Synthesis and Aggregation Behavior of Chiral Naphthoquinoline Petroporphyrin Asphaltene Model Compounds

• Published in: Chemistry : a European journal 2016-03, Vol.22 (10), p.3378-3386
• Main Authors: Schulze, Matthias, Scherer, Alexander, Hampel, Frank, Stryker, Jeffrey M., Tykwinski, Rik R.
• Format: Article
• Language: English
• Subjects: Absorption; Absorption spectra; Affinity; Agglomeration; Aggregation behavior; Asphaltenes; Asphalts; Binding; Biomarkers; Chemical behavior; Chemical compounds; Chemical synthesis; Chemistry; Circular dichroism; Conformation; cracking; Crude oil; Dichroism; Heavy petroleum; Metals; multicomponent reaction; Nickel; Petroleum; porphyrinoids; Porphyrins; Pyridines; steroids; supramolecular chemistry
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201504683

The synthesis of structurally relevant compounds that model the chemical behavior and supramolecular aggregation of the asphaltenes, the most polar and metal‐rich fraction of heavy petroleum, has been extended to include fusions of important petroleum biomarkers. The synthetic protocol features a multicomponent reaction to form a dyad composed of a fused steroidal naphthoquinoline, followed by a pyrrole cyclocondensation reaction to incorporate the dyad into a chiral triad containing a NiII‐porphyrin substituent. This synthetic protocol has been used to prepare large molecules that represent both “continental” and “archipelago” models of asphaltene composition. The steroid–naphthoquinoline–porphyrin triads have been studied by UV/Vis and circular dichroism (CD) spectroscopies, and the results suggest that the naphthoquinoline core, a tetrahydro[4]helicene, adopts a helical conformation, producing a CD signal electronically related to the characteristic Soret absorption band of the porphyrin subunit. Finally, supramolecular aspects of asphaltene aggregation have been examined on a molecular level through analysis of axial coordination of pyridine to the Ni‐porphyrin. The relative affinity of pyridine for binding to the Ni center of the porphyrin is evaluated by comparing binding propensities in a series of sterically differentiated substituted porphyrins. Combining petroleum biomarkers: A naphthoquinoline moiety (formally a tetrahydro[4]helicene) fused to a steroid is tethered to NiII‐porphyrins to form asphaltene model compounds (see figure). CD spectroscopy documents chirality in the porphyrin chromophore induced by the steroid group. Studies of axial coordination with pyridine show that changes in peripheral substitution result in significant changes in axial coordination, which is suggested to occur in natural asphaltenes.