Lithium cyclopentadienyl derivatives

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 15. Solid-state stmctures of polymeric lithium cyclopentadienyl derivatives... 

FIGURE 16. Molecular fractions of polymeric lithium cyclopentadienyl derivatives in the solid-state. The cations of the lithocene anions 47 and 48 (Ph4P+ and [(12-crown-4)2Li], respectively) have been omitted for clarity. With 55 the cation is [(THF)4Li]+. AU three are solvent-separated ion pairs. 

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

A certain jr-facial selectivity was achieved when MCpCl2 (M = Ti, Zr) fragments were coordinated to the optically active fused cyclopentadienyl ligands. For instance, reaction of ZrCpCl3 with the lithium derivative of 126 at —78 °C gave predominantly 133 which was characterized by X-ray structural analysis [152]. 

An alternative synthesis of a thermally stable cyclopentadienyl functionalized polymer involved ring bromination of poly(oxy-2,6-diphenyl-l,4-phenylene), followed by lithiation with butyl lithium to produce an aryllithium polymer. Arylation of 2-norbornen-7-one with the metalated polymer yielded the corresponding 2-norbornen-7-ol derivative. Conversion of the 7-ol to 7-chloro followed by treatment with butyl lithium generated the benzyl anion which undergoes a retro Diels-Alder reaction with the evolution of ethylene to produce the desired aryl cyclopentadiene polymer, 6. 

Better yields are obtained when polar solvents are utilized and an amine such as tetramethylethyl-enediamine is present, which also facilitates the formation of the dilithium compound. The lithium derivatives undergo a large number of reactions that can be used to produce the enormous number of ferrocene derivatives. Rather than trying to show a great number of reactions, a few of the common reactants and the substituents that they introduce on the cyclopentadienyl rings are shown in Table 21.3. 

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. 

Of course, it is not without precedence as there are already various reviews and textbooks in the area. Most closely related to this work are the reviews by Setzer and Schleyer and by Weiss °. A textbook edited by Sapse and Schleyer contains chapters on various topics of lithium chemistry. Three reviews by StaUce, Harder and by Jutzi and Burford concentrate on cyclopentadienyl organometallic derivatives, containing also the s-block derivatives. Two reviews deal with the heavier alkali metal organics, the first written by Schade and Schleyer , the second by Smith . There are several chapters on lithium organics within more general metal organic publications ... 

Highlights in the chemistry of cyclopentadienyl compounds have been reviewed.65 Trends in the metallation energies of the gas-phase cyclopentadienyl and methyl compounds of the alkali metals have been studied by ab initio pseudopotential calculations. Whereas there is a smooth increase in polarity of M-(C5H5) bonds from Li to Cs, lithium appears to be less electronegative than sodium in methyl derivatives. The difference between C5H5 and CH3 derivatives is attributed to differences in covalent contributions to the M-C bonds. In solution or in the solid state these trends may be masked by the effects of solvation or crystal packing.66 The interaction between the alkali metal ions Li+-K+ and benzene has also been discussed.67... 

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