Acid catalysis through N-protonation in undistorted carboxamides: improvement of amide proton sponge acylating ability

Acid catalysis of weakly distorted or undistorted carboxamides in acyl-migration reactions proceeding through N-protonation is a process with low probability, in contrast to O-protonation. This circumstance makes the experimental study of the selective N-protonation path difficult or even impossible. However, such a problem can be solved by means of N-acyl-N,N,N-threemethylnaphtalenediamine (amide proton sponge, AmPS), which was synthesized back in 1999; after protonation, the relatively mobile NH proton of the dimethylamino group and amide nitrogen are in close proximity in this compound that significantly increases the probability of the N-protonation pathway. As was already shown, the noted structural peculiarity leads to remarkably high mobility of the acyl group in AmPS, but the details of this phenomenon were not explored previously. In this regard, a contribution of the N-protonation pathway to the reactivity of AmPS is of particular interest. In this study, for the first time, by means of quantum-chemical calculations, the energetic profiles for the reaction of AmPS with charged and uncharged nucleophiles have been established. A range of the similar AmPSs has been studied and the structural motifs promoting the decay of the amide bond and decreasing the Gibbs activation energy (ΔG‡) for nucleophilic attack have been determined. The most effective ways to achieve the desired effects are the additional withdrawal of the electron density from the amide nitrogen by means of substituents with negative mesomeric effect and a decrease in the steric constraints through aromatic core modification. The strengthening of the intramolecular hydrogen bond by the use of the buttressing effect shows a lesser extent of reduction in ΔG‡. The proposed structural modification of the classic AmPS can facilitate the design of a new acylation agent for practical use.