Epoxidation hydroperoxide structure

Teeth whiteners, percarbamide, 623 Temperature, reaction rates, 903-12 Terminal olefins, selenide-catalyzed epoxidation, 384-5 a-Terpinene, peroxide synthesis, 706 a-Terpineol, preparation, 790 Terrorists, dialkyl peroxide explosives, 708 Tertiary amines, dioxirane oxidation, 1152 Tertiary hydroperoxides, structural characterization, 690-1... 

T he epoxidation of olefins using organic hydroperoxides has been studied in detail in this laboratory for a number of years. This general reaction has also recently been reported by other workers (6,7). We now report on the effects of five reaction variables and propose a mechanism for this reaction. The variables are catalyst, solvent, temperature, olefin structure, and hydroperoxide structure. Besides these variables, the effect of oxygen and carbon monoxide, the stereochemistry, and the kinetics were studied. This work allows us to postulate a possible mechanism for the reaction. 

Effect of Hydroperoxide Structure. The reactivity of various hydroperoxides was studied with 2-octene and 2-methyl-l-pentene (Table VII). The yield of epoxide was quantiative. The data show that the substitution of the electron-withdrawing nitro groups in the para-position of cumene hydroperoxide markedly increases the reaction rate. The order of reactivity is p-nitrocumene > cumene > tert-butyl hydroperoxide. 

A density functional study of the transition structures of Ti-catalyzed epoxidation of allylic alcohol was performed, which mimicked the dimeric mechanism proposed by Sharpless et al.5 Importance of the bulkiness of alkyl hydroperoxide to the stereoselectivity, the conformational features of tartrate esters in the epoxidation transition structure, and the loading of allylic alcohol in the dimeric transition structure model were pointed out. 

R. A. Sheldon, J. A. van Doom, Metal-catalyzed epoxidation of olefins with organic hydroperoxides. 11. The effect of solvent and hydroperoxide structure, J. Catal. 31 (1973) 438. 

All schemes presented are similar and conventional to a great extent. It is characteristic that the epoxidation catalysis also results in the heterolytic decomposition of hydroperoxides (see Section 10.1.4) during which heterolysis of the O—O bond also occurs. Thus, there are no serious doubts that it occurs in the internal coordination sphere of the metal catalyst. However, its specific mechanism and the structure of the unstable catalyst complexes that formed are unclear. The activation energy of epoxidation is lower than that of the catalytic decomposition of hydroperoxides therefore, the yield of oxide per consumed hydroperoxide decreases with the increase in temperature. 

The authors also investigated the mode of activation of these BINOL-derived catalysts. They proposed an oligomeric structure, in which one Ln-BINOL moiety acts as a Brpnsted base, that deprotonates the hydroperoxide and the other moiety acts as Lewis acid, which activates the enone and controls its orientation towards the oxidant . This model explains the observed chiral amplification effect, that is the ee of the epoxide product exceeds the ee of the catalyst. The stereoselective synthesis of cw-epoxyketones from acyclic cw-enones is difficult due to the tendency of the cw-enones to isomerize to the more stable fraw5-derivatives during the oxidation. In 1998, Shibasaki and coworkers reported that the ytterbium-(f )-3-hydroxymethyl-BINOL system also showed catalytic activity for the oxidation of aliphatic (Z)-enones 129 to cw-epoxides 130 with good yields... 

The haem peroxidases are a superfamily of enzymes which oxidise a broad range of structurally diverse substrates by using hydroperoxides as oxidants. For example, chloroperoxidase catalyses the regioselective and stereoselective haloge-nation of glycals, the enantioselective epoxidation of distributed alkenes and the stereoselective sulfoxidation of prochiral thioethers by racemic arylethyl hydroperoxides [62]. The latter reaction ends in (i )-sulfoxides, (S)-hydroperoxides and the corresponding (R)-alcohol, all In optically active forms. 

Effect of Olefin Structure. The reaction rate of the epoxidation depends on olefin structure. In general, the more alkyl substituents bonded to the carbon atoms of the double bond, the faster the reaction rate. This was shown by a reaction of 2-methyl-2-pentene, cyclohexene, and 2-octene with cumene hydroperoxide under the same conditions (Table V). The yield of epoxide was quantitative. The results indicate that 2-methyl-2-pentene reacts faster than cyclohexene and 2-octene. 

Silylation of Ti-MCM-41 materials produces highly active and selective for epoxidation ol olefins using organic hydroperoxides as oxidants, it has been found that the controlling parameters of the final catalytic activity of silylated Ti-MCM-41 materials are the hydrophobicity and the concentration of free silanol groups on the external surface of the mesopores that built up the Ti-MCM-41 structure. 

While some molybdenum complexes such as Mo03(dien) were found to be inactive,236 the rates of molybdenum-catalyzed epoxidation of alkenes were found to be independent of the structure of the complex used, after an induction period representing the time for exchange of anionic ligands by the alkyl hydroperoxide. cis-Dioxomolybdenum(VI) diolates such as (78) were isolated... 

Asymmetric epoxidation of allylic alcohols is a very reliable chemical reaction. More than a decade of experience has confirmed that the Ti-tartrate catalyst is extremely tolerantof structural diversity in the allylic alcohol substrate for epoxidation yet is highly selective in its ability to discriminate between the enantiofaces of the prochiral olefin. Today the practitioner of organic chemistry need provide only the allylic alcohol to perform the reaction. All other reagents and materials required for the reaction are available from supply houses and usually are sufficiently pure as received to be used directly in the asymmetric epoxidation process. [When purchasing f-butyl hydroperoxide in prepared solutions, however, the more concentrated 5.5-M solution in isooctane (2,2,4-trimethylpentane) should always be chosen over the 3.0-M solution.] If the considerations presented in this chapter are observed, with attention to the moderately stringent technique outlined, no difficulty should be encountered in performing this reaction. 

From these considerations, the synthesis of silsesquioxanes was optimised, by means of HTE, as a function of the activity of the catalysts obtained after titanium coordination to the silsesquioxane structures. Therefore, this approach aimed at producing any incompletely condensed silsesquioxane that would result in active catalysts after titanium coordination rather than a specific structure (like silsesquioxane ulhS). The epoxidation of 1-octene with tert-butyl hydroperoxide (TBHP) as the oxidant was chosen as test reaction for the activity of the catalysts [26]. 

A new parameter space for the synthesis of silsesquioxane precursors was defined by six different trichlorosilanes (R=cyclohexyl, cyclopentyl, phenyl, methyl, ethyl and tert-butyl) and three highly polar solvents [dimethyl sulfoxide (DMSO), water and formamide]. This parameter space was screened as a function of the activity in the epoxidation of 1-octene with tert-butyl hydroperoxide (TBHP) [26] displayed by the catalysts obtained after coordination of Ti(OBu)4 to the silsesquioxane structures. Fig. 9.4 shows the relative activities of the titanium silsesquioxanes together with those of the titanium silsesquioxanes obtained from silsesquioxanes synthesised in acetonitrile. The values are normalised to the activity of the complex obtained by reacting Ti(OBu)4 with the pure cyclopentyl silsesquioxane o7b3 [(c-C5H9)7Si7012Ti0C4H9]. 

The rates of metal-catalyzed epoxidations are also influenced by the structure of the olefin and the structure of the hydroperoxide. The relative rates of epoxidation of a series of olefins using a mixture of r-Bu02H and Mo(CO)6 paralleled quite closely those for epoxidations with organic peracids.435... 

Before discussing the structural evidence for the precatalyst 9.35, we quickly go through the proposed mechanism of epoxidation. The precatalyst 9.35 reacts with one mole each of allyl alcohol and f-butyl hydroperoxide to give 9.36, where two alkoxide ligands on the same Ti atom are substituted according to reaction 9.4. 

Several optically active, /i-unsaturated esters 1, bearing a chiral auxiliary as the ester group, were epoxidized with tor-butyl hydroperoxide in the presence of butyllithium with yields highly dependent on the structure of epoxidized compound and moderate to good diastereose-lectivity 28. (—)-(S )-S -Phenyl-.V-2-phenylvinyl-A, -tosylsulfoximide (3) was epoxidized under the same conditions to give almost quantitatively the crude epoxide 4 (which decomposes on attempted chromatography, isolated yield 20%) with complete diastereoselectivity28. 

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