Cyclooctatetraene dianion from

The 1.4-dihydro-l,4-diazocines prepared from iyn-benzene diimines (Section 1.4.1.2.) can be transformed to other derivatives by exchange of the substituents at nitrogen. For this purpose, the dipotassium salt of 1,4-diazocine is generated and then reacted with appropriate electrophiles. For example, reduction of the bistosyl derivative 3 gives a relatively stable dianion, a lOrr-electron system analogous to cyclooctatetraene dianion, which on protonation clearly gives the parent l,4-dihydro-l,4-diazocine (4, E = H) as the only product. 

Let us compare anion-radicals with dianions, which are definitely stronger bases. For example, the cyclooctatetraene dianion (CgHg ) accepts protons even from solvents such as dimethylsulfoxide (DMSO) and V,V-dimethylformamide. The latter is traditionally qualified as an aprotic solvent. In this solvent, the cyclooctatetraene dianion undergoes protonation resulting in the formation of cyclooctatrienes (Allendoerfer and Rieger 1965) + 2H+ CgHjo. It is seen that... 

The species (CH)8Mg, or we should say its THF 2.5-solvate, is readily formed from cyclooctatetraene and Mg. The NMR spectrum shows eight equivalent ring atoms and so suggests either the cyclooctatetraene dianion and Mg salt or a putative (and highly fluxional) solvated magnesacyclopentene (or more properly magnesabicyclonon-atriene). However, there is no structural data for the rf open sandwich species and the enthalpy of formation of this simple and sensible half-sandwich, or tight ion pair, cannot even be estimated. 

Eight-membered rings with two O, S or N atoms or combinations of these heteroatoms in a 1,2- or 1,4-relationship and three double bonds possess conjugated tt-electron frameworks and can be designated as dihetera[8]annulenes (78AHC(23)55). These 1,2- or 1,4-diheterocins are isoelectronic with the cyclooctatetraene dianion, and if planar represent potentially aromatic IOtt-electron systems. Considerable interest has attached to the degree of aromaticity of these compounds from both theoretical and experimental standpoints. Most theoretical treatments have led to the conclusion that 1,2-diheterocins, because of... 

The structure of a potassium salt of a cyclooctatetraene dianion has been determined by Raymond and co-workers (269). In this bright yellow complex (XlXb in Fig. 3) a diglyme molecule is coordinated to each of the potassium atoms through all three oxygen atoms. Two potassium-diglyme units lie on either side of the planar carbocyclic ring equidistant from the ring center. 

Figure 2.13. Comparison of the absorption spectra of benzene, the tropylium ion, and the cyclooctatetraene dianion (adapted from Heilbronner, 1966 and Dvorak and Michl, 1976).

Uranocene (15.19), a key discovery from 1%8, showed that the higher radius and charge of relative to the lanthanides allows stabilization of the planar, aromatic, lOrr-electron cyclooctatetraene dianion (cot ) in U(cot)2. This pyrophoric 22e compound also shows the failure of the 18e rule in the 5/ elements. 

The anion from one electron transfer is probably an intermediate. Simple alkenes do not form such stable dianions, but some conjugated derivatives do form dianions as with cyclooctatetraene forming dianion with alkali metals. 

The cyclooctatetraenyl dianion 19 (COT-2) is an aromatic 10a electron system and hence can be readily prepared from cyclooctatetraene by reduction (alkali metals or electrochemically). An early report [36] of the photochemistry of COT-2 in the presence of weak acids such as amines and terminal alkynes showed that it is protonated in the excited state (to give the monoanion) COT-2 is more basic in the excited state than in the ground state. However, in the absence of such weak acids, photolysis of COT-2 results in electron photoejection [37-39]. The electron photoejection process and subsequent chemistry has been studied in... 

In 1945 Michael J. S. Dewar suggested that the tropylium ion (the cation derived from cycloheptatriene) should also be aromatic (Figure 9). This was confirmed in 1954 since then, the dianion of butadiene and the dication of cyclooctatetraene have also been shown to be aromatic. Like benzene, all four of these ions are planar rings with six tt electrons. According to Hiickel s rule the cyclopropene cation should also exhibit aromaticity, and it does. (In this case n = 0, and 4n + 2 = 2.) The planar anion of cyclononatetraene and the dianion of cyclooctatetraene should also be aromatic (n = 2, and 4n + 2 = 10), and both of them are. 

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