Synthesis of the porphyrins from petroleum

Abstract

The presence of complex mixtures of metalloporphyrins in organic sediments has long been recognised. Recently, many new and structurally diverse petroporphyrin structures have been determined, many of which bear exocyclic rings. There is a need for synthetic samples to provide unambiguous confirmation of these petroporphyrin structures. In addition, synthetic samples would be of some value as standards for the analysis of geoporphyrins. We have developed a new approach to the synthesis of geochemically significant porphyrins bearing exocyclic rings. Tetrahydroindoles ( b-cyclohexenopyrroles), b-cycloheptenopyrroles and b-cyclooctenopyrrole have been prepared in moderate to good yield from the corresponding cyclic ketones. Reaction with lead tetraacetate affords acetoxy derivatives and these compounds have been further elaborated to give pyrrolyl- b-cycloalkenopyrroles (dipyrroles). Porphyrins with six-, seven- or eight-membered exocyclic rings have been synthesised by application of the MacDonald condensation or the tripyrrene- a, c-biladiene route. Porphyrins with six-membered exocyclic rings have been prepared in good overall yields. Eight-membered carbocyclic rings moderately inhibit porphyrin formation and seven-membered rings severely inhibit porphyrin cyclisations. Nonetheless, syntheses of the meso, β-butanoporphyrins from petroleum have been achieved. The application of b-cyclopentenopyrroles to the synthesis of porphyrins with five-membered exocyclic rings is presently under review and our initial results are reported.