Dianions, antiaromaticity

The surprising finding is that the strain in 24 and 25 brings about the formation of an intramolecular a-bond as a means of avoiding an unfavorable strained dianionic antiaromatic state. 

The fact that many 4 systems are paratropic even though they may be nonplanar and have unequal bond distances indicates that if planarity were enforced, the ring currents might be even greater. That this is true is dramatically illustrated by the NMR spectrum of the dianion of 83 (and its diethyl and dipropyl homologs). We may recall that in 83, the outer protons were found at 8.14-8.67 8 with the methyl protons at —4.25 8. For the dianion, however, which is forced to have approximately the same planar geometry but now has 16 electrons, the outer protons are shifted to about -3 8 while the methyl protons are found at 21 8, a shift of 258 We have already seen where the converse shift was made, when [16]annulenes that were antiaromatic were converted to 18-electron dianions that were aromatic. In these cases, the changes in NMR chemical shifts were almost as dramatic. Heat of combustion measurements also show that [16]annulene is much less stable than its dianion. 

We can therefore conclude that in 4n systems antiaromaticity will be at a maximum where a molecule is constrained to be planar (as in 59 or the dianion of 83) but, where possible, the molecule will distort itself from planarity and avoid equal bond distances in order to reduce antiaromaticity. In some cases, such as cyclooctatraene, the distortion and bond alternation are great enough for antiaromaticity to be completely avoided. In other cases (e.g., 96 or 103), it is apparently not possible for the molecules to avoid at least some p-orbital overlap. Such molecules show paramagnetic ring currents and other evidence of antiaromaticity, although the degree of is not as great as in molecules such as 59 or the dianion of 83. 

Benzene (aromatic) does not yield its antiaromatic dianion and naphthalene reduces only to its anion radical. 

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. 

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. 

Unlike the tub-shaped parent azocines which are antiaromatic, their 1 Orr-electron dianions (e.g. 30) are planar and aromatic in nature (7iJAi6i). The dianiones are formed by two-electron reduction of azocines. An intermediate radical-anion (29) was obtained from 3,8-dimethyl-2-methoxyazocine (28) (83JA6078) which has a strong tendency to disproportionate into dianion (30) and neutral azocine. 

The aromaticity of the dianion (1) and the antiaromaticity of the correponding dication of tetrabenzo[5.5]fulvalene have been evaluated through magnetic criteria, ll... 

Minkin VI, Glukhovtsev MN, Simkin B Ya (1994) Aromaticity and antiaromaticity electronic and structural aspects. Wiley, New York, pp 157-161. (b) Krogh-Jespersen K, Schleyer PvR, Pople JA, Cremer D (1978) J Am Chem Soc 100 4301. (c) The cyclobutadiene dianion, another potentially aromatic system, has been prepared Ishii K, Kobayashi N, Matsuo T, Tanaka M, Sekiguchi A (2001) J Am Chem Soc 123 5356... 

This unusual charge distribution in these Ann-conjugated dianions can be interpreted in terms of minimization of paratropicity (antiaromaticity) and seems to be a general property of such anions. By forcing the charge into the anthracene moiety, the phenanthrene moiety remains almost neutral and aromatic and thus the total paratropicity is reduced and destabilization due to antiaromaticity is minimized. 

In addition to probing the effect of nonplanarity on the antiaromatic character of these phenanthrene-based systems, it is also possible to analyze the effect of charge on their racemization barrier. This can be achieved by studying systems like 4-isopropyl-1,5,8-trimethylphenanthrene (ll)38. The diastereotopic isopropyl marker at the bay region position C4 yields a free-energy barrier (AG ) of 22.2 kcalmol-1 for the racemization process of the neutral compound and 15.4 kcalmol-1 for the racemization of the dianion (ll2-). Although the barrier of racemization decreases as a result of reduction, the system still maintains its helicity. 

The 13C NMR spectrum of the dianion of 43 shows three carbon peaks (5 = 120, 154 and 204 ppm) that are shifted to a lower field than those of the neutral compound. The extremely low field signal of the quaternary hub carbon (S = 204 ppm) can be explained by the strong deshielding effect of the outer antiaromatic ring current. 

The charge distribution in 656- resembles six covalently linked benzyl anions, in which each carbon of the central benzene ring is the benzylic position of the phenyl to which it is attached (Figure 23). As a result, the central benzene ring carries two charge units (the first example of a formal benzene dianion), and in order to avoid antiaromaticity it adopts a twist-boat conformation, as suggested by DFT (B3LYP/6-31G ) calculations. These... 

Pentalene has eight n electrons and is antiaromatic. Pentalene dianion, however, has ten tc electrons and is a stable, aromatic ion. 

Piekarski, A. M. Mills, N. S. Yousef, A. Dianion and dication of tetra-benzo[5.7]fulvalene. Greater antiaromaticity than aromaticity in comparable systems, J. Am. Chem. Soc. 2008,130,14883-14890. 

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