Local aromaticity of the five-membered rings in acenaphthylene derivatives

Abstract

In this paper, a detailed study of the local aromaticity in a series of cyclopenta-fused linear polyacenes (acenaphthylene derivatives) was performed using several different criteria of aromaticity. Namely, the energy effect (ef), bond resonance energy (BRE), harmonic oscillator model of aromaticity (HOMA) index, multi centre delocalization indices (MCI), electron density at ring critical points ρ(r(C)), nucleus independent chemical shifts (NICS) and ring current maps were employed. All these methods agree that the extent of aromaticity of the five-membered ring in the acenaphthylene molecule is quite small. On the other hand, it was shown that the aromatic character of the five-membered rings depends on the size of the polyacenic part and on the position of the five-membered ring along the polyacenic part of the given acenaphthylene derivatives. According to energetic, electron delocalization and geometrical indices, the aromatic character of the pentagons in some higher acenaphthylene congeners is of the same order of magnitude as the aromatic character of some hexagons in the respective molecules. Ring current maps revealed the existence of a weak paratropic (antiaromatic) circulation in the five-membered rings which, in the case of higher members of the homolog series, becomes weaker. In contrast, the NICS indices showed a significant antiaromatic character of the five-membered rings in higher members of the acenaphthylene series.

Evaluation of Aromaticity for Open‐Shell Singlet Dicyclopenta‐Fused Acenes and Polyacenes Based on a Magnetically Induced Current

“…As mentioned previously,N ICS values are often adopted as criterion of aromaticity,b ut since they are local properties, they are not fully reliable. [4,45] It is therefore beneficial to compare NICS and bond currents,a sw ell as the differenceb etween the Ra nd UN ICS values because foro pen-shell singlet systems they have so far been mostly evaluated using restricted methods, which fail to describe their electronic structures even qualitatively and are controversial in the community of aromaticity. [26] Figure 8s hows the NICS(1) and Às zz (1) components for each ring of DPA (7) and PA (7)b yu sing the Ra nd Us olutions.…”

A Phosphorus Analogue of Acenaphthylene

“…Finally, the most striking result of this work is the stability of a phosphaalkene like 5 . It is strained, not protected by steric hindrance, and the delocalization within the five‐membered ring is weak if we transpose the results of the theoretical study of its all‐carbon analogue . It appears that the methoxy substitution is sufficient to provide stability.…”

“…It is strained, not protected by steric hindrance, and the delocalization within the five-membered ring is weak if we transpose the results of the theoretical study of its all-carbon analogue. [2] It appears that the methoxy substitution is sufficient to provide stability. The activation of the P= C double bond by the complexation of the low-lying phosphorus lone pair is also quite similar to what was observed in studies of 2-phosphaphenol.…”

“…Since its discovery in 1912, acenaphthylene has been the subject of numerous studies concerning its electronic structure and chemical reactivity. A recent theoretical study concluded that the delocalization within the five‐membered ring is weak, which explains why the reactivity of the corresponding C=C double bond remains relatively high. Among the recent reactivity studies, we note an extensive investigation of its photochemical dimerization .…”

Metal‐Catalyzed C−H Bond Activation of 5‐Membered Carbocyclic Rings: A Powerful Access to Azulene, Acenaphthylene and Fulvene Derivatives

“…Acenaphthylene is a commercially available compound (Figure ). The extent of aromaticity of the five‐membered ring in the acenaphthylene molecule is very small, and both C1‐ and C2‐positions of acenaphthylene can be functionalized via Pd‐ or Rh‐catalyzed coupling reactions. However, so far, only a few examples of arylations and two examples of Pd‐catalyzed alkylation and alkynylation of acenaphthylene have been reported.…”