Sulfenate ester-sulfoxide

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

The [2,3]-sigmatropic interconversion of sulfoxides and sulfenate esters is easily reversible, and this is why chiral allylic sulfoxides typically racemize at room temperature. The sulfoxide-sulfenate ester equilibrium usually favors the sulfoxide. If a reactive thiophile that cleaves the O-S bond of the sulfenate ester is introduced into the reaction mixture, then the equilibrium is displaced and the allylic alcohol is formed with the possibility of transfer of chirality (Figure 10.10). Starting from... 

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)]. 

The main primary fragment ions of diaryl sulfoxides 13 and 14 have the structures 16a or 16b(C8H7S02 + m/z 167) and the ion m/z 152 (17) can be obtained from both by the loss of CH3 (equation 5)13. Ions 16a and 16b are formed from the sulfenate ester structure of the molecular ions of 13 and 14 through a cyclization process and a simultaneous loss of the other O Ar part. A similar ortho effect is not possible in 15 and hence its most intense ion is M+ (23% of the total ionization in comparison with 2.7 and 0.6% for 13 and 14, respectively) and its primary fragments are typical for a normal diaryl sulfoxide. 

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) . ... 

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]. 

A sulfenate ester analogous to but with methyl Instead of chlorine Is formed on MCPBA oxidation of the corresponding methallyl thlocarbamate this patent report ( ) does not mention any Intermediates or speculate on the mechanism of the reaction. Formation of the sulfenate ester Is conveniently monitored not only by IR and NMR but also by CI-MS, In the latter case because In contrast to the sulfoxides (7, 9) the sulfenates are sufficiently stable to exhibit a strong molecular Ion (7 ). 

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. 

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