Alkenes, homoallylic epoxidation

Epoxidation of Acyclic Alkenes. Stereoselective epoxidation of a series of allylic carbamate methyl esters (eq 34), homoallylic alcohols (eq 35), and acetates (eq 36) could be performed with good to excellent stereocontrol. It is believed that the directing effect of the carbamate protecting group plays an important role in dictating the level of stereocontrol. 

The Pd-catalyzed hydrogenolysis of vinyloxiranes with formate affords homoallyl alcohols, rather than allylic alcohols regioselectively. The reaction is stereospecific and proceeds by inversion of the stereochemistry of the C—O bond[394,395]. The stereochemistry of the products is controlled by the geometry of the alkene group in vinyloxiranes. The stereoselective formation of stereoisomers of the syn hydroxy group in 630 and the ami in 632 from the ( )-epoxide 629 and the (Z)-epoxide 631 respectively is an example. 

The carbonyl oxygens are responsible for the stereo-directing effects of peroxy acids on the epoxidation of allylic and homoallylic carbamoyloxyalkenes (188 —> 189)298. For other mechanistic investigations of the peroxy acid epoxidations of alkenes, see elsewhere299-301. 

The asymmetric epoxidation of homoallylic alcohols has continued to be a problematic area. A potential solution has recently been published <07JA286 07T6075>. The use of bis-hydroxamic acid 1 as a chiral ligand for a vanadium catalyst has provided both excellent yields and enantioselectivity. This method works well with both cis- and trans-alkenes. 

In contrast to allylic alcdiols, the asymmetric epoxidation of homoallylic alcohols shows the following three general characteristics (i) the rates of epoxidation are slower (ii) enantiofacial selectivity is reversed, i.e. oxygen is delivered to the opposite face of the alkene when the same tartrate ester is used and (iii) the of oiantiofacial selectivity is lower with enantiomeric excesses of the epoxy alcohols... 

The discussion to this point has focused entirely on the epoxidation of allylic (and homoallylic) alcohols catalyzed by the [Ti(OR)2(tartrate)] complex. The role of the alkene as a nucleophile towa the activated peroxide oxygen in Ais reaction has been established (see Section 3.2.6). If the alkene of the allylic alcohol is replaced by another nucleophilic poup then, in principle, oxidation of that group may occur (equation 8). In practice, oxidations of this type have b n observed and generally have been carried out with a substrate bearing a racemic secondary alcohol so that kinetic resolution is achieved. While these oxidations are not strictly within the scope of this chapter, they are summarized briefly in... 

Epoxidation of allyhc and homoallylic alcohols not part of the diene complex can be achieved using the Sharpless r-butyl hydroperoxide/vanadium acetylacetonate protocol. Dihydroxylation of alkenes adjacent to the diene complex using osmium tetraoxide-r-butyl peroxide has been reported... 

The influence of the steric congestion on the catalytic performance of Ti(iv) active centers in the epoxidation of alkenes has been probed using a range of soluble-based silesquioxane species.1934 Bis-homoallylic alcohols are diastereoselectively converted, in good yields, into tetrahydrofuranols and tetrahydropyranols, catalyzed by mono-Gp and bis-Gp Ti derivatives in the presence of B -hydroperoxide activated with 4 A molecular sieves.1935... 

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. 

Diamides of tartaric acid with primary amines have been used as chiral ligands for titanium-and zirconium-catalyzed epoxidations of homoallylic alcohols by the Sharpless method (Section D.4.5.2.4.). Such diamides are conveniently obtained from dimethyl or diethyl tartrate by reaction with the corresponding amine38. The iV,A A, /V -tetrarnethyl diamide has been used for the formation of chiral dioxolanes (Section D.1.5.1.) and in the synthesis of chiral alkenes (Section D.l.6.1.5.). 

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