Cyclopentadienid anion

Based on the above-mentioned stereochemistry of the allylation reactions, nucleophiles have been classified into Nu (overall retention group) and Nu (overall inversion group) by the following experiments with the cyclic exo- and ent/n-acetales 12 and 13[25], No Pd-catalyzed reaction takes place with the exo-allylic acetate 12, because attack of Pd(0) from the rear side to form Tr-allyl-palladium is sterically difficult. On the other hand, smooth 7r-allylpalladium complex formation should take place with the endo-sWyWc acetate 13. The Nu -type nucleophiles must attack the 7r-allylic ligand from the endo side 14, namely tram to the exo-oriented Pd, but this is difficult. On the other hand, the attack of the Nu -type nucleophiles is directed to the Pd. and subsequent reductive elimination affords the exo products 15. Thus the allylation reaction of 13 takes place with the Nu nucleophiles (PhZnCl, formate, indenide anion) and no reaction with Nu nucleophiles (malonate. secondary amines, LiP(S)Ph2, cyclopentadienide anion). 

Fig. 16. Stmctural analogy between the 1 dicarboUide dianion and the [C H ] cyclopentadienide anion (6) where O represents BH , CH ...

An interesting structural question involves the contribution of a dipolar structure which pictures the molecule as the fusion of a cyclopentadienide anion and a cycloheptatrienyl cation ... 

Because the five-membered ring is a substituted cyclopentadienide anion in some dipolar resonance stmctures, it might be expected that exocyclic groups that could strongly stabilize a positive charge would lead to a larger contribution from dipolar stmctures and enhanced stability. The stmctures 13 and 14 are cases in which a dipolar contribution would be feasible. 

Section 11.21 Species with six tt electrons that possess special stability include certain ions, such as cyclopentadienide anion and cycloheptatrienyl cation. 

The preparation of mono- and di-tm-butylcyclopentadienes 1 and 2 starting from monomeric cyclopentadiene was reported first in 1963 [23]. It was noted that the nucleophilic attack of the cyclopentadienide anion on ferf-alkyl halide has to compete with elimination reaction giving isobutene. The yield of the di- and tri-fer/-butylcyclopentadienes 2 and 3 was therefore reported to be modest to low [23, 24], Recently an elegant improvement for this synthesis using phase transfer catalysis was presented (Eq. 1), but the availability of the tri-substituted derivative... 

Bases that would be fully protonated in water, such as the cyclopentadienide anion,, behave as very weak bases in ammonia. 

More recently, a very efficient asymmetric carbolithiation of N,N-dimethyl-aminofulvene 30, leading to a chiral cyclopentadienide anion, was reported by Hayashi et al. [6] for the synthesis of chiral metallocenes (Scheme 6). By adding an aryl lithium such as 31 complexed with a chiral ligand on fulvene 30, a cyclopentadienide ion 32 bearing a stereogenic center at the a position was generated. This anion was reacted with [RhCl(nbd)]2 to yield... 

One of the most familiar of the TT-organometallic compounds is ferrocene, a neutral compound that is readily prepared from cyclopentadienide anion and iron(II).316... 

According to a semiempirical study by Malar, the different polyphospholide anions have 86—101% aromaticities of that of the cyclopentadienide anion. Chesnut and Quin reported on the basis of GIAO NMR calculations using a triple-valence quality basis set that the phospholide anion s aromaticity is 63% that of the cyclopentadienide anion. The aromatic stabilization energy (ASE) obtained by Schley-er et al. from eq 2 (X = P ) was 90% that of the cyclopentadienide anion. 

An unexpected elimination of cyclopentadienide anion results on reaction of sila-cyclohexadienes with an extremely hindered aryllithium. ... 

The pyrrolyl ligand is interesting in that it can function as both a u-bonded amido group or a yr-bonded multihapto ligand. The pyrrolyl group appears to be a weaker electron donor than the cyclopentadienide anion 90). In the tetrakis-(2,5-dimethylp3u rolyl)uranium(IV) complex a preliminary structure has been revised with a tris(monohapto) (pentahapto) structure now assigned from variable temperature PMR spectra 90). 

In organic chemistry this stabilizing effect is well known the stability of carbanions is known to be enhanced by nitro groups. The stability of the cyclopentadienide anion is increased by complexing with a typical Lewis acid so that it becomes less reactive. For example, ferrocene is not ionized in nitromethane solution. Addition of a Lewis acid such as aluminum chloride facilitates the occurrence of intramolecular race-mization (75) a process which is believed to involve ionic intermediates [16). This belief is supported by kinetic evidence and the failure of the reaction to occur in nearly inert solvents like methylene chloride and in those of high donidty. Whereas the former do not support the solvation of the cation formed in the process of ionization, the latter will react preferentially with the Lewis acid, which is then no longer available for the stabilization of the carbanion. 

Phosphorus analogues of the cyclopentadienide anion, i.e. phospholide (chapter 2.15), 1,2-diphos-pholide (chapter 3.15), 1,3-diphospholide (chapter 3.16), 1,2,4-triphospholide (Y), 1,2,3-triphos-pholide (Z), tetraphospholide (Ai), and pentaphospholide (0) ions were examined by complete... 

The divergent hyperfine patterns of the two ions can be ascribed to their 6ti aromatic substructures, cyclopentadienide anion, and cycloheptatrienylium cation, respectively. 

The tropylium ring is planar with equal C—C distances. Like benzene and the cyclopentadienide anion, the tropvlium cation is an aromatic, six-electron species. 

---