Cations antiaromaticity

Thus if and have similar signs, the anion is aromatic and the cation antiaromatic, while if and have opposite signs, the cation is aromatic and the anion is antiaromatic. The signs of the coefficients can be seen from equation (3.31). The signs are the same if the odd AH has 4n + 1 atoms and opposite if it has 4n + 3 atoms. Thus anions with 4n + 1 atoms and cations with 4n + 3 atoms are aromatic, while cations with 4n + 1 atoms and anions with 4n + 3 atoms are antiaromatic. Since the anion has one n electron more than the radical and the cation one n electron less, and since the radical has as many n electrons as conjugated carbon atoms, one can see that the ions with (4n + 2) tt electrons are aromatic, while those with 4n n electrons are antiaromatic. Thus Hiickel s rule applies equally well to odd, cyclic conjugated ions. 

The five membered cyclopentadienyl system contrasts with cycloheptatrienyl Here the cation has four tt electrons is antiaromatic very unstable and very difficult... 

The Hiickel rule predicts aromaticity for the six-7c-electron cation derived from cycloheptatriene by hydride abstraction and antiaromaticity for the planar eight-rc-electron anion that would be formed by deprotonation. The cation is indeed very stable, with a P Cr+ of -1-4.7. ° Salts containing the cation can be isolated as a product of a variety of preparative procedures. On the other hand, the pK of cycloheptatriene has been estimated at 36. ° This value is similar to those of normal 1,4-dienes and does not indicate strong destabilization. Thus, the seven-membered eight-rc-electron anion is probably nonplanar. This would be similar to the situation in the nonplanar eight-rc-electron hydrocarbon, cyclooctatetraene. 

The [S3N3]" cation is of interest as an example of an antiaromatic eight TT-electron system (Section 4.4). Ab initio molecular orbital calculations indicate that a triplet cation, with a planar ring, is more stable than the singlet cation. The [S3N3]" cation has been obtained as the norbomene adduct 5.16, but salts of the free cation have not been isolated. ... 

Similar arguments can be used to predict the relative stabilities of the cyclo-heptatrienyl cation, radical, and anion. Removal of a hydrogen from cyclohepta-triene can generate the six-77-electron cation, the seven-77-electron radical, 01 the eight-77-elec iron anion (Figure 15.6). All three species again have numerous resonance forms, but HiickeTs rule predicts that only the six-7r-electron cyclohep-tatrienyl cation should be aromatic. The seven-77-electron cycloheptatrienyl radical and the eight-77-electron anion are antiaromatic. 

Two other systems that have been studied as possible aromatic or antiaromatic four-electron systems are the cyclopropenyl anion (59) and the cyclopentadienyl cation (60). In these cases also the evidence supports, antiaromaticity, not aromaticity. With respect to 59, HMO theory predicts that an unconjugated 61 (i.e., a single canonical form) is more stable than a conjugated 59, so that 61 would... 

Anderson and coworkers [59-66] produced boron cluster cations Bj-B in molecular beams using laser vaporization and studied their chemical reactivity and fragmentation properties. The structures of B3 —IBI3 cations have been established computationally (see review [7] for details) represented in Figure 29.1. In this chapter, we are discussing stability and reactivity of Bj — B 3 cations on the basis of their multifold aromaticity, multifold antiaromaticity, and conflicting aromaticity. 

The presented consideration of the family of cationic boron clusters exemplifies that the assessment of stability and reactivity of clusters can be performed at the qualitative level using multifold aromaticity, multifold antiaromaticity, and variety of conflicting aromaticities. 

Fluorenylidene dications, such as the dications of p- and m-substituted diphenylmethylidenefluorenes, show appreciable antiaromaticity. Evidence of antiaromaticity is demonstrated through H NMR shifts, nucleus independent chemical shifts (NICS), magnetic susceptibility exaltation, A, and (anti)aromatic (de)stabilization energies, ASE. Extension of the research to indenylidenefluorene dications shows that, contrary to expectation, the indenyl cation in these dications is less antiaromatic than the fluorenyl cation. The magnitude of the antiaromaticity is evaluated through comparison to the aromaticity of related dianions and reveals that the fluorenylidene dications are more antiaromatic than the fluorenylidene dianions are aromatic. 

Is the behavior of fluorenylidene dications, such as 3 or 5 unusual Comparison of the average chemical shifts of fluorenyl cations 7 with the analogous diphenylmethyl cations shows minimal antiaromaticity in the monocations, with paratropic shifts of 0.5 ppm or less (29). The dramatic para-... 

We have not yet prepared the unsubstituted indenylidene fluorene dication 17 but the NICSO) (18) values demonstrate that the fluorenyl cation is still more antiaromatic than the indenyl cation, see Table III. 

Evaluation of Antiaromaticity for Fluorenyl and Indenyl Cations Revisited... 

To perform a similar evaluation for the indenyl cation, 18, we must examine the bis-indenylidene dication, 19, and the appropriate data for both systems is given in Table IV. While 19 is more antiaromatic than 18 by all measures, the dependence of the measure of aromaticity and presumably antiaromaticity on ring size requires that the values for 19 be divided by 2. When examined in this manner, the indenyl cation 18 is more antiaromatic than 19, in contrast to the behavior of the fluorenyl system in 7 and 3. 

The previous sections have demonstrated the antiaromaticity of a variety of indenyl and fluorenyl cations and dications but there has been no attempt to evaluate the degree of antiaromaticity. That is, are these species very antiaromatic or not particularly antiaromatic at all We need a context for the numbers and the corresponding aromatic dianions provides that context. 

That is, just as oxidation of tetrabenzo[5.5]fulvalene resulted in formation of two antiaromatic fluorenyl cations linked by a single bond, reduction should give two aromatic fluorenyl anions, linked by a single bond, 20. We call this relationship the aromaticity/antiaromaticity continuum and can apply the same measures of aromaticity to 20 as were applied to 3, to evaluate relative aroma-... 

The antiaromaticity of indenyl cations was examined in the dications of indenylidene fluorenes. Contrary to expectation, the fluorenyl cation was more antiaromatic than the indenyl cation in these systems. The sum of NICS(l)zz values was linearly related to global measures of antiaromaticity such as A, suggesting that they could be used to evaluate antiaromaticity in a local sense, for individual rings. 

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