Epoxidation catalytic methods

Although enolates, their equivalents, and otherwise stabilized carbanions would be interesting candidates for ARO of weso-epoxides, no efficient catalytic method has been developed to date. Crotti reported that 20 mol% of (salen)Cr-Cl complex 2 promoted the addition of the lithium enolate of acetophenone to cyclohexene oxide with moderate ees (Scheme 7.26) [50], However, the very low yields obtained... 

Xu, L., Sithambaram, S., Zhang, Y., Chen, C., Jin, L., Joesten, R. and Suib, S.L. (2009) Novel urchin-like CuO synthesized by a facile reflux method with efficient olefin epoxidation catalytic performance. Chemistry of Materials, 21, 1253-1259. Calvert, C., Joesten, R., Ngala, K., Villegas, J., Morey, A., Shen, X. and Suib,S.L. 

The use of epoxides has expanded dramatically with the advent of practical asymmetric catalytic methods for their synthesis. Besides the enantioselective epoxida-tion of prochiral olefins, approaches for the use of epoxides in the synthesis of enantiomerically enriched compounds include the resolution of racemic epoxides. 

If small amounts of PhSSPh were deliberately added from the beginning, the more positive reduction potential could be applied from the start. Indirect electrochemical eliminations were also performed in the case of 1,2-dihalides Ringopening reactions by the redox catalytic method were possible in the case of methylene cyclopropanes and epoxides... 

Subsequent work [55-65], in particular by Asami [56-60] and Andersson [61-65], has led to the development of catalytic methods in which a sub-stoichiometric amount of a chiral diamine such as 61 or 62 is used with an over-stoichiometric quantity of an achiral lithium amide base such as LDA (Scheme 13.29). Examples of catalytic epoxide isomerizations using the Asami diamine 61 or the Andersson... 

Although the original Sharpless epoxidation method was stoichiometric, the development of a catalytic method has allowed the reaction to be amenable to scale up. The addition of molecular sieves for the removal of trace amounts of water is important in the catalytic procedure.5 21-23... 

As was demonstrated by addition of epoxide under reaction conditions, the epoxide is not the precursor of the cis diol. The cis dihydroxylation is probably a two-step reaction, first with addition of a H202-derived oxygen atom to the double bond, followed by insertion of a Mn-coordinated oxygen atom (water or OH-). It is clear that the availability of free coordination sites in cis positions on the Mn (4b) is important for understanding the formation of cis dihydroxylated products. This is the first example of a cis dihydroxylation that is catalytic and uses Mn the route is therefore an alternative to stoichiometric permanganate reactions or to catalytic methods with more... 

Berkessel A, Andreae MRM (2001) Efficient catalytic methods for the Baeyer-Villiger oxidation and epoxidation with hydrogen peroxide. Tetrahedron Lett 42 2293-2295... 

A catalytic method which promises to find wide application in view of its mildness and ease of execution uses a catalytic amount of tetra-n-propylammonium perruthenate (TPAP) with A7-methylmorpholine -oxide (NMO) as the cooxidant. ° l4imary (and secondary) alcohols which contain a range of ftinc-tional groups (alkenes, tetrahydropyran ethers, epoxides, lactones, silyl ethers and indoles inter alia) can be oxidized without interference by the other functional group (equations 21-23). The performance of the reagent is improved further by including molecular sieves in the reaction mixture. ... 

Reaction times with the catalytic method are comparable to those obtained with the stoichiometric method and Z-disubstituted allylic alcohols can be efficiently epoxidized at —20 JC in 1 -4 hours using just 5% titanium(IV) isopropoxide and 6-7.5% tartrate (Table 4). The method is especially useful for epoxidation of alcohols that react only very slowly under normal conditions. In particular, Z-disubstituted allylic alcohols (Table 4) react at useful rates only if the reaction is warmed to —10 °C. In the catalytic procedure the absence of large quantities of titanium salts and tartrate minimize epoxide opening that would be a problem in the stoichiometric procedure at this temperature. A similar situation exists for unsymmetrical disubstituted allylic alcohols (Table 4) which are also prone to opening under the conditions of the stoichiometric method. 

An additional advantage of the catalytic methods is that reactions can be performed at higher concentration. Stoichiometric reactions are typically carried out at 0.1 M substrate to minimize reactions such as epoxide opening. Catalytic reactions can be run at concentrations of 1 M although Sharpless recommends substrate concentrations of 0.3-0.5 M32-42. 

A highly effective catalytic method for alkynylation of epoxides has recently been reported this involves the chelation-controlled alkylation of hetero-substituted epoxides with Mc3A1 and alkynyllithiums via pentacoordinate organoaluminum complexes [82]. For instance, reaction of epoxy ether, (l-benzyloxy)-3-butene oxide (75) in toluene with PhC = CLi under the influence of catalytic MesAl (10 mol%) proceeded smoothly at 0 °C for 5 h to furnish the alkynylation product l-(benzyloxy)-6-phenylhex-5-yn-3-ol (76) in 76 % yield. The yield of the product was very low (3 %) without MeaAl as catalyst under similar conditions. This is the first catalytic procedure for amphiphilic alkylation of epoxides. The participation of pentacoordinate MesAl complexes of epoxy ethers of type 75 is emphasized by comparing the reactivity with the corresponding simple epoxide, 5-phenyl-l-pentene oxide (77), which was not susceptible to nucleophilic attack of PhC s CLi with catalytic Me3Al under similar conditions (Sch. 50). 

Enantioselective oxidation of olefins is a very elegant way of introducing oxygen and in some cases also nitrogen functions into molecules. The catalytic methods with the highest industrial potential are epoxidation and dihydroxylation, and the kinetic resolution of racemic terminal epoxides (Table 3). 

Among the large variety of catalytic methods available for the epoxidation of alkenes, those based on tungstate catalysts hold a prominent place when it comes to industrial relevance. Several positive features of W-based catalyst systems explain their popularity in industry ... 

The synthesis is a catalogue of modern asymmetric catalytic methods. The epoxide 25 was resolved by a hydrolytic kinetic resolution (chapter 28) using a synthetic asymmetric cobalt complex. The asymmetric Diels-Alder reaction (chapter 26) was catalysed by a synthetic chromium... 

Enolisable aldehydes such as 101 or 103 do not give quite such good yields but the ees are still good and the diastereoselectivity in favour of the trans epoxides 102 and 104 is excellent. The secret of this method is the simple preparation of the reagent 96. In the next chapters you will see that superior catalytic methods are available for asymmetric epoxidation of allylic alcohols and of m-alkenes but they are less good for the trans disubstituted alkenes that would give 97, 102, or 104. You will also see catalytic versions of sulfur ylid epoxidation. 

Introduction Catalytic methods of asymmetric induction Part I - Sharpless Asymmetric Epoxidation The AE Method The ligands The catalyst Catalyst structure The mnemonic device The synthesis of propranolol Modification after Sharpless Epoxidation Oxidation after Sharpless epoxidation The Payne rearrangement... 

Then we changed over to the isomer allylic alcohol, to 3-methyl-2-buten-l-ol (prenol). Being a primary alcohol, it was smoothly epoxidized under both stoichiometric and catalytic Sharpless conditions. While the stoichiometric method provides only moderate yields as the dimethyl glycidol is fairly watersoluble, the catalytic method affords the double yield. The e.e. amounts to 90% in both cases. Optical purity and e.e. of the 3,3-dimethyl glycidol were determined by polarimetry and -NMR in the presence of chiral europium shift-reagent [22]. 

Chiral epoxides and vicinal diols (employed as their corresponding cyclic sulfate or sulfite esters as reactive intermediates) are extensively employed high-value intermediates in the synthesis of chiral compounds because of their ability to react with a broad variety of nucleophiles (Figs. 11.2-1 and 11.2-2). In recent years, extensive efforts have been devoted to the development of chemo-catalytic methods for their production19, 10]. Thus, the Sharpless methods allowing for the asymmetric epoxida-... 

Whereas several catalytic methods are currently available for manganese-catalyzed epoxidation with aqueous H2O2, high turnover numbers for cis-dihydroxylation reactions so far have only been achieved with osmium compounds 161-165). However, manganese-catalyzed oxidation reactions have a few inherent advantages, such as the low price of the manganese salts and complexes and their non-toxic nature. 

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