Photosensitized oxidation of naturally occurring isoprenoid allyl alcohols as a possible pathway for their transformation to thiophenes in sulfur rich depositional environments

This study presents the results of investigations into possible pathways for the transformation of allylic alcohols (phytol and farnesol) to the corresponding thiophenes in sulfur-rich depositional environments. Since the reactions between these alcohols and polysulfides were found to be slow previously, we have employed a photooxidation step for the first stage of activation. Allylic alcohols can undergo photochemical oxidation to form the α,β -unsaturated aldehydes and ketones in the oxic zone of a sediment or water column. These in turn are known to be highly reactive towards polysulfides. The presence of anthraquinone or other photosensitizers catalyses these reactions. Exposure of the allylic alcohols to sunlight in the presence of polysulfides did not produce any oxidized products. Hence, under these conditions only a very small amount of sulfur reacted with the organic substrates. On the other hand, when the photo-produced mixture was exposed to polysulfides under different conditions simulating highly reducing conditions due to intensive sulfate reducing bacterial activity, the sulfur containing products contained monomers and oligo-polymers. The polysulfide cross-linked oligo-polymers were investigated by chemical degradation using MeLi/MeI. Comparison of the thio-ethers obtained from chemical degradation of the oligo-polymers obtained from (i) synthesized farnesal or phytenal as starting molecules and (ii) phytol and farnesol photo-oxidized mixture, revealed similar distributions. Thermal treatment of the oligo-polymers released thiophenes and thiolanes,that are abundant in immature to-mature sediments enriched in organically bonded sulfur. The geochemical significance of the proposed schemes is discussed.