Selenoxides carbonyl compounds

There are many other methods for carrying out 1,2 eliminations to give olefins. Several are particularly useful and widely used. Selenoxide eliminations are frequently used to install file double bond of a, /3-unsaturated carbonyl compounds. They occur by concerted, cyclic, syn processes... 

Preparation of a.p-Unsaturated Carbonyl Compounds and Nitriles by Selenoxide Elimination... 

N-Sulfonyloxaziridines are an important class of selective, neutral, and aprotic oxidizing reagents.11 Enantiopure N-sulfonyloxaziridines have been used in the asymmetric hydroxylation of enolates to enantiomerically enriched a-hydroxy carbonyl compounds,9 11-13 the asymmetric oxidation of sulfides to sulfoxides,14 1S selenides to selenoxides,16 sulfenimines to sulfinimines,17 and the epoxidation of alkenes.18... 

Another example of alkene synthesis by the pyrolysis of selenoxide is given in Scheme 4.14. The enolate derived from 4.18 reacts with either PhSeBr or PhSeSePh to form selenide 4.19. Oxidation of 4.19 gives selenoxide 4.20, which undergoes sy -elimination to give a,P Unsaturated carbonyl compound 4.21. 

The most versatile method for introduction of the selenenyl moiety is by low temperature reaction of the enolate anion or an enolic derivative with a suitable selenium species, the precise conditions being dependent on the reactivity of both the carbonyl compound and the selenium species. Like sulfur, selenium may be introduced either in the divalent state and subsequently oxidized or, more recently, as the selenoxide directly (Scheme 14). The choice of method is determined by the subsequent reactions that need to be carried out. 

Several oxaziridines related to (14) (eq 8) have been used, most notably in the enantioselective oxidation of sulfides to sulfoxides, of selenides to selenoxides, and of alkenes to oxiranes, It is also the reagent of choice for the hydroxylation of lithium and Grignard reagents and for the asymmetric oxidation of enolates to give a-hydroxy carbonyl compounds, - A similar chiral fluorinating reagent has also been developed, ... 

Sodium periodate (sodium metaperiodate), NaI04 (mp 300 °C dec), which is commercially available, is applied mainly in aqueous or aqueous-alcoholic solutions. Like the free periodic acid, sodium periodate cleaves vicinal diols to carbonyl compounds [762], This reaction is especially useful in connection with potassium permanganate [763, 764] or osmium tetroxide [765], Such mixed oxidants oxidize alkenes to carbonyl compounds or carboxylic acids, evidently by way of vicinal diols as intermediates. Sulfides are transformed by sodium periodate into sulfoxides [322, 323, 766, 767, 768, 769, 770, 771, 772], and selenides are converted into selenoxides [773]. Sodium periodate is also a reoxidant of lower valency ruthenium in oxidations with ruthenium tetroxide [567, 774],... 

Selenoxides are useful intermediates in the preparation of a,3-unsat-urated carbonyl compounds and esters. The treatment of aldehydes, ketones, or esters with benzeneselenyl chloride, C6H5SeCl, followed by the oxidation of the selenides to selenoxides by hydrogen peroxide, peroxy acids, or sodium periodate, gives a,3-unsaturated aldehydes, ketones, or esters. Thus, dehydrogenation with the formation of a carbon-carbon double bond is accomplished under very mild conditions [167, 169] (equation 593). 

Further reaction of Aese species with carbonyl compounds and hydrolysis of the resulting alkoxide leads to p-oxidoalkyl selenoxides which have been transformed into allyl alcohols on thermal decomposition (Schemes 51, 52 and 54, entry a see Section 2.6.4.4) or reduced to p-hydroxyalkyl selenides or to alkenes (Scheme 53). P-Oxidoalkyl selenoxides derived from cyclobutanones react in a different way since Aey rearrange to cyclopentanones upon heating (Scheme 54, b. Schemes 120 and 121 and Section 2.6.4.5.3). 

This reaction allows the synthesis of (i) p,p -dienols by oxidation of p-hydroxy-y-alkenyl sel-enides or more conveniently from a-lithioalkyl selenoxides and enones (Scheme 136 and 166) (ii) p,5-dienols from l-lithio-3-alkenyl phenyl selenoxides and carbonyl compounds (Scheme 177) and (iii) 2-(r-hydroxyalkyl)-1,3-butadienes from 1-methylselenocyclobutyllithium and carbonyl compounds (Scheme 178). a,p-Unsaturated alcohols bearing a methylselenoxy or a phenylselenoxy group at the a-position do not lead on thermolysis to propargyl alcohols however, those be ng a (trifluoro-methylphenyl)selenoxy moiety at the a-position are valuable precursors of such con unds (Scheme 179). ... 

From enolates via selenoxidesA Lithium enolates derived from ketones, lactones, and esters react with PhSe-SePh or with phenylselenyl bromide or chloride (PhSeX) to form a-(phenylseleno)carbonyl compounds. These can be oxidized to the corresponding selenoxides with subsequent 5yn-elimination of benzeneselenic acid to form enones. 

An important strategy for the regio- and stereoselective preparation of a,(3-unsat-urated carbonyl compounds is via selenoxide elimination. Treatment of enolates... 

Selenoxide elimination is now widely used for the synthesis of a,p-unsaturated carbonyl compounds, allyl alcohols and terminal alkenes since it proceeds under milder conditions than those required for sulfoxide or any of the other eliminations discussed in this chapter. The selenoxides are usually generated by oxidation of the parent selenide using hydrogen peroxide, sodium periodide, a peroxy acid or ozone, and are not usually isolated, the selenoxide fragmenting in situ. The other product of the elimination, the selenenic acid, needs to be removed from the reaction mixture as efficiently as possible. It can disproportionate with any remaining selenoxide to form the conesponding selenide and seleninic acid, or undergo electrophilic addition to the alkene to form a -hydroxy selenide, as shown in... 

Although unable to metallate selenides, dialkyl amides are sufficiently strong to metallate phenylselenoacetals39,46 51) as well as methyl48,52) and phenyl 46,47,52) selenoorthoesters. They are also able to metallate selenoxides 4 9,11 53 55) and selenones 14). Finally selenoacetals are readily available 4,711,12 S6) from carbonyl compounds and selenols in the presence of a Lewis acid and selenoorthoesters have been prepared from orthoesters, selenols, and boron trifluoride etherate47,48,52). 

Both the selenoxide and the selenonium ylide routes have been applied to cyclobutyl derivatives, themselves readily available 57) from selenoacetals of cyclo-butanones on one hand and primary alkyl halides, epoxides, or carbonyl compounds on the other. 

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