Quantification of the Lewis Basicities and Nucleophilicities of 1,3,5‐Tris(dialkylamino)benzenes

Abstract

AbstractEquilibrium constants for the formation of Wheland complexes from 1,3,5‐tris(dialkylamino)benzenes and benzhydrylium ions (Ar2CH+) have been determined photometrically in dichloromethane solution at 20 °C. The Lewis basicity of the ring carbons increases in the series trimorpholinobenzene<tripiperidinobenzene<tripyrrolidinobenzene. Wheland complexes, which are formed with equilibrium constants 102<K/M−1<106 in the reactions of triaminobenzenes with carbenium ions, show temperature‐dependent dynamic 1H NMR spectra, due to rapid reverse reactions and recombination at different positions of the triaminobenzenes. Since the rates of the formation of the Wheland complexes are too high to be measured directly, they were calculated as the product of photometrically determined equilibrium constants and the rate constants for the reverse reactions, which were derived from the dynamic 1H NMR spectra. The experimentally determined equilibrium and rate constants were in good agreement with the results of DFT calculations using the SMD solvent model. The calculations show that in all cases the Wheland complexes are thermodynamically more stable than the ammonium ions formed by attack of the benzhydrylium ions at the amino group of the title compounds, which explains the exclusive formation of Wheland complexes in thermodynamically controlled reactions. With nucleophilicity parameters in the range 10<N<15 the triaminobenzenes have comparable nucleophilic reactivities as enamines, silyl ketene acetals as well as stabilized phosphonium and sulfonium ylides.