Cyclopentadienide salts

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... 

Cyclopentadienyl anion The aromatic cyclic anion C5H5. Its salts are called cyclopentadienide salts. 

The ionic nature of the cyclopentadienide salts has been confirmed in a variety of ways. When they were first prepared it was noted that it was possible to prepare other metal salts from potassium cyclopentadienide by double decomposition [1]. The ionic character was demonstrated by conductivity measurements in liquid ammonia,[10], and from spectra [11]. 

Introduction of electron-withdrawing groups attached to the cyclo-pentadiene ring greatly increases the stability and lowers the reactivity of the related cyclopentadienide anions, presumably because of enhanced delocalisation of the negative charge. An example of such an unreactive cyclopentadienide salt was first prepared in 1912 by the condensation of nitromalonaldehyde with hexane-2,5-dione in the presence of alkali [34] ... 

A variant on this method dispenses with the need of first preparing a cyclopentadienide salt. Cyclopentadiene and toluene-p-sulphonazide react together in the presence of an amine such as diethylamine to give diazocyclopentadiene [59]. Phenylated diazocyclopentadienes have also been prepared in this way, [52,53,57,60], the yields being notably better than those obtained by the original method. 

Cyclopentadienide salts, diazocyclopentadienes, cyclopentadien-ylides, cyclopentadienylidene derivatives, fulvenes and cyclopentadienium salts will now be considered severally. 

As previously mentioned, the first cyclopentadienide salt to be prepared was the potassium salt, obtained by the action of potassium on cyclopentadiene dissolved in benzene under an atmosphere of nitrogen [1]. It separated out as a yellow precipitate, insoluble in benzene. Since then many other salts have been made including a... 

Not surprisingly the cyclopentadienide anion reacts readily with electrophilic reagents. With carbon dioxide it gives a dicarboxylic acid which dimerises [1,4]. It is acylated or aroylated by acid chlorides, no catalyst being required, to give diacyl [19,20] or diaroyl [21,22] derivatives which are monomeric. The monoacetyl-cyclopentadienide ion has been obtained by reaction with ethyl acetate in tetrahydrofuran [23]. With methyl chloroformate mono-and di- methoxycarbonyl)cyclopentadienide salts are formed [24]. 

Diazocyclopentadienes react with methyl lithium or phenyl lithium to form azo-substituted cyclopentadienide salts [56,57,74]. Protonation of these salts gives rise to cyclopentadienone hydrazones e.g. 

As with cyclopentadienide salts, benzo-annelation leads to decreased stabilisation in the tropylium ring. This is illustrated by the pK + values for the tropylium, benzotropylium, 1,2,4,5-di-benzotropylium and tribenzotropylium cations which are respectively 4-75 [3,70], 1-7 [106], -3-7 [107] and --15 [108]. The latter value suggests that there is little if any special stabilisation in... 

R. D. Chambers, S. J. Mullins, A. J. Roche, J. F. S. Vaughan, J. Chem. Soc., Chem. Commun. 1995, 841-842. Direct syntheses of pentalds(trifluoromethyl)cyclopentadienide salts and related dienes. 

Reported lithium salts of NHCs remain relatively few despite the fact that they offer potential to be effective and affordable transmetallation reagents. It should be recognized that these salts would compete with Ag(I) complexes as transmetallation reagents and that some have already shown their potential (vide infra). In 1999, Arduengo et al. described the synthesis of the first complexes in which a lithium atom is coordinated to only carbon centers in the NHC-stabilized lithium cyclopentadienide salts shown in Figure 15.3 [22]. 

Alkali metal cyclopentadienide salts are important reagents in organometallic chemistry, having been used to prepare innumerable cyclopentadienyl complexes. Potassium cyclopentadienide, KCp, is usually made by deprotonation of cyclo-pentadiene with potassium metal either in organic solvents such as THF and benzene or in liquid ammonia, or by deprotonation with KH" or KOH. The analogous sodium compound, NaCp, was reported by the groups of Fischer and... 

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). 

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

Reaction of Cyclopentadienides or Their Nitrogen Analogs with Azolium and Dithiolium Salts... 

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... 

The reaction of 1,2-dithiolanes with 2- and 4-picolyllithium has been examined <96PS(112)101> and the reactions of thioanhydrides such as 94 with both thiols <95JOC3964> and amines <96TL5337> have been reported. Treatment of 1,2-dithiolium salts with lithium or thallium cyclopentadienide results in formation of a variety of bi-, tri- and tetracyclic products <96LA109>. Reaction of 95 with trimethyl phosphite gives some of the desired coupling product but also the phosphonates 96 <96PS(109)557>. 

Aryl-l,2-dithiolium salts are a source of either dithiatricyclodecadienes (29) or cyclopenta[fc]thiopyrans (30) depending on whether the initial ring opening resulting from reaction with a metal cyclopentadienide is followed by an intramolecular cycloaddition or a condensation <96LA109>. 

Reactions between salts of [m Jo-7-CBioHi3] and [Fc3(CO)i2] afford the mononuclear anionic iron compound [2,2,2-(CO)3-c/o5o-2,l-FeCBioHn], typically isolated as its [N(PPh3)2] salt (11) (Chart 4). No anionic triiron complex analogous to 5 and 7 is formed in this reaction. The anionic mononuclear iron, ruthenium and osmium complexes and the previously mentioned neutral mononuclear ruthenium dicarbollide complex 4, obtained from [Ru3(CO)i2] and /Jo-7,8-C2BgHi3, are iso-lobal with the cyclopentadienide species [Mn(CO)3(ri-C5H5)] and [Fe(CO)3 (il-CsHs)]. ... 

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