Catalysis selective epoxidation

Toward Catalysis of Selective Epoxidation and Its Applicability to Multistage Organic Synthesis... 

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). 

J. R. Monnier, The selective epoxidation of non-allylic olefins over supported silver catalysts, in R. K. Grasselli, S. T. Oyama, A. M. Gaffney, J. E. Lyons (Eds.), 3rd World Congress on Oxidation Catalysis, Elsevier, Amsterdam, 1997, Stud. Surf. Sci. Catal. 110, (1997)135. 

Key Words Ethylene oxide, Ethylene, Epoxidation, Silver, Cl promotion, Cs promotion. Promotion, Selectivity, Oxametallacycle, Adsorption, Desorption, Chemisorption, Activation energy, Ag-O bond. Reaction mechanism, Oxidation, Cyclisation, Heterogeneous catalysis, Selective oxidation, Eletrophilic oxygen. Nucleophilic oxygen. Subsurface O atoms, Ag/a-A Oj catalyst. 2008 Elsevier B.V. 

S. Chavan, W. Maes, J. Wahlen, P. Jacobs, D. De Vos, W. Dehaen, Benzimidazole-functionalized dendrons as molybdenum supports for selective epoxidation catalysis, Catal. Commun. 6 (2005) 241. 

Although the conditions of catalysis have not yet been optimized, the results are interesting in the sense that these heterogeneous molecular catalysts all show properties different from their molecular counterparts, in terms of either activity (epoxidation of olefins, MPVO reactions, silane activation) or selectivities (epoxidation for example). These features need to be further investigated, by varying the number of solid ligands in the coordination sphere of Zr preliminary experiments with epoxidation of olefins show that this parameter is important for the activity of Zr. 

Direct air epoxidation of propylene to propylene oxide suffers from selectivity problems. Epoxidation by alkyl hydroperoxide, as practiced by Arco, is based on the use of Mo(CO)g as a homogeneous catalyst. The most impressive use of homogeneous catalysis in epoxidation, however, is in the Sharpless asymmetric oxidation of allylic alcohols. In view of its importance, this enantioselective reaction is included in Chapter 9 which is devoted mainly to asymmetric catalysis. 

Several cyclodextrin ketones with a ketone attached to the secondary face of the cyclodextrin in the form of a 2,3-0-( 1,3-acetone) group (79), and some selected cyclodextrin ketones having the ketone at the primary face, were investigated for their catalysis of epoxidation of stilbenes and styrene by oxone in 1 1 acetonitrile/H20. It was found that secondary face ketones were better catalysts giving a cat uncat over 10, and more stereoselective giving up to 76% ee in (5)-styrene oxide. ... 

The ratio of yy -epoxide (shown above) to ant -eipoxide is 10—25 1 with TYZORTPT catalysis, whereas vanadjdacetylacetonate is less selective and y -chloroperoxybenzoic acid gives the reverse 1 25 ratio. It is supposed that TYZOR TPT esterifies the free hydroxyl, then coordinates with the peroxide to favor yy -epoxidation (135). This procedure is related to that for enantioselective epoxidation of other allyflc alcohols in 9—95% enantiomeric excess (135). 

When heated in the presence of a carboxyHc acid, cinnamyl alcohol is converted to the corresponding ester. Oxidation to cinnamaldehyde is readily accompHshed under Oppenauer conditions with furfural as a hydrogen acceptor in the presence of aluminum isopropoxide (44). Cinnamic acid is produced directly with strong oxidants such as chromic acid and nickel peroxide. The use of t-butyl hydroperoxide with vanadium pentoxide catalysis offers a selective method for epoxidation of the olefinic double bond of cinnamyl alcohol (45). 

The asymmetric epoxidation of enones with polyleucine as catalyst is called the Julia-Colonna epoxidation [27]. Although the reaction was originally performed in a triphasic solvent system [27], phase-transfer catalysis [28] or nonaqueous conditions [29] were found to increase the reaction rates considerably. The reaction can be applied to dienones, thus affording vinylepoxides with high regio- and enantio-selectivity (Scheme 9.7a) [29]. 

The past thirty years have witnessed great advances in the selective synthesis of epoxides, and numerous regio-, chemo-, enantio-, and diastereoselective methods have been developed. Discovered in 1980, the Katsuki-Sharpless catalytic asymmetric epoxidation of allylic alcohols, in which a catalyst for the first time demonstrated both high selectivity and substrate promiscuity, was the first practical entry into the world of chiral 2,3-epoxy alcohols [10, 11]. Asymmetric catalysis of the epoxidation of unfunctionalized olefins through the use of Jacobsen s chiral [(sale-i i) Mi iln] [12] or Shi s chiral ketones [13] as oxidants is also well established. Catalytic asymmetric epoxidations have been comprehensively reviewed [14, 15]. 

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