Sulfenate esters, allyl

Scheme 1R.1 Allylic alcohol/allyl sulfenate ester/allyl sulfoxide interconversinns.

Owing to the reversible nature of the allylic sulfenate/allylic sulfoxide interconversion, the stereochemical outcome of both processes is treated below in an integrated manner. However, before beginning the discussion of this subject it is important to point out that although the allylic sulfoxide-sulfenate rearrangement is reversible, and although the sulfenate ester is usually in low equilibrium concentration with the isomeric sulfoxide, desulfurization of the sulfenate by thiophilic interception using various nucleophiles, such as thiophenoxide or secondary amines, removes it from equilibrium, and provides a useful route to allylic alcohols (equation 11). 

Deprotonation of allylic aryl sulfoxides leads to allylic carbanions which react with aldehyde electrophiles at the carbon atom a and also y to sulfur . With benzaldehyde at — 10 °C y-alkylation predominates , whereas with aliphatic aldehydes at — 78 °C in the presence of HMPA a-alkylation predominates . When the a-alkylated products, which themselves are allylic sulfoxides, undergo 2,3-sigmatropic rearrangement, the rearranged compounds (i.e., allylic sulfenate esters) can be trapped with thiophiles to produce overall ( )-l,4-dihydroxyalkenes (equation 24). When a-substituted aldehydes are used as electrophiles, formation of syn-diols 27 occurs in 40-67% yields with diastereoselectivities ranging from 2-28 1 (equation 24) . ... 

The thermal racemization of optically active aryl allyl sulfoxides ArS(0)CHjCH=CH2 is orders of magnitude faster, and has a much lower activation energy, than that of aryl alkyl sulfoxides ArS(0)R (Bickart et ai, 1968). The reason is that the presence of the allyl group permits the sulfoxide to equilibrate with the isomeric, achiral sulfenate ester by a concerted, cyclic process (89) for whichis only about 20 kcal moM.The rates of racemiz-... 

One of the most interesting reactions in sulfur chemistry is the reversible [2,3]sigmatropic rearrangement of allyl sulfoxides to the corresponding sulfenate esters, which are achiral at sulfur. However, in the case of suitably substituted allyl sulfoxides a new chiral center may be generated at the a-carbon in this process, as shown in eq. [137]. 

It has been reported that treatment of methyl 1-methylsulfanylvinyl sulfoxides with sodium thiophenolate in methanol affords l-methylsulfanylalk-l-en-3-ols. A sequence has been proposed218 (see Scheme 45) in which the thiophilic base first causes an in situ isomerization of the vinyl sulfoxide moiety into an allylic sulfoxide which then undergoes a [2,3]-sigmatropic rearrangement and subsequent thiophilic cleavage of the intermediate sulfenic ester. 

Reaction of an allylic alcohol with PhSCI gives an unstable sulfenate ester that rearranges on heating to an allylic sulfoxide by a [2,3]-sigmatropic rearrangement involving both O and S. 

This is an unfavourable reaction, because the equilibrium lies over on the sulfoxide side. But the nucleophile traps the sulfenate ester and the methanol ensures that the alkoxide ion formed is immediately protonated so that we get another allylic alcohol. 

The product obtained is an allylic alcohol with the hydroxyl group at the other end of the allyl system from where the sulfur started—-a rearrangement has taken place. We have observed the rearrangement in this case because the P(OMe)3 has trapped the rearrangement product but, even without this reagent, allylic sulfoxides are continually and reversibly rearranging into sulfenate esters by the mechanism shown below. 

If our proposal that allylic sulfoxides rearrange reversibly to sulfenate esters is correct, then, if we make the sulfenate ester by another route, it too should rearrange to an allylic sulfoxide—and indeed it does. The sulfenate ester arising from reaction of allylic alcohols with PhSCl (phenylsulfenyl chloride) cannot be isolated instead, the allylic sulfoxide is obtained, usually in very good yield, and this method is often used to make allylic sulfoxides. 

The [2,3] sigmatropic rearrangement of sulfenate esters 3, prepared in situ on treatment of 4-morpholinesulfenyl chloride with allylic alcohols, allows easy synthesis of diastereomeric allylic sulfinamides 4 (configurations were not established)104. 

When t7. -l-(2,5-dichlorophenyl)-4-methyl-2-cyclohexenol is treated with 2,4-dinitrobenzene-sulfenyl chloride in the presence of triethylamine. the allylic sulfoxide 9 is stereoselectively formed via [2,3] sigmatropic rearrangement of the initially formed sulfenate ester 8106. 

Another useful reaction of these Diels-Alder adducts is shown in equation (54). Dihydrothiazine oxide (123) from ( . )-2,4-hexadiene can be opened with a Grignard reagent to allylic sulfoxide (124) which undergoes a stereoselective 2,3-sigmatropic rearrangement via the envelope-like transition state conformation shown, having a quasi-equatorial methyl group to afford sulfenate ester (125). Desulfurization of (125) provides E)-threo smino alcohol derivative (126) in excellent overall yield. If ( ,Z)-2,4-hexadiene is used, the E)-erythro epimer of (126) is formed cleanly. 

On the other hand the reversible nature of the allyl sulfenate-sulfoxide rearrangement can be used very efficiently for the facile interconversion of allylic sulfoxides and alcohols via their sulfenate esters (Scheme 13). 

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