Enhanced reactivities in substitution and elimination reactions in dimethyl sulphoximide

Abstract

The dielectric constants of dimethyl sulphoximide (DMSOI), tetramethylene sulphoximide (TMSOI), and dimethyl N-methylsulphoximide (N-MeDMSOI) were measured. The sulphoximides were tested as solvents for typical nucleophilic substitutions (S_N2) and base-catalysed E2 reactions. The rates for several S_N2 reactions and the activation parameters of the typical S_N2 reaction between butyl bromide and sodium azide in DMSOI revealed that it behaves almost like a polar aprotic solvent despite the presence of the protic NH group. In a mixed solvent of DMSOI and MeOH, the rates of a few S_N2 reactions were enhanced more than those in dimethyl sulphoxide–MeOH or dimethylformamide–MeOH. The rate enhancements of S_N2 reactions in various mixed solvents appeared to be related to their pK_a values. While the rate of the E2 reaction of 1-methylheptyl bromide with sodium methoxide or potassium t-butoxide in DMSOI is substantially higher (240 times) than that in methanol or t-butyl alcohol, the orientation of olefin formation in elimination from 2-substituted octanes in sulphoximides is similar to that in MeO^––MeOH. Solvolyses of some alkyl tosylates and halides in DMSOI afforded the corresponding dimethyl N-alkyl sulphoximides in moderate yields. These results indicate that the sulphoximides belong to a new type of solvent which is protic in origin but also behaves almost like a characteristic polar aprotic solvent.