13C NMR studies of reduced porphyrin compounds. Aromatic delocalization pathways

Abstract 13C NMR spectra of ms‐tetraphenylchlorins, new aminoalkyl‐ and hydroxy‐pyrroline substituted ms‐tetra‐phenylchlorins, and ms‐tetraphenylisobacteriochlorins are presented and discussed. Significant changes in the chemical shifts of the α‐pyrrole, α‐pyrroline and meso skeletal carbons are found in chlorins and isobacteriochlorins in comparison to porphyrins. In contrast, the chemical shifts of the β‐pyrrole carbons are almost unaffected by the structural modifications in chlorin and isobacteriochlorin. The chemical shifts of the α‐pyrrole carbons in the various chlorins and isobacteriochlorins are strongly affected by the substitutents of the pyrroline ring, or by the introduction of an additional pyrroline ring in isobacteriochlorins. The results show that most of the electron density is concentrated in the unreduced part of the molecule, i.e. in the pyrrole rings and, especially, on the α‐pyrrole carbons and is transferred or removed through the aromatic pathway by substituents on the pyrroline ring. These observations are supporting evidence that the α‐pyrrole, α‐pyrroline and meso‐carbons are in the aromatic pathway and favour the proposal of a 16 atom dianion as the preferred delocalization pathway in chlorins and isobacteriochlorins.