Sulfenates chirality transfer

In addition to the above-mentioned 1,3-chirality transfer, another mode of transferring centro chirality from one atom to the other is the S C transfer. This feature of the reaction has already been discussed by Mislow in his 1968 paper [40], where he delineates all important stereochemical aspects of [2,3]-sigmatropic rearrangements using the sulfoxide-sulfenate rearrangement as an example. For the sulfinate-sulfone rearrangement under consideration here, the work of Hiroi in the early 1980s should be mentioned (Scheme 16) [44,... 

The stereochemical course of sulfoxide-sulfenate rearrangements was reviewed by Hoffmann in detail. The results can be summarized as follows (i) chirality transfer from the stereocenter on sulfur to carbon is only observed for sulfoxides without a-substituents (left-hand part of Scheme 23). Then, the (Z)-isomers react preferentially via the endo transition state (185) with up to 90% chirality transfer. ( )-Sulfoxides are less selective and therefore worthless as precursors for optically active rearrangement products, (ii) Any substituent R of the sulfoxide overrides stereocontrol originating in the stereocenter on sulfur (right hand part of Scheme 23). How chirality is transmitted is here determined exclusively by the desire of R to be quasi-equatorial in the transition state (187). At the same time, this orientation of R° is tantamount to a trans configuration in the rearranged alcohol. 

A closely related stereochemical feature of the allylic sulfoxide-to-sulfenate rearrangement involves sulfur-to-carbon (S C) chirality transfer. Trapping of the sulfenate with a thiophile has the potential to relay stereochemical information from the sulfoxide chiral center to the carbinol center of the allylic alcohol product. This stereochemical transfer originates from a preference for either the exo or the endo transition state, with the latter typically predominating. Thus, as shown in Scheme 18.8. for a fixed configuration at the sulfoxide S-center and the E-alkene geometry, the exo and endo transition states provide enantiomeric... 

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... 

What do we mean by transfer of chirality Reactions in which the stereoselective formation of one stereogenic center is connected to the destruction of another stereogenic center involve transfer of chirality. Sigmatropic rearrangements are a large family of intramolecular reactions that often can be used to transfer chirality. We will look at two examples (1) the Claisen rearrangement (of allyl vinyl ethers to j8,7-unsaturated carbonyl compounds) and (2) the Mislow-Evans rearrangement (of allylic sulfoxides to allylic alcohols via sulfenate esters). 

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