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

In this work, we show two synthetic routes to substitute the N_<i>exo</i> 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_<i>exo</i> 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_<i>exo</i> 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.