Sulfoxide-sulfenate ester rearrangement

The sulfoxide-sulfenate ester rearrangement (Mislow/Evans rearrangement) where the X-Y fragment is a sulfoxide. 

This transformation presumably proceeds with initial formation of allylic sulfilimine 55, which rearranges via an envelope-like transition state to sulfenamide 56 (cf. Scheme 1-XIII). Interestingly, NMR analysis showed that this [2,3]-sigmatropic rearrangement lies totally on the side of this sulfenamide, unlike the allylic sulfoxide-sulfenate ester system, which lies predominantly to the side of the sulfoxide. Similarly, C-3 epimeric dihydrothiazine imine 58 can be converted by an identical pathway to E-erythrp vicinal diamine 59 in an efficient and totally stereoselective manner [Eq. (28)]. 

M. Johnston, R. Raines, C. Walsh, and R. A. Firestone (1980), Mechanism-based enzyme inactivation using an allyl sulfoxide-allyl sulfenate ester rearrangement, J. Amer. Chem. Soc. 102,4241-4250. 

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). 

In another synthetic application, first reported by Smith and Stirling142, the bis-2,3-(phenylsulfinyl)-l, 3-butadiene 94 has been prepared in low yield by two spontaneous sequential [2,3]-sigmatropic rearrangements of the Ws-sulfenate ester (93). More recently, the yield of this reaction (equation 42) has been improved159, and a related dienyl sulfoxide 95 has been reported (equation 43)160. This type of sulfoxide is of considerable interest in view of recent studies on Diels-Alder reactions of polysubstituted butadienes161-164. 

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) . ... 

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. 

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. 

The rearrangement product, which is less stable than the sulfoxide and is therefore never observed directly, is a sulfenate ester. It has no chirality at sulfur so, when it rearranges back to the sulfoxide, it has no memory of the configuration of the starting sulfoxide, and the sulfoxide becomes racemized. 

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. 

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. 

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