Computational design of Brønsted neutral organic superbases—[3]iminoradialenes and quinonimines are important synthetic targets

Abstract

The gas-phase proton affinities and basicities of a large number of extended π-systems possessing imino nitrogens as the most basic sites, which are parts of the [3]iminoradialene or quinonimine “spearheads”, are examined by the DFT B3LYP/6-311+G(2df,p)//B3LYP/6-31G(d) method. Some of the systems are neutral organic superbases exhibiting the gas phase proton affinities over 300 kcal mol⁻¹ and basicity values in the range of 245.3–294.7 kcal mol⁻¹. The backbone of the polycyclic π-electron networks contains either [3]radialene, methylenecyclopropene or quinoid substructures. Construction of the “tails” of planar organic superbases was at the focus of the research efforts. It is shown that the P(NR₂)₃ and C(NR₂)₂ fragments as well as the 1,3-diamino-2-methylenecyclopentene ring enable very efficient cationic resonance across the extended linear π-system, thus considerably contributing to the amplified basicity. Conclusive evidence is provided that protonation triggers aromatization of the quinoid six-membered ring (as well as that of the 1,3-diamino-2-methylenecyclopentene) particularly if they are linearly aligned. Triadic analysis has shown: (1) that the extremely large basicities are obtained, if a synergistic effect is obtained between the three contributions corresponding to the initial state, intermediate stage and final state triadic terms, and (2) that the methyl groups enhance basicity predominantly via destabilization of the principal molecular orbitals of the initial bases. It is argued that powerful organic superbases should lead to a spontaneous proton transfer, when interacting with (organic and inorganic) superacids. Formation of their ion pair might provide new interesting molecular materials.