Polycyclic annulenes

The homoaromatic interaction in other bridged annulenes has also been examined. The dications of several bridged annulenes were prepared and also studied theoretically and by NMR spectroscopy (Mullen et al., 1987 Wallraff et al., 1988). Once again homoaromatic interactions were deemed to be most important in determining the properties of these systems. Another cationic polycyclic potential homoaromatic system was investigated by Murata and Nakasuji (1980). They concluded, from NMR studies, that homoaromaticity was unimportant in the homophenalenyl cations [66], [67] and [68], (They reached the same conclusion for the corresponding anions.)... 

CONTENTS List of Contributors. Introduction to the Series An Editor s Forward, Albert Padwa. Preface, Randolph P. Thummel. Cyclooctatetraenes Conformational and ii-Elec-tronic Dynamics Within Polyolefinic [8] Annulene Frameworks, Leo A. Paquette. A Compilation and Analysis of Structural Data of Distorted Bridgehead Olefins and Amides, Timothy G. Lease and Kenneth J. Shea. Nonplanarity and Aromaticity in Polycyclic Benzenoid Hydrocarbons, William C. Herndon and Paul C. Nowak. The Dewar Furan Story, Ronald N. Warrener. Author Index. Subject Index. 

Most of the highly unsaturated monocyclic eight-membered heterocycles contain one or two nitrogen atoms and have been obtained by bond reorganization processes from strained bicyclic or polycyclic precursors. Although several of the less substituted compounds without stabilizing substituents are highly labile substances, 1,4-dihydro-1,4-diazocines qualify as dihetera[8]annulenes and display distinct aromatic properties. 

Many aromatic compounds have considerable resonance stabilization but do not possess a benzene nucleus, or in the case of a fused polycyclic system, the molecular skeleton contains at least one ring that is not a benzene ring. The cyclopentadienyl anion C5HJ, the cycloheptatrienyl cation C7H+, the aromatic annulenes (except for [6]annulene, which is benzene), azulene, biphenylene and acenaphthylene (see Fig. 14.2.2(b)) are common examples of non-benzenoid aromatic hydrocarbons. The cyclic oxocarbon dianions C Of (n = 3,4,5,6) constitute a class of non-benzenoid aromatic compounds stabilized by two delocalized n electrons. Further details are given in Section 20.4.4. 

Modulo 4 rule. Thus, the Hiickel (An + 2) rule predicts that (An) annulenes are thermodynamically much less stable than (An + 2) annulenes (e.g. compare benzene and cyclobutadiene). I. Gutman and N. Trinajstic advised to distinguish this rule from another regularity appearing also in the case of polycyclic conjugated systems, namely that the cycles containing (An -f 2) jx-electrons are stable while those with An ji-electrons are not (e.g. compare cyclo-pentadienyl anion and cation). 

Base-catalysed prototropic rearrangements of cyclic polyacetylenes have often been used to synthesize annulene and dehydroannulene systems. In such reactions, transannular carbon-carbon bond formation between proximate triple bonds occasionally takes place to give polycyclic compounds. Treatment with a base of cyclic polyacetylenes 57 and 59 gave benzenoid aromatic compounds 58 and 61, with the desired dehydro[16] and [I8]annulenes, respectively. ... 

The simplest polycycles stem from two triply-bridged points. These systems are exemplified by Trost et al. s 4,8-dihydrodibenzo[oi,g7z]pentalene (69) [87, 88], a precursor for a purturbed [12]annulene dianion, and Mislow et al. s double-bridged biphenyl derivatives (generally shown as 70) [89], where X is methylene, carbonyl, or various heteroatoms. Other longer bridged biphenyls include the triple-bridged cyclophanes 71 made by Hubert and Dale [90], their unsym-metrical relatives by Cram and Reeves [91], and the recent polyalkynyl cyclo-phane (72) made by Rubin et al. [92], a proposed fullerene precursor. 

Balaban, A.T. 1973. Chemical graphs. XVIII. Graphs of degrees four or less, isomers of annulenes, and nomenclature of bridged polycyclic structures. Rev. Roum. Chim. 18 635-653. 

We have so far applied the 4p -I- 2 rule only to monocyclic systems of various types this is quite proper since the rule has been devised formally only for monocycles ( 5.2). However, attempts have been made, in a heuristic and unsubstantiated way, to apply a modified form of the rule to polycyclic systems. This is done by considering only the peripheries of the molecules in question, any cross-links which convert the otherwise-monocyclic periphery into part of the polycyclic system being considered as small perturbations. For example, naphthalene, with its 10-re-electron system (10 = 4p + 2, with p = 2), may be considered as a perturbed [10]-annulene and is thus stable (Fig. 5-12a). 

Following the Longuet-Higgins—Salem approach, Buck studied the tendency of aromatic monocyclic compounds to bond alternation with increasing ring size. He used the Huckel model with variable P for the 7T electrons and a Morse potential for the o electrons. His distortion energies show that the o system resists distortion and the tt system favors distortions from D h to D i2)h symmetry in C H annulenes with 77 = 6, 8, 10, 12, 14, 16. The symmetry of the HOMO and LUMO is used to predict whether the C30H30 bond alternation due to the tt electrons should occur. It was subsequently shownthat this method can also be applied to the characterization of antiaromatic polycycles. 

Most of the compounds discussed in foregoing chapters have been monocyclic although some mention has been made of benzo-annelated derivatives and also, in the preceding chapter, of bridged annulenes and annulenoannulenes, all of which are bicyclic or polycyclic compounds. 

Two general methods have been used for the preparation of annulenes involving, respectively, ring-closure of diynes and photolytic ring--opening of polycyclic valence isomers of annulenes. 

Sulfonic acid Isomer distribution data for the SO3 sulfonation are available for alkylbenzenes and their halogeno derivatives phenol and anisole, and their methyl, halogeno, and hydroxy and methoxy derivatives and naphthalene, and Its methyl, and hydroxy and methoxy derivatives. SO3 sulfonation Isomer distribution data are also available for a number of polycyclic aromatic hydrocarbons and 1,6-methano[10]annulenes, including some alkyl derivatives. ... 

Pyrene (32), a polycyclic aromatic hydrocarbon with four rings, can be seen as comprising varied subunits an ethylene-bridged [14]annulene, four fused benzene rings, two fused naphthalenes, a biphenyl or a phenanthrene. It s enthalpy of formation is 225.7... 

The aromaticity of annulenes and heteroannulenes can be described with the Hiickel rule. Due to their closed-shell structures, annulenes with 4N + 2 electrons are not distorted Dnh symmetry) and show strong diamagnetic ring currents while singlet 47V annulenes are often distorted and have paratropic character. However, the Hiickel rule cannot be applied in polycyclic systems, where, for example. 

The 4/2+2 rule solved the mystery of the profound difference between benzene, [10]annulene, [14]-annulene, and [ISjannulene on one side and the 4/2 monocyclic systems, like elusive cyclobutadiene and puckered cyclooctatetraene, on the other side. Attempts were made to extend the 4/2+2 rule to polycyclic systems, for which it was not initially designed. Of numerous attempts in this direction, we will mention only that of Platt,who proposed that the 4/2+2 rule be applied to molecular periphery. It turns out that Platt s generalization of the Hiickel 4/2+2 rule is correct when one restricts attention to benzenoid hydrocarbons. For example, the perimeter rule correctly classifies pyrene (which has 16 Jt-elec-trons), perylene (which has 20 //-electrons), and coronene (which has 24 //-electrons) as aromatic as they have 14 or 18 //-electrons on the perimeter. But the perimeter rule does not give a correct answer for the non-benzenoid systems illustrated in Figure 10. The structure on the left, which has 14 //-electrons on the periphery, instead of being aromatic, as will be seen later, is in fact fully anti-aromatic . On the other hand, the structure on the right (corannulene), which has 15 //-electrons on the periphery, is not... 

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