haruta and coworkers on Au/Ti02 catalysts upon cofeeding H2 123 and also in view of the recent demonstration by Lambert and coworkers124 126 that Cu(lll) and Cu(110) surfaces are both extremely efficient in the epoxidation of styrene and butadiene to the corresponding epoxides. In fact Cu was found to be more selective than Ag under UHV conditions with selectivities approaching 100%.124-126 The epoxidation mechanism appears to be rather similar with that on Ag as both systems involve O-assisted alkene adsorption and it remains to be seen if appropriately promoted Cu124 126 can maintain its spectacular selectivity under process conditions. 

B.S. Uphade, M. Okumura, S. Tsubota, and M. Haruta, Effect of physical mixing of CsCl with Au/Ti-MCM-41 on the gas-phase epoxidation of propene using H2 and02 Drastic depression of H2 consumption, Appl. Catal. A 190, 43-50 (2000). 

The mechanisms of the cyclisation of 2 -hydroxychalcone derivatives which can lead to flavanones, flavones and aurones have been reviewed <95MI1> and the formation of 3-hydroxy- chromanones and -flavanones from l-(2-hydroxyphenyl)-2-propen-l-ones via the epoxide has been optimised <96JOC5375>. 

Xanthobacter sp. strain Py2 was isolated by enrichment on propene that is metabolized by initial metabolism to the epoxide. The monooxygenase that is closely related to aromatic monooxygenases is able to hydroxylate benzene to phenol before degradation, and toluene to a mixture of 2-, 3-, and 4-methylphenols that are not further metabolized (Zhou et al. 1999). 

C[bicarbonate] and NMR were used to demonstrate that the first product in the metabolism of propene epoxide is acetoacetate, which is then reduced to (3-hydroxybutyrate (Allen and Ensign 1996). 

Hydrolysis to the diol followed by dehydration to the aldehyde and oxidation to the carboxylic acid is used by a propene-utilizing species of Nocardia (de Bont et al. 1982). Although an ethene-utilizing strain of Mycobacterium sp. strain E44 degrades ethane-l,2-diol by this route, the diol is not an intermediate in the metabolism of the epoxide (Wiegant and de Bont 1980). 

Xanthobacter sp. strain Py2 may be grown with propene or propene oxide. On the basis of amino acid sequences, the monooxygenase that produces the epoxide was related to those that catalyzes the monooxygenation of benzene and toluene (Zhou et al. 1999). The metabolism of the epoxide is initiated by nucleophilic reaction with coenzyme M followed by dehydrogenation (Eigure 7.13a). There are alternative reactions, both of which are dependent on a pyridine nucleotide-disulfide oxidoreductase (Swaving et al. 1996 Nocek et al. 2002) ... 

Fluidized-bed reactor Epoxidation of propene Mycobacterium cells 125... 

Fig. 12.9. Structure and relative energies of four modes of hydrogen bonding in transition structures for epoxidation of 2-propen-l-ol by peroxyformic acid. Relative energies are from B3I.YP/6-311G -level computations with a solvation model for CH2C12, e = 8.9. Reproduced from / Org. Chem., 64, 3853 (1999), by permission of the American Chemical Society.

A heterogeneous olefin epoxidation catalyst containing both V and Ti in the active site was prepared by sequential non-hydrolytic grafting. The silica was exposed first to VO(OiPr)3 vapor followed by Ti(0 Pr)4 vapor. Formation of propene is evidence for the creation of Ti-O-V linkages on the surface. Upon metathesis of the 2-propoxide ligands with BuOOH, the catalyst becomes active for the gas phase epoxidation of cyclohexene. The kinetics of epoxidation are biphasic, indicating the presence of two reactive sites whose activity differs by approximately one order of magnitude. 

The epoxidation of nonconjugated olefins is slow123,124 and shows reduced enantioselectivity as compared with the epoxidation of conjugated olefins. For example, enantioselectivities from the epoxidation of (Z)-l-cyclohexyl-1-propene, 3,3-dimethyl-l-butene, and geranyl acetate are 82% (with (34)), 70% (with (34)),123 and 53% (6,7-epoxide, with (26)),124 respectively, and yields of the epoxides are modest. 

Cyano-l-propene, m30a 2-Cyanotoluene, tl 84 4-Cyanotoluene, tl 85 Cyanuric chloride, t255 l,5-Cyclododecadiene-9,10-epoxide, e4... 

The various spectroscopic techniques had revealed that Ti4+ ions in TS-1, Ti-beta and, Ti-MCM-41 are 4-coordinate in the dehydrated state. Tetrapodal Ti(OSi)4 and tripodal Ti(OH)(OSi)3 are the main Ti species. Upon exposure to H20, NH3, H202, or TBHP, they increase their coordination number to 5 or 6. On samples in which the Ti4+ has been grafted onto the silica (referred to as Ti f MCM-41), a dipodal Ti species (Ti(OH)2(OSi)2) may also be present. As a result of interaction with the oxidant ROOH (R = H, alkyl), the formation of 7)1- and p2-peroxo (Ti-O-O-), hydroperoxo (Ti-OOH), and superoxo (Ti02 ) species has been observed experimentally (Section III). A linear correlation between the concentration of the p2-hydroperoxo species and the catalytic activity for propene epoxidation has also been noted from vibration spectroscopy (133). 

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