Epoxidation metal-catalysed

Urea hydrogen peroxide adduct (UHP) was employed in metal-catalysed asymmetric epoxidation [98] and Baeyer-Villiger oxidation [99,100]. Since the presence of urea does not change the course of the reaction, this will not be described here. Conversion of epoxides to halohydrins with elemental... 

This is the first example of a reaction for which the presence of a chelating ligand was observed to facilitate rather than retard metal-catalysed epoxidation (Gao et al., 1987). It was found that the use of molecular sieves greatly improves this process by removing minute amounts of water present in the reaction medium. Water was found to deactivate the catalyst. All these developments led to an improved catalytic version that allows a five-fold increased substrate concentration relative to the stoichiometric method. Sensitive water-soluble, optically active glycidols can be prepared in an efficient manner by an in situ derivatisation. This epoxidation method appears to be competitive with enzyme-catalysed processes and was applied in 1981 for the commercial production of the gypsy moth pheromone, (-1-) disparlure, used for insect control (Eqn. (25)). 

A review of the metal-catalysed ring opening of achiral epoxides by achiral carbon-, sulfur-, nitrogen- and halogen-containing nucleophiles and kinetic resolution of racemic epoxides has been published.24 The review also discusses the reactions of chiral bases with epoxides that give allylic alcohols. 

Michael-aldol reaction as an alternative to the Morita-Baylis-Hillman reaction 14 recent results in conjugate addition of nitroalkanes to electron-poor alkenes 15 asymmetric cyclopropanation of chiral (l-phosphoryl)vinyl sulfoxides 16 synthetic methodology using tertiary phosphines as nucleophilic catalysts in combination with allenoates or 2-alkynoates 17 recent advances in the transition metal-catalysed asymmetric hydrosilylation of ketones, imines, and electrophilic C=C bonds 18 Michael additions catalysed by transition metals and lanthanide species 19 recent progress in asymmetric organocatalysis, including the aldol reaction, Mannich reaction, Michael addition, cycloadditions, allylation, epoxidation, and phase-transfer catalysis 20 and nucleophilic phosphine organocatalysis.21... 

Transition-metal-catalysed epoxidations work only on allylic alcohols, so there is one limitation to the method, but otherwise there are few restrictions on what can be epoxidized enantioselectively. When this reaction was discovered in 1981 it was by far the best asymmetric reaction known. Because of its importance, a lot of work went into discovering exactly how the reaction worked, and the scheme below shows what is believed to be the active complex, formed from two titanium atoms bridged by two tartrate ligands (shown in gold). Each titanium atom retains two of its isopropoxide ligands, and is coordinated to one of the carbonyl groups of the tartrate ligand. The reaction works best if the titanium and tartrate are left to stir for a while so that these dimers can form cleanly. 

Rings containing Oxygen. - The catalytic epoxidations of alkenes with hydroperoxides have been surveyed,as have transition-metal-catalysed stereo-controlled epoxidations. A major review of oxiran chemistry (956 references) has been published as a chapter in Saul Patai s series on the Chemistry of Functional Groups (1980). ... 

Other Metal-Catalysed Epoxidations of Unfunctionalised Olefins... 

Although the metal-catalysed epoxidation of olefins using peroxides has been known for a long time it is only this year that the first reports of asymmetric epoxidations have appeared. " The asymmetric epoxidation of allylic alcohols... 

Other than the L-DOPA, there have been two other significant processes developed for industrial applications. The first of these uses a Sharpless asymmetric epoxidation, one of the most widely applied asymmetric transition metal catalysed transformations, to convert allyl alcohol (41) into (5)-glycidol (43), a valuable chiral building block, developed by ARCO Chemical Company (Scheme 4.12) [29]. Most of the successful applications of transition metal mediated asymmetric... 

Reviews cover transition-metal-catalysed enantioselective a-heterofunctionalization of carbonyl compounds (103 references)," and organocatalysed enantioselective a-hetero-functionalization from 2006 to 2009, demonstrating straightforward syntheses of useful a-amino acids and alcohols, epoxides, 1,2-diols, and a-sulfenylated, a-selenenylated, and a-halogenated carbonyl derivatives." ... 

Metal-catalysed Epoxidation. Many of the reports of metal-catalysed formation of epoxides are associated with the development of more economic and efficient industrial routes to simple epoxides. A review has appeared on the manufacturing processes for production of ethylene oxide by direct gas-phase oxidation of olefins. Further reports on this process include the rather unusual use of potassium chloride and quartz catalysts. The more usual silver-catalysed epoxidation of ethylene has been studied in detail both from a kinetic and from a stereochemical point of view. The latter... 

Nevertheless, the two methods developed for formation of ferrilactone complexes show wider substrate tolerance than either the thermal or photo-lytic reactions and avoid problems that can occur at higher temperature, such as decarbonylation, decarboxylation or metal-catalysed hydrogen shifts [265]. They both give comparable yields of ferrilactone under the conditions described (see Table 8). The one exception is the case of vinyl epoxide (59), when four equivalents of Fe2(CO)9 were required [206]. This is probably due to coordination of the reactive species to the oxygens of the cw-dibenzoyl system reducing the effective concentration of tetracarbonyliron in solution. Conversely, the ultrasound reaction presumably takes place at the surface of the solid Fe2(CO)9 where the local concentration of Fe(CO)4 is much higher and this is reflected in the yields obtained. 

Reviews.—Recent reviews involving olefin chemistry include olefin reactions catalysed by transition-metal compounds, transition-metal complexes of olefins and acetylenes, transition-metal-catalysed homogeneous olefin disproportionation, rhodium(i)-catalysed isomerization of linear butenes, catalytic olefin disproportionation, the syn and anti steric course in bi-molecular olefin-forming eliminations, isotope-elfect studies of elimination reactions, chloro-olefinannelation, Friedel-Crafts acylation of alkenes, diene synthesis by boronate fragmentation, reaction of electron-rich olefins with proton-active compounds, stereoselectivity of carbene intermediates in cycloaddition to olefins, hydrocarbon separations using silver(i) systems, oxidation of olefins with mercuric salts, olefin oxidation and related reactions with Group VIII noble-metal compounds, epoxidation of olefins... 

Transition-metal catalysed asymmetric synthesis has again seen further developments over the past year. Of particular note are stereoselective aldol condensations, and asymmetric epoxidation reactions. Hydrogenation methods have seen few significant advances, and are considered only briefly this year. The important area of carbon-carbon bond-forming reactions continues to attract considerable attention although much work still uses palladium, iron, or copper, nickel and titanium chemistry is also being increasingly used, a trend that will no doubt continue. 

In addition to a general review, specific reviews of autoxidation reactions, and oxidation of C=C and C=N groups have been published. By considering the consequences of initial metal-hydroperoxide complex formation in metal-catalysed autoxidation reactions it has proved possible to predict the critical metal concentration level at which the catalyst-inhibitor transition occurs. Epoxides are de-oxygenated by Na-Hg in the presence of C5HB)aXa] (M = Mo or W, Xg = Clg or O) the active catalyst is believed to be MCpa (Scheme 4). ... 

In summary, the formation of optically active compounds through hydrolysis reactions is dominated by biocatalysis mainly due to the availability and ease of use of a wide variety of esterases, lipases and (to a lesser extent) acylases. Epoxide ring-opening (and related reactions) is likely to be dominated by salen-metal catalysts while enzyme-catalysed nitrile hydrolysis seems destined to remain under-exploited until nitrilases or nitrile hydratases become commercially available. 

The requirement for the presence of an adjacent alcohol group can be regarded as quite a severe limitation to the substrate range undergoing asymmetric epoxidation using the Katsuki-Sharpless method. To overcome this limitation new chiral metal complexes have been discovered which catalyse the epoxidation of nonfunctionalized alkenes. The work of Katsuki and Jacobsen in this area has been extremely important. Their development of chiral manganese (Ill)-salen complexes for asymmetric epoxidation of unfunctionalized olefins has been reviewed1881. 

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