Anti‐Aromatic versus Induced Paratropicity: Synthesis and Interrogation of a Dihydro‐diazatrioxacirculene with a Proton Placed Directly above the Central Ring

• Published in: Angewandte Chemie Vol. 132; no. 13; pp. 5182 - 5188
• Main Authors: Pedersen, Stephan K, Eriksen, Kristina, Nataliya N Karaush‐Karmazin, Minaev, Boris, Ågren, Hans, Baryshnikov, Gleb V, Pittelkow, Michael
• Format: Journal Article
• Language: English
• Published: Weinheim Wiley Subscription Services, Inc 01.03.2020
• Subjects: Aromatic compounds; Chemistry; Columnar structure; Conjugation; Crystal structure; Energy levels; Interrogation; Magnetic resonance spectroscopy; Naphthalene; NMR; NMR spectroscopy; Nuclear magnetic resonance; Protons; Ring currents; Spectroscopy; Spectrum analysis
• ISSN: 0044-8249; 1521-3757
• DOI: 10.1002/ange.201913552

We present a high‐yielding intramolecular oxidative coupling within a diazadioxa[10]helicene to give a dihydro‐diazatrioxa[9]circulene. This is the first [n]circulene containing more than eight ortho‐annulated rings (n>8). The single‐crystal X‐ray structure reveals a tight columnar packing, with a proton from a pendant naphthalene moiety centred directly above the central nine‐membered ring. This distinct environment induces a significant magnetic deshielding effect on that particular proton as determined by 1H NMR spectroscopy. The origin of the deshielding effect was investigated computationally in terms of the NICS values. It is established that the deshielding effect originates from an induced paratropic ring current from the seven aromatic rings of the [9]circulene structure, and is not due to the nine‐membered ring being antiaromatic. UV/Vis spectroscopy reveals more efficient conjugation in the prepared diazatrioxa[9]circulene compared to the parent helical azaoxa[10]helicenes, and DFT calculations, including energy levels, confirm the experimental observations.