Epoxidation of non-functionalized olefins

Katsuki has studied asymmetric epoxidation of non-functionalized olefins catalyzed by chiral Mn(salen) complex. Recently they proposed that the ligands of Mn(salen) complexes take non-planar stepped conformation and the direction of the folding ligands is strongly related to the sense of chirality in the asymmetric epoxidation (Eq. (7.26)) [71]. On the basis of this proposal, conformational con-... 

Shi and co-workers <97JOC2328, 97JA11224> have optimized their chiral dioxirane protocol for the asymmetric epoxidation of non-functionalized rra/i.v-olefins (e.g., 44), such that the chiral ketone 42 can be used in catalytic quantities with potassium peroxomonosulfate (Oxone) as the stoichiometric oxidant. The key to preserving the lifetime of the chiral auxiliary is pH control during the reaction the optimum range was found to be 10.5 or above, which is conveniently maintained with potassium carbonate. 

The applicability of the Sharpless asymmetric epoxidation is however limited to functionalized alcohols, i.e. allylic alcohols (see Table 4.11). The best method for non-functionalized olefins is the Jacobsen-Kaksuki method. Only a few years after the key publication of Kochi and coworkers on salen-manganese complexes as catalysts for epoxidations, Jacobsen and Kaksuki independently described, in 1990, the use of chiral salen manganese (111) catalysts for the synthesis of optically active epoxides [276, 277] (Fig. 4.99). Epoxidations can be carried out using commercial bleach (NaOCl) or iodosylbenzene as terminal oxidants and as little as 0.5 mol% of catalyst. The active oxidant is an oxomanganese(V) species. 

The in situ generated peroxocomplexes were tested for the catalytic epoxidation of various olefins, such as allyhc alcohols, homoallylic alcohols and non-functionalized olefins. The results of these H2O2 oxidations in an alcohol-water system are summarized in Table 2 for the hydrophilic catalyst A, and in Table 3 for the lipophihc material C. Especially for the more reactive alkenes, the turnover number comes close to the maximum of 300. The epoxide selectivity generally exceeds 90%, with minimal solvolysis. With catalyst A, some substrates gave a lower selectivity. For instance, the product distribution for cyclohexene is 65% epoxide, 27% of allylic oxidation products and only 4% of the diol. The epoxycyclohexane selectivity increases to 91% with the hydrophobic material C. 

Table 3. Epoxidation of allylic alcohols and non-functionalized olefins with aqueous H2O2 in the presence of LDH-[p-toluenesulphonate, W04 ]. ...

The premiere method for producing chiral oxiranes from non-functionalized olefins is the Jacobsen asymmetric epoxidation, which utilizes a chiral manganese salen complex as a catalyst. Since Jacobsen s first report in 1990, intensive study in this area has generated a plethora of reaction conditions and catalyst type.s, as well as questions regarding the mechanistic parameters. The course of the oxygen transfer itself remains a matter of much debate. Norrby and Ackermark <97AG(E) 1723> maintain support for the intermediacy of a metallaoxetane species... 

In the last few years, several non-heme iron complexes have been identified as functional models for non-heme iron dioxygenases (85-88). These model complexes are able to catalyze the cis-dihydroxylation of olefins as well as the epoxidation of olefins using H2O2 as the primary oxidant. Table V presents the results of olefin oxidation by some representative mononuclear and dinuclear non-heme iron complexes in combination with H2O2. 

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