Classical Annulenes

Another question arising from classical annulene chemistry is whether the porphyrin perimeter can be extended. The construction rules for appropriate systems are simple ... 

In this section we describe the reduction of some classical annulenes , [8]-annulene 3, [12]annulene 4, [16]annulene 5 and [18]annulene 6. In all of these systems the reduction is accompanied by changes in the molecular frame. We shall concentrate on the NMR spectra of the diamagnetic anions and the EPR spectra of the [4n- -l]-radical anions, both of which serve as probes for the delocalization and planarity of these r-systems. The Hiickel theory [58], which only considers the 71-energy, neglecting the c-frame, can only describe planar, strain-free r-perimeters therefore, it is important to analyze the structure of the annulenes and their anions. 

Consequently, the reduction of [4n]annu-lenes should be observed at relatively positive potentials with small AE separations for the dianion formation, while the reduction of the [4n + 2]annulenes should occur at more negative potentials with large AE separations for the dianion formation. This is exactly what is observed in (see Table 1). Although benzene [37, 38], the classical Hiickel aromatic with 4 1 + 2 = 6 jr-electrons, is reduced at —3.42 V (vs. Ag/AgCl), the reduction of the [Sjannulene COT occurs at —1.81V. Similarly, the [16]annulene (3) [67, 68] is more easily reduced than the corresponding [ISjannulene (4) [69], although the reduction of the larger jr-systems should be more favorable for electrostatic reasons. 

Semiempirical methods which involve an explicit quantum mechanical treatment of the -electrons by PPP-type theories, coupled with a classical a-com-pression energy, have been employed by Paldus et al. to investigate the general question of Peierls distortion in polyenes and in large [n]-annulenes.22-25 The results show that the r-energy tends to be... 

The three classical Kekule structures (already alluded to in section III.E) of naphthalene are shown in Scheme 36a. Two of them are designated as Ki and K2 and represent the annulenic resonance along the perimeter of the naphthalene, while the third one, Kc, has a double bond in the center and transforms as the totally symmetric irreducible representation, Ag of the Dzh group. The Ki and K2 structures are mutually interchangeable by the i, C2, and ov symmetry operations of the point group, much as in the case of benzene. An in-phase combination transforms, therefore, as Ag, whereas an out-of-phase one transforms as B2u. These symmetry adapted wave func-... 

Figure 7.7a shows the four classical Kekule structures of anthracene (16,19,24). Two of the structures involve resonance in the central benzenic ring and are therefore labeled as KiB and K2B. The other two involve annulenic resonance along the molecule perimeter, and are labeled accordingly as K1A and K2a- The structures of the types A and B form two symmetry subsets, and within each subset, the two structures are mutually transformable by the D2h symmetry operations (i, C2, and ov). Therefore, as shown in Fig. 7.7b, within each subset there will be a positive combination that transforms as Ag and a negative one that transforms as B2u. 

Tetraacetylenes such as 115 and 116 contain the 1,5-hexadiyne group as a bridging element. Since the base-catalyzed isomerization of this unit to hexa-l,3-dien-5-yne (6) constitutes the basic reaction of Sondheimer s annulene chemistry [75], it appeared attractive to attempt to apply this classic reaction of planar aromatic chemistry to a layered precursor and create three-dimensional relatives of Sondheimer s dehydroannulenes. Indeed, both 115 and 116 could be isomerized to their fully conjugated isomers 129 and 130, respectively, by treatment with potassium tert-butoxide in tert-butanol, the original Sondheimer conditions (Scheme 28). From the X-ray structure obtained for 130, it was concluded that both hydro-... 

Benzene is [6]annulene, cyclic, with a continuous ring of overlapping p orbitals. There are six pi electrons in benzene (three double bonds in the classical structure), so it is a (41V+2) system, with N = 1. Hiickel s rule predicts benzene to be aromatic. 

Cyclooctatetraene is [8]annulene, with eight pi electrons (four double bonds) in the classical structure. It is a (41V) system, with N = 2. If Htickel s rule were applied to cyclooctatetraene, it would predict antiaromaticity. However, cyclooctatetraene is a stable hydrocarbon with a boiling point of 153 °C. It does not show the high reactivity associated with antiaromaticity, yet it is not aromatic either. Its reactions are typical of alkenes. 

Oxidation of a,(J-unsaturated aldehydes (2, 261). Woo and Sondheimer6 prepared methyl [18]annulenecarboxylate (2) by the method of Corey et al. (2, 263, ref. 42) by treating [18]annulenecarboxaldehyde (1) in THF and methanol with hydrogen cyanide and manganese dioxide (44% yield). In this case the classical method of oxidation by Jones reagent failed owing to complete destruction of the annulene system. 

The annulenes are that series of monocyclic polyolefins (C H ) containing a complete system of contiguous double bonds. While benzene (the best known member of this class of compounds) has been in evidence for some time it is only of late that interest in the higher members has become apparent. This interest has its origins in the LCAO-MO theory of re-elec-tron systems as formulated by E. Hiickel (in particular the "Hiickel rule relating aromatic stability to structure). Although the non-classical chemistry of the benzenoid hydrocarbons had previously been the subject of some conjecture, Httckel s theoretical studies provided the first satisfactory explanation of the peculiar stability of this class of compounds and, incidently, the elusiveness of cyclobutadiene. 

Like many other quasi-classical methods applied within the domain of quantum mechanics, the free electron theory explained the gross features of the phenomenon and provided an attractively simple physical picture. The method did not of course, suggest any parallels with the quantum mechanically based HMO theory (which predicted an alternation in 7c-electron properties among the annulenes). 

A potentially anti-homoaromatic species (79, see Table 16) has been generated by protonation of l,6-methano-[10]annulene at the 2-position 224>. As expected, 1,3 overlap is not significant and the compound can be formulated as a classical carbonium ion. In fact the abnormally large H(2a) to H(2a) coupling constant of —24.9 Hz indicates that the 1,3 distance has been maximized in order to avoid the expected destabilization, and thus there is no evidence for a paramagnetic ring current in this system. 

An ingenious synthesis of steroidal annulenes has appeared. The close relationship of annulenes to the classical aromatic substance naphthalene stimulated the synthesis of l,6-methano-[10]annuleno-steroids, which are analogues of equilenin (97). The y,5-cyclopropyl-a -unsaturated ketone (98) was the initial target. Treatment of the cyclopropyl ketone (98) with acetic anhydride and methyl orthoformate in the presence of an acid catalyst then gave (99). Conversion of the cycloheptatriene (99) into the requisite 10 c-electron system (100) was then accomplished by dehydrogenation. ... 

In addition to the classical [6]annulene, benzene, neutral in-plane trishomoaromatic "benzene" frameworks (such as that depicted in Fig. 5) were also studied.Although not yet synthesized, these compounds were collectively studied using a variety of computational techniques. Unique from benzene, the six over-... 

ReUance on simple classical electron coimting rules corresponds to an approximate description of the clamped cycle in terms of its annulene perimeter, and this is clearly insufficient. We can propose three perturbed perimeter models, whereby the simple perimeter annulene analogy is considered as the zeroth-order solution (Model 0) in a perturbative treatment. Model I includes the non-perimeter bonds perturbatively. Model II includes the perimeter heteroatoms perturbatively. Model III includes both (see Scheme 5). By means of these pictorial perturbative models (I, II and III) it is shown below that, when first-order corrections to the angular momentum character and orbital energies of the perimeter annulene (Model 0) are taken into account, it is possible, within the ipsocentric model, to give a unified rationalisation for the survival of the original ring current in XHXH clamped monocycles, and its extinction in HC = CH-clamped monocycles, even at the simple Hiickel level of theory. 

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