With substituted monocyclopentadienyls

Dimethyl- and diethylzinc reacted with dimethylpyrrole- and mesityl-substituted cyclopentadiene ligands to give monocyclopentadienyl(methyl)zinc and -ethyl(zinc) compounds. These products then formed, Scheme 20, crystalline adducts with tetramethylethylenediamine (TMEDA) such as 24, whose solid-state structure is shown in Figure 11. 

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],... 

Monocyclopentadienyl titanium derivatives are the most active precursors for catalysts possessing high syndiospecific polymerisation activity for styrene and ring-substituted styrenes. The polymerisation activity of biscyclopentadie-nyl titanium compounds activated with methylaluminoxane is lower than that of other soluble titanium-based catalysts [73]. 

In retrospect, it is ironic to it that when I met Ernst Schumacher in 1969 (he was then Professor at the University of Bern in Switzerland) we did not talk about the experiments he did at Zurich in the same building where I was at that time. Instead, his interest focussed on our work on borazine transition metal compounds and we discussed in some detail whether it would be possible to incorporate metal atoms like chromium or molybdenum between the layers of hexagonal boron nitride (BN) in a similar way as it can be done with graphite. In the course of these discussions I did not mention that, after I had moved to Zurich, we had begun to investigate the reactivity of nickelocene towards both nucleophilic and electrophilic substrates. The reason was that we were still at the beginning, and while we had been able to prepare a series of monocyclopentadienyl nickel complexes from Ni(C5H5)2 and Lewis bases, our attempts to obtain alkyl- or acyl-substituted nickelocenes by the Friedel-Crafts reaction failed. 

There are a number of examples of chiral ferrocene derivatives prepared by reaction of substituted cyclopenta-diene anions with FeCl2 or a monocyclopentadienyl iron species rather than using the strategy of introduction and modification of a side chain on ferrocene itself. Addition... 

Probably substitution of the cyclopentadienyl ligand on alcoholysis and hydrolysis, and its transfer in reactions of di- and monocyclopentadienyl-titanium derivatives with FeClj or iron(II) acetylacetonate, follows a mechanism common for 77-ligand exchange in other metal 77-complexes (see later). 

For the preparation of unsymmetrical ferrocenes, two ways by which only mono-substituted derivatives are produced have been suggested. One route starts from iron tetracarbonyl and a substituted cyclopenta-diene the other from monocyclopentadienyl iron dicarbonyl bromide, which on treatment with a substituted cyclopentadienyl lithium is finally converted into the corresponding mono-substituted ferrocene. Experience shows the first method to be more suitable for the preparation of aryl, and the latter method for the preparation of alkyl, derivatives (57). Corresponding work already carried out on substitution in Ru(C5H6)2 and Os CsH5)2 has also been fruitful. It is found that in the Friedel-Crafts reaction with acetyl or benzoyl chloride there is a distinct predominance of mono- over disubstitution as the atomic weight of the central atom increases (47, 72). 

The hydrides of the early lanthanides may be stabilised by bulky substituted cyclopentadienyl ligands, such as 1,3-di-tert-butylcyclopentadienyl and penta-methylcyclopentadienyl. The late lanthanide hydrides [(C5H4R)2LnH(L)]2, Ln = Lu, Er and Y, may be obtained with less bulky cyclopentadienyl ligands. Only monocyclopentadienyl hydrides of scandium and yttrium of general formula [(CjMe4R)LnH(L)] have been reported [6]. Most of the lanthanide hydrides... 

---