Oxiranes catalytic processes

Obviously, in a relatively small work such as this it is not possible to be comprehensive. Preparations of bulk, achiral materials (e.g. simple oxiranes such as ethylene oxide) involving key catalytic processes will not be featured. Only a handful of representative examples of preparations of optically inactive compounds will be given, since the emphasis in the main body of this book, i.e. the experimental section, is on the preparation of chiral compounds. The focus on the preparation of compounds in single enantiomer form reflects the much increased importance of these compounds in the fine chemical industry (e.g. for pharmaceuticals, agrichemicals, fragrances, flavours and the suppliers of intermediates for these products). 

Hi. Role of additive. There are some reports in the literature of the beneficial effect of powerful donor solvents such as DBU on the reactivity and enantioselectivity of HCLA-mediated oxirane rearrangements for both stoichiometric and catalytic processes. However, this effect is not general (see above) and the role of such additives is still unclear. In one study, the influence of the concentration of DBU on the relationship between the ee s of catalyst and the product for the enantioselective isomerization of cyclohexene oxide mediated by substoichiometric amount of HCLA 56a (20 mol%) in the presence of LDA (2 equiv) has been investigated. At high DBU concentration (6 equiv), the enantiomeric... 

The only known example of such a catalytic process was reported in 1998 for enan-tioselective arylation of both linear and small ring cis oxiranes. The oxirane is added to a precomplexed mixture of phenylithium (1.6 equivalents) and a homochiral Schiff base of type 107 or 108 (5 mol%) to afford the S,R) ring opening product in moderate to excellent yield with ee s up to 90% (Scheme 48) . 

A homogeneous catalytic process, developed by Oxirane, uses a molybdenum catalyst that epoxidizes propylene by transferring an oxygen atom from tertiary butyl hydroperoxide. This is shown by 8.28. The hydroperoxide is obtained by the auto-oxidation of isobutane. The co-product of propylene oxide, /-butanol, finds use as an antiknock gasoline additive. It is also used in the synthesis of methyl /-butyl ether, another important gasoline additive. The over-... 

A transition metal catalyzed synthesis of ethers by carbene insertion into the O—bond has been reported. Not only saturated but also unsaturated alcohols can be utilized in this catalytic process. ° Intermolecular and intramolecular oxirane ring opening reactions by alkoxides and phenoxides also provide efficient and stereospecific preparations of acyclic and cyclic ethers. The procedures have been surveyed in detail. ... 

Ojima and co-workers have undertaken extensive research into the formation of AT-acyl-a-amino acids via amidocarbonylation chemistry [5]. Their focus includes the generation of A -acyl-a-amino acids directly from allyl alcohols, oxiranes, or olefins using homogeneous binary catalyst systems, particularly cobalt octacarbo-nyl - Group VIII transition-metal complex combinations. New catalytic processes feature ... 

High oxidation state molybdenum 0x0 complexes are weU-estabhshed catalysts for the epoxidation of alkenes by aUg l hydroperoxides, such as in the production of 2-methyl-oxirane (Halcon process). Chlorodioxo(Ti -pentamethylcyclopentadienyl)molybdenum(VI) provides an organometaUic example of a catalytically active system. The epoxidation reaction is stereoselective, as shown by the selective formation of tram- and ds-l,2-diphen-yloxirane from the respective E- and Z-aUcenes, and can be apphed to highly substituted alkenes see Scheme 42. Studies on this system have shown that the degradation of the catalyst involves oxidative poisoning to an imreactive peroxo complex.l l... 

The living nature of ethylene oxide polymerization was anticipated by Flory 3) who conceived its potential for preparation of polymers of uniform size. Unfortunately, this reaction was performed in those days in the presence of alcohols needed for solubilization of the initiators, and their presence led to proton-transfer that deprives this process of its living character. These shortcomings of oxirane polymerization were eliminated later when new soluble initiating systems were discovered. For example, a catalytic system developed by Inoue 4), allowed him to produce truly living poly-oxiranes of narrow molecular weight distribution and to prepare di- and tri-block polymers composed of uniform polyoxirane blocks (e.g. of polyethylene oxide and polypropylene oxide). 

The first example of such a process was reported in 1994 by Asami, who noticed that LDA was less reactive than HCLA 53 toward oxirane and thus proposed its use as a co-base in a catalytic cycle . Based upon this seminal result, the system has been extended to other HCLAs and various co-bases have been tested Selected results for the asymmetric rearrangement of cyclohexene oxide mediated by sub-stoichiometric quantities of HCLA are collected in Table 4. 

A structurally unusual 3-blocker that uses a second molecule of itself as the substituent on nitrogen is included here in spite of the ubiquity of this class of compounds. Exhaustive hydrogenation of the chromone (13-1) leads to a reduction of both the double bond and the carbonyl group, as in the case of (11-2). The car-boxyhc acid is then reduced to an aldehyde (13-2) by means of diisobutylaluminum hydride. Reaction of that intermediate with the ylide from trimethylsulfonium iodide gives the oxirane (13-3) via the addition-displacement process discussed earlier (see Chapters 3 and 8). Treatment of an excess of that epoxide with benzylamine leads to the addition of two equivalents of that compound with each basic nitrogen (13-4). The product is then debenzylated by catalytic reduction over palladium to afford nebivolol (13-5) [16]. The presence of four chiral centers in the product predicts the existence of 16 chiral pairs. 

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. 

Most polymerizations of cyclic monomers are ionic processes. Coordination catalysts are effective only for some heterocycles (oxirane and its derivatives, lactones). Ziegler-Natta catalysts can only be used for cycloalkene polymerization by metathesis heterocycles act as a catalytic poison. Smooth radical polymerization of hydrocarbon monomers with ring strain is unsuccessful [304], The deep-rooted faith that ring strain represents a major contribution to the driving force in ring opening (polymerization) has to be revised [305, 306]. 

Cyclododecanone has been synthesized from epoxycyclododecane on a Pd catalyst.Comprehensive work has been carried out on the hydrogenolysis and isomerization of methyloxirane on various metals. The results have been compared with those for oxacycloalkanes with larger rings.The transformations of 1,1-dimethyloxirane and 1-methylepoxycyclopentene have been followed on Pd, Pt, Rh, Cu, and Ni catalysts. The mechanisms of the catalytic reactions have been dealt with in detail. It has been demonstrated that the isomerization of the oxiranes on metals is the primary process, occurring in parallel with hydrogenolysis. The pathway of the reaction depends on the nature of the metal. Deuteration has been utilized to establish the role of hydrogen. 

Oxirane process. A method of making ethylene glycol by catalytic oxidation of ethylene to the diacetate, which is then hydrolyzed to ethylene glycol. 

The subtle mechanistic nuances between air-CaaD and CaaD were discovered by affinity labeling with (if)-oxirane-2-carboxylate and subsequent crystallographic analysis (Figure 15(b)). ° The carboxylate side chain of (/f)-oxirane-2-carboxylate enables it to bind at the active site of air-CaaD, in a mode that results in the covalent modification of Pro-1 and the loss of catalytic activity. The affinity-labeling reaction is stereospecific, because the (5)-enantiomer of oxirane-2-carboxylate does not alkylate the enzyme. The rate of inactivation is also hindered by the presence of substrate. T aken together, these findings support the active site nature of the inactivation process and the critical contribution of Pro-1 to activity. 

Catalytic and photochemical processes using mixed metal carbonyls are also represented. Tin-cobalt carbonyl compounds have been used as catalysts for ring-opening of oxiranes by secondary and tertiary alcohols 121 and the photochemical reactions of Pt2M4(CO)jg (M=Os, Ru) with cycloocta-1,5-diene under UV irradiation 122,123,124 jjgve been reported. A new rhenium-cobalt complex has been characterised 126 and Pt(cod)2 has been used to prepare new complexes via its reaction with 0 3(CO) jQ(FlCMe)2 26. 

According to Scheme 21.12, the multistep process involves first ethylene carbonate production from oxirane (EO) and carbon dioxide, the commercial catalytic technology. The carbon dioxide utilised herein is the byproduct from a nearby oxirane plant. Then, step A produces dimethyl carbonate and monoethyleneglycol (MEG) by catalytic transesterification of ethylene... 

The competitive process, the Oxirane process, starts either from isobutane or from ethylbenzene. This starting materials are converted to hydroperoxides by catalytic oxidation with air or oxygen to give tert-butyl hydroperoxide or ethylbenzene hydroperoxide. The hydroperoxides oxidize the propene in the presence of catalysts to give propylene oxide, and as byproducts either tert-butyl alcohol (2.8 t/t PO), which is converted to methyl-tert-butyl ether, or 1-phenyl-ethanol (2.5 t/t PO), which is converted to vinylbenzene (styrene). 

The most satisfactory results so far in this area are to be found in a report of the reaction of benzaldehyde with trimethylsulphonium iodide and aqueous base according to equation (10), where, under optimum conditions and catalysed by (23 R = C2H5), up to 97% enantiomer excess in the oxirane product can be achieved. Possible rationalizations have been discussed for these processes, and the role of the )S-hydroxyethyl substituent at the quaternary centre seems to be significant in both catalytic efficiency and enantioselectivity. Further advances in this field can be expected. 

The difference between a catalytic and a stoichiometric reaction is illustrated by the selective oxidation of ethylene to ethylene epoxide, where we compare the silver-catalyzed ethylene epoxidation with the stoichiometric epichlorohydrine process. Ethylene epoxide (oxirane) has industrial importance as a starter material for the production of ethylene glycol (antifreeze) and many other products (poly ethers, polyurethanes). 

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