GEOCHEMICAL ORIGINS OF SEDIMENTARY BENZOPORPHYRINS AND TETRAHYDROBENZOPORPHYRINS

Complex mixtures of metalloporphyrins are associated with organic-rich sediments. Although some of these petroporphyrins have been correlated with known biological pigments, the origins of certain structures are obscure. Benzoporphyrins and tetrahydrobenzoporphyrins, two minor families of geological tetrapyrroles, have been isolated from numerous oil shales and petroleums but the origins of these molecular fossils are poorly understood. A number of possible pathways for the geochemical formation of benzo- and tetrahydrobenzoporphyrins are discussed. The data presently available favors a Diels-Alder cycloaddition mechanism between putative divinylchlorophyll and quinone precursors for the formation of these compounds. As an aid to these studies, total syntheses of benzo- and naphthoporphyrins have been carried out and the spectroscopic properties of these extended chromophores are discussed. Further studies are proposed to help distinguish between the various possible pathways for the geochemical genesis of tetrahydrobenzo- and benzoporphyrins.