Annulene resonance structures

Hydrocarbons containing one or more triple bonds in addition to double bonds have been excluded from the tile, as have been radicals (e.g. the allyl radical C3H5 ) and aromatic molecules, i.e. molecules for which more than one unexcited resonance structure (Kekule structure) can be written. Consequently, hydrocarbons such as phenyl-substituted polyenes, or annulenes — bridged or unbridged—have not been included. 

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. 

The nature of multiple bonding between germanium and the heavier chalcogens in the complexes (/74-Megtaa)GeE (E = Se, Te) is best described as an intermediate between the Ge+— E and Ge=E resonance structures. The preparation of these complexes involves the addition of the elemental chalcogen to (/74-Mestaa)Ge, which is synthesized by the metathesis of GeCl2(l,4-dioxane) and Li2[Mestaa] (Mestaa = octamethyldibenzotetraaza[14]annulene dianion). The molecular structures of both complexes are shown in Figures 5 and 610. 

For the double bonds to be completely conjugated, the annulene must be planar so the p orbitals of the pi bonds can overlap. As long as an annulene is assumed to be planar, we can draw two Kekule-like structures that seem to show a benzene-like resonance. Figure 16-3 shows proposed benzene-like resonance forms for cyclobutadiene and cyclooctatetraene. Although these resonance structures suggest that the [4] and [8]annulenes should be unusually stable (like benzene), experiments have shown that cyclobutadiene and cyclooctatetraene are not unusually stable. These results imply that the simple resonance picture is incorrect. 

Hexadehydrodibenzo[12]annulene aimelated by a CPDN moiety 373 was synthesized by Tobe as outlined in Scheme 6.90. Theoretical calculations and experimental NMR and IR spectra indicate that 373 is weakly aromatic owing to the contribution of the diatropic [ISjannulenone resonance structure [221]. 

The outer protons of 1,3,7,13-tetradehydro-[18]annulene 107>, 16, (see Table 4) resonate in a range encompassing the chemical shift of the hexadehydro-[18]annulene, and, as expected, the two inner protons of this compound are shielded, and absorb as two quartets at t 7.66 and t7.80. The structure given in Table 4 is the most likely 10 ) of the four structures possible ) for a tetradehydro-[18]annulene containing three cis and two trans double bonds. 

A tridehydro-[26]annulene, 27, has been reported 118>, but the structure is only one of the many possible, as the positions of the triple bonds and the stereochemistry of the double bonds are as yet undetermined. The n.m.r. consists of a very broad multiplet at t 2.0 to 4.5, and on cooling to — 60 °C no discrete inner or outer proton resonances axe observed. It is concluded that this compound, although formally a Htickel An + 2) system, is not aromatic. As noted previously, the... 

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