Selenium dioxide alkenes

Selenium dioxide is a useful reagent for allylic oxidation of alkenes. The products can include enones, allylic alcohols, or allylic esters, depending on the reaction conditions. The mechanism consists of three essential steps (a) an electrophilic ene reaction with Se02, (b) a [2,3]-sigmatropic rearrangement that restores the original location of the double bond, and (c) solvolysis of the resulting selenium ester.183... 

Selenium dioxide reveals a useful stereoselectivity when applied to trisubstituted gem-dimethyl alkenes. The products are predominantly the Zs-allylic alcohol or... 

Selenium dioxide is also an oxygen donor to alkenes. In this case, however, the initial reaction of the double bond is with the selenium center followed by two pericyclic steps. After hydrolysis of the organo-selenium intermediate, the result is a hydroxylation at the allylic carbon position65. Thus, limonene (2) yields racemic p-mentha-l,8(9)-dien-4-ol66. The high toxicity of selenium intermediates and prevalence of many rearrangements has limited the widespread use of the reagent in synthesis. 

Silylation followed by selenium dioxide oxidation converted 13 into 14. Epoxidation of the derived TES ether proceeded by addition of oxygen to the more open face of the alkene, leading to 15. Ozonolysis followed by diastereoselective one-carbon homologation provided 17. This set the stage for intramolecular epoxide opening by the carboxylate, to give 2, in which all of the stereogenic centers of tetrodotoxin have been established. 

Oxidation of the allylic carbon of alkenes may lead to allylic alcohols and derivatives or a, 3-unsaturated carbonyl compounds. Selenium dioxide is the reagent of choice to carry out the former transformation. In the latter process, which is more difficult to accomplish, Cr(VI) compounds are usually applied. In certain cases, mixture of products of both types of oxidation, as well as isomeric compounds resulting from allylic rearrangement, may be formed. Oxidation of 2-alkenes to the corresponding cc,p-unsaturated carboxylic acids, particularly the oxidation of propylene to acrolein and acrylic acid, as well as ammoxidation to acrylonitrile, has commercial importance (see Sections 9.5.2 and 9.5.3). 

Allylic Hydroxylation and Acyloxylation. Selenium dioxide is a standard, commonly used stoichiometric oxidant to convert alkenes to allylic alcohols.668-671 In aqueous solution it reacts as seleniuos acid (84), whereas in other hydroxylated solvents (alcohols, carboxylic acids) the alkyl selenite (85) and the mixed anhydride (86), respectively, are the actual reagents ... 

Selenium dioxide, Se02, is a widely used reagent for the allylic oxidation of alkenes and ketones. The subject has been extensively covered by recent review articles56,291 293 and only a short summary will be given in this chapter. 

Selenium dioxide is a most useful reagent for the oxidation of ketones or aldehydes to a-dicarbonyl compounds along with a,)3-unsaturated carbonyl compounds as by-products.291,293 The carbonyl compound probably reacts in its enol form in a way similar to that of alkene oxidation (equation 130).358... 

Allylic oxidation is carried out by addition of one equivalent of selenium dioxide. First Se02 will react with the alkene in a [4 + 2] cycloaddition reminiscent of the ene reaction. The initial product is an allylic selenic acid 40, which undergoes - like an allylic sulfoxide -allylic rearrangement to give an unstable intermediate, which decomposes rapidly to the allylic alcohol 42.16... 

The selenium-dioxide mediated allylic oxidation of alkenes was explored by means of 2H and 13C KIEs to clarify the mechanism of ene step.85 Changes of isotopic composition were determined for unreacted 2-methyl-2-butene 33 in reaction with Se02 at 25°C in ferf-butyl alcohol (Equation (49)). 

The most useful application of 3 is its use in photochemical [2+2] cycloadditions with alkenes and alkynes at the carbon-carbon double bond to afford bicyclo[4.2.0]octane-2,5-diones and bicyclo[4.2.0]oct-7-ene-2,5-diones in good to excellent yield.6 Since selenium dioxide oxidation of the resulting adducts furnishes the corresponding 3-ene-2,5-diones, diketone 3 can be regarded as a 1,4-benzoquinone equivalent leading to [2+2] cycloadducts at the carbon-carbon double bond. 

Selenium dioxide oxidation of alkenes with a hydrogen in an a-position involves the formation of the allyl selenic ester (X = OH) by an ene reaction. [2,3] Sigmatropic rearrangement of the allyl selenic ester to the selenium(II) ester and its hydrolysis also resulted in the formation of allylic alcohols. The oxidation of alkenes with selenium dioxide is covered in Section D.4.10. 

Transition metal catalysts not only increase the reaction rate but may also affect the outcome of the oxidation, especially the stereochemistry of the products. Whereas hydrogen peroxide alone in acetonitrile oxidizes alkenes to epoxides [729], osmic acid catalyzes syn hydroxylation [736], and tungstic acid catalyzes anti hydroxylation [737]. The most frequently used catalysts are titanium trichloride [732], vanadium pentoxide [733,134], sodium vanadate [735], selenium dioxide [725], chromium trioxide [134], ammonium molybdate [736], tungsten trioxide [737], tungstic acid [737],... 

Selenium dioxide, Se02 (mp 315 °C, sublimes), and selenious acid, H2Se03, which is obtained by the evaporation of an aqueous solution of Se02 [507, 50S], are very selective oxidants. They are capable of mild dehydrogenation to form double bonds [375] and can oxidize alkenes and acetylenes to vicinal dicarbonyl compounds [509, 510] and allylic ethers to aldehydes [511]. The most important applications are conversions of alkenes into allylic alcohols [5i2] of benzylic, methyl, or methylene groups into carbonyl groups [513, 514, 5i5] and of carbonyl compounds into a-... 

Another kind of anti hydroxylation is the reaction of alkenes with hydrogen peroxide in the presence of vanadium pentoxide, molybdenum trioxide, selenium dioxide, and especially tungsten trioxide (tungstic acid) [131, 144]. Cyclohexene is thus converted into DL-/rarts-l,2-cyclohex-anediol [131],... 

Alkenes may also be transformed into vicinal dicarbonyl compounds. The treatment of alkenes with selenium dioxide, although possible, does not give satisfactory yields [509]. Better results are obtained when unsaturated compounds such as oleic acid are oxidized with potassium permanganate buffered with acetic anhydride [861],... 

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