Cyclopentadienide anion stability

Because the five-membered ring is a substituted cyclopentadienide anion in some dipolar resonance stmctures, it might be expected that exocyclic groups that could strongly stabilize a positive charge would lead to a larger contribution from dipolar stmctures and enhanced stability. The stmctures 13 and 14 are cases in which a dipolar contribution would be feasible. 

Section 11.21 Species with six tt electrons that possess special stability include certain ions, such as cyclopentadienide anion and cycloheptatrienyl cation. 

According to a semiempirical study by Malar, the different polyphospholide anions have 86—101% aromaticities of that of the cyclopentadienide anion. Chesnut and Quin reported on the basis of GIAO NMR calculations using a triple-valence quality basis set that the phospholide anion s aromaticity is 63% that of the cyclopentadienide anion. The aromatic stabilization energy (ASE) obtained by Schley-er et al. from eq 2 (X = P ) was 90% that of the cyclopentadienide anion. 

In organic chemistry this stabilizing effect is well known the stability of carbanions is known to be enhanced by nitro groups. The stability of the cyclopentadienide anion is increased by complexing with a typical Lewis acid so that it becomes less reactive. For example, ferrocene is not ionized in nitromethane solution. Addition of a Lewis acid such as aluminum chloride facilitates the occurrence of intramolecular race-mization (75) a process which is believed to involve ionic intermediates [16). This belief is supported by kinetic evidence and the failure of the reaction to occur in nearly inert solvents like methylene chloride and in those of high donidty. Whereas the former do not support the solvation of the cation formed in the process of ionization, the latter will react preferentially with the Lewis acid, which is then no longer available for the stabilization of the carbanion. 

An especially important example is that of 1,3-cyclopentadiene, which is acidic because its conjugate base (cyclopentadienide anion) is greatly stabilized by electron delocalization. The anion is formed easily from the hydrocarbon and methyllithium ... 

Considerable effort has been invested in attempts to explore the total replacement of CH groups by nitrogen in cyclopentadienide anions or benzene. The ability of aryl groups to stabilize free radicals, cations or anions is due to extension (conjugation) of re-orbitals. More than a century ago, it was observed that alkylbenzenes afford benzoic acid on oxidation and that the stability of arylazoles toward oxidants and acids increased with the number of N atoms, so that the oxidants attack the benzene ring in phenylpyrazole, and that benzotriazole survives refluxing with aqua regia. 

An energy difference for the isodesmic reaction [Eq. (28)] indicates a quantitative measure of the extent of electron delocalization (6-31G ) (62). The energy difference for the C, structure of the silacyclopentadienide anion is only 2.2 kcal/mol, which is much smaller than 73.4 kcal/mol for the carbon analog, a planar cyclopentadienide anion (C2 ). On the basis of these calculations, it is concluded that the ground-state structure of the silacyclopentadienide anion has only ca. 3% of the resonance stabilization exhibited by the cyclopentadienide anion. However, the previous calculation (3-21G//STO-2G) on the 2 planar structure shows that the resonance energy is 23 kcal/mol, which means ca. 25% as aromatic as the carbon analog (65). 

While the first, third, and fourth structures can lose protons from sp carbons to give resonance-stabilized anions, only the second structure can make a cyclopentadienide anion. It will lose a proton most easily of these four structures which, by definition, means it is the strongest acid. 

The more resonance-stabilized cyclopentadienide anion is readily alkylated with alkyl halides, tosylates, or epoxides. The sodium reagent often gives best results [36]. The initially formed 5-alkylcyclopentadiene rapidly isomerizes at room temperature under the reaction conditions to the 1 -allyl isomer in good yield and isomeric purity. 

A convincing demonstration of the stability of cyclopentadienide anion can be found in the acidity of cyclopentadiene. 

The relative stability of the anions derived from cyclopropene and cyclopentadiene by deprotonation is just the reverse of the situation for the cations. Cyclopentadiene is one of the most acidic hydrocarbons known, with a pK of 16.0. The pA s of triphenylcyclopropene and trimethylcyclopropene have been estimated as 50 and 62, respectively, using electrochemical cycles (see Section 6.1). The unsubstimted compound would be expected to fall somewhere between and thus must be about 40 powers of 10 less acidic than cyclopentadiene. MP2/6-311-i-G(2fi(/,2pfi() and B3LYP/6-3ll+G(2df,2pd) calculations indicate a small destabilization of the cyclopropenyl anion, relative to the cyclopropyl anion. Thus the six Tr-electron cyclopentadienide anion is enormously stabilized relative to the four ir-electron cyclopropenide ion, in agreement with the Htickel rule. 

The first clear correlation between the formation of the stable cyclopentadienide anion and the fact that this ion had a sextet of iT-electrons was made in 1928 "in regard to its ability to provide the electrons for the stable sextet - cyclopentadiene can do so only by the appropriation of the electrons of one of its hydrogen atoms, it is this circumstance which gives to the hydrocarbon and its derivatives properties analogous to those of an acid, and confers stability on the corresponding anion " [9]. 

Introduction of electron-withdrawing groups attached to the cyclo-pentadiene ring greatly increases the stability and lowers the reactivity of the related cyclopentadienide anions, presumably because of enhanced delocalisation of the negative charge. An example of such an unreactive cyclopentadienide salt was first prepared in 1912 by the condensation of nitromalonaldehyde with hexane-2,5-dione in the presence of alkali [34] ... 

Synthesis of cyclopentadienyl compounds of metals from cyclopentadiene depends on the strongly acidic properties of cyclopentadiene. This acidity may be attributed to the ready formation of the highly resonance-stabilized cyclopentadienide anion, CsHs , in which there are six delocalized ir electrons. This method is limited primarily to the active metals of Groups lA and IIA and to their strongly basic compounds. The compounds produced are principally the ionically bonded metal cyclopentadienides. 

Di(l-azulenyl)(6-azulenyl)methyl cation (24+) represented in Figure 17 exemplifies the cyanine-cyanine hybrid (20). Di(l-azulenyl)methylium unit in 24+ acts as a cyanine terminal group. The tropylium substructure stabilizes the cationic state (24+). Reduction of 24+ should afford the neutral radical 24, which is stabilized by capto-dative substitution effect, because 24 is substituted with azulenes in the donor and acceptor positions. The anionic state (24") is also stabilized by contribution of the cyclopentadienide substructure, which should exhibit the third color change in this system. 

Diphenylfulvene is very readily converted to the distonic radical anion (29 ). The aromatic character of the 6-7i-cyclopentadienide ring explains the stability of the distonic species... 

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