Two Different Clar’s Sextets: Clar’s Rule Does Not Necessarily Contradict the Global Pathway of Conjugated Macrocycles

Abstract

Two contradictory theories, Clar’s sextet and resonance theory, explain the stability of conjugated macrocycles using localized and delocalized models, respectively. To reconcile these theories and gain better insights, PAH-containing porphyrinoids were chosen as a representative class of conjugated macrocycles. Two types of Clar’s sextets were identified and proposed for the first time based on their interaction with global conjugated pathways: (i) shared sextets that integrate with and perturb the global resonance pathway, and (ii) independent sextets that are separate from the pathway and potentially stabilize or do not perturb it. To test our hypothesis, we synthesized precise regioisomers with and without an independent sextet to exclude variables such as steric hindrance, different PAH characteristics, and other elements. This task is challenging due to the extensive synthetic efforts needed for regioisomeric PAH-containing porphyrinoids. We focused on 2,3-vinylnaphthiporphyrin, which features shared sextets, because its o-regioisomers are readily accessible. Two distinct regioisomers (1,2-vinylnaphthiporphyrins) with an independent sextet were synthesized along with unexpected nonaromatic N-fused porphyrinoids. Our analysis shows that 1,2-vinylnaphthiporphyrins with an independent sextet are more aromatic than 2,3-vinylnaphthiporphyrin (nonaromatic). This sextet analysis was also applied to other reported and yet-to-be-synthesized PAH-containing porphyrinoids, and these results are consistent with our current study.