Is Schleyer’s Predicted All-Cis [10]Annulene an Unstable Framework?

In this work we describe the synthesis of a long sought after planar [10]annulene. Remarkably, the efforts also led us to discover a completely unexpected and highly aromatic cyclopropa mono-trans[10]annulene. The parent [10]annulene (cyclodecapentaene) is non-planar and thus non-aromatic because the aromatic stabilization energy is not sufficient to overcome the strain involved in expanding CCC bond angles from the standard 120° to the necessary 144°. Interestingly, it was theorized by Schleyer (10.1021/ja953126i) that planar structures could be attained by adding cyclopropane (or cyclobutane) rings to the periphery. We opted to target the dicyclopropa[10]annulene derivative that was proposed as it was a formal cyclopropanation of our previously found stable dehydrocyclopropa[10]annulene (10.1038/s44160-022-00135-z, 10.26434/chemrxiv.14458653.v1). It was also presumed that the symmetry present within the structure would make the Kekulé structures degenerate, thus improving aromaticity substantially. However, we were uncertain if the structure would be kinetically stable and wished to attempt a synthesis to investigate it further. In this paper we describe our evolving synthetic strategy that ultimately led us to the preparation of the dicyclopropa[10]annulene. Interestingly we noted that the framework itself was rather unstable and degraded quite rapidly. We also observed that the final preparation of the desired material led to a significant side product that was identified as a cyclopropa mono-trans[10]annulene. This side product was completely unexpected and found to be more kinetically persistent than the original target.