Zirconocene complexes

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.

Zirconocene complexes containing two indenyl or tetrahydroindenyl groups bridged with short links such as —CH2—CH2— or —Si(CH3)— can produce isotactic polymers of higher a-olefins (32). To synthesize syndiotactic PO, bridged zirconocene complexes with rings of two different types are required, one example of which is isopropyl(cyclopentadienyl)(l-fluorenyl)zirconocene. These complexes are used for the synthesis of syndiotactic PB... 

The metallocene dichloride of zirconium and hafnium 20b and 20c were also prepared and underwent reduction with potassium to give monomeric metallocene monochloride complexes 21b and 21c (Eq. 8) [39b]. The structure of the zirconocene complex 21 b in the crystal showed a conformation which suggests a less steric strain as compared to 21a due to zirconium s larger atomic size. As a consequence of the coordinative unsaturation an unusually short Zr —Cl bond length was found. 

The zirconocene complex Zr(Si2Cp)2Cl2 (90) is a versatile starting material for a variety of zirconocene complexes with zirconium in the oxidation state IV, III,... 

Table 2. Activation energy for ring rotation in l.l -disubslitued zirconocene complexes...

ChinkovM, Marek I (2005) Stereoselective Synthesis of Dienyl Zirconocene Complexes. 10 133-166... 

Szymoniak J, Bertus P (2005) Zirconocene Complexes as New Reagents for the Synthesis of Cyclopropanes. 10 107-132... 

Spectroscopic and structural data for some of the hydrido zirconocene complexes described above have been reported and discussed in references cited in this section (see also [75, 76]). 

The regioselective hydrozirconahon of internal unsymmetrical alkenes remains a challenge, as it could considerably expand the use of zirconocene complexes. Little is known about the mechanism of zirconium migration along an alkyl chain. 

Then, contrary to what was reported previously, the olefin dissociates from the zirconium metal complex. This conclusion was further supported by other experimental observations. However, it cannot be completely excluded that competition between dissociative and direct rearrangement pathways could occur with the different isomerization processes studied up to now. Note that with cationic zirconocene complexes [Cp2Zr-alkyl], DFT studies suggest that Zr-alkyl isomerizations occur by the classical reaction route, i.e. 3-H transfer, olefin rotation, and reinsertion into the Zr-H bond the olefin ligand appears to remain coordinated to the Zr metal center [89]. 

The regiochemistry of the hydrozirconation of disubstituted stannyl- [24, 167-170] and silyl- [171] acetylenes and boron- [118, 172-175] and zinc- [34, 126] alkynyl derivatives result in the formation of 1,1-dimetallo compounds. Hydrozirconation of alkynyliodonium salts affords alkenylchlorozirconocenes with the Zr-C bond geminal to the iodonium moiety [176]. These zirconocene complexes allowed the preparation of ( )-trisubstituted olefins (Scheme 8-20). 

Enones and enoates undergo 1,2-reduction [115, 191]. Lipshutz et al. reported the effective protection of carbonyl functions by the triisopropylsilyl acyl silane group (TIPS), which allowed the selective conversion of alkenes or alkynes to the corresponding zirconocene complexes [24]. The aldehyde could subsequently be regenerated by desilylation with TBAF [186]. 

Bimetallic Transition Metal-Zirconocene Complexes from Zirconium Hydrides... 

The bis-zirconocene complex CpjClZrCHjCHjZrCpjCl has been isolated upon double hydrozirconation of acetylene with 1 [102]. Recently, the preparation of a heterogeneous bis-zirconocene catalyst was succesfully achieved from zirconocene dichloride complexes containing alkenyl or alkynyl substituents [224]. 

C02-Bridged bimetallic zirconocene complexes have been formed from 1 and metallocarboxylic acids [229]. Reachon of 1 with metal enolates Cp(CO)3WCHR COX (X = OEt, Me, Ph) gives Cp(CO)3WCH(R )CH(R)OZrCp2(Cl). The structure for R = H and R = Me was solved by an X-ray analysis and the chemical reactivity of these organometallic products have been studied [230]. 

Fig. 1 Proposed bimolecular zirconocene complex leading to reductive coupling...

Waymouth and coworkers used chiral zirconocene complexes such as 56 with Et3Al as the stoichiometric reductant to enantioselectively desymmeter-ize oxabicyclic compounds (Scheme 9) [29]. A reductive coupling mechanism to give 57 followed by (i-alkoxidc ring opening and transmetallation is consistent with the experimental results. Neither direct insertion of the alkene into the M - C bond nor nucleophilic attack mechanisms can be ruled out, however [12]. 

Erker and co-workers in 1983 found that Cp2Zr(CO)2 (2) was formed, along with a mixture of other organozirconium products, when oligomeric bis(cyclopentadienyl)dihydridozirconium, (Cp2ZrH2), was stirred in toluene under 148 atm of CO at room temperature for 1 week. While Cp2Zr(CO)2 (2) could be isolated in 30% yield, the product of interest was a novel trimeric (7j2-formaldehyde)zirconocene complex (56). 

The isotacticities and activities achieved with nonbridged metallocene catalyst precursors were low. Partially isotactic polypropylene has been obtained by using a catalyst system of unbridged (non-ansa type) metallocenes at low temperatures [65]. A chiral zirconocene complex such as rac-ZrCl2(C5H4 CHMePh)2 (125) is the catalyst component for the isospecific polymerization of propylene (mmmm 0.60, 35% of type 1 and 65% of type 2 in Scheme Y) [161]. More bulky metallocene such as bis(l-methylfluorenyl)zirconium dichloride (126) together with MAO polymerized propylene to isotactic polypropylene in a temperature range between 40 and 70°C [162]. 

One interesting recent addition is a silene—zirconocene complex proposed in the reaction shown in Scheme 1.16 [94]. 

Scheme 1.37. Common patterns in the migratory insertion of zirconocene complexes with CO, isonitriles, and metal alkynylates.

Scheme 1.58. Zirconium and alkene migration of diene-zirconocene complexes.

Conversion to Ketone— and Ketene Zirconocene Complexes and Reactions Thereof 5.3.2.1 Ketone zirconocene complexes... 

Scheme 5.8. Formation and reactions of benzophenone-zirconocene complex.

Scheme 5.9. (CH3)3AI-mediated formation of a ketone-zirconocene complex and reactions.

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. 

Cationic r 2-acylzirconocenes [Cp2Zr(COR)(L)+] have been reported to show attractive reactivity [39]. A cationic r 2-acylzirconocene can be generated in quantitative yield by treating a cationic zirconocene complex [Cp2Zr(R)(L)+] (R = alkyl or alkenyl) with carbon monoxide (<23 °C, 1 atm) (Scheme 5.36). The cationic acylzirconocene complexes undergo regiose-... 

Reaction of saturated acylzirconocene chlorides with (CH3)2Cu(CN)Li2 gives the secondary alcohol (73%), and D20 work-up of the reaction mixture gives the a-deuterio alcohol. This observation suggests the formation of a ketone—zirconocene complex (Scheme 5.40 see also Section 5.3.2.1). Thus, for the reaction of a,p-unsaturated acylzirconocene chlorides with R2Cu(CN)Li2, initial formation of an unsaturated ketone—zirconocene complex followed by 1,3-rearrangement of the zirconocene moiety to an oxazirconacyclopentene, which is a ketone carbanion equivalent, has been proposed (Scheme 5.41). 

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