Cycloheptatrienylium cation

Quantum mechanical considerations predict that a jr-electron system containing six electrons should be particularly stable.205 Examples of conspicuously stable six electron systems are benzene and the cyclopentadienyl anion. The cycloheptatrienylium cation is also stable, presumably for the same reason.206-207... 

The divergent hyperfine patterns of the two ions can be ascribed to their 6ti aromatic substructures, cyclopentadienide anion, and cycloheptatrienylium cation, respectively. 

The non-systematic name for the cycloheptatrienylium cation is the tropylium cation. 

Figure 15.7 Reaction of cycloheptatriene with bromine yields cycloheptatrienylium bromide, an ionic substance containing the cycioheptatrienyl cation. The electrostatic potential map shows that all seven carbon atoms are equally charged and electron-poor blue).

Volume 75 concludes with six procedures for the preparation of valuable building blocks. The first, 6,7-DIHYDROCYCLOPENTA-l,3-DIOXIN-5(4H)-ONE, serves as an effective /3-keto vinyl cation equivalent when subjected to reductive and alkylative 1,3-carbonyl transpositions. 3-CYCLOPENTENE-l-CARBOXYLIC ACID, the second procedure in this series, is prepared via the reaction of dimethyl malonate and cis-l,4-dichloro-2-butene, followed by hydrolysis and decarboxylation. The use of tetrahaloarenes as diaryne equivalents for the potential construction of molecular belts, collars, and strips is demonstrated with the preparation of anti- and syn-l,4,5,8-TETRAHYDROANTHRACENE 1,4 5,8-DIEPOXIDES. Also of potential interest to the organic materials community is 8,8-DICYANOHEPTAFULVENE, prepared by the condensation of cycloheptatrienylium tetrafluoroborate with bromomalononitrile. The preparation of 2-PHENYL-l-PYRROLINE, an important heterocycle for the synthesis of a variety of alkaloids and pyrroloisoquinoline antidepressants, illustrates the utility of the inexpensive N-vinylpyrrolidin-2-one as an effective 3-aminopropyl carbanion equivalent. The final preparation in Volume 75, cis-4a(S), 8a(R)-PERHYDRO-6(2H)-ISOQUINOLINONES, il lustrates the conversion of quinine via oxidative degradation to meroquinene esters that are subsequently cyclized to N-acylated cis-perhydroisoquinolones and as such represent attractive building blocks now readily available in the pool of chiral substrates. 

The name of a cationic ligand is used without change, e.g. cycloheptatrienylium and (1-amino-JV,A,JV-trirnethylmethanaminium). 

The crucial structural feature which underlies the aromatic character of benzenoid compounds is of course the cyclic delocalised system of six n-electrons. Other carbocyclic systems similarly possessing this aromatic sextet of electrons include, for example, the ion C5Hf formed from cyclopentadiene under basic conditions. The cyclopentadienide anion is centrosymmetrical and strongly resonance stabilised, and is usually represented as in (7). The analogous cycloheptatrienylium (tropylium) cation (8), with an aromatic sextet delocalised over a symmetrical seven-membered ring, is also demonstrably aromatic in character. The stable, condensed, bicyclic hydrocarbon azulene (Ci0H8) possesses marked aromatic character it is usually represented by the covalent structure (9). The fact that the molecule has a finite dipole moment, however, suggests that the ionic form (10) [a combination of (7) and (8)] must contribute to the overall hybrid structure. 

As we have mentioned in Chap. II, it is possible to prepare stable, fully ionised crystalline salts of carbocations provided the positive charge is sufficiently delocalised over the cationic moiety and the nucleophilicity of the anion is sufficiently low. Within this family of compounds, we will limit our attention to two major groups of salts with which most of the fundamental studies on cationic polymerisation of olefins has been carried out the triphenylmethylium (trityl) and the cycloheptatrienylium (tropylium) salts. It has been a common practice to assume that if these salts are fully ionised in the solid state, they will also be completely dissociated in solution. Thus, the equilibria... 

When the vacant p orbital is part of a completely conjugated cyclic system, there can be further stabilization resulting from aromaticity. (See Section 8.3 for further discussion.) The cyclopropenylium ion and the cycloheptatrienylium ion fit into this category. When cyclic conjugation is antiaromatic, as in the cyclopentadienylium ion, there is net stabilization relative to a methyl cation, but significantly less than for the aromatic analogs. ... 

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