Thermal Behavior of Free‐Base and Core‐Modified Bicyclo[2.2.2]octadiene‐Fused Porphyrins

Abstract

AbstractMultistep thermal fragmentation of quadruply bicyclo[2.2.2]octadiene‐fused porphyrins giving tetrabenzoporphyrins was examined in detail. After the first extrusion of an ethylene molecule from the porphyrin derivative, the opposite bicyclo[2.2.2]octadiene moiety preferentially underwent the second retro‐Diels–Alder reaction to give an opp‐dibenzoporphyrin derivative rather than an adj‐dibenzoporphyrin derivative. These two benzoporphyrin derivatives then decomposed to give a tribenzoporphyrin derivative in similar rates. The temperature regions of these fragmentations could not be distinguished by thermogravimetric analysis. In contrast, the third and the fourth fragmentations obviously occurred stepwise. There was a temperature region where the tribenzoporphyrin derivative preferentially existed. In the case of the 21,23‐dithiaporphyrin derivative, opp‐21,23‐dithiadibenzoporphyrin, possessing benzo moieties fused at the pyrrole parts of the core‐modified porphyrin chromophore was predominantly formed during the fragmentation. In the case of the 21‐thiaporphyrin derivative, an ethylene molecule was extruded selectively from the bicyclo[2.2.2]octadiene moiety adjacent to the thiophene part to give 21‐thiabenzo[q]porphyrin and then 21‐thiabenzo[g,q]porphyrin derivatives. In these cases, the last ethylene extrusion also occurred very slowly. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)