Substituted troponimines: when aromatization of the conjugate acid leads to very strong neutral organic superbases

Abstract

In the present study, N-methyl-2,4,6-cycloheptatriene-1-imine (troponimine) with (Me)₂N- and MeO-substituents has been investigated theoretically to evaluate its suitability as a new neutral organic superbase. To introduce the aforementioned system, thermochemical parameters such as gas basicity and proton affinity have been calculated along with pK_BH⁺ values in acetonitrile. The results revealed that all of the studied troponimine derivatives were more basic than 1,8-bis(dimethylamino)-naphthalene whose gas phase basicity of 237 kcal mol⁻¹ is considered to be the borderline between ordinary bases and superbases. However, the most important finding was that some of the proposed troponimines were much stronger bases than the well-established cyclopropenimine superbases. The origin of troponimine basicity was elucidated through NICS calculations which revealed that the troponimines become aromatic upon protonation. AIM analyses revealed that a hydrogen bond could also contribute to the basicity if a suitable substituent is located adjacent to the imine group of the troponimine.