Bis zirconocenes

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]. 

Hydrocarbonyl compounds, lanthanide complexes, 4, 4 ( -Hydrocarbyl)bis(zirconocene), preparation, 4, 906 Hydrocarbyl-bridged cyclopentadienyl-amido complexes, with Zr(IV), 4, 864 Hydrocarbyl complexes bis-Cp Ti hydrocarbyls reactions, 4, 551 structure and properties, 4, 551 synthesis, 4, 542 cobalt with rf-ligands, 7, 51 cobalt with rf-ligands, 7, 56 cobalt with ]4-ligands, 7, 59 cobalt with rf-ligands, 7, 71 heteroleptic types, 4, 192 homoleptic types, 4, 192 into magnetic metal nanoparticles via ligand stabilization, 12, 87 via polymer stabilization, 12, 87 into noble metal nanoparticles... 

Protonation of (/u.-alkenyl)bis(zirconocene) complexes under nonnucleophilic conditions takes a different regio-chemical course than the B(C6p5)3 addition (equation 27). The X-ray crystal structure of (60) is consistent with distorted square-pyramidal pentacoordinated geometry at the central atom Cl. 

A stable organometalhc C2v methane derivative (59) was obtained starting from the (/u.-vinyl)bis(zirconocene) complex with tris(pentafluorophenyl) borane. ... 

The synthesis of mono- and bis-zirconocene complexes of [2.2]paracyclophanes with formal triple bonds in the bridges, corresponding to zirconacyclopropenes such as (48), has been... 

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... 

Zirconocene dichloride (bis[cyclopentadienyl]zirconium dichloride) [1291-32-3] M 292.3, m 242-245 , 248 . Purified by recrystn from CHCI3 or xylene, and dried in vacuum. H NMR (CDCI3) 8 6.52 from MeaSi. Store in the dark under N2 as it is moisture sensitive. [IR, NMR, MS Aust J Chem 18 173 7965 method of J Am Chem Soc 81 1364 7959 and references in the previous entry.]... 

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

A concise total synthesis of the indole alkaloid dihydrocorynantheol (101) (Scheme 19), that features two RCM steps and a zirconocene-catalyzed carbo-magnesation [68], is a further example of Martin s interest in applying RCM as a key reaction for the construction of alkaloid frameworks [69]. The first RCM step was applied to bis-allyl amide 96. The resulting intermediate 97 was directly subjected to carbomagnesation and subsequent elimination to deliver 98 in 71% yield from 96. Amide 98 was then transformed into acrylamide 99 in... 

Along similar lines, Schwartz and Gell later reported that tertiary phosphines would also induce reductive elimination in bis(i7-cyclopenta-dienyl) (cyclohexylmethyl) (hydrido)zirconium resulting in high yields of zirconocene bis(phosphine) complexes (53-55). Carbon monoxide was found to readily react with a benzene solution of Cp2Zr(PMePh2)2... 

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). 

Schwartz s Reagent3 is available commercially (from the Aldrich Chemical Company, Inc.) although it is quite expensive. Two literature preparations of this important reagent are available. The first utilizes LiAI(OtBu)3H to reduce zirconocene dichloride.4 The second method utilizes sodium bis(2-methoxyethoxy) aluminum hydride (RED-AL) as the reducing agent.2a The disadvantages of these procedures have been discussed.3... 

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]. 

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],... 

The use of organomagnesium reagents as terminal reductants in zirconocene-catalyzed diene reductive cyclization permits derivatization of the resulting bis(magnesiomethyl)cycloalkanes. However, the use of other stoichiometric reductants is likely to afford catalytic systems that exhibit complementary selectivity profiles. Molander reports the... 

To complete the range of geometric isomers of terminal and non-terminal dienes and trienes available, systems nominally derived from inaccessible (Z)-alkenylzirconocenes are desirable. Fortunately, insertion of the various carbenoids discussed above into mono- or bis(alkynyl) zirconocenes 64 and 65 affords dienyne products 66 [38], which are readily reduced to the desired ( ,Z,2)-trienes (Scheme 3.15) [45—47]. Insertion of the f5-alkynyl carbenoid 62 allows a convenient access to (Z)-enediynes 67. 

The sequential double migratory insertion of CO into acydic and cydic diorganozircono-cene complexes through acylzirconocene and ketone—zirconocene species provides a convenient procedure for preparing acyclic and cyclic ketones (Scheme 5.6) [8], Thus, the bi-cydic enones from enynes can be obtained through CO insertion into zirconacyclopen-tenes followed by a subsequent rearrangement (Scheme 5.7). The scope and limitations of this procedure have been described in detail elsewhere [8d]. This procedure provides a complementary version of the well-known Pauson Khand reaction [9]. 

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). 

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. 

Collins and co-workers have also reported on an enantioselective catalytic Diels—Alder cycloaddition, in which zirconocene and titanocene bis(triflate) complexes were used as catalysts [104], The influence of the solvent polarity on the observed levels of stereoselectivity is noteworthy. For example, as shown in Scheme 6.34, with 108 as the catalyst, whereas in CH2C12 (1 mol% catalyst) the endo product was formed with 30% ee (30 1 endoxxo, 88% yield), in CH3N02 solution (5 mol% catalyst) the enantioselectivity was increased to 89% (7 1 endoxxo, 85% yield). Extensive 1H and 19F NMR studies further indicated that a mixture of metallocene—dienophile complexes was present in both solutions (-6 1 in CH2C12 and -2 1 in CH3N02, as shown in Scheme 6.34), and that most probably it was the minor complex isomer that was more reactive and led to the observed major enantiomer. For example, whereas nOe experiments led to ca. 5 % enhancement of the CpH proton signals of the same ring when Hb in the minor complex was irradiated, no enhancements were observed upon irradiation of Ha in the major complex. 

The reaction of the trimethylphosphane-stabilized bis(trimethyl)silyl zirconocene 66 (Rj = R2 = SiMe3) with (HBEt2)2 not only gives the anti van t Hoff/Le Bel compound 69, but also the dimeric species Cp[g-(r 1 T 5-C5H4)]ZrC(SiMe3)=C(H)(SiMe3) 2, which... 

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. 

Lewis adds based on zirconocene have been employed as catalysts in several reactions. The metallic species used have mainly been the bis(triflate) and the Cp2ZrCl2/AgC104 reagent. 

The isomerization of an O-silyl ketene acetal to a C-silyl ester is catalyzed by a cationic zirconocene—alkoxide complex [92], This catalysis was observed as a side reaction in the zirconocene-catalyzed Mukaiyama aldol reactions and has not yet found synthetic use. The solvent-free bis(triflate) [Cp2Zr(OTf)2] also catalyzes the reaction in nitromethane (no reaction in dichloromethane), but in this case there may be competitive catalysis by TMSOTf (cf. the above discussion of the catalysis of the Mukaiyama aldol reaction) [91] (Scheme 8.51). 

Organometallic Chemistry of Titanocene and Zirconocene Complexes with Bis(trimethylsilyl)acetylene as the Basis for Applications in Organic Synthesis... 

Novel Titanocene and Zirconocene Reagents with Bis(trimethylsilyl)acetylene... 

Figure 10.1. Novel titanocene and zirconocene reagents incorporating bis(trimethylsilyl)acetylene.

No defined complexes could be isolated from reactions of complex 1 with acetone Me2C=0. Complexes 2a and 2b react with acetone to give the zirconafuranone 2c, which is an interesting zirconocene precursor in view of its extremely good solubility in hydrocarbon solvents and because of its ability to dissociate into the alkyne complex [2f], It is also possible to cleanly substitute the bis(trimethylsilyl)acetylene unit so as to obtain the complex 47, or, alternatively, to substitute the acetone with formation of the zirconafuranone 95 (Fig. 10.14) [2f],... 

Scheme 10.9. Synthesis of titanocene (1, 3, rac-5) and zirconocene (4, rac-6) complexes with bis(trimethylsilyl)acetylene without additional ligands.

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