Steric hindrance as a key factor on proton transfer in the σ-adduct forming reactions of o -substituted anilines with 1,3,5-trinitrobenzene in dimethylsulfoxide

Kinetic and equilibrium studies are reported of the reactions of 1,3,5-trinitrobenzene (TNB) with a series of o-substituted anilines in dimethyl sulfoxide (DMSO) in the presence of 1,4-diazabicyclo2.2.2.]octane (DABCO). The pK _a values in DMSO for the aniline derivatives were measured using the proton-transfer equilibrium with 2,4-dinitrophenol. Kinetic studies are compatible with a two-step process involving initial nucleophilic attack on TNB by amine to give a zwitterionic intermediate which may transfer an acidic proton to DABCO to yield the anionic product. The results indicate steric hindrance to proton transfer in reactions involving 2,6-disubstituted anilines.