Investigation of the Tautomerism of ¹⁵N‐Labeled Hydroporphyrins by Dynamic NMR Spectroscopy

The barrier height for tautomerism of the inner protons of porphyrin is significantly influenced by specific reduction of the pyrrole rings. In the tetraphenyl derivatives of chlorin and bacteriochlorin an aromatic ring current was established 1 H NMR spectroscopically; in contrast, in the tetraphenyl derivative of isobacteriochlorin it is virtually absent. The activation barrier in the hydrogenation of a pyrrole ring is ca. 20 kJ mol −1 higher. In the hydrogenation of two adjacent rings a system is formed in which two symmetric double minimum potentials of differing barrier height have been demonstrated for the first time for an intramolecular proton transfer.