Substitution Cyclopentadienyls

Substitution Cyclopentadienyls.— The question of nucleophilic attack at metal or ligand, alluded to in the previous two paragraphs, is also relevant to cyclopentadienyl derivatives. The reaction of nickelocene with triethyl phosphite, in dioxan solution, follows a third-order rate law and has a low activation energy and large negative activation entropy. The rate law 

The rate law and rate constants for photolysis of Ti(7x-C5H5)2S4 in benzene, dioxan, and dimethylformamide have been determined radio-chemically. The rate law is simple first-order there is some solvent participation in the reaction in dimethylformamide.  

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Metallocene Catalysts. Polymerization of cycloolefins with Kaminsky catalysts (combinations of metallocenes and methylaluminoxane) produces polymers with a completely different stmcture. The reactions proceeds via the double-bond opening in cycloolefins and the formation of C—C bonds between adjacent rings (31,32). If the metallocene complexes contain bridged and substituted cyclopentadienyl rings, such as ethylene(hisindenyl)zirconium dichloride, the polymers are stereoregular and have the i j -diisotactic stmcture. 

At about 159°C it turns pink and adopts the sandwich structure, expected for [M(C5H5)2] compounds, and this is retained in the gaseous phase and in hydrocarbon solutions. Using substituted cyclopentadienyls a variety of analagous sandwich compounds have been prepared and their magnetic properties indicate that the... 

The scope of the present paper is limited to those cyclopentadienyl ligands that contain more than two bulky substituents and transition metal complexes derived thereof in order to be able to focus on the specific effects of these ligand systems. A selection of some mono-substituted cyclopentadienyl ligands will be treated also. Among the numerous reviews highlighting special aspects of cyclopentadienyl... 

Two commonly used synthetic methodologies for the synthesis of transition metal complexes with substituted cyclopentadienyl ligands are important. One is based on the functionalization at the ring periphery of Cp or Cp metal complexes and the other consists of the classical reaction of a suitable substituted cyclopentadienyl anion equivalent and a transition metal halide or carbonyl complex. However, a third strategy of creating a specifically substituted cyclopentadienyl ligand from smaller carbon units such as alkylidynes and alkynes within the coordination sphere is emerging and will probably find wider application [22]. 

Phenyl-Substituted Cyclopentadienyl Ligand 3.1 Ligand Synthesis... 

One of the first substituted cyclopentadienyl ligands systematically used in organotransition chemistry was those containing phenyl groups [53], There was... 

Table 1. Energy barriers to ring rotation in unsubstituted and substituted cyclopentadienyl transition metal complexes.

The crystal and molecular structure of the 44, 45, and C5-vinylferro-cenyl-thymidine show that the substituted cyclopentadienyl ring is essentially co-planar with the nucleobase (164). DFT calculations indicate that, irrespective of the extent of saturation in the bridging C2-unit in ethynyl-, vinyl- or ethyl-ferrocenyl-C5-thymidine, a similar amount of spin density is transferred to the nucleobase (Fig. 48). The reduction potentials for these compounds are shifted little compared to the parent ferrocenyl derivatives. 

Metallocenes with substituted cyclopentadienyl rings. Metallocenes with methylated rings were among the first heavy alkaline earth metallocenes to be structurally characterized, but many other substituents have been incorporated into bis(cyclopentadienyl) complexes. Under this classification are included compounds with indenyl ligands, which in... 

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). 

An interesting catalytic ruthenium system, Ru(7/5-C5Ar4OH)(CO)2H based on substituted cyclopentadienyl ligands was discovered by Shvo and coworkers [95— 98]. This operates in a similar fashion to the Noyori system of Scheme 3.12, but transfers hydride from the ruthenium and proton from the hydroxyl group on the ring in an outer-sphere hydrogenation mechanism. The source of hydrogen can be H2 or formic acid. Casey and coworkers have recently shown, on the basis of kinetic isotope effects, that the transfer of H+ and TT equivalents to the ketone for the Shvo system and the Noyori system (Scheme 3.12) is a concerted process [99, 100]. 

Kagan et al. were the first to report the corresponding enantioselective catalytic hydrogenation using chiral metallocene derivatives [94, 95]. By using menthyl- and neomenthyl-substituted cyclopentadienyl titanium derivatives in the presence of activators (Scheme 6.5) [96], these authors observed low ee-values (7-14.9%) for the catalytic hydrogenation of 2-phenyl-l-butene into 2-phenylbutane. In contrast, no enantiomeric excess was obtained with the corresponding zirconocene derivatives. 

Substituent effects, 9 510 Substituted alkylhydrazines, 13 572-573 Substituted amide waxes, 26 221 Substituted cyclopentadienyl uranium complexes, 25 440-441 Substituted cyclopentadienyl (Cp)... 

The possible mechanism for the formation of C—C coupled (1) and decoupled (2) bimetallic complexes from Cp2M has been investigated [1 lb,c]. We followed a stepwise procedure to arrive at a model that was practical and at the same time realistic. In the first stage, the substituted cyclopentadienyls were replaced by Cp and the substituents on acetylides and butadiynes were replaced by H. The relative energies showed that, the C—C coupled structure 1 for M = Ti when L = Cp and R = H is more stable than 2 by 3 kcal/mol, while 2 is calculated to be 14.8 kcal/mol... 

Substituted cyclopentadienyl ligands are extremely effective for a variety of polymerisation reactions, but they are not unique. Both from a point of view of scientific interest and from a patent point of view, in attempts to acquire one s... 

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