Arylsulfonyl leaving groups

Polar C=Y double bonds (Y = NR, O, S) with electrophilic carbon have been added to suifinic acids under formation of sulfones. As in the preceding section one must distinguish between carbonyl groups and their derivatives on the one hand, and carboxylic acids (possessing leaving groups at the electrophilic carbon) on the other. Aldehydes " of sufficient reactivity—especially mono-substituted glyoxals - —and their aryl or arylsulfonyl imines have been added to suifinic acids (in a reversible equilibrium) to yield a-hydroxy or a-amino sulfones the latter could also be obtained from the former in the presence of primary amines (equation 26). 

By employing arylsulfonyl groups as suitable leaving groups, a highly stabilised carbocation could be generated using potassium fluoride supported on alumina, which then allowed for easy interception by the thus-formed nucleophilie enamine intermediate from the aldehyde and L-proline (Scheme 5.24). 

Other leaving groups have also been demonstrated to be effective. A Pd-catalyst was effective in the desulfonative homocoupling of arylsulfonyl chlorides (25). Aryl triflates participate in Ni- and Pd-cataly2 d cross-coupling (24 5) and homocoupling reactions (26-55), for example see equation 11 (26). 

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