Highly Strained, Fully π-Conjugated Porphyrin Cyclophanes: Template-free Synthesis and Global Aromaticity

Abstract

Porphyrin-based nanohoops, nanorings, and cages with fully π-conjugated structures are highly sought after, but their synthesis remains challenging. Herein, we report the template-free synthesis of a highly strained, bithiophene-bridged porphyrin cyclophane (1) from a porphyrin quinone via a stereoselective nucleophilic addition followed by intermolecular Yamamoto coupling strategy. X-ray crystallographic analyses of 1 and its dication 1²⁺ reveal significantly distorted cyclophane-like geometries, with calculated strain energies of 51.2 and 80.7 kcal/mol, respectively. While the neutral compound 1 exhibits localized aromaticity, the dication 1²⁺ is globally aromatic, with the porphyrin unit displaying weak antiaromaticity. Additionally, the dication 1²⁺ undergoes nucleophilic addition with chloride, relieving strain. This work presents a novel synthetic strategy for highly strained, fully π-conjugated systems with intriguing electronic properties and chemical reactivity.