Allylic compounds Selenium dioxide

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). 

Methylmagnesium N-cyclohexyliso-propylamide, 189 By oxidation at an allylic carbon Selenium dioxide, 272 By reduction of a,0-unsaturated carbonyl compounds Sodium borohydride, 278 Sodium dithionite, 281 Other methods r-Butyllithium, 58 Butyllithium-Potassium f-butoxide,... 

Allylic oxidation. Selenium dioxide is the classical reagent for allylic hydroxylation (1,994-996), but it has the drawback that selenium compounds are about as toxic as arsenic compounds. However, SeOa need be used in only catalytic amounts if used in conjunction with H2O2 (507 ). Thus -pinene (1) can be converted into /runi-pinocarveol (2) in 49-55% yield by oxidation with Se02-H2O2 in /-butyl alcohol. This product has been prepared in somewhat higher yield... 

Treatment of double-bond compounds with selenium dioxide introduces an OH group into the allylic position (see also 19-14). This reaction also produces conjugated aldehydes in some cases.Allylic rearrangements are common. There is evidence that the mechanism does not involve free radicals but includes two pericyclic steps (a and... 

Vapor phase chromatography was used to analyze allylic oxidation products. The column, inch X 35 feet, was packed with 20% UCON 5100 on Celite (50-70). NMR analyses were performed using a Varian A-60 instrument. Molecular weights were determined by cryoscopic methods (benzene and acetic acid solvents). Organoselenium compounds were analyzed for selenium by the method of Gould (2), which involves wet digestion of the sample, followed by iodometric titration of the selenium dioxide produced. 

Scheme 2 shows Rapoport s synthesis [15]. The cinnamic acid derivative 3 prepared from m-methoxy benzaldehyde [20] was ethylated by diethyl sulfate to give ethyl cinnamate derivative 4, followed by Michael addition with ethyl cyanoacetate to afford compound 5. Compound 5 was converted to lactam 6 by the reduction of the cyano group and subsequent cyclization. Selective reduction of the lactam moiety of 6 was achieved by treatment with trimethy-loxonium fluorob orate followed by sodium borohydride reduction. Amine 8 was obtained by the reductive methylation of amine 7. Amine 8 was converted to compound 9 by methylene lactam rearrangement [21], followed by selenium dioxide oxidation to provide compound 10. Allylic rearrangement of compound 10 and subsequent hydrolysis gave compound 12. The construction of the decahydroisoquinoline structure began with compound 12,... 

Selenium-mediated allylic oxidations producing allylic alcohols have been discussed above however, in some cases oxidation proceeds further to give the a, -unsaturated carbonyl compounds directly, or mixtures of alcoholic and ketonic products. That the regioselectivity observed in these allylic oxidation reactions closely resembles that found in classical selenium dioxide oxidations is in accord with initial formation of the intermediate allylic alcohol before in situ oxidation to the carbonyl compound. This process was studied by Rapoport and was explained mechanistically as an elimination of the intermediate allylic selenite ester via a cyclic transition state, analogous to Ssi (rather than 5n20 solvolysis (Scheme 21). Of the two possible transition states (78) and (79), the cyclic alternative (78) was preferred tecause oxidation exclusively yields trans aldehydes. 

Selenium dioxide (SeOi) oxidation Selenium dioxide is an excellent oxidizing agent for the oxidation of allylic and benzylic C-H fragments to allylic or benzylic alcohol. It also oxidizes the aldehydes and ketones to 1,2-dicarbonyl compounds (i.e. oxidation of active methylene groups to carbonyl groups). 

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-... 

Diphenylpyridazine is reduced with sodium and ethanol, affording the 1,2-dihydro derivative.133 Since 3,6-diphenyl-4,5-dihydropyridazine is more stable than the isomeric 1,2-dihydro compound, the latter is isomerized in the presence of alkali to the 4,5-dihydro compound.134 The 1,2-diethoxycarbonyl-l,2-dihydropyridazine is formed by selenium dioxide oxidation135 or via allylic bromination-dehydrobromination136137 of the corresponding 1,2,3,6-tetrahydro compound. 1-Ethoxycarbonyl- or 1,2-diethoxycarbonyl-l,2-dihydropyridazines were obtained similarly from alkali treatment of 1,2-diethoxycarbonylhexahydropyridazines.138... 

The cuprous iodide procedure has also been used in a simplified synthesis of dl-sirenin (11) from the diazo compound (9). Thus, when (9) is slowly added to a stirred suspension of an equivalent amount of cuprous iodide in THF at 35°, the bicyclic ester (10) is obtained in about 50% yield.3 This ester has been converted into d/-sirenin (11) by Rapoport5 in two steps allylic oxidation with selenium dioxide followed by reduction of the aldehyde and ester functions with mixed hydride. 

Allylic functionalization of 187 could not be effected under a variety of conditions (e.g., A-bromosuccinimide, lead tetraacetate, selenium dioxide), nor could an additional double bond be introduced under dehydrogenation conditions (e.g., chloranil). Osmium tetroxide was ineffective in hydroxylation of 187 however, potassium permanganate treatment gave the diol 189 although in only 5.5% yield [originally reported as 33% (67)]. Compound 189 could be converted into 190 again in low yield (4.5%). The latter was shown to be identical with yet another degradation product (68, Scheme 11) of securinine by comparison of IR spectra and GLC behavior. 

Selenium dioxide is probably the most common reagent used to selectively oxidize an alkyl fragment, and is particularly effective for converting allylic or benzylic C—H fragments to the corresponding allylic alcohol.522 In the most common applications, carbonyl derivatives are oxidized to 1,2-dicarbonyl compounds and allylic hydrocarbons to alcohols or esters. 23... 

Allylic oxidation by selenium dioxide 61.3.3-5.2 Oxidations by tellurium-oxo compounds... 

From a practical viewpoint, direct C-1 hydroxyktion of vitamin D3 is a highly desirable reaction. However, direct allylic oxidation of vitamin D3, for example, with selenium dioxide/t-butylhydroperoxide [scheme (31)] produces only small amounts (< 5%) of 1 -hydroxylated compounds. 

Review. Oxidations with selenium dioxide have been reviewed (634 references). The reagent is useful for oxidation of carbonyl compounds to 1,2-dicarbonyl compounds, for oxidation of olefins to allylic alcohols, and for benzylic oxidations. It has been used for dehydrogenations, but high-potential quinones are probably more useful for this reaction. ... 

A related reaction of selenium in its +4 oxidation state (as selenium dioxide, Se02) allows us to make allylic alcohols and enals from simple alkenes. The overall reaction is the simple oxidation shown in the margin, but the route by which the compound gets there involves two successive pericyclic reactions. 

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