Stability cyclopentadienyl anion

Whereas alkylation of activated methylene systems by classical methods produces a mixture of mono- and dialkylated products, with the latter frequently predominating, phase-transfer catalytic procedures permit better control and it is possible to obtain only the monoalkylated derivatives. Extended reaction times or more vigorous conditions with an excess of the alkylating agent lead to dialkylated products or, with dihaloalkanes, carbocyclic compounds as the technique mimics dilute concentration conditions, e.g. the resonance stabilized cyclopentadienyl anion, generated under solidiliquid two-phase conditions, or under liquiddiquid conditions, reacts with 1,2-dihaloethanes to form spiro[2,4]hepta-4,6-diene (70-85%) [1-3]. Reaction with dichloromethane produces bis(cyclopenta-2,4-dien-l-yl)methane (60%) [4],... 

The acidity of cyclopentadiene provides convincing evidence for the special stability of cyclopentadienyl anion. 

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). 

The structure of cyclopentadienylthallium(I) has been the subject of controversy and while the arguments have not been entirely satisfactorily settled, the evidence now available indicates that the compound is primarily ionic in the solid state but possibly mainly covalent in the gaseous phase. The former conclusion at least is reasonable in view of the well-known stability of the cyclopentadienyl anion. Cyclopentadienylthallium(I) has... 

Although the details will not be shown, it is easy to compute the resonance energies to determine the stabilities of rings with five carbon atoms (the cyclopentadiene, Cp, ring). When this is done, it is found that Cp > Cp > Cp+, which is in agreement with the fact that there is an extensive chemistry associated with the cyclopentadienyl anion. 

Given a sufficiently electronegative carbon atom, no carbonyl or conjugated carbonyl group is heeded for addition to take place. For example, a carbon atom that is part of the cyclopentadiene system is able to stabilize the negative charge quite well. The cyclopentadienyl anion is comparatively stable.8 8... 

The first metal-olefin complex was reported in 1827 by Zeise, but, until a few years ago, only palladium(II), platinum(Il), copper(I), silver(I), and mercury(II) were known to form such complexes (67, 188) and the nature of the bonding was not satisfactorily explained until 1951. However, recent work has shown that complexes of unsaturated hydrocarbons with metals of the vanadium, chromium, manganese, iron, and cobalt subgroups can be prepared when the metals are stabilized in a low-valent state by ligands such as carbon monoxide and the cyclopentadienyl anion. The wide variety of hydrocarbons which form complexes includes olefins, conjugated and nonconjugated polyolefins, cyclic polyolefins, and acetylenes. 

Iridium(I), stabilized by the ir-cyclopentadienyl anion, forms a stable, diamagnetic complex [Ir(C6H6)(C6Ht)] with cyclopentadiene (91) by the reaction ... 

By considering the n MOs of the cyclopentadienyl system (C5H5) to result from an interaction between cri-butadiene n MOs and an sp1 hybridized C atom, explain the stability of the cyclopentadienyl anion and the instability of the cyclopentadienyl cation. 

Ferrocene is only one of a large number of compounds of transition metals with the cyclopentadienyl anion. Other metals that form sandwich-type structures similar to ferrocene include nickel, titanium, cobalt, ruthenium, zirconium, and osmium. The stability of metallocenes varies greatly with the metal and its oxidation state ferrocene, ruthenocene, and osmocene are particularly stable because in each the metal achieves the electronic configuration of an inert gas. Almost the ultimate in resistance to oxidative attack is reached in (C5H5)2Co , cobalticinium ion, which can be recovered from boiling aqua regia (a mixture of concentrated nitric and hydrochloric acids named for its ability to dissolve platinum and gold). In cobalticinium ion, the metal has the 18 outer-shell electrons characteristic of krypton. 

Fig. 6 Reaction energy profile for reactions 34a/34b (A), 35a/35b (B), and 36a/36b (C). (A) and (B) Aromatic stabilization of the transition state is greater than that of benzene or cyclopentadienyl anion, respectively. (C) Anti-aromatic destabilization (positive ASE) of the transition state is less than that of cyclobutadiene the high barrier results from the additional contribution by angular and torsional strain at the transition state.

Fig. 4.23 Hiickel s rule says that cyclic n systems with An + 2 n electrons ( = 0, 1, 2,. .. An + 2 = 2, 6, 10,. ..) should be especially stable, since they have all bonding levels full and all antibonding levels empty. The special stability is usually equated with aromaticity. Shown here are the cyclopropenyl cation, the cyclobutadiene dication, the cyclopentadienyl anion, and benzene formal structures are given for these species - the actual molecules do not have single and double bonds, but rather electron delocalization makes all C/C bonds the same...

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. 

We have seen that aromatic stabilization leads to unusually stable hydrocarbon anions such as the cyclopentadienyl anion. Dianions of hydrocarbons are rare and are usually... 

The pyrrole anion, C4H4N -, is a 6 n electron species that has the same electronic structure as the cyclopentadienyl anion. Both of these anions possess the aromatic stability of 6 n electron systems. 

The Fmoc group is acidic because the resulting anion is similar to the cyclopentadienyl anion, which is resonance-stabilized and is aromatic. 

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