Zirconium allyls

The corresponding zirconium allyl derivatives have also been investigated6,7, but, in general, do not have any particular advantages over the titanium reagents. In addition, the starting materials are often more expensive and more difficult to purify. 

Substituents in the allyl group of a catalyst have a marked effect on the polymerization efficiency (9,12). This is shown in Table IV for the polymerization of ethylene with chromium and zirconium allyls and for the polymerization of methyl methacrylate with chromium allyls. Introducing a methyl group into the allyl ligand increases the activity by a factor of 2 to 7. In some polymerizations of ethylene Cr(2-Me-allyl)3 compounds are ten times more effective than the simple allyl derivatives. The introduction of... 

In addition, internal unsaturations may be found these arise most probably via C-H activation and zirconium allyls. A mechanism for their formation is shown in Scheme 8.43 this also explains the formation of H2 in such polymerisation systems [128]. 

Table 7 presents the results for the ethylene and propylene polymerization with the zirconium allyl complexes 8 and 10. For both complexes, below an Al Zr ratio of 400 neither ethylene nor propylene was polymerized. At atmospheric pressure, no polymerization of propylene was detected. 

Monocyclopentadienyl zirconium allyl compounds are prepared in moderate yield at RT by adding a dilute ether or THE solution of the appropriate allyl Grignard to a stirred ether solution of rj -Cp ZrCl prepared in situ ... 

We next turned our attention to the Clark zirconium allylation for the synthesis of C8-C9 bond (Scheme 6). Clark had previously shown that in s(tM-generated allyl zirconium nucleophiles could selectively produce anfz-hydroxypropionates of a variety of aliphatic and aromatic aldehydes. In our lab, we found that reaction of aldehyde 15 with the same allyl zirconium nucleophile did selectively generate anti-hydroxypropionates. Unfortunately, the epoxide failed to control the facial selectivity of this addition thus, a 1 1 mixture of separable diastereomers 18 and 19 was formed. This reaction represents our first successful, albeit limited, synthesis of the desired intermediate 18. We next attempted to optimize the output of diastereomer 18 by modifying the Clark reaction conditions defined as THF or PhCH3 solvent with no additives. 

Zirconium—ally complexes also have catalytic properties. Tetraally zirconium [12090-34-5] on a siUca substrate catalyzes ethylene polymerization (265). Supported on sihca, ZrR (R = allyl or neopentyl) catalyzes olefin isomerization (266). 

I.3.3.3.8.2.I.I. With Titanium(IV) and Zirconium(IV) Reagents Nonhetero-Substituted Allylic Reagents... 

Recently some information became available on a new type of highly active one-component ethylene polymerization catalyst. This catalyst is prepared by supporting organometallic compounds of transition metals containing different types of organic ligands [e.g. benzyl compounds of titanium and zirconium 9a, 132), 7r-allyl compounds of various transition metals 8, 9a, 133), 7r-arene 134, 185) and 71-cyclopentadienyl 9, 136) complexes of chromium]. 

The (pentamethylcyclopentadienyl)zirconium amidinate unit also served as a platform for the synthesis and characterization of remarkable cationic and zwitterionic allyl zirconium complexes derived from trimethylenemethane (TMM). A direct synthetic route to the neutral precursors was found in the... 

The zirconium TMM complexes also react with organic electrophiles, including unactivated ones, according to Scheme 99 under formation of the corresponding substituted allyl complexes. ... 

Ally] ethers and iodobenzene have also been added using a zirconium complex. Similarly, allyl ethers and allyl chlorides have been added. ... 

In the case of the allyl compounds, displacement of the allyl group seems to be required. Thus the reaction of Zr(allyl)4 with pyridine follows a fundamentally different course to the corresponding benzyl compound. In this instance the reaction is irreversible, and an allyl group is displaced to give a trivalent zirconium compound (9) as shown by the equation... 

Examinations of the far-infrared spectra of solutions of Zr (allyl) 3CI and Zr (allyl) 2CI2 (9) suggest that the former exists in solution as the dimer (X), whereas the latter has the monomeric structure (XI). A broad intense peak at 244 cm-1 can be assigned to zirconium-bridging chlorine stretching mode. This band is completely absent from the spectrum of the dihalide and is replaced by a very strong band at 342 cm-1 due to the nonbridging chlorine. 

In contrast to the zirconium compounds the chromium halides are less active than pure Cr(allyl)3 Cr (allyl) Cl2 seems to be completely inactive. The only other element known in which halide substitution of the corresponding allyl compound results in activation is hafnium, suggesting the effect may be confined to group IVa. 

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