Vinyl oxidation/oxidative cleavage

Oxidative cleavage of an alkene with Klvln04 gives a carboxylic acid if the alkene has at least one vinylic hydrogen (Section 7.9). 

Efficient cleavage of A.A-dimethylhydrazones to yield the parent ketones (>90%) is effected under neutral conditions with tetra-n-butylammonium persulphate [40], The procedure is particularly useful for compounds with acid-sensitive substituents, e.g. vinyl or ketal groups. Similarly, reaction times for the oxidative cleavage of semicarbazones with sodium nitrite or nitrate/trimethylsilyl chloride are reduced by the addition of benzyltriethylammonium chloride [41]. 

While secondary alcohols are now relatively easy to prepare in enantiomerically-pure form, secondary amines have been more challenging. Larry Overman of UC Irvine reports (J. Am. Chem. Soc. 125 12412, 2003) the catalytic rearrangement of primary allylic alcohols such as 4 to the corresponding protected vinyl amine 5 with excellent . Hydrolysis of the amine 5 gives the GABA aminotransaminase inhibitor 6. Unnatural amino acids can be prepared by oxidative cleavage of the protected vinyl amines. 

To complete the synthesis, the ketone of 2 was homologated to the alkene 12. Selective oxidative cleavage of the two vinyl groups followed by reduction provided the diol, the less encumbered alcohol of which was protected to deliver the fully-differentiated ester 13. Oxidation followed by ester cleavage then gave 3. 

Formation of 1,2-dioxetanes is usually characteristic of alkenes activated by heteroatom in the vinylic position. Most dioxetanes are unstable and decompose with concomitant chemiluminescence. The products are carbonyl compounds resulting from oxidative cleavage of the double bond. 

Palladium-catalyzed allylic oxidations, in contrast, are synthetically useful reactions. Palladium compounds are known to give rise to carbonyl compounds or products of vinylic oxidation via nucleophilic attack on a palladium alkene complex followed by p-hydride elimination (Scheme 9.16, path a see also Section 9.2.4). Allylic oxidation, however, can be expected if C—H bond cleavage precedes nucleophilic attack 694 A poorly coordinating weak base, for instance, may remove a proton, allowing the formation of a palladium rr-allyl complex intermediate (89, path by694-696 Under such conditions, oxidative allylic substitution can compete... 

In previous studies, i,e. concurrent carbonyl-oxygen exchange in the hydrolysis of esters, acid hydrolysis of orthoesters and oxidation of acetals by ozone, the configuration of the tetrahedral intermediate was determined by the application of the principle of stereoelectronic control. There could be some ambiguity in these experiments as the theory of stereoelectronic control is used to predict both the stereochemistry of the tetrahedral intermediate as well as its breakdown. The oxidation cleavage of vinyl orthoesters can therefore be considered a more powerful experimental technique in that respect because the configuration of the hemi-orthoester... 

The carbonyls Fe(CO)5 and [CpFe(CO)2]+ (2) form stable cationic complexes with alkenes, which are used for both protection and activation of alkenes [1]. [CpFe(CO)2]+ (2 abbreviated as Fp+) is prepared by the reaction of cyclopentadienyl anion (1) with Fe(CO)5, followed by oxidative cleavage with bromine, and used for the protection of alkenes. The electron density of the double bond is decreased by the coordination of [CpFe(CO)2]+ and hence this bond is activated to nucleophilic attacks. Introduction of nucleophiles, such as the carbon nucleophile of malonate, to cyclopentene becomes possible via the formation of the complex 3, and the stable tftmv-er-alkyliron complex 4 of cyclopentane is prepared. The vinyl ether complex 6 is obtained easily from the a-bromoacetal 5, and reacts with an enolate of ketone 7 as an... 

Base-catalysed conjugate addition of nitropropane to methyl vinyl ketone occurred smoothly to give the nitroketone. Formation of the salt with sodium methoxide was followed by oxidative cleavage of the C-N linkage with ozone. The product was a 1,4-diketone which was isolated without further aldol reaction by this route. 

This procedure consists of the synthesis of a precursor, methoxymethyl vinyl ether, an a-hydroxy enol ether, and the intramolecular hydrosilylatlon of the latter followed by oxidative cleavage of the silicon-carbon bonds. The first step, methoxymethylation of 2-bromoethanol, is based on Fujita s method.7 The second and third steps are modifications of results reported by McDougal and his co-workers. Dehydrobromination of 2-bromoethyl methoxymethyl ether to methoxymethyl vinyl ether was achieved most efficiently with potassium hydroxide pellets -9 rather than with potassium tert-butoxide as originally reported for dehydrobromination of the tetrahydropyranyl analog.10 Potassium tert-butoxide was effective for the dehydrobromination, but formed an adduct of tert-butyl alcohol with the vinyl ether as a by-product in substantial amounts. Methoxymethyl vinyl ether is lithiated efficiently with sec-butyllithium in THF and, somewhat less efficiently, with n-butyllithium in tetrahydrofuran. Since lithiation of simple vinyl ethers such as ethyl vinyl ether requires tert-butyllithium,11 metalation may be assisted by the methoxymethoxy group in the present case. 

Addition of oiganditfaiuin and organomagnesium reagents to such vinyl(alkoxy)silanes, followed by catalyzed coupling of the new organometallic with either vinyl or allyl bromide, leads,after oxidative cleavage, to allylic or homoallylic alcohols, respectively (Scheme 9). 

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