Carbanions cyclopentadienide

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. 

A systematic study of some monosubstituted solvent-separated cyclopentadienide salts of Group 1 cations (Li, Na, K, Cs) in DMSO revealed the bonding preferences for the fulvenoid (4) over the cyclopentadienyl isomer (5). Three of the substiments contained an a-carbonyl group formyl, acetyl and benzoyl. From the viewpoint of the anions, there is a conflict between the inherent greater stability of oxyanions over carbanions and the aromaticity associated with the 6 jr-electron carbocyclic system. The authors demonstrated that the fulvenoid interaction is favored by strong electron-accepting substituents and... 

Whereas sodium indenide displays a stereoselectivity similar to phenylzinc chloride (see Table 26), sodium cyclopentadienide and 3-trimethylsilylindene act like soft carbanions. Both substrates 10 and 12 undergo allylic substitution with net retention of configuration to 1127 and 1328, respectively, via attack on the ally ligands of the intermediate 7t-allylpalladium complexes anti to the metal. Again, the ratio of the two possible diastereomers of structure 13 is unknown. 

Direct reaction between sodium or potassium and alkyl and aryl halides is complicated by exchange and coupling reactions, which can lead to mixtures of products. These complications can be reduced by rapid stirring and the use of finely divided metal or amalgams. Phenylsodium can be made in this way PhCl -h Na->PhNa-h NaCl. Acidic hydrocarbons react with alkali metals in ether solvents. Cyclopentadiene, for example, affords sodium cyclopentadienide in tetrahydrofuran (p. 279). Triphenylmethylpotassium is obtained as a deep red solution from triphenylmethane and potassium in 1,2-dimethoxyethane. These carbanions, in which the negative charge is delocalized over several carbon atoms, do not attack ethers, in contrast to the simple alkyl or aryl carbanions present in methylsodium or phenylpotassium. 

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