Arylthio groups, electronic effects

The mode of the diastcrcofacial selectivity is completely reversed in the case of reactions with A -methyl A-acyliminium precursors 4176. Now the nitrogen atom of the A-acyliminium ion is not able to chelate with the tin atom and the lower diastereoselectivity is explained by the less rigid nonchelation-controlled transition state 5. An electronic effect, such as n-iz attraction between the electron-deficient carbonyl group of the acyliminium ion and the electron-rich phenyl group of the phcnylthio substituent R, may account for the somewhat higher diastereoselectivity in the case of arylthio substituents R. 

Cycloaddition of sulfenes to a,)8-unsaturated sulfides that contain electron-donor substituents, such as alkyl and arylthio groups, has proved to be a valuable synthetic pathway for the production of thietane 1,1-dioxides. Methylsulfonylsulfene (84) is more reactive than the regular sulfenes because of the activating effect by the strong electron-acceptor properties of the sulfonyl group. If, in addition, a strong electron-donor group, such as the alkylamino function, is provided by the unsaturated sulfide molecule, the cycloaddition proceeds much more readily. 

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