Dearomatic Sulfur‐Shifted Ene Reaction of 3‐Thiiranylbenzo[b]thiophenes and Ketenes: Further Studies and Experimental Evidence

To better understand the dearomatic sulfur‐shifted ene reaction mechanism and to provide direct experimental evidence for the new reaction, 3‐thiiranylbenzo[b]thiophenes are rationally designed and used to investigate the stereostructure of the ene products and mechanistic insights. These compounds allow to control the reaction to stop at their ene products, preventing further aromatization due to the presence of only weakly acidic allylic C‐H bonds in their ene products. Single‐crystal struc‐ture analysis, DFT calculations, structural analysis of the ene products, and control experiments indicate that the dearomatic sulfur‐shifted ene reactions of 3‐thiiranylbenzo[b]thiophenes and ketenes are kinetically controlled, resulting in the for‐mation of more stable trans ene products via endo‐attack chair‐like six‐membered cyclic transition states. The structural analysis of the ene products suggests that the occurrence of aromatization depends on the acidity of the generated allylic C‐H moiety in the ene products. This investigation provides further mechanistic insights and direct experimental evidence for the dearomatic sulfur‐shifted ene reaction of arylthiiranes and alkoxy/aryloxyketenes.