Isomerization, alkenes epoxidation

Conjugated dienes can be epoxidized to provide vinylepoxides. Cyclic substrates react with Katsuki s catalyst to give vinylepoxides with high ees and moderate yields [17], whereas Jacobsen s catalyst gives good yields but moderate enantiose-lectivities [18]. Acyclic substrates were found to isomerize upon epoxidation (Z, )-conjugated dienes reacted selectively at the (Z)-alkene to give trans-vinylepoxides (Scheme 9.4a) [19]. This feature was utilized in the formal synthesis of leuko-triene A4 methyl ester (Scheme 9.4b) [19]. 

The rotation about the carbon-carbon bond in intermediate 30 is fast compared to cyclization. As a result isomeric alkenes are transformed to oxiranes of the same, predominantly trans stereochemistry. A practical process for the epoxidation of a wide range of alkenes was described in the presence of biacetyl as sensitizer to yield epoxides in about 90% yield.268... 

Although a large number of asymmetric catalytic reactions with impressive catalytic activities and enantioselectivities have been reported, the mechanistic details at a molecular level have been firmly established for only a few. Asymmetric isomerization, hydrogenation, epoxidation, and alkene dihydroxylation are some of the reactions where the proposed catalytic cycles could be backed with kinetic, spectroscopic, and other evidence. In all these systems kinetic factors are responsible for the observed enantioselectivities. In other words, the rate of formation of one of the enantiomers of the organic product is much faster than that of its mirror image. 

Although cis-trans isomerization of epoxides is not formally associated with this section, it is clearly a potential problem in the conversion of an alkene to an epoxide. There are several catalysts for this process. 

Epoxidations with peroxy-acids are highly stereoselective and take place by cis addition to the double bond of the alkene. For example, oleic acid 26 gave cis-9, 0-epoxystearic acid 27, whereas elaidic acid 28 gave the isomeric traw -epoxide 29 (5.41). 

For an alkene that shows ds,trans isomerism, epoxidation is also stereospecific the stereochemistry of the product depends on the stereochemistry of the starting alkene. Epoxidation of ds-2-butene, for example, yields only the meso compound cis-2,3-dimethyloxirane, and epoxidation of frans-2-butene yields only the enantiomers of... 

As a further application of the reaction, the conversion of an endocyclic double bond to an c.xo-methylene is possible[382]. The epoxidation of an cWo-alkene followed by diethylaluminum amide-mediated isomerization affords the allylic alcohol 583 with an exo double bond[383]. The hydroxy group is eliminated selectively by Pd-catalyzed hydrogenolysis after converting it into allylic formate, yielding the c.ro-methylene compound 584. The conversion of carvone (585) into l,3-disiloxy-4-methylenecyclohexane (586) is an example[382]. 

Olefin isomerization can be catalyzed by a number of catalysts such as molybdenum hexacarbonyl [13939-06-5] Mo(CO)g. This compound has also been found to catalyze the photopolymerization of vinyl monomers, the cyclization of olefins, the epoxidation of alkenes and peroxo species, the conversion of isocyanates to carbodiimides, etc. Rhodium carbonylhydrotris(triphenylphosphine) [17185-29-4] RhH(CO)(P(CgH )2)3, is a multifunctional catalyst which accelerates the isomerization and hydroformylation of alkenes. 

Recently, we have demonstrated another sort of homogeneous sonocatalysis in the sonochemical oxidation of alkenes by O2. Upon sonication of alkenes under O2 in the presence of Mo(C0) , 1-enols and epoxides are formed in one to one ratios. Radical trapping and kinetic studies suggest a mechanism involving initial allylic C-H bond cleavage (caused by the cavitational collapse), and subsequent well-known autoxidation and epoxidation steps. The following scheme is consistent with our observations. In the case of alkene isomerization, it is the catalyst which is being sonochemical activated. In the case of alkene oxidation, however, it is the substrate which is activated. 

The protocol developed by Jacobsen and Katsuki for the salen-Mn catalyzed asymmetric epoxidation of unfunctionalized alkenes continues to dominate the field. The mechanism of the oxygen transfer has not yet been fully elucidated, although recent molecular orbital calculations based on density functional theory suggest a radical intermediate (2), whose stability and lifetime dictate the degree of cis/trans isomerization during the epoxidation <00AG(E)589>. 

In rearrangement reactions that lead to isomerization, an important discrimination must be made between epoxides of aromatic compounds, e.g., benzene oxide (10.1, Fig. 10.1), and epoxides of alkenes. As a class, epoxides of aromatic compounds (also known as arene oxides) are markedly un-... 

In collaboration with Prof. Herman Ammon (University of Maryland), MOLPAK/WMIN computational methods [6-12] have been employed successfully to predict the crystal densities of (i) several isomerically pure monomethylated PCU alkene dimers [13] and (ii) several polycyclic epoxides [14]. The crystal densities calculated from X-ray crystal structure data are compared with the corresponding calculated values. Attempts have been made to address discrep>-ancies that may exist between computed densities and those derived from A-ray data by using advanced theoretical methods. 

Styrenes [103], conjugated aT-dienes [107], and aT-enynes [108] are also epoxidized with ketones 57 in high ees (Table 5, entries 9-14). No isomerization of the epoxides was observed therefore only c/x-epoxides were obtained from cis-olefins. Alkenes and alkynes appear to be effective directing groups to favor the desired transition states T and V (Fig. 19). 

Based on this model, the regiochemical outcome of both oxabicycHc alkenes and vinyl epoxides can be explained by the relative stability of the two cr-intermediates. With oxabicyclic alkenes intermediate 41 is generated directly and then goes on to give product, whereas with the vinyl epoxides a more highly strained intermediate 40 is first produced which must first isomerize to 41 prior to product formation. The regiochemical outcome with each substrate may therefore arise from the commonahty of a cr-in-termediate 41 to both reaction pathways (Scheme 9.13). 

Conversion of bromohydrins to alkenes. The 14,15-epoxide (1) of arachidonic acid methyl ester has been converted into the 11,12-epoxide (5) in two steps. The first is conversion of 1 into a mixture of isomeric bromohydrins (2 and 3) with KBr. The mixture is then epoxidized under the conditions of Sharplcss (5, 75 76) to give, after chromatographic purification, 4, the epoxide of 2. Conversion of 4 into 5 presented a problem, but was eventually achieved by treatment of the w c-bromo Inflate with P[N(CHj)2]j as a Br acceptor. 

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