Generating Antiaromaticity: Thermally-selective Skeletal Rearrangements at Interfaces

Antiaromatic polycyclic conjugated hydrocarbons (PCHs) are attractive research targets in modern organic chemistry in view of their interesting structural, electronic and magnetic properties. Unlike aromatic compounds, the synthesis of antiaromatic PHs is challenging as a result of their high reactivity and lack of stability, stemming from the small energy gap between their highest occupied and lowest unoccupied molecular orbitals. In this work, we describe a strategy toward the introduction of antiaromatic units in PHs via thermally selective intra- and intermolecular ring-rearrangement reactions of dibromomethylene-functionalized molecular precursors upon sublimation on a hot Au(111) metal surface, not available in solution chemistry. The synthetic value of these reactions is proven by 1) the integration of pentalene segments into acene-based precursors which undergo intramolecular ring-rearrangement; 2) the formation of π-conjugated ladder polymers, linked through cyclobutadiene connections, prior to ring-rearrangement and [2+2] cycloaddition reactions of indenofluorene-based precursors. The elucidation of the reaction products of the title reactions are investigated by scanning tunneling and non-contact atomic force microscopy investigations, and the mechanistic insights are unveiled by state-of-the-art computational studies.