A theoretical analysis of the fundamental stepwise six-electron oxidation of porphyrinogen to porphyrins: the energetics of porphodimethene and artificial porphyrin intermediates

Density functional calculations have been carried out on a series of model hypothetical intermediates in the six-electron oxidation of porphyrinogen (5,10,15,20,22,24-hexahydroporphyrin) to porphyrin. Two possible reaction pathways have been investigated: the conventional one, supposed to be followed both in the chemical synthesis and the biosynthesis of porphyrins, and the unconventional one, which has been discovered on studying the oxidation of a stable form of porphyrinogen, namely meso-octaalkylporphyrinogen. The energetics of both pathways have fully been investigated for the free porphyrinogen. The conventional route is strongly preferred with respect to the unconventional one. The metal-assisted six-electron oxidation of porphyrinogen to porphyrin was also investigated by density functional calculations on several nickel and cobalt model complexes. The metal does not seem to be able to force the system to follow a different route; the conventional route is even more preferred with respect to the unconventional one.