Cyclooctatetraene, dianion

The crystal structure of the potassium salt of 1,3,5,7-tetramethylcyclootatetraene dianion has been determined by X-ray dififaction. ° The eight-membered ring is planar, with aromatic C—C bond lengths of about 1.41 A without significant alternation. The spectroscopic and structural studies lead to the conclusion that the cyclooctatetraene dianion is a stabilized delocalized structure. 

Problem 15.7 Cyclooctatetraene readily reacts with potassium metal to form the stable cycio-octatetraone dianion, C8H82. Why do you suppose this reaction occurs so easily What geometry do you expect for the cyclooctatetraene dianion ... 

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. 

Besides l,4-dihydro-l,4-diazocines (see Section 1.4.) there are other 1,4-diheterocines that are isoelectronic with the cyclooctatetraene dianion. These systems have primarily found interest as potential 107t-aromatic systems. No extensive delocalization is expected theoretically and calculations predict nonaromatic systems for both 1,4-dioxocin (1) and 1,4-dithiocin (see Section l.ll.).3-4-7... 

Uranium complexes analogous to these compounds were also described, but with the cot (cyclooctatetraene dianion) ligand rather than the Cp or Cp ones. Both the dianionic U(IV) [(cot)U(dddt)2]2 [47] and monoanionic U(V) [(cot)U(dddt)2] [48] complexes were isolated and structurally characterized (Fig. 5). Spectacular distortions of the US2C2 metallacycles were rationalized on the basis of DFT calculations, which reproduced the spectacular folding of the endo US2C2 metalla-cycle when the dianionic species undergoes an oxidation. 

On the basis of topological criteria, Balaban predicted in 1965 the aromaticity of compounds with 10-jr-electron systems in eight-membered rings with two heteroatoms (analogous to the cyclooctatetraene dianion).250 Schroth and co-workers were among the first to investigate the systems experimentally, and they synthesized compounds 210—213 (Scheme... 

Ogliaruso et al. (1966) formed the ten 7r-electron monohomocycloocta-tetraene dianion [148]. In the same way that cyclooctatetraene dianion can be formed by donation of two electrons to cyclooctatetraene, their synthesis was the two-electron donation to the monomethylene adduct. H NMR studies support the assignment of the homoaromatic form [148] and exclude the classical form [149]. 

Cyclooctatetraene was reduced electrochemically to cyclooctatetraenyl dianion. In DMF the product is mostly (92%) 1,3,5-cyclooctatriene at —1.2 V. If the potential is lowered the main product is 1,3,6-cyclooctatriene. Previous experiments, in which the anion radical was found to be disproportionated, were explained on the basis of reactions of the cyclooctatetraene dianion with alkali metal ions to form tightly bound complexes, or with water to form cyclooctatrienes. The first electron transfer to cyclooctatetraene is slow and proceeds via a transition state which resembles planar cyclooctatetraene102. 

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... 

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... 

Another example concerns the initial electronic reduction of a-nitrostilbene (Todres et al. 1982, 1985, Todres and Tsvetkova 1987, Kraiya et al. 2004). The reduction develops according to direction a in Scheme 2.9 if the mercury cathode as well as cyclooctatetraene dianion are electron sources and according to direction b if the same stilbene enters the charge-transfer complexes with bis(pyridine)-tungsten tetra(carbonyl) or uranocene. For direction b, the charge-transfer bands in the electronic spectra are fixed. So the mentioned data reveal a great difference in electrochemical and chemical reduction processes a and b as they are marked in Scheme 2.9. 

The behavior of the same azoxybenzene is studied in homogeneous conditions— when the dipotassium salt of cyclooctatetraene dianion (CgHgKj) acts as a dissolved electrode. In this case, the reduction of azoxybenzene stops at the very first stage, that is, after the transfer of one electron only (Todres et al. 1975). The initial one-electron reduction produces the azoxybenzene anion-radicals, which are not reduced further despite the presence of residual electron donor in the solution. The ESR method does not reveal these anion-radicals although one-electron oxidation by phenoxyl radicals quantitatively regenerates azoxybenzene and produces the corresponding potassium phenolate molecules in a quantitative yield. Treatment with water leads to a 100% yield of azobenzene (Scheme 2.14). 

The diamagnetic complex is not reduced further by the cyclooctatetraene dianion. This prevents the conversion of the azoxybenzene anion-radicals into azodianions. Potassium cation plays an important role in this limitation of the reduction process, which, generally, proceeds readily (the... 

Di(thiocyano)thiophene also presents a different behavior at a mercury-dropping electrode and in the case of treatment by dipotassium salt of cyclooctatetraene dianion THF is the same solvent for both cases (Todres et al. 1979). The reaction between di(thiocyano) derivative and CjHgKj taken in equimolecular amounts leads to the formation of potassium salt of 2-mercapto-5-thiocyanothiophene (potassium mercaptide), potassium cyanide, and cyclooctatetraene (see Scheme 2.17). Potassium mer-... 

However, the analogs containing the nitro group exhibit different behavior in reactions at the surface of the electrode and on reduction by the cyclooctatetraene dianion. The difference is depicted in Scheme 2.22 (Todres 1980). 

In the case of cyclooctatetraene, an electron prefers the isotopicaUy heavier material. At 173 K, the equilibrium constant for -F C Dg = CgHg + CgDg was found to be 1.16 (Stevenson 2007). However, when this anion-radical reacts with cyclooctatetraene dianion (not with the anion-radical), the transferred electrons prefer the isotopicaUy lighter material (Stevenson et al. 1990,1992) as follows CgHg -F = CgHg -F CgDg. The semiempirical quantum chemical consideration led to the... 

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... 

SINDOl calculations, which successfully reproduced both the geometry and the aromaticity of the cyclooctatetraene dianion, predict a high degree of bond localization and nonplanarity for 1,4-dioxocin and its derivatives (84JOC4475). 

By use of a similar method other more stable 1-substituted phosphiranes are prepared, 1-phenylphosphirane (123) being the most stable. When the cyclooctatetraene dianion is treated with dichlorophenylphosphine the bicyclic 1-phenylphosphirane derivative (124) is formed. At 70 °C in CHC13 it rearranges into the bicyclic phosphabicyclo[4.2.1]heptane derivative (125). Oxidation and photochemical reaction give the tricyclic phospholene 1-oxide derivative (126 equation (76)) (66JA3832). 

Examine the geometry of cyclooctatetraene dianion. Is it planar If not, describe its shape. Are all the carbon-carbon bond lengths the same If so, are they the same length as those in benzene If not, do they alternate between single and double bonds Do your observations suggest that cyclooctatetraene dianion is aromatic Is this in accord with Hiickel s rule ... 

By exciting the red-orange cyclooctatetraene dianion 1 in the presence of cyclooctatetraene in our photoelectrochemical cell (n-TiC>2/NH3/Pt), we were able to observe photocurrents without detectable decomposition of the anionic absorber (2). Presumably, a rapid dismutation of the photooxidized product inhibited electron recombination, producing a stable hydrocarbon whose cathodic reduction at the counter electrode regenerates the original mixture essentially quantitatively (eqn 3). 

Calculations using GIAO and IGAIM methods have enabled the chemical shifts of methyl cation and anion to be determined by gauging the shielding of the nucleus by neighbouring electrons the experimental data for cyclopentadienide ion, benzene, tropylium ion, and cyclooctatetraene dianion have also been reproduced and the data have been correlated by a simple model that also accounts for the large downfield shifts found for carbenes.14... 

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. 

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