Carbon-13 nuclear magnetic resonance spectra of porphin and some related compounds

Abstract

Carbon-13 NMR studies of the pyrrole-derived pigments continue to be of biological and theoretical interest. For example, the nature of the rearrangement process central to the biogenesis of the type-III porphyrinogen skeleton has been delineated using this technique, the critically important meso carbon assignments in protoporphyrin-IX being established using carbon-13 labelled compounds from an impressive synthetic programme. On the theoretical side, carbon-13 NMR arguments for the 16-membered π-delocalisation pathway, 4 against which we have argued, continue to be advanced. In this letter we report the carbon-13 NMR spectra of porphin (l) (the basic porphyrin skeleton, free from extraneous substituent and some derivatives (Figure); the spectrum of porphin in particular is of fundamental importance in our longer term aim to obtain a unique and unambiguous method assignment of all carbon atoms in porphyrins, and as a basis for any future theoretical developments. Moreover, these shifts provide yet more evidence against the 16-membered pathway for π-electron delocalisation in the porphyrin nucleus.