Cyclopentadienides

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). 

A standard method for prepanng sodium cyclopentadienide (CsHsNa) is by the reaction of cyclopentadiene with a solution of NaNH2 in liquid ammonia Write a net ionic equation for this reaction identify the acid and the base and use curved arrows to track the flow of electrons... 

Nickel plays a role in the Reppe polymeriza tion of acetylene where nickel salts act as catalysts to form cyclooctatetraene (62) the reduction of nickel haUdes by sodium cyclopentadienide to form nickelocene [1271 -28-9] (63) the synthesis of cyclododecatrienenickel [39330-67-1] (64) and formation from elemental nickel powder and other reagents of nickel(0) complexes that serve as catalysts for oligomerization and hydrocyanation reactions (65). 

TC-Cyclopentadienyl Nickel Complexes. Nickel bromide dimethoxyethane [29823-39-9] forms bis(cydopentadienyl)nickel [1271 -28-9] upon reaction with sodium cyclopentadienide (63). This complex, known as nickelocene, 7T-(C3H3)2Ni, is an emerald-green crystalline sandwich compound, mp 173°C, density 1.47 g/cm. It is paramagnetic and slowly oxidi2es in air. A number of derivatives of nickelocene are known, eg, methylnickelocene [1292-95-4], which is green and has mp 37°C, and bis( 7t-indenyl)nickel [52409-46-8], which is red, mp 150°C (87,88). 

In the early 1960s it was recognized (6,187) that there were bonding similarities between the pentagonal face of the isomeric nido-Q fri ]f, ions and the well-known cyclopentadienide ion (Cp ) (Fig. 16). The isomeric nido-Q fri ]f, ions, which are commonly known as dicarboUide ions, and... 

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

Dicyclopentadienedicarboxylic Acid. Sodium cyclopentadienide (10) can be carboxylated to give dicyclopentadienedicarboxyHc acid (11)... 

The cooled contents of the 2S0-ml. flask containing ferrous chloride (Note 6) are added to the cold sodium cyclopentadienide solution while passing a stream of nitrogen through both flasks. The combined mixture is stirred for 1.25 hours at a temperature just below reflux. Solvent is removed by distillation, and the ferrocene is extracted from the residue with several portions of refluxing petroleum ether (b.p. 40-60°). The product is obtained by evaporation of the petroleum ether solution. Ferrocene may be purified by recrystallization from pentane or cyclohexane (hexane, benzene, and methanol have also been used) or by sublimation. The 3ueld is 31-34 g. (67-73%) (Note 7), m.p. 173-174°. 

Under the best conditions, sodium cyclopentadienide gives pale yellow or orange solutions. Traces of air lead to red or purple solutions, as does insufficiently purified solvent, without, however, lowering the reaction yield appreciably. If 1,2-di-methoxyethane is used, in which sodium cyclopentadienide is less soluble than in tetrahydrofuran, white crystals may be obtained at this point. 

Ferrous chloride may be substituted by ferric chloride directly, with a corresponding reduction in yield, since the sodium cyclopentadienide solution will reduce ferric chloride. 

The relative stability of the anions derived from cyclopropene and cyclopentadiene by deprotonation is just the reverse of the situation for the cations. Cyclopentadiene is one of the most acidic hydrocarbons known, with a of 16.0. The plCs of triphenylcyclo-propene and trimethylcyclopropene have been estimated as 50 and 62, respectively, from electrochemical cycles. The unsubstituted compound would be expected to fall somewhere in between and thus must be about 40 powers of 10 less acidic than cyclopentadiene. MP2/6-31(d,p) and B3LYP calculations indicate a small destabilization, relative to the cyclopropyl anion. Thus, the six-7c-electron cyclopentadienide ion is enormously stabilized relative to the four-7c-electron cyclopropenide ion, in agreement with the Hixckel rule. 

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. 

Fenocene has an even more interesting stmcture. A central iron is ir-bonded to two cyclopentadienyl ligands in what is aptly described as a sandwich. It, too, obeys the 18-electron rule. Each cyclopentadienyl ligand contributes five electrons for a total of ten and iron, with an electron configuration of [Ar]45 34i contributes eight. Alternatively, fenocene can be viewed as being derived from Fe " (six valence electrons) and two aromatic cyclopentadienide rings (six electrons each). 

Cyclopentadienylsodium is ionic. Its anion is the aromatic cyclopentadienide ion, which contains six tt electrons. 

Ring bridges of these types are found in alkaline earth cyclopentadienides such as [Ca(CsH5)2] and are characteristic of the electrostatic nature of the bonding. 

Novel, mixed alkyl cyclopentadienides have also been prepared for the heavy lanthanides ... 

Reaction of Cyclopentadienides or Their Nitrogen Analogs with... 

Condensation Reaction of Cyclopentadienides with Azolium Salts. 130... 

As described in Section II,A,1, the condensation of azolium salts with aza derivatives of cyclopentadienide (pyrroles, imidazoles, pyrazoles) also plays... 

Another type of diazafulvalenes was obtained by the condensation of sodium cyclopentadienide with l,2-dimethyl-3,5-dimethoxy- and -3,5-bis(methylsulfanyl)pyrazolium salts (94AP385). Under mild conditions displacement of MeZ groups in each case led to the diazafulvalenes 58. A... 

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