Lash has synthesized the simplified component of a porphyrin 1, which lacks two of the pyrrole rings.1 This compound should act as a modified annulene, and the NMR and x-ray structure support that notion. The NMR shows a multiplet at -2.52 ppm for the internal protons and the external protons show up at 9.88 and 9.96 ppm. The x-ray structure exhibits a nearly planar structure, with the C-C distances around the macrocycle varying from 1.379 to 1.418 Å. Interestingly, the UV-vis of 1shows a Soret-band at 401 nm, indicative of porphyrin-like behavior.
It is a simple thing to do some computations on a model of 1, and so I have computed (at B3LYP/6-311+G(d,p)) the structure and NMR of 2, shown in Figure 1. This compound is strictly planar. The C-C distances about the macrocycle vary from 1.386 to 1.415 Å, in excellent agreement with the experiment and indicating little bond alternation. The NICS values at the center of the macrocycle and 1 Å above this point are -12.6 and -11.9 ppm, supporting the aromatic annulene structure. Further, the chemical shifts of the interior and exterior protons are computed to be -7.6 (interior) and 11.4 ppm (exterior) – in fair agreement with experiment. Nonetheless, simple computations provide support for the notion that this compound, and related porphyrins have a dominant annulene character.
Figure 1. B3LYP/6-311+G(d,p) optimized structure of 2.
(1) Lash, T. D.; Jones, S. A.; Ferrence, G. M., "Synthesis and Characterization of Tetraphenyl-21,23-dideazaporphyrin: The Best Evidence Yet That Porphyrins Really Are the Annulenes of Nature," J. Am. Chem. Soc., 2010, 132, 12786-12787, DOI: 10.1021/ja105146a