Cyclopentadienyl anion aromaticity

Lithium cyciopentadienide (s), LiCp, may be formally protodelithiated to cyclopen-tadiene (Iq) by a simple Li/H exchange with an accompanying reaction enthalpy of 182.4 kJmoPL The large endothermicity is not unexpected for the nominally aromatic cyclopentadienyl anion. By comparison, for formal protodelithiation of solid LiOH to liquid H2O is 199.1 kJmol . ... 

Cyclopentadiene is unusually acidic because loss of a proton converts the nonaromatic diene to the aromatic cyclopentadienyl anion. Cyclopentadiene contains an sp3 hybrid (—CH2—) carbon atom without an unhybridized p orbital, so there can be no continuous ring of p orbitals. Deprotonation of the —CH2— group leaves an orbital occupied by a pair of electrons. This orbital can rehybridize to a p orbital, completing a ring of p orbitals containing six pi electrons the two electrons on the deprotonated carbon, plus the four electrons in the original double bonds. 

The proton to be removed has a pi< of about 25 because its conjugate base is an aromatic cyclopentadienyl anion (we discussed this in Chapter 8). The ElcB elimination takes place with a secondary or tertiary amine as the base. Spontaneous loss of CO2 from the eliminated product gives an amine, and you will meet this class of compounds again shortly in Chapter 25 where we discuss the Fmoc protecting group. 

Cyclopentadienyllithium. The tt charge in the aromatic cyclopentadienyl anion is distributed equally to all five carbon atoms. A lithium counterion should thus electrostatically favor a central location (Csv, 26a) over the tt face. The same conclusion is reached on the basis of MO considerations (46). The six interstitial electron interactions involving the three cyclopentadienyl TT orbitals and those of corresponding symmetry on lithium (one of these is shown in 26b) also favor structure 26a. 

Active Figure 15.5 An orbital view of the aromatic cyclopentadienyl anion, showing the cyclic conjugation and six tt electrons in five p orbitals. The electrostatic potential map further indicates that the ion is symmetrical and that all five carbons are electron-rich (red). Sign in at www.thomsonedu.com to see a simulation based on this figure and to take a short quiz. 

The calculations support unequivocally the notion that dianions of germoles and siloles are highly delocalized and that the degree of aromaticity in these molecules is nearly as large as in the classic aromatic cyclopentadienyl anion.[10]... 

According to structural, thermodynamic, electronic, and magnetic criteria the metallole dianions (and also their dilithio complexes) show indications of cyclic delocalization. The degree of "aromaticity" in silole and germole dianions is of the same magnitude as in the classic aromatic cyclopentadienyl anion... 

It is a [l,S]-sigmatropic rearrangement. The figure T in the square brackets shows that the same atom is at one end of the new a bond as was at one end of the old a bond. One atom has moved in a 1,5 manner and these are often called [l,5]-sigmatropic shifts. This is often abbreviated to [1,S]H shift to show which atom is moving. This particular example is important because sadly it prohibits a most attractive idea. The aromatic cyclopentadienyl anion is easily formed, stable, and readily alkylated. This sequence of alkylation and Diels—Alder reaction looks very good. 

Figure 8.27 Aromatic cyclopentadienyl anion contribution in the TSs of 1,5-Ef shifts, natural (NBO) charges at...

Spiro[3,4]octa-l,5,7-triene (249) has been prepared by modification of the carboxylic acid (250). The triene was characterized chemically by reduction to the saturated hydrocarbon and by its Diels-Alder reaction with JV-phenyltriazoline-3,5-dione. Any spiroconjugative interaction in the triene is not shown on its u.v. spectrum, which has at 262 nm, the same as observed for the diene (251). The triene is unstable, rearranging at — 5°C (t = 90 min) to 6-vinylfulvene and a dihydropentalene derivative. The ring-opening of (249) to 6-vinylfulvene is unusually rapid for a 3,3-disubstituted cyclobutene derivative, and other similarly strained compounds do not show this reactivity. The ring-opening may well proceed via a biradical intermediate with stabilization of the zwitterionic form of the biradical by formation of the aromatic cyclopentadienyl anion. 

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 . ... 

Attack by Nu at C4 generates a fused, aromatic cyclopentadienyl anion framework. 

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