Selenoxides, alkenes from

The regioselectivity of selenoxide elimination is similar to that observed for sulfoxides in that it takes place preferentially towards allylic, propargylic and benzylic hydrogens to form conjugated alkenes with CH3 > CH2 CH, e.g. the conjugated diene (103) was obtained from selenoxide (102 equation 40). ... 

Selenoxide elimination. The yield of alkenes from alky) phenyl selenoxides and alkyl methyl selenoxides under usual conditions (30% H2O2, O3, Oa) tends to be rather low because of formation of the original selenide. Much higher yields are obtained if the selenides are oxidized with /-butyl hydroperoxide (4 equiv.) in the presence of basic alumina (8 equiv.) in THF at 55°. No epoxida-tion is observed under these conditions. A less satisfactory method is ozonization in CH2CI2 in the presence of 1-3 equiv. of triethylamine. ... 

The conversion of the polystyrene-supported selenyl bromide 289 into the corresponding acid 290 allowed dicyclohexylcarbodiimide (DCC)-mediated coupling with an amidoxime to give the 1,2,4-oxadiazolyl-substituted selenium resin 291 (Scheme 48). Reaction with lithium diisopropylamide (LDA) and allylation gave the a-sub-stituted selenium resin 292, which was then used as an alkene substrate for 1,3-dipolar cycloaddition with nitrile oxides. Cleavage of heterocycles 293 from the resin was executed in an elegant manner via selenoxide syn-elimination from the resin <2005JC0726>. 

Mono- and 1,2-di-substituted alkenes react with PhSeQ/Hg(SCN)2 in benzene (0.5-96 h, at 20 C), giving -tra/if-phenylselenoalkyl isothiocyanates in 70-94% yields.2 Terminal alkenes generally give the product with the selenium terminal (an exception is the prc uct from Bu CH H2) internal alkenes show the expected stereochemistry (cis to threo, trans to erythro). Oxidation to selenoxides could be achieved cleanly only with ozone, and the products cis eliminate in the usual manner to give predominantly the vinylic isothiocyanates (Scheme 73). 

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 selenosulfonates (26) comprise another class of selenenyl pseudohalides. They are stable, crystalline compounds available from the reaction of selenenyl halides with sulftnate salts (Scheme 10) or more conveniently from the oxidation of either sulfonohydrazides (ArS02NHNH2) or sulftnic acids (ArS02H) with benzeneseleninic acid (27) (equations 21 and 22). Selenosulfonates add to alkenes via an electrophilic mechanism catalyzed by boron trifluoride etherate, or via a radical mechanism initiated thermally or photolytically. The two reaction modes produce complementary regioselectivity, but only the electrophilic processes are stereospecific (anti). Similar radical additions to acetylenes and allenes have been reported, with the regio- and stereochemistry as shown in Scheme 11. When these selenosulfonation reactions are used in conjunction with subsequent selenoxide eliminations or [2,3] sigmatropic rearrangements, they provide access to a variety of unsaturated sulfone products. 

In separate experiments, it was shown that alkenes lacking the OH-trap, such as the geranylse-leninic acid (3), prepared from the corresponding diselenide, cleanly rearrange to linalool (4) (75 %)8 moreover the allyl phenyl selenoxide 5 gave the -alcohol 6 (71 %) exclusively. 

The first example of a catalytic approach to the selenium promoted conversion was reported by Torii, who described an oxyselenenylation-deselenenylation process using catalytic amounts of diphenyl diselenide [115]. The electrophilic species was produced from the diselenide by electrochemical oxidation in the presence of the alkene 233 in methanol or in water. As indicated in Scheme 36, the addition product is electrochemically oxidized to afford the selenoxide which by elimination gives the allylic ether or alcohol 234 and the phenylselene-nic acid which continues the cycle by adding again to the alkene 233. 

The asymmetric elimination has so far generally been carried out by enan-tioselective deprotonation using chiral bases [15]. Therefore, if an elimination from optically active selenoxides gave optically active alkenes, the reaction may provide a new methodology for asymmetric elimination to form a carbon-carbon double bond under mild conditions. A few examples of asymmetric selenoxide elimination are described below. 

Oxyselenenylation-oxidative deselenenylation (oxyselenenylation-selenoxide elimination) sequence provides the double-bond transpositioned allylic alcohols and ethers from alkenes. Oxyselenenylation of alkenes and its asymmetric... 

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). 

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... 

P-Hydroxy selenides are conveniently prepared from epoxides by treatment with sodium phenylse-lenide (Scheme 32) and by the addition of benzeneselenenic acid and its derivatives to alkenes (Scheme 33), - -" although in some cases these reactions are not regioselective. Useful phenylseleno -etherification and -lactonization reactions have been developed which can be regioselective (equation 42 and Schemes 34 and 35). -" " Selenide- and selenoxide-stabilized carbanions have been used in addition reactions with aldehydes and ketones, - and the reduction of a-seleno ketones also provides a route to P-hydroxy selenides. ... 

The oxidative elimination of primary selenides, readily available from the corresponding alcohol by treatment with an arylselenocyanate and tributylphosphine, is an attractive approach to the synthesis of terminal alkenes." However these reactions are relatively slow when compared with other selenoxide eliminations allowing side reactions, in particular the addition of the areneselenenic acid to the newly formed double bond (Scheme 23), to compete. It has been found that arylselenides with electron-withdrawing substituents fragment more readily, giving improved yields of products, in particular the use of o-nitrophenyl and 2-pyridyl selenides has been recommended (Scheme 37). Often for the elimina-... 

Oxidation to the selenoxide 39 and thermal elimination (chapter 32) produces a new alkene 40 that forms an epoxide 41 on the outside face again. At this point every carbon atom in the six-membered ring is functionalised, five oxygen-based and one nitrogen, and this all started with symmetrical cyclohexadiene. Each functional group was selectively introduced from an alkene. 

Using a similar strategy diene (124) has also been used to synthesize A -acetylneuraminic acid (129) via cycloadduct (126 Scheme 36)7 In contrast to the scheme used for KDO, the alkene resulting from the selenoxide elimination is oxidized to the aldehyde and the NeuAc side chain is installed by a modification of the Homer-Emmons reaction. Osmylation and reduction of the side chain followed by deprotection provides NeuAc (129). 

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