Cyclopentadiene cyclopentadienyl anion from

It is appropriate here to compare the acidity of cyclopentadiene, which has pATa 16, considerably more acidic than most hydrocarbon systems and comparable to water and alcohols. Removal of one of the CH2 protons from the non-aromatic cyclopentadiene generates the cyclopentadienyl anion. This anion has an aromatic sextet of electrons, two electrons being contributed by the negatively charged carbon (see Section 2.9.3). 

Neodymium cyclopentadienyl complexes are also obtained in-situ by the addition of Cp-derivatives (e.g. indene, cyclopentadiene, pentamethyl-cyclopentadiene, tetramethylcyclopentadiene, di-tert-butylcyclopentadiene, methylcyclopentadiene and fluorene) to standard Nd-catalyst systems such as NdV/MAO. It can be assumed that the respective cyclopentadienyl-anions are formed by proton abstraction from the Cp-derivatives. In the homopolymerization of BD the addition of Cp-derivatives results in an increase of the 1,2-content of about 4-10%. In addition, the in-situ formed Nd Cp-derivatives... 

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 ... 

The cyclopentadienyl anion is readily formed from cyclopentadiene by a Brpnsted-Lowry acid-base reaction. 

CarbenoU synthesis of hydrocarbons. Katz et al. - reported a simple synthesis of the previously rare valence isomer of benzene, benzvalene (3), from cyclopentadiene (1). The diene is first converted into the cyclopentadienyl anion (2) by treatment with methyllithium in dimethyl ether then a solution of methylene chloride and methyl-lithium in diethyl ether is added at —45°. Benzvalene (3) was obtained in 24, yield together with a trace (6.4%) of benzene. 

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). 

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. 

To form an aromatic ion from cyclopentadiene, it is necessary to convert the CH2 group to a CH group in which the carbon becomes sp hybridized and has two electrons in its unhybridized 2p orbital. The resulting cyclopentadienyl anion is aromatic. Its aromatic character may also be represented by an inscribed circle with a minus sign (Figure 21.12). 

Endocyclic dienes ranging in ring size from four to seven have also been used in the intramolecular Diels-Alder reaction. Cyclobutadienes are prepared from the corresponding metal complexes [37]. Cyclopentadienes are most readily prepared by alkylation of cyclopentadienyl anion [38, 39]. They are also available by Michael addition to fulvene derivatives and by aminal exchange [40, 41]. 

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 . ... 

Flgure15.6 The aromatic six-ir-electron cyclopentadienyl anion can be formed by removing a hydrogen ion (H ) from the CH2 group of 1,3-cyclopentadiene. Similarly, the aromatic six-Tr-electron cycloheptatrienyl cation can be generated by removing a hydride ion (H ) from the CH2 group of 1,3,5-cycloheptatriene. 

The ferrocene molecule serves as a more involved example from organometallic chemistry. As can be seen from Figure 2(b), ferrocene is a representative of the group known as sandwich compounds two parallel cyclopentadienyl rings enclose one iron atom. From both cyclopentadiene moieties, CsHfi, one proton has been removed, respectively, leaving behind two cyclopentadienyl anions, CsHs". The iron is a two-fold positively charged cation the whole molecule is, therefore, electrically neutral. The ROSDAL encoding can be as follows ... 

Certain five-membered rings can also be aromatic. An example of this is the cyclopentadienyl anion. If cyclopentadiene is treated with a suitable base, a proton is removed from the sp carbon and the electron pair from the C-H bond remains with that carbon. The hybridization of the carbon becomes sp and the pair of electrons is contained within the p-orbital. At this point, the ring is flat, each atom in the ring has a p-orbital and there are six electrons in the n-system (four from double bonds and two from the filled p-orbital). The fact that this anion is... 

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