Zirconium alkyl

Mixed-Metal Systems. Mixed-metal systems, where a zirconium alkyl is formed and the alkyl group transferred to another metal, are a new apphcation of the hydrozirconation reaction. These systems offer the advantages of the easy formation of the Zr—alkyl as well as the versatiUty of alkyl—metal reagents. For example, Cp2ZrRCl (R = alkyl or alkenyl) reacts with AICI3 to give an Al—alkyl species which may then be acylated with... 

The alkylation of 15 (path b, Scheme 15)40 has been performed according to conventional procedures using lithium or Grignard reagents. The preliminary isolation of 15 is not essential thus, the synthesis of the zirconium alkyl or aryl derivatives can be carried out in situ directly from 3 (path a, Scheme 15). The h NMR spectra of 71-74 all imply C2V symmetry, as revealed by the CH2 (one pair of doublets), Bu (two singlets), and MeO (one singlet) patterns. All the alkyl derivatives 71-73 are thermally stable up to 80°C in CgDg. The aryl derivative... 

Polymerization of Ethylene by Zirconium Alkyl Halides in Toluene at 80°C. Concentration 3.00 X 10 3 mole liter-1 Ethylene Partial Pressure 10 atm.Hydrogen Partial Pressure 10 atm (9, 16)... 

The 7r-back donation stabilizes the alkene-metal 7c-bonding and therefore this is the reason why alkene complexes of the low-valent early transition metals so far isolated did not catalyze any polymerization. Some of them catalyze the oligomerization of olefins via metallocyclic mechanism [25,30,37-39]. For example, a zirconium-alkyl complex, CpZrn(CH2CH3)(7/4-butadiene)(dmpe) (dmpe = l,2-bis(dimethylphosphino)ethane) (24), catalyzed the selective dimerization of ethylene to 1-butene (Scheme I) [37, 38]. 

For application in organic synthesis, the regiochemistry of insertion of carbenoids into un-symmetrical zirconacydes needs to be predictable. In the case of insertion into mono- and bicydic zirconacydopentenes where there is an wide variety of metal carbenoids insert selectively into the zirconium—alkyl bond [48,59,86], For more complex systems, the regiocon-trol has only been studied for the insertion of lithium chloroallylides (as in Section 3.3.2) [60]. Representative examples of regiocontrol relating to the insertion of lithium chloroal-lylide are shown in Fig. 3.2. 

More recently, Landis et al. studied the polymerisation kinetics of 1-hexene with (EBI)ZrMe( t-Me)B(C5F5)3 64 as catalyst in toluene [EBI = rac-C2H4(Ind)2]. Catalyst initiation was defined as the first insertion of monomer into the Zr-Me bond, 65 (Scheme 8.30). Deuterium quenching with MeOD was used to determine the number of catalytically active sites by NMR. The time dependence of the deuterium label in the polymer was taken as a measure of the rate of catalyst initiation. This method also provides information of the type of bonding of the growing polymer chain to zirconium, as n-or sec-alkyl, allyl etc. Hexene polymerisation is comparatively slow, with high regio- and stereoselectivity there was no accumulation of secondary zirconium alkyls as dormant states [96]. 

However, in a comprehensive study, Seebach and coworkers have shown the usefulness of titanium and zirconium alkyls as selective nucleophilic reagents.106,107 Typically compounds of the type (RO)3MR (M = Ti, Zr) are reacted with aldehydes or ketones to yield asymmetric alkoxides (equation 32),107 The method has been extended to allow enantioselective addition using chiral organotitanium reagents where the chirality is imposed by the initial alkoxide substituents.107,108... 

Another site-selective reaction of 35 with a,/3-unsaturated carboxylic acid derivatives in the presence of a Lewis acid has been reported (Scheme 25) [35]. The intermediate 37 possesses two nucleophilic sites the zirconium alkyl and the enol ether moieties. Starting from 37, two competing pathways lead accordingly to cyclopropane (38, path a) and/or cyclobutane (39, path b) derivatives. The... 

The insertion of metal carbenoids into zirconacyclopentenes that are not a-substituted on the alkyl and the alkenyl sides of the Zr shows only a 2.2 1 selectivity in favor of the alkyl side. In the case of insertion into zirconacyclopentenes in which there is an Q -substituent on the alkenyl, a wide variety of metal carbenoids selectively insert into the zirconium alkyl bond. On the contrary, steric hindrance of the alkyl side leads to complete selectivity in favor of insertion into the alkenyl side. The selectivity cannot be systematically attributed to steric factors see Steric Effect) especially when insertion into the more hindered side is observed. In this case, the observed selectivity can be correlated with the highest... 

It is generally assumed that MAO firstly undergoes a fast ligand exchange reaction with the metallocene dichloride ( - Me). Either CH or CH3 is further abstracted from the complex by an Al-center from MAO, thus forming a cationic species (step 1). In the step 2, the monomer (alkene) coordinates to the empty orbital of the complex. Step 3 is related to the insertion see Insertion) of the alkene into the zirconium-alkyl bond which generates a new free... 

Zirconium imido complexes, generated by thermolysis of zirconium alkyl amides and bis(amides)104 or tetrazenes105 in the presence of excess norbornene, gave the [2 + 2] cycloaddition products with exo configuration, e.g., 11 (X-ray)104. 

Detailed 11 and 13C lf) assignments were used to deduce the geometry for the substituted salicylaldimine derivatives of zirconium alkyls, Zr(L)R2, where R = CH2Ph or Cl I2 Bu, H2L = derivatives of 2,2 -diamino-6,6 -dimethylbi-phenyl.56... 

Equations 8.1721 and 8.18 indicate that zirconium alkyls do undergo cleavage by electrophiles (see Section 8-4-1), such as protic acids or halogens (X2), to give the corresponding hydrocarbon or alkyl (alkenyl) halide. Note that retention of configuration occurs at the carbon attached to the metal as the reaction proceeds to the organic product. 

Table i. Bis(cyclopentadienyl)zirconium(alkyl) Chlorides Prepared From Alkenes and ( / -Cp)2ZrHCl... 

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