An Acceptor-Substituted <i>N</i>‑Heterocyclic <i>ortho</i>-Quinodimethane:\nPushing the Boundaries of Polarization\nin Donor–Acceptor-Substituted Polyenes

We report the synthesis, isolation, and characterization\nof a stable\ndonor–acceptor substituted <i>ortho</i>-quinodimethane\n(<i>o</i>QDM). This system with an imidazolidine scaffold\nas the donor can also be referred to as acceptor-substituted <i>ortho</i>-<i>N</i>-heterocyclic quinodimethane (<i>o</i>NHQ). We have examined the extent of polarization of the\nconjugated π-system using single-crystal X-ray diffraction,\nNMR and UV/vis spectroscopy, cyclic voltammetry, and DFT computations.\nThe bond lengths in the phenyl linker do not exhibit the alternation\ntypical of <i>o</i>QDMs. In addition, the <sup>13</sup>C\nand <sup>15</sup>N NMR shifts suggest significant charge separation,\nan interpretation supported by the diatropic ring current determined\nby NICS<sub>ZZ</sub>(<i>r</i>) computations, which is characteristic\nof aromatic compounds. DFT calculations show that polarization is\nan electronic effect that is amplified by steric influences. More\nstrikingly, the oxidation and reduction potentials of the push–pull\nsubstituted <i>o</i>QDM are virtually identical to those\nof authenticated anionic and cationic derivatives. The results therefore\nindicate that an aromatic zwitterionic structure represents the electronic\nstructure more accurately than a neutral quinoidal Lewis structure,\nwhich indicates that the acceptor-substituted <i>o</i>NHQ\nis a rare example of an organic zwitterion in which the centers of\ncharge are in conjugation. The ambiphilic reactivity of the acceptor-substituted <i>o</i>NHQ, which is evidenced by the dehydrogenation of ammonia\nborane and the addition of phenylacetylene via heterolytic C–H\nbond cleavage, further supports its notation as an organic zwitterion\nand is reminiscent of frustrated Lewis pairs (FLPs). Thus, the acceptor-substituted <i>o</i>NHQ can be considered to be an intramolecular carbogenic\nFLP in terms of its reactivity.