Anti‐Aromatic versus Induced Paratropicity: Synthesis and Interrogation of a Dihydro‐diazatrioxa[9]circulene with a Proton Placed Directly above the Central Ring

Abstract

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.

A Fully Conjugated Planar Heterocyclic [9]Circulene

“…No dehydrogenation of 13 , to give 9 , was observed under these conditions. It should be noted that 13 contains the same circulene motif as that being presented in our previously reported dihydro-diazatrioxa[9]­circulene derivative . Dehydrogenation of 13 , to give the target [9]­circulene ( 9 ), was achieved quantitatively by treatment with DDQ.…”

“…All individual aromatic rings constructing the circulenes have negative values, due to the diatropic ring current, and the central ring has positive NICS values due to the paratropic ring current. The presence of paratropic ring currents in the central nonaromatic rings of 9 and 10 can be explained by our previously reported concept of topologically induced paratropicity …”

“…The presence of paratropic ring currents in the central nonaromatic rings of 9 and 10 can be explained by our previously reported concept of topologically induced paratropicity. 30 Concerning the NICS(1) and NICS(0) (1.8, 3.6, respectively) of the central nonaromatic ring of 9, it should be noted that these are significantly lower than the corresponding values of the related diazadioxa [8]circulene, indicating a contribution from the antiaromatic cyclooctatetraene in the latter.…”

“…It should be noted that 13 contains the same circulene motif as that being presented in our previously reported dihydro-diazatrioxa [9]circulene derivative. 30 Dehydrogenation of 13, to give the target [9]circulene (9), was achieved quantitatively by treatment with DDQ. During the synthesis of 12 from 11, the doubly condensed product 10 was also observed in trace quantities (see the SI for details).…”

When are Antiaromatic Molecules Paramagnetic?

“…5 A large number of studies have shown that the degree of aromatic-city can be determined from magnetic shieldings, 6 ring-current strengths 7 and other molecular magnetic properties. [8][9][10][11] Electron delocalization and pathways of magnetically induced current densities provide information about molecular behaviour in the presence of an external magnetic field (B), which is important when designing novel magnetic materials. 12 The magnetic susceptibility (χ) also called magnetizability is a measure of interactions between matter and an external magnetic field (B).…”