Zirconocene alkyls

Stable transition-metal complexes may act as homogenous catalysts in alkene polymerization. The mechanism of so-called Ziegler-Natta catalysis involves a cationic metallocene (typically zirconocene) alkyl complex. An alkene coordinates to the complex and then inserts into the metal alkyl bond. This leads to a new metallocei e in which the polymer is extended by two carbons, i.e. 

In 1978, Schwartz and Gell found that CO would induce reductive elimination of alkane in various zirconocene alkyl hydride complexes with concurrent formation of Cp2Zr(CO)2 (2) (52,53). It was postulated that CO initially coordinates to the 6-e complex 23 forming the coordina-tively saturated species 24 which can then reductively eliminate alkane and/or rearrange to a zirconocene acyl hydride intermediate. When R = cyclohexylmethyl, methylcyclohexane reductively eliminated and Cp2Zr(CO)2 was isolated in 25% yield. 

It has since been shown that if less coordinating anions are used, then cationic zirconocene alkyls may serve as highly active single-component catalysts. Hence, treatment of Cp2ZrMe2 with... 

Cationic zirconocene alkyl complexes of general type [Cp2Zr(R)(L) ]+ (n = 0, 1, L = labile ligand) see Labile) react readily with pyridines and other A-heterocycles by coordination and metalation ortho C H activation) to yield... 

The closely related cationic zirconocene alkyl derivatives [(l)2ZrMe]+ and [(21)2ZrMe]+were investigated by quantum-chemical methods. 

In most cases, complexes of primary alkyl ligands are more stable than the isomeric complexes of secondary or tertiary alkyl ligands. For example, Reger has shown that the secondary butyl iron complex in Equation 3.18 isomerizes to the corresponding primary n-butyl complex, and that the isopropyl palladium complex in Equation 3.19 isomerizes to the more stable -propyl isomer. Likewise, secondary zirconocene alkyl complexes isomerize to the linear isomers (Equation 320), as shown many years ago by Schwartz, and Labinger and Bercaw have recently shown that the sec-butyl complex of zirconocene, generated by the hydrozirconation of cis-2-butene, isomerizes in several hours to the corresponding n-butyl complex. ... 

The relative reactivities of dihydrogen, arenes, and alkanes toward a-bond metathesis parallels the relative reactivities of these reagents toward oxidative addition. a-Bond metathesis between dihydrogen and zirconocene alkyl complexes occurs (Equation 6.49), - but metathesis between alkanes or arenes and these complexes does not. Examples of a-bond metatheses with arenes are less common than those ivith dihydrogen, but they are more common and occur faster than those with alkanes. For example, Cp ScMe reacts with benzene to form Cp ScPh (Equation 6.50) faster than Cp ScMe reacts with labeled methane. 

Otiier reductive eliminations that form C-H bonds do occur after initial dissociation of a ligand. For example, the elimination of carborane in Equation 8.14, - the elimination of ketone in Equation 8.15, and the elimination of aldehyde in Equation 8.16 aU occur after dissociation of a phosphine ligand. In contrast, die reductive elimination of alkane from the zirconocene alkyl hydride complex in Equation 8.17 occurs after association of ligand. ... 

Several examples of p-chloride elimination are shown in Equations 10.24-10.29. Reaction of vinyl chloride with cationic zirconocene-alkyl complexes (Equations 10.24a and 10.24b) forms propylene and the corresponding zirconocene-chloride complex as the initial products. Tlie final products result from polymerization of the propylene by the starting zirconocene-alkyl species, and generation of a dinuclear cationic chloride species from the resulting cationic chloride complex and a zirconocene dichloride by a less-defined pathway. The propylene is formed by the process shown in Equation 10.24b. Insertion of vinyl chloride into the zirconocene methyl, followed by p-chloride elimination from the p-chloroalkyl intermediate generates propylene and a cationic chloride complex. 

In the light of Yasuda s successful use of neutral lanthanocene alkyl complexes as initiators, several authors, including Collins et al., Soga et al., Gibson et al., and Hocker et al., rationalized that isolobal single-component, cationic zirconocene alkyl complexes should also polymerize MMA without the need for a second Zr center. This is indeed the case, and nearly all contemporary reports in this area now employ single-site cationic alkyl and cationic enolate initiators. Figure 23.9 collects many of the complexes examined to date and summarizes the resultant PMMA tacticities. 

Yang, X., Stem, C.L., and Marks, T.J. (1991) "Cation-like" homogeneous olefin polymerization catalysts based upon Zirconocene alkyls and tris(pentafluorophenyl) borane, J. Am. Chem. Soc., 113, 3623-3625. 

Highly active and more well-defined catalyst systems have become accessible by reaction of a zirconocene alkyl precursor with one of several cationization reagents that contain a trityl or dimethylanilinum cation capable of abstracting an alkyl anion from this precursor. An inert tetra(perfluorophenyl) borate or related anion coordinates only weakly to the resulting alkyl zirconocenium cation in an inner-sphere ion pair of type A (Fig. 3) [26, 28, 29]. 

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