Illustrating atropisomerism in the porphyrin series using NMR spectroscopy

The existence of biphenyl-type atropisomerism in porphyrins has been known for more than twenty-five years. Over the last two decades, different examples of this type of stereoisomerism have been reported in the literature with regard to the use of synthetic metalloporphyrin complexes as models of heme-containing enzymes. In this article, we present a clear $^1$H NMR study to illustrate the concept of atropisomerism in the porphyrin series. The example is based on a porphyrin substituted in the meso positions by sulfonatomesityl groups such that the two methyls in both ortho positions restrict the rotation about the aryl-porphyrin C-C bond and the sulfonate group in the meta position clearly resolves the $^1$H resonances of the two ortho methyls. This analysis allows visualization and unambiguous check of the statistical distribution of the different atropisomers