Tropylium cation. See

While benzene was the first aromatic system studied, the formulation of HtickePs rule and the theory behind it created an impetus to prepare non-benzenoid species such as the tropylium cation and cyclopentadienyl anion that also obeyed Huckel s rule to see if these species were also aromatic. This required that the properties of aromatic compounds be defined. 

As we can see, this structure can be viewed as a substituted form of the tropylium cation discussed in (a). 

When cycloheptatriene is treated with a reagent that can abstract a hydride ion, it is converted to the cycloheptatrienyl (or tropylium) cation. The loss of a hydride ion from cycloheptatriene occurs with unexpected ease, and the cycloheptatrienyl cation is found to be unusually stable. The NMR spectrum of the cycloheptatrienyl cation indicates that all seven hydrogen atoms are equivalent. If we look closely at Fig. 14.12, we see how we can account for these observations. 

Figure 31.1 shows the structures of some of the calixarene and calixpyrrole derivatives that were found to self-assemble into dimeric capsules (see structures 2-5). The tetraureacalix[4]arenes (2) were found to from dimeric capsules with internal volumes estimated to be in the range of 160-200 [lib]. These dimeric capsules were found to encapsulate solvent molecules like chloroform and benzene and small ammonium salts [11]. In addition, it was demonstrated with the aid of diffusion NMR [16] that such dimers have a high affinity for charged systems, and it was found that tropylium cation and cobaltocenium cation have higher affinities than benzene and ferrocene, respectively, for the cavity of the dimer of 2 [17]. These and other studies demonstrated that indeed diffusion NMR is an excellent means to probe encapsulation [17, 18], a fact that was instrumental in the study of the large hexameric capsules of resorcin[4]arenes and pyrogallol[4]arenes as will be demonstrated below [18-20]. 

Hiickel pointed out that, on the basis of molecular orbital theory, monocyclic conjugated polymethines have filled shells of tt-electrons when the number of TT-electrons is An + 2, where n is an integer. These systems may be expected to be stable. The rule may be illustrated by reference to Fig. 2.1. If = 0, then a system with 27r-electrons should be stable. Such a situation is found in the cyclopropenyl positive ion, which has been isolated as the hexachloroanti-monate. For n = the prediction is that the cyclopentadienyl anion, benzene and the cycloheptatrienyl (tropylium) cation are stable. This is certainly in accord with experience. The stability of benzene is well known, the cydo-pentadienyl anion is readily formed by the action of potassium metal on cyclopentadiene, and the cycloheptatrienyl cation is one of the most stable carbonium ions known. Huckel s rule also predicts that some of the larger cyclic conjugated systems are stable, e.g. those with 10,14 and 18 rr-electrons. However, the situation is complicated by steric problems (see for example Garratt, 1971) and need not be considered further here. 

The tropylium and the cyclopropenyl cations are stabilized aromatic systems. These ions are arumatic according to Hiickel s rule, with the cyclopropeniiun ion having two n electrons and the tropyliiun ion six (see Section 9.3). Both ring systems are planar and possess cyclic conjugation, as is required for aromaticity. 

The infrared and Raman spectra of tropylium bromide in hydrobromic acid solution have no common bands, which means that the cation exists in a highly symmetrical form in this solution (see Section 9-8), At higher pH, reversible formation of the hydroxy compound occurs ... 

Another characteristic of electrophilic reactions of pseudoazulenes is the application of numerous cations as the electrophile, for example, diazonium salts and Vilsmeier- Haack s reagent (see Table VI), tropylium ion,135 triphenylmethyl cation,"4 pyrylium ion,119 and dithiolium ion.166 Very stable cations are formed (e.g., 120) addition of base releases the substituted pseudoazulene (see example in Eq. 10). Generally reactions of this type are thermodynamically favored (see also Section IV,B). The site of substitution... 

Scheme 7.1 Preparation of cationic tropylium metal tricarbonyls and their reactions with anionic nucleophiles (2, 4, 6, 8 M = Cr 3, 5, 7, 9 M = Mo for R and X- see text)...

Two bridged, ionic 10 n systems directly analogous to the bridged l,6-methano-[10]annulene discussed earlier, have been studied. Thus the cation 61 and anion 62 (see Table 13) bear a similar relationship to 1,6-methano-[10]annulene, 28, as do the tropylium ion and cydopentadienyl anion to benzene. The n.m.r. spectrum of cation 6117>203> is characterized by low-field absorptions of the peripheral protons (t 0.4 to 1.7) and by... 

Azulenes are basic compounds (see Section 4.1.1). The formation of carbocyclic azulenium cations in the presence of strong adds is a transformation to tropylium salts that is accompanied by a large hypsochromic shift and the color changes to yellow (55FCF(3)334, p. 380 66CZ691). Azulenes fused to heterocyclic nitrogen bases, which are primarily N-protonated, do not behave uniformly. 

Increasing delocalization, as in the cyclohexadienyl cation, is further stabilizing. This structure is simply protonated benzene, and it serves as a model for the intermediate in electrophilic aromatic substitution (see Section 10.18). Similarly, benzyl ion is quite stable for a formally 1° ion. Aromaticity effects are clear, as in the much greater stability of the six it electron tropylium ion vs. the four tt electron cyclopentadienyl ion (see Section 2.4.1 for a discussion of aromaticity). 

In the union of heptatrienate cation with CH3" (Fig. 7.9e), the NBMO of the cation and the 2p AO of CH3" are degenerate. The interaction between the orbitals is consequently large, so the HOMO of the product, octatetraene, lies well below the nonbonding level. In tropylium, on the other hand, the LUMO is antibonding (see Section 3.13). The interaction of this antibonding MO with the 2p AO of CH3 on union of the two (Fig. 7.9f) is consequently much less. The HOMO of heptafulvene therefore lies much closer to the nonbonding level than does the HOMO of octatetraene. The ionization potential of heptafulvene must therefore be much less than that of octatetraene. 

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