Regio- and Enantiospecific On-Surface Synthesis of Iso [4]triangulene via Ethyl Cyclization

Abstract The dehydrogenative cyclization of alkyl substituents on surface enables polycyclic aromatic hydrocarbon synthesis, while controlling the selectivity of this process is challenging. Here, we report a regio- and enantiospecific intramolecular cycloaromatization on Cu(111). Cyclization of syn-5,10,15-triethyltruxene quantitatively yields iso[4]triangulene (I4TA), a nonalternant isomer of [4]triangulene, with complete central chirality transfer to the surface-adsorbed prochiral I4TA. In contrast, anti-5,10,15-triethyltruxene yields a defective product on which one heptagon is missing. Combining scanning tunneling microscopy, noncontact atomic force microscopy, and density functional theory calculations, we elucidate the full reaction pathway involving successive dehydrogenation and cyclization, and the origin of selectivity. Furthermore, the central hexagon of I4TA features distinct π-bond character, which governs its ground-state electronic configuration and local aromaticity. This work provides access to the first member of iso[n]triangulene family and establishes a new paradigm for stereospecific cycloaromatization of alkyl groups, transferring the central chirality of tailored precursors into the planar chirality of surface-adsorbed products.