Pseudomolecular rearrangement of O-ethyl N-methyl toluene-4-sulphonimidate to N-ethyl-N-methyltoluene-4-sulphonamide and its relevance to the nucleophilic properties of neutral sulphonamides

Abstract

Kinetic studies are reported for the pseudo-molecular rearrangement of O-ethyl N-methyl toluene-4- sulphonimidate to N-ethyl-N-methyltoluene-4-sulphonamide in organic solvents at 34–100 °C. Without catalysts, the rearrangement follows the equation rate =k_rearr[substrate]², which is indicative of an intermolecular S_N2 transalkylation via an ion-pair intermediate: it is accompanied by concurrent E2 elimination to N-methyltoluene-4-sulphonamide. The rearrangement is catalysed by electrophilic reagents such as alkyl halides, Znl₂, and HBr where rate =K₂[substrate][catalyst]. For alkyl halides, a two-step mechanism via an ionic intermediate applies in which formation of the intermediate by an S_N2 reaction between the substrate and alkyl halide is rate limiting. Other catalysts effect rearrangement by forming alkyl halides in an initial rapid reaction with the substrate. The results are discussed in relation to the ambident nucleophilic properties of sulphonamides. It is suggested that, like carboxylic acid amides and phosphinylamides, alkylation occurs most readily at the O-atom of neutral sulphonamides to give a sulphonimidate (kinetic product), which then rearranges in the presence of electrophilic catalysts to give an N-substituted sulphonamide (thermodynamic product). Rearrangement is normally too fast for the isolation of O-alkyl sulphonimidates, but O-aryl analogues can be obtained.