Cyclopentadiene ligand

Zeijden [112] used chiral M-functionalized cyclopentadiene ligands to prepare a series of transition metal complexes. The zirconium derivative (82 in Scheme 46), as a moderate Lewis acid, catalyzed the Diels-Alder reaction between methacroleine and cyclopentadiene, with 72% de but no measurable enantiomeric excess. Nakagawa [113] reported l,T-(2,2 -bis-acylamino)binaphthalene (83 in Scheme 46) to be effective in the ytterbium-catalyzed asymmetric Diels-Alder reaction between cyclopentadiene and crotonyl-l,3-oxazolidin-2-one. The adduct was obtained with high yield and enantioselectivity (97% yield, endo/exo = 91/9, > 98% ee for the endo adduct). The addition of diisopropylethylamine was necessary to afford high enantioselectivities, since without this additive, the product was essentially... 

Dimethyl- and diethylzinc reacted with dimethylpyrrole- and mesityl-substituted cyclopentadiene ligands to give monocyclopentadienyl(methyl)zinc and -ethyl(zinc) compounds. These products then formed, Scheme 20, crystalline adducts with tetramethylethylenediamine (TMEDA) such as 24, whose solid-state structure is shown in Figure 11. 

In addition to phosphine ligands, a variety of other monodentate and chelating ligands have been introduced to functionalized polymers [1-5]. For example, cyclo-pentadiene was immobilized to Merrifield resins to obtain titanocene complexes (Fig. 42.13) [102]. The immobilization of anionic cyclopentadiene ligands represents a transition between chemisorption and the presently discussed coordinative attachment of ligands. The depicted immobilization method can also be adopted for other metallocenes. The titanocene derivatives are mostly known for their high hydrogenation and isomerization activity (see also Section 42.3.6.1) [103]. 

Further experiments (159), including gas titrations, reactivity studies, and spectroscopic evidence, led to the formulation of the intermediate, earlier postulated (158) to be an oxygenated cobaltocene adduct, as the organic peroxide structure 63, in which the dioxygen bridge once again links the cyclopentadiene ligands in an exo fashion. This complex... 

With CO, a fast addition reaction and transfer of the acetyl group from the molybdenum to the rj4-1,3-cyclopentadiene ligand is observed, and the red endo isomer of 94, [(>75-C5H5)Mo(CO)2to4-end0-C5H6COCH3)] (96), is formed in high yields [Eq. (52)]. Trimethylphosphite releases the y4-i,3-... 

Substitution reactions are also observed with the dienes la-lc, lg and It [Eq. (54)]. The >/4-l,3-cyclopentadiene ligand is displaced by the dienes, and the acetyl group immediately transfers to C-l or C-4 of the r/4-diene ligands in the intermediates 78a-78c, 78g, and 78t. Carbonyl- 5-cyclopentadienyl- /3 2-(Z)-2-hexen-l-yl-5-onemolybdenum (76a) and the corresponding derivatives are obtained in high yield (124). With the unsymmetrically substituted dienes lb and lc, two different isomers each, 76b, 76b, 76c and 76c are formed... 

Brown and Ikeda [25, 26] have reported thermal and photo-assisted decomposition of Cp2Ti(N)3)2 to deposit TiN. The fact that these films are generally contaminated with a large amount of carbon indicates that the cyclopentadiene ligands do not form volatile leaving groups such as HCp or fulvalenes. On the other hand, cyclopentadi-enyl compounds of titanium have been successfully applied to the preparation of TiC films by MOCVD [27]. 

Cyclopentadiene ligands display many types of fluxionality. A particularly novel type has recently been discovered in bis(cyclopentadienyl)beryllium. X-ray data show the molecule to possess a slip sandwich structure and this structure persists in the solution and vapor phases. A recent NMR study based on partially... 

A third alternative simply uses ferrocenes as walls in cyclophane-like structures.The ferrocenyl units typically are linked via flexible hydrocarbon chains appended to cyclopentadiene ligands. In other words, covalent carbon-carbon bonds rather than coordinate-covalent metal-nitrogen, - carbon, -oxygen, or -phosphorous bonds are used. 

The unusual complex (98), which involves a bridging c clopentadienyl-cyclopentadiene ligand, is obtained when QHgNa reacts with PtCla. The Pt-Pt distmice, at an early stage in the refinement, is 2.581(4) A. ... 

The series of alumohydride complexes of dicyclopentadienylyttrium [32 - 36], -samarium [37] and -lutetium [38 - 40] has been synthesized by Bulychev and coworkers. The main routes to these products are the reactions of lithium and sodium alumohydrides or AIH3 with Cp2LnCl and similar chlorides. The structure of compounds formed in these reactions depends on the molar ratio of initial reagents, the basicity of used solvent and the existence of substituents in the cyclopentadiene ligands. In most cases the bimetallic dimer [Cp2LnAlH4(B)]2 (B = Et20, THF, Et3N) is formed. 

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