Carbanions cyclopentadienyl anion

Stabilization by an Aromatic Ring. Certain carbanions are stable because they are aromatic (see the cyclopentadienyl anion p. 52, and other aromatic anions in Chapter 2). 

The operation of (d) is seen in cyclopentadiene (14) which is found to have a pKa value of 16 compared with 37 for a simple alkene. This is due to the resultant carbanion, the cyclopentadienyl anion (15), being a 6n electron delocalised system, i.e. a 4n + 2 Hiickel system where n = 1 (cf. p. 18). The 6 electrons can be accommodated in three stabilised n molecular orbitals, like benzene, and the anion thus shows quasi-aromatic stabilisation it is stabilised by aromatisation ... 

Such equilibrium constants, Veq. are known only for highly conjugated carbanions, such as in cyclopentadienyl anion in water or triphenylallyl anion in DMSO3. Some values are known for equilibrium constants and enthalpies of equation 1 in the gas phase. Additional energies are available for many compounds by computation—with modern methods, computed energies for equation 2 are reliable to a few kcalmol-1. 

Because the cyclopentadienyl anion (six pi electrons) is aromatic, it is unusually stable compared with other carbanions. It can be formed by abstracting a proton from cyclo-pentadiene, which is unusually acidic for an alkene. Cyclopentadiene has a pKa of 16, compared with a pKa of 46 for cyclohexene. In fact, cyclopentadiene is nearly as acidic as water and more acidic than many alcohols. It is entirely ionized by potassium terf-butoxide ... 

When we say the cyclopentadienyl anion is aromatic, this does not necessarily imply that it is as stable as benzene. As a carbanion, the cyclopentadienyl anion reacts readily with electrophiles. Because this ion is aromatic, however, it is more stable than the corresponding open-chain ion. 

Condensed aromatic rings fused to a cyclopentadienyl anion are known to stabilize the carbanion. X-ray crystallographic structures have been obtained for Ph2CH and Ph3C enclosed in crown ethers. Carbanion 21 has a lifetime of several minutes (hours in a freezer at —20 °C) in dry Thf.113... 

Other factors contribute, but often to a much lesser degree. Inductive and field effects from electronegative atoms can also stabilize the carbanion somewhat. Hydrogen bonding is very rare for carbanions. Aromaticity greatly stabilizes carbanions, but the only common example is cyclopentadienyl anion C5H5". 

Cyclopentadiene is an acidic hydrocarbon. In 1928 English chemist Christopher Ingold suggested that this was because the cyclopentadienyl anion had an aromatic sextet of electrons. This was the first case of aromatic character being attributed to an ion. An interesting derivative made from this very stable carbanion vizs ferrocene (discovered in 1951). 

As a result of its aromaticity, the cyclopentadienyl anion is an unusually stable carbanion. This is why cyclopentadiene has an unusually low pA a. In other words, it is the stability conveyed by the aromaticity of the cyclopentadienyl anion that makes the hydrogen much more acidic than hydrogens bonded to other sp carbons. 

Both the carbanion and carbocations are stable provided they contain [An+ 2) K electrons. For example, cyclopentadienyl anion, cyclopropenium cation, and tropyhum cation exhibit unusual stability. Stable carbanions do, however, exist. In 1984 Ohnstead presented the lithium crown ether salt of the diphenylmethyl carbanion from diphenylmethane, butyllithium, and 12-crown-4 at low temperatures. Addition of n-butyUithium to triphenyhnethane in THF at low temperatures followed by 12-crown-4 resulted in a red solution and the salt complex precipitated at —20°C. The central C-C bond lengths are 145 pm with the phenyl ring propelled at an average angle of 31.2° (Scheme 3.11). 

In the cartoon below the resonance structure on the right sacrifices one "cyclopentadienyl anion" to generate the fluorenyl carbanion hence, there should be little net loss in aromatidty. This "no net loss of aromaticity" has recently been exploited by Marder and coworkers in designing NLO-phores with large h3q>erpolarizabilities. ... 

A systematic study of some monosubstituted solvent-separated cyclopentadienide salts of Group 1 cations (Li, Na, K, Cs) in DMSO revealed the bonding preferences for the fulvenoid (4) over the cyclopentadienyl isomer (5). Three of the substiments contained an a-carbonyl group formyl, acetyl and benzoyl. From the viewpoint of the anions, there is a conflict between the inherent greater stability of oxyanions over carbanions and the aromaticity associated with the 6 jr-electron carbocyclic system. The authors demonstrated that the fulvenoid interaction is favored by strong electron-accepting substituents and... 

Among transition metals, zirconium is the most important for hydrometallation of alkynes. The available 16 electron complex Cp2ZrHCl (Cp = cyclopentadienyl) 122 (zirconocene hydrochloride) adds stereoselectively syn to alkynes to give the -vinyl zirconium species 126. The initial interaction is the donation of two electrons by the triple bond 123 to make the reasonably stable 18-electron n-complex (or r 2-complex) 124. Typically for a transition metal Ji-complex, a ligand is now transferred from the metal to one end of the tr-bond 125 while the metal itself forms a stable 16-electron o-complex at the other 126. The H transfer is intramolecular so the metal and H atoms must add to the same side of the triple bond. The zirconium atom transfers the least stable anion among its ligands. This is obviously II as Cl is much more stable. The metal prefers to take the terminal position because the resulting o-complex is more stable as it is a o-complex of the less-substituted carbanion.27... 

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