π-extended pyrenes: from an antiaromatic buckybowl to doubly curved nanocarbons with gulf architectures

Abstract

The synthesis of π-extended pyrenes keeps attracting considerable attention. In particular, frameworks containing nonbenzenoid rings might display intriguing properties. Here, we report a practical synthetic pathway to access a new buckybowl (1), which is composed of four five-membered rings externally fused to a pyrene core. The buckybowl 1 exhibits antiaromaticity involving 22 π-electrons, a rapid bowl-to-bowl interconversion, and a small band gap. Furthermore, this buckybowl could be subjected to Scholl cyclodehydrogenation to prepare the doubly curved nanocarbons (2_rac and 2_meso), which exist as two diastereomers, as demonstrated by X-ray crystal structure determination. Variable temperature ¹H NMR measurements reveal that 2_meso can isomerize into 2_rac under thermal conditions, with an activation free energy of 27.1 kcal mol⁻¹. Both the enantiomers of 2_rac can be separated by chiral HPLC and their chiroptical properties are thoroughly examined. In addition, the nanocarbon 2_meso with two gulf architectures facilitates host–guest chemistry with a variety of guests, including PDI, TDI, C₆₀ and C₇₀.