Olefins epoxide generation

The addition of an oxygen atom to an olefin to generate an epoxide is often catalyzed by soluble molybdenum complexes. The use of alkyl hydroperoxides such as tert-huty hydroperoxide leads to the efficient production of propylene oxide (qv) from propylene in the so-called Oxirane (Halcon or ARCO) process (79). 

Other examples involve the immobilization of ruthenium porphyrin catalysts [74]. While Severin et al. generated insoluble polymer-embedded catalysts 16 by co-polymerizing porphyrin derivatives with ethylene glycol dimethacrylate (EGD-MA) [74 a], Che et al. linked the ruthenium-porphyrin unit to soluble polyethylene glycol (PEG) 17 [74b]. Both immobilized catalysts were employed in a variety of olefin epoxidations with 2,6-dichloropyridine N-oxide (Gl2pyNO), providing similar conversions of up to 99% and high selectivities (Scheme 4.9). 

These catalysts, 11-13, show good enantioselectivity ranging from 80 to 95% ee in the epoxidation of conjugated cfs-di- and tri-substituted olefins. Epoxidation of "good substrates such as 2,2-dimethylchromene derivatives proceeds with excellent enantioselectivity (>95% ee). Since the results obtained with these first-generation Mn-salen catalysts have been reviewed [21,33], only typical examples are shown in Table 6B.1. These reactions are usually carried out in the presence of donor ligand [34] such as 4-phenylpyridine A -oxide with terminal oxidants such as iodosylbenzene and sodium hypochlorite as described above. However, the use of some other terminal oxidants under well-optimized conditions expands the scope of the Mn-salen-... 

HMPA at 65 C for 3 hours affords the corresponding trans and cis olefins, respectively. Backside attack of Mc3SiK on the cis (and trans) epoxides generates the threo- (and erythro-) jS-alkoxysilanes, respectively, followed by syn elimination (Peterson elimination) to give the olefin with inverted stereochemistry. 

To date, the observed reactivity of Fe =0 species toward olefins does not lend unequivocal support for this idea. Although Fe =0 species derived from 2 or 3 were found to react with cyclooctene generating epoxide in about 50% yield, oxidation of olefins with FI202 catalyzed by 2 or 3 was not very efficient under argon (20% conversion of oxidant to product or less) and showed evidence for a significant contribution from radical autooxidation when carried out in air [28]. Furthermore, the Fe =0 complex supported by a 15-membered macrocyclic ligand was inactive in olefin epoxidation, but became quite active only in the presence of excess oxidant (PhIO). A PhIO Fe =0 adduct was proposed as the oxidant instead [64]. 

Epoxidation.2 The reaction of tetralin hydroperoxide (cumene, cyclohexyl, and /-butyl hydroperoxides were also used, but to a lesser extent) with cyclohexyl metaborate (This volume)3 in the presence of an olefin results in formation of an epoxide from the olefin and generation of tetralol. The new reagent is very similar in behavior to penoxyacids. Transient hydroxonium ions, OH+, are postu-... 

Scheme 1.8. Reactivity and stereoselectivity of oxomanganese(IV) and oxomanganese(V) (generated by Mn(III) and oxidants in situ) in olefin epoxidation.

Similarly, [Mo(=0)(OSiOR3)4] reacts with SBA-15 to generate an isolated Mo oxo species [(=SiO)Mo(=0)(OSiOR3)3l (Scheme 44), which is an active olefin epoxidation catalyst. ... 

With regard to the biomimetic non-heme iron complexes, the work devoted to develop catalysts that perform catalytic alkane hydroxylation has resulted in a large number of iron complexes, which generate Fe =0 iron-oxo species characterized by different spectroscopic techniques. There is now direct evidence that the involvement of high-valent iron-oxo species leads to stereospecific alkane hydroxylation, while hydroxyl radicals contribute to non-selective oxidations. The impressive work performed by Que and co-workers has demonstrated that olefin epoxidation and cis-dihydroxylation are different facets of the reactivity of a common Fe -OOH intermediate, whose spin state can be modulated by the electronic and steric properties of... 

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