Non-alternant non-benzenoid kekulenes: the birth of a new kekulene family

Abstract

"Kekulene" is a doughnut-like shaped polycyclic aromatic hydrocarbon consisting of cyclically arrayed benzene rings. It has attracted a great deal of theoretical interest because it is regarded as an ideal model to study conjugation circuits of π electrons, i.e. whether they delocalize locally in benzene rings or globally throughout the molecule. Though kekulene was synthesized in 1978, it was the only known compound of this class of compounds for a long time. Recently, new kekulene-related molecules, septulene, which is a non-alternant benzenoid hydrocarbon, and a tetracyclopentatetraphenylene (TCPTP) derivative belonging to non-alternant non-benzenoid hydrocarbons, were synthesized. This article presents theoretical and experimental aspects of kekulene-related molecules focusing on the viewpoint of conjugation circuits by classifying them into three types: benzenoid kekulenes including kekulene itself and septulene, yet unknown anti-kekulene and non-alternant non-benzenoid kekulenes represented by TCPTP.

Graph Theory of Aromatic Stabilization

“…atoms are predominantly fullerenes. 146 I h (IPR-1)-C 60 (113) is rather comparable in %TRE to one of the lowest-energy non-IPR isomers, C 2v (1809)-C 60 (112). However, the former is by far the most abundantly isolated fullerene but the latter is never isolable although it is generated in the gas phase.…”

“…137,160162,165 For example, the plot of BRE for 5/5 π-bonds in C 2v (1809)-C 60 (112) against the number of π-electrons (N π ) is presented in Figure 43. This plot is more or less similar in shape to the TREvs-N π plot for the same molecule in Figure 42.…”

“…Therefore, we immediately noticed that π-bonds shared by two pentagons are extremely reactive with large negative BREs. Hereafter, m/n (112), and I h (IPR-1)-C 60 (113). Red and orange lines denote π-bonds with BREs smaller than ¹0.100 and 0.000 «¢«, respectively (see section 11.2).…”

“…It is impressive that two 5/5 bonds in 112 are markedly stabilized in the polyanions. BREs for these π-bonds change from ¹0.213 «¢« in (112) and I h (IPR-1)-C 60 (113). Neutral states are denoted by asterisks.…”

“…In order to examine such a unique behavior of fullerenes in more detail, the TRE-vs-N π plots for C 2v (1809)-C 60 (112) and I h (IPR-1)-C 60 (113) are shown in Figure 42. 165,166 These plots are not bilaterally symmetric because fullerenes are not alternant hydrocarbons.…”