1,3-Cyclopentadiene thallium complex

In 1971, a preparation of 1 from 2 using thallium cyclopentadienide was reported (I) but the toxicity of thallium and the mass of the reagent needed render this procedure unsuitable for large-scale preparations. An improved method was reported by Bruce et al. (3,4), using cyclopentadiene, ruthenium trichloride hydrate (3), and triphenylphosphine, which gives the desired complex in high yield [Eq. (2)]. The primary advantage of this latter method is formation of the complex in one pot. 

In a first approximation supra-Cp metal complexes can be prepared the same way as normal or other Cp-metal and organo-metal bonds in general. The methods used most often (see Appendix) are the metathesis reaction [Eq. (1)] followed in number by oxidative additions (Eq. (2)] and metallation/deprotonation reactions [Eq. (3)]. The latter is especially important for the cyclpentadienyl alkali metal compounds. A useful variation of reaction (3) is the formation of CpTl in an acid/base reaction from cyclopentadiene and thallium ethoxide [Eq. (3b)]. This represents a convenient route to cyclopentadienylthallium compounds, which are also valued (in place of Cp alkalis) as mild Cp-transfer reagents for the synthesis of difficultly isolable cyclopentadienyl derivatives (77). 

In this group, too, the first observations were made by Thiele 192), who found that cuprous and silver ions are precipitated from aqueous ammoniacal solution by cyclopentadiene, but gave no analytical data. Thallium was later found to be precipitated similarly. Copper (I), however, also yields a white crystalline cyclopentadienyl-triethylphosphine complex, C5H5CuP(C2H5)3, which is stable to water 215) this was, in fact, analogous to an earlier method described by van Peski and Melsen. Attempts to make a complex by the Grignard reagent method or by the action of cyclopentadienyl sodium on cuprous halides were unsuccessful. 

Cyclopentadienylthallium is a superior reagent for many syntheses. It is easily prepared and stored, it may be handled in air, and the insolubility of thallium halide leads to high yields of product that may be filtered easily and worked up. The reagent has been used for preparing a variety of actinide, lanthanide, and transition metal cyclopentadiene complexes. 

A better route is the one shown in Scheme 11. The benzene complex 104 forms readily in near quantitative yield upon heating Ru(III) chloride and 1,3-or 1,4-cyclohexadiene in EtOH. The introduction of the Cp ligand is by hali-de/Mcp exchange. The classic procedure to 105 required a stoichiometric quantity of thallium or silver [103], but a very recent, improved procedure shows that cyclopentadiene/K2C03/ abs. EtOH can be used successfully [104]. Although beyond the scope of this review, it is interesting to note that the much less electrophilic complexes [(arene)Ru(C5Me5)] [PFg] are accessible in a one-pot procedure from RuClj [105]. The sequence Birch reduction/complexa-... 

The mixture of double bond isomers of l,2,3,4-tetramethyl-5-(trifluoro-methyl)cyclopentadiene (Cp H) is a clear, colorless liquid with a sweet ole-finic odor similar to that of pentamethylcyclopentadiene. Cp H is an air-stable compound but it should be stored in the refrigerator. Although gas chromatographic analysis shows two peaks, the presence of all three of the possible cyclopentadiene isomers is indicated by H NMR spectroscopy as three quartets for the ring protons are observed at 3.26 (Jhf = 10.0 Hz), 2.99 (Jhh = 6.7 Hz), and 2.66 ppm (Jhh = 6.8 Hz) with a relative ratio of approximately 1 16 2.4 for isomers I, II, and III, respectively. Resonances between 4.8 and 5.0 ppm are attributed to products that result from an elimination mechanism rather than cyclization in the last step of the synthesis. These impurities boil very close to that of Cp H, but they are removed by careful fractional vacuum distillation as part of the forerun. The IR spectrum (neat liquid) exhibits significant bands at 2977(vs), 2937(vs), 2884(s), 2864(s), 2751(w), 1658(m), 1599(s), 1446(s), 1384(vs), 1357(vs), 1280(s), 1258(vs), 1164(vs), 1099(vs), 1012(vs), and 686(m) cm . The syntheses of organotransi-tion metal complexes are usually carried out with the 91% purity Cp H product. Because alkali metal and thallium salts of Cp are unstable, organo-transition metal complexes are prepared from Cp H itself, as are many complexes of pentamethylcyclopentadiene and cyclopentadiene."... 

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