Lithium cyclopentadienyl complexes

The first example of lithium-NHC complexes, in which the lithium is coordinated only to carbon centres, was reported by Arduengo and coworkers.10 Stable NHCs were reacted with lithium 1,2,4-n-A(trimethylsilyl) cyclopentadienide to give 2 (Fig. 2). A single crystal X-ray structure reveals a complex in which the lithium centre is coordinated in a r 5-fashion to the cyclopentadienyl ring, with a single cr-interaction between the lithium and carbene centre. The lithium centre lies 2.155(4) A from the carbene centre hence has a closer contact than in the previous example, possibly as a result of the carbene interacting with only one lithium centre. 

X-ray analysis of complex 7 showed that two of the metal-chloride bonds are shorter than the other two Th(l) - Cl(l) = 2.770(2)A, Th(l) - Cl(2) = 2.661(2)A, Th (1) - Cl(3) = 2.950(2)A, and Th(l) - Cl(4) = 2.918(2)A. The longer Th-Cl distances are those belonging to the chlorine atoms encountered in the threefold bridging positions and connected to the lithium atoms. The other two chlorine atoms are coordinated only to one lithium atom. All the Th-Cl distances are longer than those observed for terminal Th-Cl distances (Th-Cl = 2.601 A for Cp2 ThCl2 or 2.65A for Cp2 Th(Cl)Me). Ansa-chelating bis(cyclopentadienyl) complexes of uranium have been prepared as presented in Scheme 1. Bums et al. have described an efficient high yield procedure for the required U(1V) complexes [44]. 

Table 1.13 Cyclopentadienyl lithium-lanthanoid complexes (continued). 

The base-free compounds of CpjLn type are known for all metals of the being considered block including radioactive promethium (Table III.3.). The cyclopentadienyl complexes of trivalent Sc, Y, La, Ce, Pr, Nd, Sm and Gd prepared in 1954 by Wilkinson and Birmingham in the reaction of anhydrous metal chlorides with CpNa were the first REM organoderivatives [119]. Later this method has been modified and applied to Eu, Tb, Dy, Ho, Er, Tm, Yb and Lu [88, 91, 120, 121]. Up to now it remains to be the main way to the tricyclopentadienyl REM complexes. Besides CpNa, cyclopentadienides or substituted cyclopentadienides of lithium and potassium are used in these reactions [31,95, 100,101, 111, 116] ... 

Monomeric carbene complexes with 1 1 stoichiometry have now been isolated from the reaction of 4 (R = Bu, adamantyl or 2,4,6-trimethylphenyl R = H) with lithium l,2,4-tris(trimethylsilyl)cyclo-pentadienide (72). The crystal structure of one such complex (R = Bu) revealed that there is a single cr-interaction between the lithium and the carbene center (Li-C(carbene) 1.90 A) with the cyclopentadienyl ring coordinated in an if-fashion to the lithium center. A novel hyper-valent antimonide complex has also been reported (73). Thus, the nucleophilic addition of 4 (R = Mes R = Cl) to Sb(CF3)3 resulted in the isolation of the 1 1 complex with a pseudo-trigonal bipyramidal geometry at the antimony center. 

The cyclopentadienyl group is another interesting ligand for immobilization. Its titanium complexes can be transformed by reduction with butyl lithium into highly active alkene hydrogenation catalysts having a TOF of about 7000 h 1 at 60 °C [85]. Similar metallocene catalysts have also been extensively studied on polymer supports, as shown in the following section. 

Covalently bonded chiral auxiliaries readily induce high stereoselectivity for propionate enolates, while the case of acetate enolates has proved to be difficult. Alkylation of carbonyl compound with a novel cyclopentadienyl titanium carbohydrate complex has been found to give high stereoselectivity,44 and a variety of ft-hydroxyl carboxylic acids are accessible with 90-95% optical yields. This compound was also tested in enantioselective aldol reactions. Transmetalation of the relatively stable lithium enolate of t-butyl acetate with chloro(cyclopentadienyl)-bis(l,2 5,6-di-<9-isopropylidene-a-D-glucofuranose-3-0-yl)titanate provided the titanium enolate 66. Reaction of 66 with aldehydes gave -hydroxy esters in high ee (Scheme 3-23). 

A new development in silsesquioxane ehemistry is the eombination of sil-sesquioxanes with cyclopentadienyl-type ligands. Reeently, several synthetie routes leading to silsesquioxane-tethered fluorene ligands have been developed. The scenario is illustrated in Seheme 47. A straightforward aeeess to the new ligand 140 involves the 1 1 reaction of 2 with 9-triethoxysilylmethylfluorene. Alternatively, the chloromethyl-substituted c/oxo-silsesquioxane derivative 141 can be prepared first and treated subsequently with lithium fluorenide to afford 140. Compound 141 has been used as starting material for the preparation of the trimethylsilyl and tri-methylstannyl derivatives 142 and 143, respeetively, as well as the novel zirconoeene complex 144. When activated with MAO (methylalumoxane), 144 yields an active ethylene polymerization system. 

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