To investigate the formation process of high carbon number (>C32) sedimentary porphyrins, heating experiments of several porphyrins were performed. Chromic acid oxidation of the heating products of protoporphyrin IX dimethyl ester afforded 2-methyl-3-n-propylmaleimide as the predominant product among the side-chain extension products formed. On the other hand, saturated substituents of etioporphyrin were also extended on heating to slowly form normal and branched homologs. These results may suggest that the transalkylation of porphyrin side chains proceeds mainly by a regioselective mechanism involving alkyl radical addition to a vinyl group of chlorophylls or their diagenetic products.
Etioporphyrin, one of the most common sedimentary porphyrins, was heated at 300 to 450 °C to investigate the diagenetic formation of benzoporphyrins. Chromic acid oxidation of the resulting products afforded maleimides, as well as phthalimides, indicating the formation of benzopyrrole units in the substrate. Isomer ratios of dimethylphthalimides formed during heating were in accordance with those obtained by chromic acid oxidation of the Cretaceous/Tertiary sediments at Rumoi, Hokkaido, which suggests that alkylporphyrins could be transformed into benzoporphyrins in sediments.
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