Tris cyclopentadiene, synthesis

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... 

See also Scheme 17 for the template synthesis of tri(te/t-butyl)-Cp from a dimeric tantalum alkylidyne complex and 3,3-dimethyl-1-butyne. Schemes 8 and 9 show the syntheses following method 2. The 1,3,5-substi-tuted tri(/m-butyl)cyclopentadiene is obtained as the single isomer from both routes. 

The template synthesis for the intermediate tris- and tetrakis(meth-ylthio)-Cp s as the chloro modification is described in Scheme 21. Synthesis of pentakis(methylthio)-Cp from unsubstituted cyclopentadiene (method 2) is shown in Scheme 13. 

Scheme 10. Reaction sequence for the synthesis of tri-, tetra-, and penta(isopropyl)cyclo-pentadienes (34,35) from di(isopropyl)-Cp (59) via a series of metallations/alkylations. Tri(isopropyl)-Cp is obtained in a 4 1 mixture of the 1,2,4- and 1,2,3-substituted isomers. The C—H acidic tetra(isopropyl) isomers are separated from the 5,5 -geminal dialkylated forms by their transformation in the sodium salt and evaporation of the nonmetallated components. Subsequent hydrolysis yields l,2,3,4-tetra(isopropyl)cyclopentadiene in an iso-merically pure form. Similarly, l,2,3,4,5-penta(isopropyl)-Cp is purified after separation of the 1,2,3,S,5 derivative as the main alkylation product. For more details and the respective yields, see Refs. 34 and 35.

Methyl lithium and butyl lithium are widely used for the synthesis of other organolithium compounds. For example, lithium cyclopentadienides are generally prepared by the reaction of the cyclopentadiene with butyl lithium. In contrast, the amido-alkali metal compounds are becoming increasingly important in the synthesis of organoalkali metal compounds with the heavier alkali metals. For example, l,2,4-tris(trimethylsilyl)-l,3-cyclopentadiene reacts with potassium bis(trimethylsilyl)amide to form potassium l,2,4-tris(trimethylsilyl)cyclopentadienide. ... 

A second one-step synthesis of ferrocene is achieved by the direct action of metal halides on cyclopentadiene in the presence of a base such as di- or tri-ethylamine. This method also can be applied to the preparation of cobalt, nickel, copper, and palladium cyclopentadienyl 7r-complexes, although the yields with these metals are not as high as with iron. 

Zirconacyclopentene 1-5 could also be applied in synthetic reaction (Scheme 1.8). For example, in the presence of CuCl and iodine, oxidative demetallation of 1-5 gives cyclobutene derivatives [34]. Reaction of 1-5 with acid chloride gives tri-subslimted cyclopentadiene derivatives [35]. This research group also reported zirconocene-mediated cyclization of alkyne, ethylene, and two molecules of aldehyde toward synthesis of 2-alkenyl tetrahydrofuran [36]. 

---