Reductive desulfonylation reactions lead to the replacement of a carbon-sulfur bond in the sulfonyl group with a carbon-hydrogen bond. Because the sulfonyl group is by definition attached to two carbons, however, reduction to two sets of products is possible. Mechanistic studies of reductions employing metal amalgams as the reducing agent suggest that upon electron transfer to the sulfone, fragmentation to a sulfinate anion and the more stable organic radical occurs. Immediate reduction of the radical and protonation then occur to afford the sulfur-free product derived from the more stable radical. Thus, S-alkyl bonds are cleaved in preference to S-aryl or S-alkenyl bonds.
Samarium(II) iodide may be used to reductively cleave α-keto sulfones; in the presence of hexamethylphosphoramide (HMPA), SmI is able to effect reductive elimination of α-functionalized sulfones (see equation () below).
Tin hydrides reduce α-keto and allylic sulfones. The mechanisms of these processes involve the addition of a tin-centered radical to the substrate followed by elimination of a sulfinyl radical, which abstracts a hydrogen from a molecule of tin hydride to propagate the radical chain. Protonation of the organotin intermediates thus formed (by sulfinic acid generated in situ) leads to reduced products. Addition of a stoichiometric amount of proton source allows the use of tin hydride in catalytic amounts. Although desulfonylations of allylic sulfones are site selective (providing only products of allylic transposition), they are not stereoselective and afford mixtures of double bond isomers. The mechanism of desulfonylation of α-keto sulfones is similar.
Transition-metal-mediated reductive desulfonylations rely on the generation of an intermediate π-allyl complex, which undergoes nucleophilic attack by hydride or another nucleophile to afford reduced products. Nucleophilic attack generally occurs at the less substituted position of the π-allyl moiety, although site selectivity depends strongly on the substrate and reaction conditions. Palladium(0) complexes are the most commonly used precatalysts.