Acidic epoxidation systems

The observation that addition of imidazoles and carboxylic acids significantly improved the epoxidation reaction resulted in the development of Mn-porphyrin complexes containing these groups covalently linked to the porphyrin platform as attached pendant arms (11) [63]. When these catalysts were employed in the epoxidation of simple olefins with hydrogen peroxide, enhanced oxidation rates were obtained in combination with perfect product selectivity (Table 6.6, Entry 3). In contrast with epoxidations catalyzed by other metals, the Mn-porphyrin system yields products with scrambled stereochemistry the epoxidation of cis-stilbene with Mn(TPP)Cl (TPP = tetraphenylporphyrin) and iodosylbenzene, for example, generated cis- and trans-stilbene oxide in a ratio of 35 65. The low stereospecificity was improved by use of heterocyclic additives such as pyridines or imidazoles. The epoxidation system, with hydrogen peroxide as terminal oxidant, was reported to be stereospecific for ris-olefins, whereas trans-olefins are poor substrates with these catalysts. 

The use of the ionic liquid [bmim][BF4] further improved the Burgess epoxidation system [70]. Chan and coworkers found that replacement of sodium bicarbonate for tetramethylammonium bicarbonate and performing the reaction in [hmim][BF4] allowed for efficient epoxidation of a number of different olefins, including substrates affording acid-labile epoxides (such as dihydronaphthalene (99% yield) and 1-phenylcyclohexene (80% yield)). 

Homopolymerization, epoxides aluminate-Lewis acid catalyst system, 11, 602 via aluminum-porphyrin-Lewis acid catalysts, 11, 599 aluminum-tetradentate ligand catalyst system, 11, 601 anionic polymerization, 11, 598 cationic aluminum catalyst system, 11, 603 cationic polymerization, 11, 598 characteristics, 11, 597 zinc-based catalyst system, 11, 605 Homopolymers, cyclic olefins, 11, 716... 

Hou, C.T. 2006. Monooxygenase system of Bacillus megaterium ALA2 Studies on linoleic acid epoxidation products. J. Am. Oil Chem. Soc., 83, 677-681. 

This chapter is concerned with the reactions of epoxides with Lewis acids in the absence of nucleophilic substances. These reactions can lead to a wide variety of rearranged structures depending upon the environment of the epoxide system. The ring-opening reactions of epoxides involving external nucleophiles are discussed in Chapter 3. 

Djordjevic et al.464 have described the synthesis and characterization of amino acid complexes MoO(02)2Leq(OH2) (192) and the X-ray structures of the Gly, Ala, and Pro derivatives. Chiral ligands such as (R)/(S)-(194), (R)/(S)-(195), and (RR,S)/(R,SR)-(196) form Mimoun complexes, MoO(02)2L and MoO(02)2L(OH2). IR and 31P NMR data, as well as the X-ray structure of pentagonal bipyramidal MoO(02)2(R,R,.S -196)(OH2), indicate the presence of equatorial phos-phoryl donors.465,466 Enantioselectivity in the stoichiometric epoxidation of pro-chiral olefins was marginal (<10%) except in the case of the binaphthyl derivatives (L - 195) where e.e. s of up to 39% were recorded.4 Related structures are observed for phosphine oxide458,467 and chelate complexes such as MoO(02)2 OE(iPr)2CH2CH2OMe (197, E = P, As) 458 468 An efficient bipha-sic catalytic epoxidation system based on MoO(02)2(OPR3) (R = -dodecyl) has been developed and the activity of related complexes assessed.460 Earlier attempts to produce aqueous oxidants included the synthesis of water-soluble bpy derivatives.469,470... 

We investigated the side reactions of CECs with acetic acid and its aqueous solutions at 10-40 °C. The reaction mixtures served as models of the epoxidation systems. The course of the reactions was followed by the determination of epoxy and carboxyl groups in the reaction mixture. Equimolar ratios of the reagents were applied [18]. 

Several other groups have reported effective dioxirane systems employing Oxone as the terminal oxidant. For example, Armstrong et al. have developed a spirocyclic iV-carbethoxy-azabicyclo[3.2.1]octanone precatalyst, which affords up to 91.5% ee in the epoxidation of stilbenes (eq 102). Shing et al. have developed an arabinose-derived ketone and employed this in the enantioselective synthesis of the Taxol side chain however, enantioselectivities for the epoxidation were only up to 68% (eq 103). Bortolini et al. have also described the epoxidation of alkenes with the stoichiometric keto bile acid-Oxone system, a range of ee values were observed over several substrate types but up to 98% was observed for the epoxidation of tran -stilbene, although the yield was only 50% (eq 104). ... 

Copolymerization of caprolactone with epoxides opens a wide range of various functionalities that can be accessed. These functional epoxides can be based on substituted epichlorohydrin [63] or on other molecules. In our laboratory, we recently presented an epoxide system based on 2-methyl-4-pentenoic acid, which is esterified in a first step with a desired functional moiety and then epoxidized to obtain a functional epoxide that is compatible with caprolactone copolymerization [44,62], This toolbox approach allows for the functionalization with different functional moieties on demand. One advantage of these statistical copolymers is that the copolymer composition can be controlled by the reactivity of the monomers, and therefore, defined copolymers can be obtained (Figure 9.7). 

Startg. alcohol in ether treated with p-methoxybenzyl trichloroacetimidate and a little (0.3 mol%) trifiuoromethanesulfonic acid at room temp, for 10 min - product. Y 77%. The method is mild, applicable to sterically hindered hydroxyl groups, and tolerates a wide range of both acid- (epoxide, bisspiroketal, acetonide, silyl) and base-sensitive (ester, mesyl, silyl) functions, even in 1,2- and 1,3-diol systems furthermore, there is no epimerization at the a-position of carbonyls or P-elimination of P-hydroxy compds. F.e.s. N. Nakajima et al.. Tetrahedron Letters 29,4139-42 (1988). 

While Oxone has been commonly used to generate dioxiranes from ketones, Shi s studies have shown that epoxidation with ketone 2 or 5c can be carried out with a nitrile and H2O2 as the primary oxidant, giving high enantioselectivity for a variety of olefins. Peroxyimidic acid 55 is likely to be the active oxidant that reacts with the ketone to form dioxirane 10. Mixed solvents, such as CH3CN-EtOH-CH2Cl2, improve the conversions for substrates with poor solubilities. This epoxidation system is mild and provides conversion and enantioselectivity similar to that using Oxone as oxidant. 

Some authors have denied the existence of mechanisms involving only ionic or only complex forms. Thus, Fiala and Lidarik studied the benzyldimethylamine-catalyzed reaction of 1,2-epoxy-3-benzoatepropane with benzoic acid in xylene. The order in acid, epoxide, and catalyst are 0.5,1, and 0.6, respectively. Like Kakiuchi and Tanaka for l,2-epoxy-3-phenoxypropane/benzoic acid systems, they found that the relation log k = a log D + b (where D is the dielectric constant of the system)... 

The behavior of an acid-epoxy system largely depends on the nature of the catalyst. Thus, Pishnamazzade and Mamishov studying the reaction of l,2-epoxy-3-allyloxypropane with various carboxylic acids, found that the direction of the epoxide ring opening depends on the Lewis acid or on the ion exchanges they used as catalyst. When they saturated an epoxy-acid mixture with hydrogen chloride in the presence of the catalyst, two reactions were observed ... 

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