Rate-Limiting Proton Transfer in ?-Complex Formation between 1,3,5-Trinitrobenzene and N-Methylaniline in Dimethyl Sulfoxide. The Differing Reactivities of Aromatic and Aliphatic Amines in ?-Complex Formation and in SNAr Processes

Article abstract of DOI:10.1021/jo00369a022

?-Complex formation between 1,3,5-trinitrobenzene (TNB) and N-methylaniline (NMA) in the presence of 1,4-diazabicyclo<2.2.2>octane (DABCO) has been studied in dimethyl sulfoxide (Me2SO) solution.The reaction is catalyzed solely by DABCO in a linear fashion, indicating that deprotonation of the zwitterionic intermediate (PH) formed along the reaction coordinate is rate-limiting.Rate and equilibrium constants for the constituents steps have been evaluated.The rate constant for reversion of PH to reactants, k_-1, is substantially higher for aromatic amines than for aliphatic amines of comparable basicity.It is suggested that this is a result of stabilization of the transition state for the reactions of aromatic amines through charge delocalization.With NMA as the nucleophile, k_-1 is an order of magnitude higher than in the case of aniline, reflecting the effect of release of steric compression in PH on reversion to reactants in the former case.The effect of addition of tetraethylammonium chloride on rates and equilibria in the title reaction was also determined.The overall equilibrium constant for the reaction, K, increases ca. 13-fold as the Et4NCl concentration is increased from 0.03 to 0.30 M.This effect is shown to arise mainly from a decrease in the reverse second order rate constant, k_r².Evidence is presented for the effect of heteroconjugation of the type DABCOH⁺ + Cl^- <*> DABCOH⁺...Cl^- in the processes.The observation of rate-limiting proton transfer in ?-complex formation in this and other systems strengthens the idea that proton transfer can be rate-limiting in nucleophilic aromatic substitution (S_NAr) reactions.The present work sheds further light on the apparent dichotomy regarding the differing reactivities of aromatic vs. aliphatic amines as nucleophiles in ?-complex formation.Our results also provide a rationale for the observation that under certain conditions for S_NAr reaction involving aromatic and aliphatic amines of the same basicity, the reactions involving the aromatic amine is base catalyzed, while the aliphatic one is not