Alkenes oxidative rearrangements

Hydroboration-oxidation (Sections 6 11-6 13) This two step sequence achieves hydration of alkenes in a ste reospecific syn manner with a regiose lectivity opposite to Markovnikov s rule An organoborane is formed by electro philic addition of diborane to an alkene Oxidation of the organoborane inter mediate with hydrogen peroxide com pletes the process Rearrangements do not occur... 

Because of Us high polarity and low nucleophilicity, a trifluoroacetic acid medium is usually used for the investigation of such carbocationic processes as solvolysis, protonation of alkenes, skeletal rearrangements, and hydride shifts [22-24] It also has been used for several synthetically useful reachons, such as electrophilic aromatic substitution [25], reductions [26, 27], and oxidations [28] Trifluoroacetic acid is a good medium for the nitration of aromatic compounds Nitration of benzene or toluene with sodium nitrate in trifluoroacetic acid is almost quantitative after 4 h at room temperature [25] Under these conditions, toluene gives the usual mixture of mononitrotoluenes in an o m p ratio of 61 6 2 6 35 8 A trifluoroacetic acid medium can be used for the reduction of acids, ketones, and alcohols with sodium borohydnde [26] or triethylsilane [27] Diary Iketones are smoothly reduced by sodium borohydnde in trifluoroacetic acid to diarylmethanes (equation 13)... 

As explained in the Introduction, alkene oxides (10.3) are generally chemically quite stable, indicating reduced reactivity compared to arene oxides. Under physiologically relevant conditions, they have little capacity to undergo rearrangement reactions, one exception being the acid-catalyzed 1,2-shift of a proton observed in some olefin epoxides (see Sect. 10.2.1 and Fig. 10.3). Alkene oxides are also resistant to uncatalyzed hydration, thus, in the absence of hydrolases enzymes, many alkene oxides that are formed as metabolites are stable enough to be isolated. 

The reaction is initiated by oxidative addition of the halide to a palladium(O) species generated in situ from the Pd(II) catalyst. The arylpalladium(II) intermediate then forms a complex with the alkene, which rearranges to a a complex with carbon-carbon bond formation. The er-complex decomposes with regeneration of Pd(0) by /i-climination. 

Similar oxidative rearrangements to produce carbonyl compounds have been carried out with thallium(III) nitrate557 or peroxytrifluoroacetic acid-BF3.558 The Jones reagent (Cr03—H2S04) in the presence of a catalytic amount of Hg(II) is also effective.559 Chromyl chloride oxidation, when combined with reductive workup, is a simple, convenient one-step method to convert 2,2-disubstituted 1-alkenes to aldehydes.560,561... 

The [2+2] Mechanism Already in 1977 Sharpless proposed a stepwise [2+2] mechanism for the osmylation of olefins in analogy to related oxidative processes with d°-metals such as alkene oxidations with CrO,Cl2 [23, 24], Metallaoxetanes [25] were suggested to be formed by suprafacial addition of the oxygens to the olefinic double bond. In the case of osmylation the intermediate osmaoxetane would be derived from an olefm-osmium(VIII) complex that subsequently would rearrange to the stable osmium(VI) ester. 

Scheme 2 Mechanistic proposals for the osmium tetraoxide oxidation of alkenes (RA = rearrangement) [3 + 2]- vs. stepwise [2 + 2]-reaction.

Stereoselectivity differences were found between alkane and alkene oxidation in the presence of TS-1, which suggested that the oxidations proceeded via different mechanisms. Stereo-scrambling was present during alkane oxidation on TS-1, without any radical clock rearrangement, suggesting that the radicals formed may have had a very short lifetime or that their movements were restricted such that no rearrangement could occur. 

A related oxidative rearrangement of cephem dioxides has been reported in which an alkene is oxidized stereospecifically with rearrangement to the allylic alcohol in good yield by simple exposure to a palladium/caibon catalyst, as depicted in equation (12). Adventitious oxygen preadsotbed on the catalyst seems the likely oxida The reaction fails on the parent ccphem or its monoxide, or on the free acid of the dioxide. This reaction would seem to hold some promise for furdier utility in the cephem field and odier related systems. 

The oxidative rearrangement of allylic alcohols to a -unsaturated kelmies or alddiydes is one of the most widely used synthetic reactions in this group, and forms part of a 1,3-carbonyl tran sition sequence. Scheme 7 shows this reaction and the related conversion of the allylic alcdiol to an a,p-epoxy carbonyl compound. Chromate reagents induce some allylic alcohol substrates to undergo a directed qmxidation of the alkene without rearrangement, but this reaction is beyond the scope of the present discussion. 

The interesting sequence depicted in Scheme 13 for a sequential oxidative rearrangement and hydrox-ylation of citral shows some potential for this reaction in nonaromatic alkenes. This transformation affords an elegant, single-step approach to the 6,8-dioxabicyclo[3.2.1]octane skeleton, although the stereoselectivity for the two induced centers is poor. ... 

In contrast to lead tetraacetate, simple addition to the double bond does not occur as a side re-action. While allylic rearrangement is common and mixtures of products are frequently obtained, the reaction often proceeds in very high yield and is simple to carry out the alkene is simply heated in an appropriate solvent with mercury(II) acetate until reaction is complete. Mercury(II) acetate has also been us for dehydrogenation, particularly in the steroid field. One interesting example incorporating simultaneous dehydrogenation and allylic oxidative rearrangement is seen in the reaction of abietic acid (37 equation 16). ... 

The oxidative rearrangement most widely used in synthesis is the oxidative 1,2-shift of an alkene or enol, which is shown in the formal sense in equation (33). The alkene may be electron deficient such as an unsaturated ketone, or electron rich such as an enol, enol ether or enamine. 

The oxidative rearrangement of cyclic alkenes and ketones often leads to ring expansion or ring contraction reactions. The reagents generally used for this purpose are hypervalent main group oxidants such as thallium(III), lead(lV), iodine(III) and selenium(IV), alAough palladium(II) has been used as well. 

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