Cyclopentadienyl anion, formation

The lithium salt of a substituted cyclopentadienyl anion has been used in reaction with phosphorus trichloride for carbon-phosphorus bond formation.70 The resultant simple displacement product ultimately undergoes dimerization and loss of four (from the dimer) equivalents of HC1 (Equation 4.25). 

The condensation of aldehyde 2 with cyclopentadiene yields the fulvene 20 which can be transformed to the respective cyclopentadienyl anion 21 by addition of a hydride anion (Scheme 1.5.8). The anion 21 opens up the possibility of the formation of sandwich and half-sandwich type of complexes with suitable metal fragments. 

Tetracarbonyl(trifluoromethyl)iron(II) iodide reacts with C-C bond formation to give per-fluoroethene. Tetracarbonyl(perfluorohexyl)iron(II) iodide gives several products, but no per-fluorohexene resulting from /1-elimination has been found.148 However, reductive defluorination of perfluoro(methylcyclohexane) has been reported with dicarbonyl(ty5-cyclopcn-tadienyl)iron(III).212 The defluorination is accompanied by substitution of fluorine with the cyclopentadienyl anion and proton abstraction from the solvent, the latter is well known in the chemistry of fluoroaromatics with the cyclopentadienyl anion. 

The carbonyls Fe(CO)5 and [CpFe(CO)2]+ (2) form stable cationic complexes with alkenes, which are used for both protection and activation of alkenes [1]. [CpFe(CO)2]+ (2 abbreviated as Fp+) is prepared by the reaction of cyclopentadienyl anion (1) with Fe(CO)5, followed by oxidative cleavage with bromine, and used for the protection of alkenes. The electron density of the double bond is decreased by the coordination of [CpFe(CO)2]+ and hence this bond is activated to nucleophilic attacks. Introduction of nucleophiles, such as the carbon nucleophile of malonate, to cyclopentene becomes possible via the formation of the complex 3, and the stable tftmv-er-alkyliron complex 4 of cyclopentane is prepared. The vinyl ether complex 6 is obtained easily from the a-bromoacetal 5, and reacts with an enolate of ketone 7 as an... 

The formation of azulenes by cyclocondensation of cyclopentadienyl anions is a prime example of a synthesis with pyrylium salts (Hafner 1973 [8], cf 307). Thus, 4,6,8-trimethylazulene 46 is produced by the reaction between 2,4,6-trimethylpyrylium salts and sodium cyclopentadienide, resulting in ring-opening at C-2 (45) followed by ring closure of the cyclopentadienyl system 47 which is a probable intermediate. 

The north pole of the fullerene was methylated by reaction with methyl cuprate. This methylated fullerene, 86, was then converted to cyano-fullerene 87 to prevent formation of the cyclopentadienyl anion by deprotonation in subsequent steps. Derivative 87 was treated with a phenyl cuprate to yield phenyl-substituted 88 with an electronically isolated [lOJcyclophenacene. Removal of the cyano group and oxidation gave the penta-oxygenated derivative 90, which could be easily... 

Anions are similarly examined, and the cyclopentadienyl anion (118) has six 7t-electrons and meets all criteria for aromaticity. It is aromatic, very easy to form, and quite stable. Formation of 118 from cyclopentadiene (120) is an acid-base reaction. It is known that 120 has a relatively low pK that reflects the special aromatic stability of the aromatic conjugate base. The pK of cyclo-pentadiene is 14-15 (compare that with a pK of 15.8 for water).This contrasts sharply to the cycloheptatrienyl anion (119), which has 4n 7i-electrons, is not aromatic, and is particularly unstable and difficult to form. As with 118, formation of 119 is an acid-base reaction from cycloheptatriene, 121. The pKg of 121 is about 36,1 however, which reflects the great difficulty in forming the antiaromatic conjugate base. 

The formation of spiro [4,2] hepta-1,4-dienes (97) and (98) by the reaction of dichloromethyl-lithium with 6-substituted fulvenes proceeds by a two-step process. Initial addition affords the cyclopentadienyl anion (96), which undergoes rapid 1,3-elimination, with a distinct preference for closure to the thermodynamically less stable cis-isomer (97) (Scheme 15). Only in the case of (96 R = Bu ) does the trans-cyclopropane (98) predominate. Such observations can be rationalized in terms of Eliel s predictions that a gauche conformation such as (96a) will be favoured over (96b) due to an attractive force between the halogen atom and the adjacent alkyl (or... 

Spiro[3,4]octa-l,5,7-triene (249) has been prepared by modification of the carboxylic acid (250). The triene was characterized chemically by reduction to the saturated hydrocarbon and by its Diels-Alder reaction with JV-phenyltriazoline-3,5-dione. Any spiroconjugative interaction in the triene is not shown on its u.v. spectrum, which has at 262 nm, the same as observed for the diene (251). The triene is unstable, rearranging at — 5°C (t = 90 min) to 6-vinylfulvene and a dihydropentalene derivative. The ring-opening of (249) to 6-vinylfulvene is unusually rapid for a 3,3-disubstituted cyclobutene derivative, and other similarly strained compounds do not show this reactivity. The ring-opening may well proceed via a biradical intermediate with stabilization of the zwitterionic form of the biradical by formation of the aromatic cyclopentadienyl anion. 

In a manner analogous to the formation of the other hydridometal complexes, the tricarbonylhydridovanadate anion is easily produced from r)5-cyclopentadienyl-vanadiumtetracarbonyl under basic phase-transfer catalytic conditions and it has been used in the reduction of nitro compounds and the reductive dehalogenation of a wide range of halides [ 12]. 

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