Aromatic dianions

Fully unsaturated azocines are 7r-equivalent heterocyclic analogs of cy-clooctatetraene. Addition of two electrons to the completely unsaturated azocine (34) can lead to a dianion 35 and removal of a proton from a dihy-droazocine (36) to the monoanion 37. Both the mono- and the dianions are lOTT-electron systems, corresponding to 7r-equivalent and 7r-excess analogs of cyclooctatetraenide [84CHEC-I(7)653], Aromatic dianions related to 35 have been fully characterized by and NMR (87TL2517). 

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. 

The solid aromatic dianion salt of l,2-di-[ C][8]annulene has not been found to scramble the Cs even on heating to over 600 °C for 2hs. ° This behaviour of the cyclooctatetraene dianion is in contrast with that of neutral aromatic systems, which readily automerize in the gas phase. Apparently, when sufficient energy is applied... 

Ullmann and Buncel attempted to deprotonate A -carene-2,5-dione (4) and the benzannelated derivative 5 in the hope of generating the possibly aromatic dianions 6 and 7. However, the dianions (and the monoanions) could not be isolated. On the grounds of deuterium exchange experiments, the strain energy of benzocyclopropene (1) was estimated to at least 45.5 kcal/mol higher than that of cyclopropane. 

In some cases a whole series of dehydrogenation reactions may proceed sequentially to yield aromatic or highly conjugated products. An example of this is seen in the aerial oxidation of the nickel(n) complex of the macrocycle formed by the template condensation of biacetyl bishydrazone with formaldehyde. The product of the oxidation is the fully aromatic dianionic macrocyclic complex (Fig. 9-24). 

The nonplanar cyclooctatetraene (38, COT) and its planar aromatic dianion (382-) have been extensively studied because these systems represent a textbook example for the application of the Htickel 4w + 2 rule57. Recently, Rabideau and coworkers were able to establish the first crystal structure of a dilithiated COT system by reducing dibenzo[a,e]cyclooctatetraene (39) in TMEDA to its dianion (392 )58. The X-ray of 392-... 

Of the known cyclic oxocarbon acids, the systems based on squaric (68) and croconic (69) acids have been most widely studied. The loss of two protons from these acids gives rise to aromatic dianions as shown in equations (18) and (19), and these can coordinate to metal anions in a variety of ways. Unidentate coordination (70,77) is known for both systems but is not common. Simple bidentate chelate coordination (78) is also relatively uncommon but is observed in a number of croconate complexes. The squarate anion adopts this mode only with larger cations, such as the group 2 and lanthanide metals, and then only in association with additional bridging interactions. Bridging coordination modes dominate the chemistry of these anions, some of which are shown here (71-76), (79-81). The various modes of coordination can usually be distinguished by IR spectroscopy, and the use of NMR spectroscopy has also been investigated. 

Hydrogenation of the imine functions of the ligand has been achieved either with platinum oxide catalyst or by electrochemical means to produce a new series of fully saturated (excluding the benzene rings) nickel(II) complexes as well as species that appear to contain aromatic dianionic ligands. 

Methoxyazocines exhibit a two-electron polarographic wave corresponding to conversion to the lOir-electron aromatic dianion (7UA161). The HMO delocalization energy for the planar azocinyl dianion has been calculated to be -5.1 /S, compared to -3.7 /S for cyclotetraenide dianion. Voltammetric data for 2-methoxy-3,8-dimethylazocine in acetonitrile-tetramethylammonium fluoroborate have been interpreted in terms of the direct formation of the dianion at the electrode followed by rapid protonation of the strongly basic dianion (76ACS(B)773). In contrast, the reduction of cyclooctatetraene under these conditions takes place in two steps. 

An unprecedented lithium-induced cyclization reaction of tribenzocyclotriyne (50) to the hydrocarbon 51 has recently been reported [106]. The proposed aromatic dianion 52 cyclizes and than abstracts protons from the solvent tetrahy-drofuran (THF) and, after further reduction with lithium metal, the resulting intermediate is quenched with methanol to give 51 in 60% yield (see Scheme 23). 

On treatment with potassium metal, m-bicyclo[6.1.0]nona-2,4,6-triene gives a monocyclic aromatic dianion. The trans isomer under similar conditions give a bicyclic radical anion that does not undergo further reduction. Explain how the stereochemistry of the ring junction can control the course of these reductions. 

Butyllithium is a very strong base. Removal of two protons occurs readily because an aromatic dianion with 10 TT electrons is formed. 

We have seen that aromatic stabilization leads to unusually stable hydrocarbon anions such as the cyclopentadienyl anion. Dianions of hydrocarbons are rare and are usually much more dilficult to form. Cyclooctatetraene reacts with potassium metal, however, to fonn an aromatic dianion. 

In most of the recent work, cleaning baths were found to be sufficient to ensure a satisfactory reaction. A series of aryl and hetero-aryl radical anions and aromatic dianions were prepared. scheme describes the experimental set-up. In the presence of N,N -tetramethylethane- or N,N -tetramethyl-propanediamine, non-ethereal solvents can be used.77/78... 

We can distinguish two broad families of tetradentate macrocyclic ligands the rigid, planar, aromatic, dianionic porphyrinato and phthalocyaninato ligands (Scheme 11 see Porphyrins and Expanded Porphyrins as Receptors, and Supramoiecular Phthalocyanine-Based Systems, Moiec-ular Recognition), and the family of aliphatic (or partially unsaturated) tetraaza macrocycles (Scheme 12) and related... 

Explain the following (a) Cyclononatetraenyl anion is planar (in spite of the angle strain involved) and appears to be aromatic, (b) Although [16]annulene is not aromatic, it adds two electrons readily to form an aromatic dianion. 

Nonaromatic cyclic polyenes can form aromatic dianions and dications... 

Cyclic systems of 4n tt electrons can be converted into their aromatic counterparts by two-electron oxidations and reductions. For example, cyclooctatetraene is reduced by alkali metals to the corresponding aromatic dianion. This species is planar, contains fully delocalized electrons, and is relatively stable. It also exhibits an aromatic ring current in NMR. 

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