Leveraging N-exo substituents to tune the donor/acceptor properties of mesoionic imines (MIIs)

Abstract

In this work, we show two synthetic routes to substitute the N_exo position of mesoionic imines (MIIs). By Buchwald–Hartwig amination, 5-amino-1,2,3-triazoles can be arylated at the said position, showing the versatility of amino-triazoles as building blocks for MIIs. The reaction of MIIs with electrophiles (MeI, fluoro-arenes) highlights the nucleophilic nature of MIIs as even at room temperature aromatic C–F bonds can be activated with MIIs. By combining experimental methods such as Tolman/Huynh-electronic-parameter and crystallographic interpretations with theoretical calculations, we establish that MIIs expand the nucleophilicity scale of N-donors. Contrary to the flanking substituents on the triazole scaffold, the N_exo substituent heavily influences the donating ability of MIIs: electron-withdrawing substituents will dramatically decrease the donor strength of the MII ligand. We have now established ways to functionalise not only the triazole backbone but also the N_exo position. More importantly, we show here how the substitution pattern influences the electronic structure of MIIs. Such electronic tunability should make MIIs suitable for use in various fields of chemistry.