Monocyclopentadienyl compounds synthesis

Syndiotactic polystyrene was first obtained only recently by Ishihara et al. [5] in polymerisation with a homogeneous catalyst derived from a transition metal compound such as monocyclopentadienyltitanium trichloride and methylalu-minoxane in toluene. Since then, several authors have reported on the synthesis of syndiotactic polystyrene promoted by different catalysts based on metal hydrocarbyls such as benzyl compounds, half-sandwich metallocenes (e.g. monocyclopentadienyl, monopentamethylcyclopentadienyl and monoindenyl metal derivatives), metal alkoxides, metallocenes and some other compounds. These catalysts are commonly derived from titanium or zirconium compounds, either activated with methylaluminoxane or aluminium-free, such as those activated with tris(pentafluorophenyl)boron, and promote the syndiospecific polymerisation of styrene and substituted styrenes [5-10,21,48-70], Representative examples of the syndiospecific polymerisation of styrene using catalysts based on various titanium compounds and methylaluminoxane are shown in Table 4.2 [6,52,53,56,58],... 

The synthesis of the first half-metallocene bis(borohydrides) of lanthanide elements has been achieved by a reaction of the tetraisopropylcyclopentadienyl sodium salt NafCsHPr ) with Ln(BH4)3(THF)3 (Ln = Sm, Nd) at room temperature to afford the monocyclopentadienyl complexes (CsHPr 4)Sm(BH4)2(THF) and (CsHPr 4)Nd(BH4)2(THF) in 50-60% yield. These complexes are thermally stable and melt without decomposition at around 170 °C.307 Analogous compounds have also been synthesized with the use of the [CsMe4Prn] ligand.308... 

Bis(cyclopentadienyl)iron derivatives, ferrocenes, are remarkably stable against heat and air and undergo various kinds of chemical reactions. They are usually prepared by the reaction of FeCh with an alkali metal salt of cyclopentadienyl or cyclopentadiene in the presence of base [51]. Although ferrocene derivatives basically have chemical reactivities similar to those of aromatic compounds, they have found only limited applications to organic synthesis so far. However, because chiral ferrocenylphosphines are capable of having both planar chirality and an asymmetry in the side chain in their rigid framework, they have been used recently in a number of asymmetric reactions. In this section, synthesis and developments of monocyclopentadienyl complexes and ferrocenylphosphines are described. The general chemistry of ferrocenes and half metallocene complexes is reviewed elsewhere [52-53]. 

In search for new active catalysts of the olefin polymerization over the last years several REM monocyclopentadienyl complexes containing a-bonded ligands have been synthesized (Table 1.7) [39, 58, 136-138]. Neutral derivatives of CpLnX2 as well as heterobimetallic complexes [Li(B)][CpLnCl3] have been utilized in these synthesis as the initial compounds. It should be noted that in all cases, except CpHo(C=CPh)2 obtained in 1975 [139], pentamethylcyclopentadienyl derivatives have been used. 

Actinides. Monocyclopentadienyl derivatives are less common and the majority of the reported complexes are Lewis-base adducts of the type CpAnXs-Ln (X = halide). Their steric and electronic unsaturation make their synthesis challenging. The first CpUCl3 2L compounds in ethereal solvents (DME or THF) were prepared by reacting UCI4 with thallium cyclopentadienide (Marks and Ernst 1982). 

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