Antiaromatic 1,5‐Diaza‐s‐indacenes

Abstract

Abstracts‐Indacene is a classical non‐alternant hydrocarbon that contains 12 π‐electrons in a cyclic π‐conjugation system. Herein, we report its nitrogen‐doped analogue, 1,5‐diaza‐s‐indacene. 1,5‐Diaza‐s‐indacenes were readily prepared from commercially available 2,5‐dichlorobenzene‐1,4‐diamine through a two‐step transformation consisting of a palladium‐catalyzed Larock cyclization with diaryl acetylenes followed by hydrogen abstraction. The thus obtained 1,5‐diaza‐s‐indacenes exhibited distinct antiaromaticity, as manifested in clear bond‐length alternation, a forbidden HOMO–LUMO transition, and a paratropic ring current. As compared to the parent s‐indacene, the 1,5‐diaza‐s‐indacenes showed higher electron‐accepting ability owing to the presence of imine‐type nitrogen atoms. The 1,5‐diaza‐s‐indacene core is effectively conjugated with the peripheral aryl groups, which enables fine‐tuning of the absorption spectra and redox properties. The two possible localized forms of 1,5‐diaza‐s‐indacene were compared in terms of their energetic aspects.