Substitution Reactions of Alkanesulfonyl Derivatives: Direct Substitution vs. Elimination-Addition Mechanisms in Substitution Reactions of Alkyl α-Disulfones

Article abstract of DOI:10.1021/ja00395a024

The reactions of a series of alkyl and aralkyl α-sulfones, RSO2SO2R ( R = Me, n-Bu, i-Pr, ArCH2) with a variety of nucleophiles in aqueous dioxane have been examined.Both rates of reaction and whether a given reaction takes place by an elimination-addition (sulfene intermediate) or a direct substitution (attack of nucleophile on SO2 group of α-sulphone) mechanism have been determined.The great majority of substitution reactions of alkyl α-disulfones take place via an elimination-addition mechanism (eq 3a), with formation of a sulphene from the α-disulphone being rate determining.Only when nucleophile is one, like azide ion, that is weakly basic while still being a good nucleophile is a direct substitution the preferred pathway.Even with azide the reaction pathway changes to elimination-addition when the acidity of the hydrogens on the carbon adjacent to the sulfonyl group is increased sufficiently, as in (PhCH2SO2)2.Comparison of rates of elimination of α-disulphones (R'CH2SO2)2 with rates of base-catalyzed hydrogen exchange of the corresponding trifluoromethyl sulfones R'CH2SO2CF3 indicates that formation of sulfenes from α-disulfones involves either an irreversible E1cB or a very E1cB-like E2 mechanism, a conclusion that is also supported by the observed variation of the rate of elimination of RR'CHSO2SO2R'' with changes in R and R'.Comparison of the behavior of an alkyl α-disulfone with that of the corresponding alkanesulfonyl chloride reveals that changing Y in RCH2SO2Y from RSO2 to Cl causes direct substitution to be able to compete much more effectively with elimination-addition.Kinetic studies show that this arises because, for a given nucleophile, (a) elimination-addition is 5-10 times slower for the alkanesulfonyl chloride than for the α-disulfone while (b) the rate of direct substitution is 5-10 times faster for the sulfonyl chloride.The origin of these rate differences is discussed and explained.