Aromatic rings cyclopentadienyl anion

Todorov, 1. Sevov, S. C. Heavy-metal aromatic rings Cyclopentadienyl anion analogues Sn5 and Pb in the zintl phases NagBaPb, NagBaSn, and NagEuSn. Inorg. Chem. 2004,43, 6490-6494. 

Look back again at the definition of aromaticity in the previous section "... a cyclic, conjugated molecule containing 4n + 2 tt electrons. Nothing in this definition says that the number of tt electrons mnst he the same as the number of atoms in the ring or that all the atoms in the ring mnst he carbon. In fact, both ions and heterocyclic compounds, which contain atoms of different elements in their ring, can also he aromatic. The cyclopentadienyl anion and the cycloheptatrienyl cation are perhaps the best known aromatic ions, while pyridine and pyrrole are common aromatic heterocycles. 

Pyrrole has a planar, pentagonal (C2 ) stmcture and is aromatic in that it has a sextet of electrons. It is isoelectronic with the cyclopentadienyl anion. The TT-electrons are delocalized throughout the ring system, thus pyrrole is best characterized as a resonance hybrid, with contributing stmctures (1 5). These stmctures explain its lack of basicity (which is less than that of pyridine), its unexpectedly high acidity, and its pronounced aromatic character. The resonance energy which has been estimated at about 100 kj/mol (23.9 kcal/mol) is intermediate between that of furan and thiophene, or about two-thirds that of benzene (5). 

The CC bond distances in cyclopentadienyl anion, C5H5, are all equal, because the anion is aromatic (see Chapter 12, Problem 10). Electrophiles that interact electrostaticaUy with the anion, such as Na", interact equally with all five carbons, and do not disturb the anion s aromatic character. On the other hand, electrophiles that make covalent bonds, such as H", might interact more strongly with one particular carbon and destroy the aromaticity of the ring. 

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). 

Structures that are also aromatic are the cyclopropenyl cation (2 jt electrons n = 0) and the cyclopentadienyl anion (6 n electrons n = 1). Although we do not wish to pursue these examples further, they are representative of systems where the number of jr electrons is not the same as the number of carbon atoms in the ring. 

Azulene can be written as fused cyclopentadiene and cycloheptatriene rings, neither of which alone is aromatic. However, some of its resonance structures have a fused cyclopentadienyl anion and cycloheptatrienyl cation, which accounts for its aromaticity and its dipole moment of 1.0 D. 

The aromatic complex can be a neutral t °-benzene derivative or an anionic ri -cyclopentadienyl ring. Substituents on these aromatic rings can greatly influence the effectiveness of these catalysts. For example, with benzene derivatives the unsubstituted benzene rings give lower ees and the use of hexamethylbenzene results in lower catalytic activities whilst the cumenyl or mesityl rings give optimum catalyst systems. The two types of chiral bifunctional hnkers that have been most practical are anionic ones based on monosulfonated diamines and amino alcohols. 

The situation in pyrrole is slightly different. Pyrrole has a tt electron system similar to that of the cyclopentadienyl anion. It has four 5p -hybridized carbons, each of which has a p orbital perpendicular to the ring and contributes one tt electron. The nitrogen atom is also 5p -hybridized and its lone pair electrons occupies a p orbital. Therefore, there is a total of six tt electrons, which makes pyrrole an aromatic compound. 

Many aromatic compounds have considerable resonance stabilization but do not possess a benzene nucleus, or in the case of a fused polycyclic system, the molecular skeleton contains at least one ring that is not a benzene ring. The cyclopentadienyl anion C5HJ, the cycloheptatrienyl cation C7H+, the aromatic annulenes (except for [6]annulene, which is benzene), azulene, biphenylene and acenaphthylene (see Fig. 14.2.2(b)) are common examples of non-benzenoid aromatic hydrocarbons. The cyclic oxocarbon dianions C Of (n = 3,4,5,6) constitute a class of non-benzenoid aromatic compounds stabilized by two delocalized n electrons. Further details are given in Section 20.4.4. 

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. 

It should be noted, however, that the concept of o-aromaticity is fairly limited in its applicability—to just three-member rings—certainly in contrast to the notion of aromaticity. For example, aromaticity can be nsed to gronp componnds with differing ring sizes (benzene, naphthalene, cyclopentadienyl anion, tropylinm cation, etc.), but no one is suggesting that cyclopentane, which has 10 o-electrons, exhibits... 

Nevertheless, the NICS values appear to readily classify standard molecules into three discrete categories. Aromatic molecules possess NICS values that are negative. The values at the center of the six-member rings of benzene and naphthalene and anthracene are -9.7 and -9.9, respectively. Charged aromatic molecules also have negative NICS values the values for cyclopentadienyl anion and tropy-lium cation are -14.3 and -7.6 ppm, respectively. Nonaromatic compounds like cyclohexane and adamantane have NICS values near zero. Lastly, antiaromatic molecules such as cyclopentadiene and planar Z>4 cyclooctatetrane have NICS values that are positive, 27.6 and 30.1 ppm, respectively. 

The cyclopentadienyl anion is a cyclic and planar anion with two double bonds and a non-bonded electron pair. In this way it resembles pyrrole. The two n bonds contribute four electrons and the lone pair contributes two more, for a total of six. By HiickeTs rule, having six n electrons confers aromaticity. Like the N atom in pyrrole, the negatively charged carbon atom must be sp hybridized, and the nonbonded electron pair must occupy a p orbital for the ring to be completely conjugated. 

The cyclopentadienyl anion and the tropylium cation both illustrate an important principle The number of n electrons determines aromaticity, not the number of atoms in a ring or the number of p orbitals that overlap. The cyclopentadienyl anion and tropylium cation are aromatic because they each have six it electrons. 

Free-base corrole (e.g., 2.6), like porphyrin, contains an 18 n-electron pathway and, as such, can sustain a diamagnetic, aromatic ring current (Scheme 2.1.1). Corroles are much stronger acids (and weaker bases) than porphyrins. Thus, unlike porphyrin, corrole forms a stable anion when treated with aqueous alkali. This anion, best represented by structure 2.7, is also an aromatic 18 Tt-electron system, which may account in part for its special stability. In any event, the neutral form of the macrocycle may be regenerated on acidification. Thus, the corrole-to-porphyrin relationship in terms of acidity allows for comparison to be made between these two systems and their smaller six n-electron counterparts benzene and cyclopentadienyl anion (Figure 2.1.2). In other words, cyclopentadienyl anion may be regarded as a contracted benzene just as corrole anion may be considered as being a contracted porphyrin . 

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... 

Figure 10.5. Cyclopentadienyl anion, (a) Two electrons in p orbital of one carbon one electron in p orbital of each of the other carbons, (b) Overlap of p orbitals to form n bonds, (c) tt clouds above and below plane of ring total of six tt electrons, the aromatic sextet.

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