Ansa-zirconocene

Inspired by the ability of cationic ansa-zirconocene complexes to effect stereocontrolled alkene polymerization reactions, Jordan has recently reported the stereoselective insertion of simple alkenes into both the (ebi)Zr(r 2 -pyrid-2 -yl) and (ebthi) Z r (r 2 -pyr id - 2 -yl) systems [113]. As shown in Scheme 6.36, treatment of rac-(ebi)ZrMe2 114 with nBu3NH+BPh4 in the presence of 2-picoline affords the (ebi)Zr(q2-pyrid-2-yl) complex 115 (the derived B(C6F5) derivatives may also be prepared and are in fact reported to be more convenient to use). 

ANOVA (analysis of variance), commercial experimental design software compared, 8 398t Anoxic conditions, defined, 3 757t ansa-metallocenes, 16 90, 94 ansa-zirconocene catalysts,... 

C2-symmetrical, 16 114 ansa-zirconocene complexes, 16 107 ANSI-approved standards, 15 760. 

Zinifex Ltd., 26 564 Zinin reduction, 2 490-491 Zircaloy tubes, 17 573 ansa-Zirconocene catalysts, C2-symmetrical, 16 114 ansa-Zirconocene complexes, 16 107 Zircon, 26 621-622... 

Short branches, specifically ethyl branches up to about 2 mol%, are formed in the polymerization of ethylene by meso-ansa zirconocenes containing unsubstituted cyclo-pentadienyl and indenyl ligands [Melillo et al., 2002]. Ethyl branches form by an isomerization process in which the usual P-hydride transfer to monomer is immediately followed by reinsertion of the vinyl-terminated polymer into the formed ethyl-zirconium bond. 

Figure 12. Chromatograms of the resolution of (a) the ansa-zirconocene derivative, (b) the adamantan derivative, (c) 2,2 -dihydroxy-6,6 -dimethylbiphenyl, (d) phenyl vinyl sulfoxide, (e) abscisic acid, and (f) laudanosine. Column, 25 x 0.46 cm (i.d.) flow rate, 0.5 ml/min. Chromatograms (a) and (e) are reproduced, with permission, from Ref. 61 (Copyright 1996, Chemical Society of Japan) and Ref. 62 (Copyright 1988, Elsevier Science B.V.),...

The Zr-F(CAr) interaction is rather weak (see below). It is easily cleaved by the addition of a suitable two-electron donor such as CO or an isonitrile. The formation of the dipolar [(carbonyl)hafnocene-C4II6 B(C6F5)3] betaine system (131)133 from 128a (M = Hf), or the adduct 132 that was obtained by treatment of 128 (M = Zr) with PMe3, are typical examples.136 The ansa-zirconocene-derived betaine 130 forms similar... 

The (butadiene) ansa-zirconocene-derived complexes 130 (RCp, R = H, Me, cyclohexyl, isopropyl, tert-butyl) are active catalysts for the polymerization of methylmethacrylate.149 The isotacticity of the resulting PMMA increases rapidly with increasing bulk of the attached substituents.150 The initial intermediate (156) of the polymerization reaction was observed by NMR spectroscopy124,149 (Scheme 54). 

Butadiene, dimerization, 275 Butadiene ansa-zirconocene-derived... 

Many ansa zirconocenes are sensitive to light (199,200). An interconversion into the Cs-symmetric meso form is observed until an equilibrium is reached between the two diastereomers. The final rac/meso proportion and the interconversion rate depend on the structures of the different metallocenes (Table VIII, p. 130). The colorless tetrahydroindenyl compounds are stable, but the substituted bis(indenyl)zirconocenes, which are the most... 

Scheme 5.7 Three examples of ansa-zirconocene dichlorides directing the polymerization of propene, in the presence of MAO, to isotactic, syndiotactic and atactic polypropylene (PP)...

Twofold inter- and intramolecular PKRs have found interesting applications in synthesis of ansa-zirconocenes. An early example illustrated the use of this approach for the synthesis of cyclopentadiene anellated[2,2]para-cyclophanes 157-158. The reaction of several paracyclophanedienes (155) with alkynes gave the corresponding twofold cycloaddition products 156, which were transformed into cyclopentadienyl anions orthogonally attached to the bridges of the paracyclophane (Scheme 46) [158]. 

On the other hand, most studies on twofold PKRs deal with the use of acyclic and cyclic diynes for the synthesis of phenyl or alkyl bridged ansa-zirconocenes such as 161. Scheme 47 shows an example of the reaction of a di-endiyne (159) that gives a twofold PKR, forming compound 160 [159,160]. 

Scheme 49 Catalytic double PKR followed by a retro-Diels-Alder reaction for the synthesis of ansa-zirconocenes...

Chiral ansa zirconocenes produce highly isotactic polypropylene Kaminsky, Brintzinger... 

Soga et al. have reported the synthesis of ansa zirconocenes on silica by synthesizing a precursor of the bridge anchored using SiOz-OH groups on the surface of silica (Table 20) [168]. 

The diastereoselectivities that can be achieved in the transformations of chiral, non-racemic substrates can be improved in certain cases by use of a chiral catalyst [10]. Because of the effect of double diastereoselection the correct absolute configuration of the catalyst is important. Hoveyda et al. chose the chiral ansa-zirconocene derivative 31, which can be synthesized according to a method described by Brintzinger (Scheme 3) [11]. 

Figure 10 Precursors for soluble polymerization catalysts ansa-zirconocene complex (left), constrained-geometry titanium complex (middle) and nickel diimine complex (right).

Discussion Point DPS The previously unexpected observation of side-chain branches in diimine-nickel catalyzed polyethylene formation is explained by the reaction scheme represented in Figure 15. Propose related chain migration schemes which explain i) the chain straightening , i.e. the incorporation of propylene methyl substituents into the backbone of polypropylene chains produced by these catalysts, ii) the 2,co-concatenation of higher a-olefins by some Ni-based catalysts, and Hi) the introduction of stereoerrors in isotactic polypropylene by chain-migration of chiral ansa-zirconocene catalysts. 

Another reaction can be used to generate a two-carbon bridge between the Cp hgands. In tetrahydrofuran (THF), the reaction of ZrCLi with two equivalents of n-BuLi, begun at —78 °C and completed at 25 °C, results in a black suspension of ZrCl2 and LiCl, which reacts with fulvenes to provide a high yield of ansa zirconocene dichloride (equation 4). 

In relation with the abihty of cationic ansa zirconocene complexes to affect stereocontroUed alkene polymerization reactions, it is possible to insert simple alkenes into the [(ebi)Zr(jj -pyrid-2-yl)]+ and [(ebthi)Zr(j7 -pyrid-2-yl)]+ systems. The derived three-membered azazirconacycles are formed with high levels of diastereoselectivity, depending on the nature of alkene substituents. Thus, the enantiopure ebthi complex catalyzes asymmetric bond-forming reactions (equation 72). 

Using the parent zirconocene-butadiene complex as a representative example, a typical bonding situation in these types of molecules is presented in Scheme 48. For 297, equilibration between the s-trans and the s-cis isomers occurs with a barrier of 23 kcal mol 1 at 283 K. The 72-olefin complex is believed to be a high-energy intermediate on the interconversion reaction surface. Significantly, structural data indicates that the s-cis complexes are best described as Zr(iv) compounds with a er2, ir ligand.158,175 The dynamic NMR measurements have also been extended to ansa-zirconocene and hafnocene butadiene complexes.176 Moreover, photoelectron spectroscopy has been used to determine the relative energetics of the two isomers for // -metallocenes.177... 

Photochemical [2 + 2]-cycloaddition of unbridged bis(alkenylindenyl)zirconocene complexes 975 is a unique approach leading to formation of 1,2-cyclobutylene-bridged highly efficient and complete within 2-3 h with nearly quantitative conversions. Upon activation with MAO, these ansa-zirconocenes are effective catalysts for ethylene/l-octene co-polymerizations at elevated temperatures. 

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