In the pursuit of further elucidation of just what the concepts “aromatic” and “antiaromatic” mean, Schleyer and Bunz reported the preparation and characterization of a novel antiaromatic compound that is isolable.1
Bunz synthesized the redox pair of compounds 1 and 2 that differ in the electron count in the pi-system. The former (1) has 14 π electrons and should be aromatic, while the latter (5) has 16 π electrons and should be antiaromatic. The NMR spectrum of both compounds was measured and compared to the computed signals of the parent compounds 3 and 4. The signals match very nicely. The structures of 1 and 2 were further confirmed by x-ray crystallography. 1 and 2 can be interconverted by redox reactions and 2 is stable in air, only slowly oxidizing to 1.
The NICS(0)πizz values computed for 3 and 4 are shown in Figure 1. (See ref 2 for a discussion on this NICS method and also Chapter 2 of my book.) These values are quite negative for each ring of 3, consistent with its expected aromatic character. On the other hand, the NICS value for each ring of 4 is more positive than the corresponding ring of 3, with the value in the center of the pyrazine ring being positive. These NICS values indicate that 4 is certainly less aromatic than 3, and perhaps even expresses antiaromatic character.
Figure 1. NICS(0)πzz values for 3 and 4 computed at PW91/6-311G**.
Interestingly, hydrogenation of 3 to give 4 is -14.0, indicating that while 3 appears to be a normal aromatic compound, 4, if it is antiaromatic, exhibits some energetic stabilization. They identify this stabilization as a result of the interaction between the dihydropyrazine ring and the thidiazole ring, evidenced in the exothermicity of the isodemic reaction:
So while 4 may be antiaromatic, it appears to be energetically reasonably stable. It is important to keep in mind though that 4 is not the most stable tricycle isomer; in fact, 5 is 7 kcal mol-1 lower in energy than 4.
Schleyer and Bunz conclude that antiaromaticity may “not result in a prohibitive energetic penalty.”
(1) Miao, S.; Schleyer, P. v. R.; Wu, J. I.; Hardcastle, K. I.; Bunz, U. H. F., "A Thiadiazole-Fused N,N-Dihydroquinoxaline: Antiaromatic but Isolable," Org. Lett. 2007, 9, 1073-1076, DOI: 10.1021/ol070013i
(2) Fallah-Bagher-Shaidaei, H.; Wannere, C. S.; Corminboeuf, C.; Puchta, R.; Schleyer, P. v. R., "Which NICS Aromaticity Index for Planar π Rings Is Best?," Org. Lett., 2006, 8, 863-866, DOI: 10.1021/ol0529546.