Free radical sulfur ring-opening of thianaphthene in the presence of indene

Sulfur ring-opening of thiophenes present in heavy oils is a challenge, particularly for non-catalytic thermal processes. In this work, sulfur ring-opening of thianaphthene was studied in a thermal conversion process (400 °C, 2 MPa) in the absence of direct hydrogen supply, but in the presence of indan and indene, molecules that are capable of hydrogen transfer. Hydrogen transfer reactions resulted in the formation of 2,3-dihydrothianaphthene from thianaphthene and it was anticipated that ring-opening of the saturated heterocycle would readily take place. Yet, when thianaphthene reacted with indan, only trace evidence of ring-opening was found, but when thianaphthene reacted with indene, 2-methylbenzenethiol and 2-ethylbenzenethiol were found. Reaction pathways that involved homolytic bond dissociation of the saturated heterocycle in 2,3-dihydrothianaphthene were unimportant at 400 °C. Ring-opening appeared to be facilitated by the olefinic group, which interacted with the sulfur to weaken the carbon–sulfur bond and stabilize the reaction intermediate.