Unsaturated Oligoquinanes and Related Systems

Abstract

Fully unsaturated oligoquinanes comprise a class of fused five-membered ring compounds which contain extended p-systems with interesting electronic properties. This class of rigid molecules contains planar, bowl-and even ball-shaped structures, and their strain energy increases for each additionally fused five-membered ring. The first three members of this family, fulvene, pentalene and acepentalene have been synthesized or at least generated, whereas all higher members were only approached or studied by computational methods. In this review the class of fully unsaturated oligoquinanes -ranging from fulvene to C 20 -fullerene -is presented with respect to their syntheses and their properties. Also, related molecules with similar structural features will be discussed, which have not been highlighted in previous volumes of this series [1,2].

Oligounsaturated Five‐Membered Carbocycles – Aromatic and Antiaromatic Compounds in the Same Family

“…The failure to detect monomeric pentalene (2) is in accord with the observation of de Mayo et al that 1methylpentalene (37) dimerizes above À140 C [29]. The formation of phenylacetylene 44 was unexpected, and it is as yet unclear as to whether it arises by migration of two hydrogens in the aryne 41 or the intermediate carbene 42, or whether it is a secondary product formed from pentalene (2).…”

“…In addition to their high strain, these polyenes possess interesting electronic properties [25]. Although several groups have been and still are engaged in efforts to synthesize some of these fully unsaturated oligoquinanes [2], only three members of the series, 2 [26,27], 3 [28], and 4 [4], have as yet actually been prepared or at least spectroscopically characterized as short-lived transients. Other members of the series have been approached synthetically, but the increasing ring strain in the series from the bicyclic pentalene 2 to C 20 -fullerene 4 makes the higher members less and less likely to be even observable [2].…”

“…This concept has actually been successfully applied in the synthesis of the antiaromatic pentalene 2 [27], but it is unlikely to be practicable for higher members of this family due to the drastic increase in ring strain. While fulvene (27) and pentalene 2 possess almost planar geometries [2], acepentalene (3) and higher fully unsaturated oligo-quinanes 4, 34-36 have a considerable inherent curvature, which increases with an increasing number of annelated five-membered rings. This is confirmed by high level ab initio calculations (see below).…”

“…In the last couple of decades, knowledge about fully unsaturated oligoquinanes, the class of fused-ring compounds consisting of five-membered rings only, has experienced an exceptional growth ( Figure 1) [2,3]. Only recently has the existence of C 20 -fullerene 4, which is the smallest conceivable fullerene and can be considered as the Mount Everest of oligoquinanes, been proved [4].…”

Pentaleno[1,2-a:4,5′]diacenaphthylenes: Uniquely Stabilized Pentalene Derivatives

“…Inquiries into the linear oligoquinane analogs have arisen owing to the structural similarity to the more well-known acenes, which are composed of multiple linearly fused six-membered rings. The limiting factor for studying oligoquinanes so far is their inherent instability . The smallest oligoquinane is pentalene 1 (Figure ), a structural unit that is unstable except for sterically protected derivatives, when used as a ligand for metal complexes, or when annulated to other aromatic rings. − While the native pentalene is known to readily dimerize above −140 °C, , a growing list of aryl-fused pentalenes − (e.g., 2 ) are known to be quite stable owing to the delocalization of the pentalene double bonds into the adjoined aromatic subunits.…”