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Synthesis of a Highly Aromatic and Planar Annulene
preprintsubmitted on 21.04.2021, 06:43 and posted on 21.04.2021, 12:57 by Karnjit Parmar, Christa S. Blaquiere, Brianna Lukan, Sydnie Gengler, Michel Gravel
As the next neutral structure following Hückels rule, a planar and aromatic annulene is ideal to study the link between ring size and aromaticity. However, the puckered geometry of the parent annulene suggests that the aromatic stabilization energy is not sufficient to overcome the ring strain that exists when the system is forced into planarity. It has been shown computationally that this ring strain can be alleviated through the addition of two or more cyclopropane rings to the periphery, thereby creating theoretically aromatic structures. An alternative strategy to eliminating the issue of ring strain was demonstrated experimentally with the successful preparation of the highly aromatic 1,6-didehydroannulene. However, the system rapidly cyclizes at -40°C to a naphthalene diradical due to the close proximity of the in-plane p-orbitals present in the system. Here we show that cyclopropanating one side of the unstable annulene successfully prevents the destabilizing cross-ring interaction while maintaining a highly aromatic structure. Remarkably, the formed annulene is bench stable and can be stored for extended periods of time.