Cyclopentadiene stable anion from

The cyclopentadienide ion, C H , is a common organic anion that forms very stable complexes with metal cations. The anion is derived by removing a proton from cyclopentadiene, QH, with strong base. The molecule has a five-memhered ring of carbon atoms, with four carbon atoms attached to only one proton and one carbon atom bonded to two. Draw the Lewis... 

The crucial structural feature which underlies the aromatic character of benzenoid compounds is of course the cyclic delocalised system of six n-electrons. Other carbocyclic systems similarly possessing this aromatic sextet of electrons include, for example, the ion C5Hf formed from cyclopentadiene under basic conditions. The cyclopentadienide anion is centrosymmetrical and strongly resonance stabilised, and is usually represented as in (7). The analogous cycloheptatrienylium (tropylium) cation (8), with an aromatic sextet delocalised over a symmetrical seven-membered ring, is also demonstrably aromatic in character. The stable, condensed, bicyclic hydrocarbon azulene (Ci0H8) possesses marked aromatic character it is usually represented by the covalent structure (9). The fact that the molecule has a finite dipole moment, however, suggests that the ionic form (10) [a combination of (7) and (8)] must contribute to the overall hybrid structure. 

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 ... 

Within the context of stable carbenium salts initiatron, we already examined a very interesting and successful study on the block copolymeriation of a-methylstyrene with cyclopentadiene performed by Vairon and Villesange (see Sect. V-A-4-b). The preparation of the product required three basic operations (i) the living anionic polymerisation of a-methylstyrene to give monodisperse macromolecules, (ii) transformation of their end groups into stable carbocationic moieties, and (iii) initiation of the polymerisation of cyclopentadiene from these active ends under conditions of minimal transfer and termination reactions. Thus, the macroinitiators in the second polymerisation were generated by a controlled anionic polymerisation and allowed tiie synthesis of a triblock near-isomolecular copolymer. 

The cation possesses a more favourable geometry than 1,6-methano-[10]annulene, being able to incorporate the 10 -system into a more planar structure. It is evidently more stable than the tropylium ion, as it can be formed from its neutral hydrocarbon precursor 57 by a hydride-transfer reaction with tropylium tetrafluoroborate 136>. The anion, on the other hand, involves substantial steric strain and is not as planar as either the cation or the neutral bridged [lOJannulene. Whereas formation of the cation from 57 was seen to be most favourable, the anion precursor 58 is much less acidic than cyclopentadiene, although in the presence of... 

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). 

Metallocenes are derived from the cyclopentadiene anion, (CsH5) , shown in Scheme 6.8(a). These are stable units, in which a delocalised orbital, similar... 

The all-c is cyclononatetraenide anion extracts a proton from cyclopentadiene to give the cyclopentadicnide anion [61,66]. Ilquili-bration of the cyclononatetraenide ion with cyclopentadiene suggests that it is more thermodynamically stable than the cyclopentadicnide anion, and that the pK of the cyclononatetraene lies between 16 and 21, since no proton exchange takes place with indene [66]. 

Anions are similarly examined, and the cyclopentadienyl anion (118) has six 7t-electrons and meets all criteria for aromaticity. It is aromatic, very easy to form, and quite stable. Formation of 118 from cyclopentadiene (120) is an acid-base reaction. It is known that 120 has a relatively low pK that reflects the special aromatic stability of the aromatic conjugate base. The pK of cyclo-pentadiene is 14-15 (compare that with a pK of 15.8 for water).This contrasts sharply to the cycloheptatrienyl anion (119), which has 4n 7i-electrons, is not aromatic, and is particularly unstable and difficult to form. As with 118, formation of 119 is an acid-base reaction from cycloheptatriene, 121. The pKg of 121 is about 36,1 however, which reflects the great difficulty in forming the antiaromatic conjugate base. 

Cyclopentadiene is a muci stronger acid than propene. The difference in acidity is enormous. Look carefully at the structure of the cyclopentadienyl anion. Here too, we have a planar, cyclic, and fully conjugated system. The molecular orbitals can be derived from a Frost circle (Fig. 13.29).There are six electrons to put into the molecular orbitals, and, as in the tropylium ion or benzene, they fully occupy the lowest molecular orbital and the set of degenerate bonding molecular orbitals. The cyclopentadienyl anion can be described as aromatic, and for an anion, this species is remarkably stable. Do not fall into the trap of expecting this anion to be as stable as benzene. 

As described above, Kealy and Pauson [9] published in Nature in 1951 that ferrocene is prepared by the reaction of ferrous chloride with Grignard reagents. The purpose of the reaction was to synthesize liable fulvalene. However, the product was very stable. From the analytical data, it is thought that it is the (T-bond compound of two cyclopentadiene rings and iron or the complex of two aromatic cyclopentadienyl anions and Fe . ... 

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