Polycyclic aromatic hydrocarbons (PAHs) containing non-benzoid rings display desirable optical, electronic and magnetic properties for a wide variety of applications, e.g. organic electronics and spintronics. Great efforts have been made over the last decades to face the challenge, their synthesis still poses. Besides various in-solution strategies including transition-metal catalysis and intramolecular cyclodehydrogenations, thermally induced on-surface reactions emerged as a powerful tool to make unique and structurally diverse PAHs accessible.

Herein, we present the synthesis of a novel cyclooctatetraene (COT) derivative 2 as a possible precursor for a nanographene containing non-benzoid rings. To introduce the COT scaffold, a simple synthetic route via a McMurry coupling of diketone 1 was established. We investigated the on-surface reactivity of 2 on Au(111) at elevated temperatures. Instead of a planarization and cyclodehydrogenation towards 3 we discovered a selective ring contraction of the eight-membered ring towards derivative 4 bearing a six-membered core. Following these findings, we looked into the reaction behavior of 2 under pyrolysis conditions. Thereby, the ring contracted product 4 was also isolated in good yields. Both on-surface studies as well as pyrolysis investigations were successfully expanded to different substrates.

Scheme 1. Synthesis of cyclooctatetraene <b>2</b> and following ring contraction induced by pyrolysis or on an Au(111) surface.

Scheme 1. Synthesis of cyclooctatetraene 2 and following ring contraction induced by pyrolysis or on an Au(111) surface.

Jan Freudenberg

Appearances