Zirconocene ethylene

Fig. 1. The structure of the ethylene-zirconocene complex (SiH2Cp2)ZrCHj-C2H4. The corresponding titanocene has basically the same structure, except that the Ti-C distances are obviously different from the Zr-C distances.

It is much more difficult to control the pair-selectivity and regioselectivity of the intermolecular cyclic carbozirconation. The fundamentally dynamic and reversible nature of most of the microsteps in these reactions is primarily responsible for the often capricious nature of these reactions. Nevertheless, considerable progress has been made recently. In particular, the use of (ethylene)zirconocene in place of (l-butene)zirconocene has been shown to provide convenient and selective procedures, as shown in Scheme 39 13,13c,212,226,227 q [lcsc procedures, however, are still not fully satisfactory, especially in terms of regioselectivity, and additional developmental works are desirable. 

The Zr-catalyzed ethylalumination of alkynes under certain conditions1,9 (Scheme 4) and ethylmagnesation of alkenes10 11 (Scheme 5) represent some of the earliest examples of the catalytic carbozirconation proceeding via zirconacycles. In Scheme 5, the carbometallative ring expansion of (ethylene)zirconocene to produce a... 

In these early studies, however, the concept of c-bond metathesis most probably did not exist, and the results were presented just as observed facts. Mainly in the 1990s, a wide variety of c-bond metathesis reactions of both three- and five-membered zirconacycles were reported. In Scheme 1.4, the reaction of the five-membered zirconacycle with EtMgBr via c-bond metathesis followed by another c-bond metathesis (p-H abstraction) produces the ethylmagnesation product along with ethylene-zirconocene [51], Some representative examples of c-bond metathesis reactions of three-membered zirconacycles are shown in Scheme 1.69. These are examples of stoichiometric c-bond metathesis reactions from which the products have been identified. 

The full ab-initio molecular dynamics simulation revealed the insertion of ethylene into the Zr-C bond, leading to propyl formation. The dynamics simulations showed that this first step in ethylene polymerisation is extremely fast. Figure 2 shows the distance between the carbon atoms in ethylene and between an ethylene carbon and the methyl carbon, from which it follows that the insertion time is only about 170 fs. This observation suggests the absence of any significant barrier of activation at this stage of the polymerisation process, and for this catalyst. The absence or very small value of a barrier for insertion of ethylene into a bis-cyclopentadienyl titanocene or zirconocene has also been confirmed by static quantum simulations reported independently... 

Fig. 2. Time-evolution of the methyl/ethyl C-C distances for both the zirconocene and the corresponding titanocene catalyst. The two curves starting at around 3.2 A represent the distance between the methyl carbon atom and the nearest-by ethylene carbon atom in the zirconocene-ethylene and the titanocene-ethylene complex, respectively. The two curves starting at around 1.35 A reflect the ethylene internal C-C bond lengths in the two complexes.

Fig. 3. Time evolution of the distance between the Zr atom and each of the three hydrogen atoms belonging to the methyl group (the original methyl group bonded to the Zr) in the zirconocene-ethylene complex. The time-evolution of one of the hydrogen atoms depicted by the dotted curve shows the development of an a-agostic interaction. Later on in the simulation (after about 450 fs) one of the other protons (broken curve) takes over the agostic interaction (which is then a 7-agostic interaction).

Property Ethylene-bridged zirconocenes Ziegler-Natta... 

Ethylene-bridged bis-indenyl zirconocene dichloride-methylalu-moxane system. 

Metallocene catalysis has been combined with ATRP for the synthesis of PE-fr-PMMA block copolymers [123]. PE end-functionalized with a primary hydroxyl group was prepared through the polymerization of ethylene in the presence of allyl alcohol and triethylaluminum using a zirconocene/MAO catalytic system. It has been proven that with this procedure the hydroxyl group can be selectively introduced into the PE chain end, due to the chain transfer by AlEt3, which occurs predominantly at the dormant end-... 

Negishi et al. reported the regioselective synthesis of diisoalkyl derivatives from monosubstituted alkenes in yields ranging from 58-95%, Scheme 8, from the in situ prepared ethylene complex Cp2Zr(C2H4).35 The zirconocene-ethylene complex presumably undergoes alkene insertion to furnish a zirconacyclopentane which further reacts with diethylzinc to yield the diisoalkylzinc compound. 

Heterogeneous tandem catalysis involving at least one of the components being supported has also been reported [178, 179]. For example, calcosilicate has recently been used as an effective carrier for simultaneous immobilisation of a dual-functional system based on a bis(imino)pyridine iron compound and a zirconocene to form a heterogeneous catalyst precursor. On activation with triethylaluminium, ethylene was converted to LLDPE the layered structure of the calcosilicate was used to account for the improved thermal stability and higher molecular weights of the LLDPE formed [179],... 

Clear formation of ketene—zirconocene complexes upon treatment of acylzirconocene chlorides with a hindered amide base indicates that the carbonyl group of the acylzirconocene chloride possesses usual carbonyl polarization (Scheme 5.10). However, these zirconocene—ketene complexes are exceptionally inert due to the formation of strongly bound dimers [13a], Conversion of the dimer to zirconocene—ketene—alkylaluminum complexes by treating with alkylaluminum and reaction with excess acetylene in toluene at 25 °C has been reported to give a cyclic enolate in quantitative yield. Although the ketene—zirconocene—alkylaluminum complex reacts cleanly with acetylene, it does not react with ethylene or substituted acetylenes [13b]. Thus, the complex has met with limited success as a reagent in organic synthesis. 

Chiral C2-symmetric ansa-metallocenes, also referred to as bridged metallocenes, find extensive use as catalysts that effect asymmetric C—C bond-forming transformations [4]. In general, bridged ethylene(bis(tetrahydroindenyl))zirconocene dichloride ((ebthi)ZrCl2) 1 or its derived binaphtholate ((ebthi)Zrbinol) 2 [5] and related derivatives thereof have been extensively utilized in the development of a variety of catalytic asymmetric alkene alkylations. 

Many chiral, enantiomerically pure zirconocenes are known [20], In order to induce an asymmetric reaction, chiral zirconocenes have to be prepared, of which the most common are [(EBTHI)ZrCl2] EBTHI = r 10-ethylene-l,2-bis(tetrahydroindenyl), see Scheme 8.47 for the corresponding bis(triflate) and Erker s [(NMI)2ZrCl2] (NMI = r 5-neomenthyhn-dene) [21] (see Scheme 8.37). The [(EBTHI)ZrCl2] complex is commercially available as a racemate or in enantiomerically pure form (for a resolution procedure, see the supplementary material of [22]), and the precursor [(EBI)ZrCl2] is available as a racemate. 

Ethylene and styrene derivatives react with various propargylic silyl ethers in the presence of zirconocene(II) to afford allenic products in high yield (Scheme 5.7). For example, substrate 67 was transformed into the trisubstituted allene hydrocarbon 68 in 93% yield under the reaction conditions [20]. In the synthesis of various tetraalkylated allenes, in which several of the alkyl substituents also contained triple bonds, allowing these substrates to be cydized intramolecularly into aromatic com-... 

After activation with MAO (molar ratios [Al] [Zr] = 1000) the polymerization of ethylene has been successfully carried out using the zirconocene functionalized dendrimer at 40 bar ethylene pressure and 70 °C. We obtained high activity and productivity values for the ethylene polymerization and polymers with very high molecular masses in the range of 2 x 10 g/mol. The polydispersity of the polymer is quite low (3.0) indicating the single site character of the catalytically active species. Optimization of this system and study of the mechanism are stiU under investigation. Nevertheless, these preliminary results reveal the suitability of polyphenylene dendrimers as supports for zirconocene catalysts. 

We note that there are NMR-based kinetic studies on zirconocene-catalyzed pro-pene polymerization [32], Rh-catalyzed asymmetric hydrogenation of olefins [33], titanocene-catalyzed hydroboration of alkenes and alkynes [34], Pd-catalyzed olefin polymerizations [35], ethylene and CO copolymerization [36] and phosphine dissociation from a Ru-carbene metathesis catalyst [37], just to mention a few. 

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