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Reports: ND251785-ND2: Elucidation of Reactions Mediated by Sulfidic Carbonate and Clay Depositions: The Search for New Organic Reactions Mediated by Natural Materials

Salvatore D. Lepore, PhD, Florida Atlantic University

Introduction. One of the key goals of the present project was to determine if plant-derived porphyrin compounds transform to their related macrocyclic cousins found in petroleum by non-biological processes. We searched for these chemical reactions in depositions of sulfidic carbonates and clays since these natural materials have been previously thought to mediate a limited number of chemical transformations. As in the previous research period, we chose pyropheophorbide-a (pPBIDa) as the ideal porphyrin starting material to search for clay-mediated reactions. Our previous studies have suggested that pPBIDa is converted to cyclopheophorbide-a-enol (Cyclo) (Figure 1) in the sulfidic carbonate marls of Florida Bay.[1] We also expanded our search for reactivity to other porphyrin starting materials. Another goal was to explore other new chemical transformations made possible by carbonate and clays with the purpose of adapting these reactions for use in organic synthesis. As described in the following section, we have recently discovered a unique carbonyl reduction reaction that may offer important advantages in chemoselective small molecule synthesis.  

Figure 1. Search for a soil-mediated reaction to give Cyclo led to an unexpected selective hydrogenation.   Experimental Setup. In this grant period, more than 250 mixtures of porphyrins, carbonates, clays, and/or sulfides in various combinations were made and incubated in seawater. With few exceptions, all reactions were flushed for several minutes with argon, sealed and incubated in the dark at room temperature. A series of 8 combinations with pPBIDa (a free acid) was also incubated at 30ºC. Incubations times ranged from 2 days to 1 month. In the following paragraphs we provide a brief description of the materials used in this study.   Porphyrin substrates. In addition to the pPBIDa substrate mentioned in the Introduction, several others were also examined: the methyl ester of pyropheophorbide-a (pPBIDa-ME); pheophorbide-a (PBIDa) and its methyl ester (PBIDa-ME); and, mesoporphyrin-IX dimethyl ester.   Carbonate and clay soils. The following media were used in an attempt to bring about the desired pPBIDa-to-Cyclo conversion (see Figure 1) separately on in various combinations: natural aragonite (Bahamian); ACS reagent grade calcium carbonate; magnesium carbonate; sodium hydrogen sulfide; ammonium sulfide; and sodium bentonite clay. In addition, well-characterized clay samples from the American Colloid Company of Skokie Illinois were also used: sodium Volclay (both medium and high ion exchange capacity) and calcium Volclay.   Product analysis. The results of our reaction screening were primarily characterized by UV/Vis spectroscopy. Analysis by HPLC was made possible by our previous developments (coPI) using synthetically prepared Cyclo[2] and a highly optimized stationary phase.[3]  Results. The majority of our experiments led to mainly degradation products with little recovery of porphyrin starting materials. For example, in one series of 70 combinations, 31 of the product mixtures contained only degradation products. The minority of these reactions (15) gave good recovery porphyrin starting materials. We observed some reactivity in the presence of natural soils in 24 cases leading to mixtures of unknown porphyrin products in very low yields. No combination gave the desired target product (Cyclo).   New chemoselective transformations. In a series of experiments, pPBIDa, pPBIDa-ME, PBIDa , and PBIDa-ME were individually incubated with aragonite, sodium bentonite, and sodium hydrogen sulfide (NaHS). In each case, we observed small amounts of hydrogenation products in which the exocyclic vinyl group was converted to an ethyl group. In the case of pPBIDa-ME, this transformation has been reported to occur though not in the presence of soil and under rather different conditions (hydrogen sulfide, sodium dodecyl sulfate, room temperature for 4 days).[4]   An unprecedented reaction. In the course of our search for a soil-mediated mechanism leading to Cyclo, we have discovered an unexpected carbonyl reduction reaction. Specifically, the ketone carbonyl in exocyclic ring (ring V) of pPBIDa was converted to a secondary alcohol to give 131-OD-pPBIDa (Figure 1). This product was identified based on HPLC retention times and UV/Vis comparisons to known pigments. Our current yields for this unprecedented soil-mediated ketone reduction are rather low (<10%).       Some general conclusions. Our attempt to bring about the conversion of pPBIDa to Cyclo using natural sediments free of biological agents proved more difficult than first envisioned. Theoretically, a sulfide-mediated mechanism for this conversion (Dieckmann condensation) still appears plausible. However, in the case of natural sediments numerous other physicochemical parameters exist which may or may not be involved. For example, in the natural cases, the porphyrin precursor(s) may still me associated with the lipid membrane system of chloroplasts or chlorosomes. Active redox cycling within the sediment may affect ionizations and ion-coupling requisite to the cyclization occurring. Basically, the natural situation within sediments is extremely complex.   Future Prospects. We are presently repeating experiments to better understand our recently discovered carbonyl reduction reaction. Another goal will be to optimize the reaction for improved yield and scalability. With more of this unique product (131-OD-pPBIDa) in hand, it will be characterized using NMR to determine the relative stereochemistry. As described in our original proposal, attempts will be made to apply new reactions discovered using sulfidic clays to organic small molecules. Depending on the outcome, other small molecule substrates will be designed for application in this reaction to establish its generality and utility.   References


[1].   Louda, J. W., Loitz, J. W., Rudnick, D. T.; Baker, E. W. Early diagenetic alteration of chlorophyll-a and bacteriochlorophyll-a in a contemporaneous marl ecosystem. Org. Geochem. 2000, 31, 1561.

[2].   Khalesi (M.-R.)M.; Louda J. W. Hemisynthesis of 132,173-Cyclomesopheophorbide-a-enol. Tetrahedron Lett. 2011, 52, 1078.
[3].   Moretzaei-Rad, M.; Louda, J. W. Polystyrene-Divinylbenzene (PS-DVB), a mild stationary phase for the chromatographic purification of the unstable 132, 173-cyclopheophorbide-a-enol. J. Liquid Chromatogr. & Rel. Technol. 2007, 30, 1361.

[4].   Pickering, M.D.; Keely, B. J. Low temperature formation of mesopyropheophorbide a  from pyropheophorbide a under conditions simulating anoxic natural environments. Geochim. Cosmochim. Acta 2011, 75, 533.