Cyclopentadienyl derivatives bases

The broader subject of the interaction of stable carbenes with main-group compounds has recently been reviewed. Accordingly, the following discussion focuses on metallic elements of the s and p blocks. Dimeric NHC-alkali adducts have been characterized for lithium, sodium, and potassium. For imidazolin-2-ylidenes, alkoxy-bridged lithium dimer 20 and a lithium-cyclopentadienyl derivative 21 have been reported. For tetrahydropyrimid-2-ylidenes, amido-bridged dimers 22 have been characterized for lithium, sodium, and potassium. Since one of the synthetic approaches to stable NHCs involves the deprotonation of imidazolium cations with alkali metal bases, the interactions of alkali metal cations with NHCs are considered to be important for understanding the solution behavior of NHCs. 

FIGURE 17. Donor-base-coordinated monomeric lithium cyclopentadienyl derivatives in the solid-... 

In scientific and patent literature on diene polymerization the following Nd(III)-salts are most often cited as catalyst precursors halides, carboxylates, alcoholates, phosphates, phosphonates, allyl compounds, cyclopentadienyl derivatives, amides, boranes and acetylacetonates. The catalyst systems based on these Nd-sources are reviewed in the following subsections. 

One catalyst system which allows for the copolymerization of BD and St is based on NdV and is activated by MAO. It is important to note that this catalyst system does not contain a halide donor. By the addition of cy-clopentadienyl derivatives (e.g. cyclopentadiene, indene, anthracene etc.) St incorporation is increased. It may be speculated that by the influence of MAO a proton is abstracted from the cyclopentadienyl derivatives added and that the resulting cyclopentadienyl-type anions coordinate to the active Nd sites [498,499]. In this way the activity pattern of Nd is changed and copolymerization of BD and St is made possible. As shown in the attached Table 28 increases of St incorporation occur simultaneously with decreases of cis-1,4-contents. 

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). 

Whereas amino groups are used most frequently as mframolecular Lewis bases (Tables 5-7), stable unsupported intermolecular complexes are typically accomplished by carbene-type donors, such as carbenes, isonitriles and ylides (Table 8). To my knowledge, no unsupported intermolecular adduct of ethers, amines or phosphanes have been structurally characterized so far (except for the more ionic cyclopentadienyl derivatives ). 

These intramolecular hydroaminations lead to the formation of chiral products. Thus, several studies have been devoted to developing ligands for enantioselective, intramolecular hydroaminations of olefins catalyzed by lanthanide complexes. ° Selectivities of these reactions with chiral cyclopentadienyl derivatives have been modest. Selectivities have been higher with catalysts containing non-cyclopentadienyl ligands. Some of tlie most selective catalysts are the yttrium complexes of the bis(thiolate) ligands reported by Livinghouse, the scandium and lutetium complexes of 33 -disubstituted binaphtholates reported by Hultzsch, and the BINAM-based bisamidates of Schafer.- A representative cyclization catalyzed by a member of each of these classes of catalyst are shown in Equation 16.64. 

Ricci, G Panagia, A. R Porri, L. Polymerization of 1,3-dienes with catalysts based on mono- and bis-cyclopentadienyl derivatives of vanadium. Polymer 1996, 37, 363-365. 

The only method for the preparation of a cyclopentadienyl derivative of copper is the reaction of cupric oxide with cyclopentadiene in the presence of triethylphosphine. The product of this reaction is CpCu PEts (32). Attempts to prepare CpCu by reaction of cuprous halides with cyclopentadiene in the presence of base, or with CpNa or CpMgBr have failed (32). 

On the other hand, treatment of 68 or 70 with base affords a cyclopentadienide anion that can be coordinated to Pd 71 and 72. A number of these cyclopentadienyl derivatives have been prepared by metathetical reactions with allylic complexes of Pd, [Pd(/i-Cl)(allyl)]2, and some X-ray structures have been determined. In general,... 

Ferrocenyl-based polymers are established as useful materials for the modification of electrodes, as electrochemical biosensors, and as nonlinear optical systems. The redox behavior of ferrocene can be tuned by substituent effects and novel properties can result for example, permethylation of the cyclopentadienyl rings lowers the oxidation potential, and the chaige transfer salt of decamethylfer-rocene with tetracyanocthylene, [FeCpJ]" (TCNE], is a ferromagnet below = 4.8 K, and electrode surfaces modified with a pentamethylferrocene derivative have been used as sensors for cytochrome c These diverse properties have provided an added impetus to studies on ferrocene dendrimers. 

---