Cyclopentadiene anion

The allyl-substituted cyclopentadiene 122 was prepared by the reaction of cyclopentadiene anion with allylic acetates[83], Allyl chloride reacts with carbon nucleophiles without Pd catalyst, but sometimes Pd catalyst accelerates the reaction of allylic chlorides and gives higher selectivity. As an example, allylation of the anion of 6,6-dimethylfulvene 123 with allyl chloride proceeded regioselectively at the methyl group, yielding 124[84]. The uncatalyzed reaction was not selective. 

Many other TT-organometallic compounds have been prepared. In the most stable of these, the total number of electrons contributed by the ligands (e.g., four for allyl anions and six for cyclopentadiene anion) plus the valence electrons on the metal atom or ion is usually 18, to satisfy the effective atomic number rule.31 ... 

From the diagrams shown in Figure 5.20, would you expect the lowest energy spectral band to be at higher energy for the cyclopropene cation, cyclopentadiene anion, or benzene ... 

Aromatic cyclic ions cyclopentadiene anion, cycloheptatriene cation (6 electrons)... 

Controlled one-electron reductions transform l,2,3,4-tetraphenyl-l,3-cyclopentadiene or 1,2,3, 4,5-pentaphenyl-l,3-cyclopentadiene into mixtures of the dihydrogenated products and the corresponding cyclopentadienyl anions (Famia et al. 1999). The anion-radicals initially formed are protonated by the substrates themselves. The latter are thermodynamically very strong acids because of their strong tendency to aromatization. As with the cyclopentadiene anion-radicals, they need two protons to give more or less stable cyclopentadienes. The following equations represent the initial one-electron electrode reduction of l,2,3,4,5-pentaphenyl-l,3-cyclopentadiene (CjHAtj) and explains the ratio and the nature of the products obtained at the expense of the further reactions in the electrolytic pool ... 

Pyrans 151a (R = Ph) and 151c gave with excess butyllithium at -30 to - 120°C 515-like anions, which isomerized to substituted cyclopentadiene anions and then, either dimerized or reacted with the reagent (Scheme 33).423... 

Compounds containing methylene groups activated by both a cationic ring and another electron-withdrawing group easily form stable anhydro-bases, e.g. (636) — (637), (638) — (639). Stabilization is also achieved by utilization of the aromatic character of the cyclopentadiene anion or the pyrrole anion compounds of type (640 Z = NR, O, S) and (643) readily lose protons to give the mesomeric anhydro-bases (as 641 <- 642) which are called pseudoazulenes. 

Soft carbon nucleophiles other than malonate derivatives follow similar patterns of regioselectivity, as shown by the addition of cyclopentadiene anion (equation 211)>os and the highly acidic 4-hydroxy-6-methyl-2-pyrone (equation 212).393... 

The cyclopentadiene anion is stabilized by five equivalent resonance structures. The anion is an aromatic anion by virtue of it being a six-jr-electron system. The indenyl anion is stabilized by a total of seven resonance contributors. However, they are nonequivalent and all but one require that the aromatic cloud of the benzene ring is disrupted. Thus, while the negative charge is well delocalized, the resonance stabilization is less than that of the cyclopentadiene system. Thus the proton is not as easily removed, making indene a weaker acid. 

Fe2 + and six electrons from each of the two cyclopentadiene anions. Nickel has two electrons more than iron so nickellocene [(cp)2Ni] has 20 electrons. The additional electron in nickellocene are in an antibonding orbital. The bond order of nickellocene is higher than that of... 

It is a [1,5]-sigmatropic rearrangement. The figure T in the square brackets shows that the same atom is at one end of the new o bond as was at one end of the old c bond. One atom has moved in a 1,5 manner and these are often called [1,5]-sigmatropic shifts. This is often abbreviated to [1,5]H shift to show which atom is moving. This particular example is important because sadly it prohibits a most attractive idea. The cyclopentadiene anion is.very stable (Chapter 8) and can easily be alkylated. The sequence of alkylation and Diels-Alder reaction looks very good. 

Fig-1- Basic structures of the metal hydride complexes exhibiting the QEC effect, (a) Trihydride, (b) cA-dihydride, (c) dihydrogen, and (d) hydrido dihydrogen complexes. The symbol Cp denotes variously substituted cyclopentadiene anions. 

Addition of a C3 unit (ethyl a-bromopropionate) to cyclopentadiene anion forms the start of a synthesis of viburtinal (400), isolated from Viburnum spp. 

A 6n electrocyclic closure could be implicated in the photocyclisation reaction reported for the N-amino pyridinium ion (257) and some of its ring substituted derivatives to give (258). This type of reactivity was also seen for compound (233). A summary of the previously published work and of new results for the 67t photocyclisation of photochromic ethylenes substituted by derivatives of cyclopentadiene anion and pyridinium cation has appeared.The basic skeleton involved in the rearrangement is shown in structure (259) which is in photochemical equilibrium with (260). 

Table 13.9 shows the EDA results for metallocenes of the alkali metals. A qualitative orbital correlation for the interaction between a singly charged metal atom E with a singly charged cyclopentadiene anion Cp is shown in Eig. 13.4. The six E-Cp bonding electrons occupy the lowest lying aj and the degenerate ej orbitals. 

Although theoretically we can view this hydrocarbon as a cyclopentadienate anion substituted on a tropylium ring, experimentally this structure has not been isolated. It appears that it polymerizes readily, due either to low stability or to a more favorable polymerized structure. 

Die Stabilisierung der negativen Ylid-Ladung gelingt auch durch Ein-beziehung in das 67r-Aromatensystem eines Cyclopentadien-anions. 

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

A full report has been published on the preparation of fused valerolactones from cyclopentadiene anions by condensations with epoxides, carboxylation, and finally cyclization (Scheme 26). Diels-Alder reactions of 3-hydroxy-2-pyrones usually lead to benzene derivatives by decarboxylation of the intermediate lactone. It has now been shown that these lactones can be isolated in good yields by suppression of the CO2 loss by carrying out the cycloadditions under a high pressure of carbon dioxide. 

In the ferrocene molecule, the cyclopentadiene anion reacts like an aromatic organic molecule. Because ferrocene is quite stable, it has been possible to perform reactions characteristic of an aromatic system on the ring without destroying the bonding to the metal, reaction (8). An extensive organic... 

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