Stable Antiaromatic [16]Triphyrin(2.1.1) with Core Modification: Synthesis Using a 16π Electrocyclic Reaction

Antiaromatic porphyrinoids have attracted significant attention owing to their unique electronic properties and potential applications. However, synthesis of antiaromatic contracted porphyrinoids is challenging owing to the inherent instability associated with smaller ring sizes. In this study, we report the synthesis and characterization of the first stable trioxa[16]triphyrin(2.1.1), a novel 16π antiaromatic contracted porphyrinoid. We utilized a core modification approach to stabilize the [16]triphyrin(2.1.1). X-ray crystallographic analysis revealed a nearly planar structure. Electrochemical studies demonstrated reversible oxidation behavior and a small HOMO-LUMO gap, which was consistent with its antiaromatic nature. Chemical oxidation yielded an aromatic [14]triphyrin(2.1.1) dication, highlighting the antiaromaticity-aromaticity switching capability of this system. This synthesis involved the discovery of a key intermediate, dihydrotrioxatriphyrin(2.1.1), which underwent oxidative dehydrogenation to yield the target compound. Theoretical calculations suggested that dihydrotrioxatriphyrin(2.1.1) formed via a rare 16π electrocyclic reaction. The successful synthesis and characterization of this stable trioxa[16]triphyrin(2.1.1) underscores the potential of the core modification strategies for the rational design of novel antiaromatic systems with tunable properties. Moreover, the discovery of the rare 16π electrocyclic reaction advances the understanding of high-order pericyclic processes and may inspire new synthetic strategies for complex macrocyclic compounds.