Zirconocene-alkyl complexes

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. 

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... 

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. 

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. 

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. 

The aforementioned observations have significant mechanistic implications. As illustrated in Eqs. 6.2—6.4, in the chemistry of zirconocene—alkene complexes derived from longer chain alkylmagnesium halides, several additional selectivity issues present themselves. (1) The derived transition metal—alkene complex can exist in two diastereomeric forms, exemplified in Eqs. 6.2 and 6.3 by (R)-8 anti and syn reaction through these stereoisomeric complexes can lead to the formation of different product diastereomers (compare Eqs. 6.2 and 6.3, or Eqs. 6.3 and 6.4). The data in Table 6.2 indicate that the mode of addition shown in Eq. 6.2 is preferred. (2) As illustrated in Eqs. 6.3 and 6.4, the carbomagnesation process can afford either the n-alkyl or the branched product. Alkene substrate insertion from the more substituted front of the zirconocene—alkene system affords the branched isomer (Eq. 6.3), whereas reaction from the less substituted end of the (ebthi)Zr—alkene system leads to the formation of the straight-chain product (Eq. 6.4). The results shown in Table 6.2 indicate that, depending on the reaction conditions, products derived from the two isomeric metallacyclopentane formations can be formed competitively. 

The alkylation of the zirconocene thioacetaldehyde complex 81a with excess Mel also yielded an 5-methylated product (122) [Eq. (26)]. In contrast, the protonation by methanol in excess gave ethanethiol, trimethyl-phosphine, and dimethoxyzirconocene, presumably via a [Zr(OMe)(SEt) (t75-C5H5)2] intermediate.70... 

Besides hydrozirconation of terminal triple bonds Cp2Zr(H)Cl (16) also reacts with double bonds.8 The mechanism is similar to that described for alkynes. After coordination of alkene 5 to the Zr center giving -complex 23 the terminal double bond inserts into the Zr-H bond to form the stable (T-alkyl complex 24. The bulky zirconocene moiety again adds to the end-position of the terminal double bond. 

The relevance of Ln/Al heterobimetallic complexes for the emulation of zirconocene-based polymerization catalysis [13-15] was later on also stressed by the Lanthanide Ziegler-Natta Model [16]. Accordingly, lanthanidocene alkyl complexes were not only successfully employed for clarifying major initiation, propagation, and termination steps (Scheme 3) [17,18]. 

The cocatalyst has various functions. The primary role of MAO as a cocatalyst for olefin polymerization with metallocenes is alkylation of the transition metal and the production of cation-like alkyl complexes of the type Cp2MR+ as catalytically active species (91). Indirect evidence that MAO generates metallocene cations has been furnished by the described perfluorophenyl-borates and by model systems (92, 93). Only a few direct spectroscopic studies of the reactions in the system CP2MCI2/MAO have been reported (94). The direct elucidation of the structure and of the function of MAO is hindered by the presence of multiple equilibria such as disproportionation reactions between oligomeric MAO chains. Moreover, some unreacted trimethylaluminum always remains bound to the MAO and markedly influences the catalyst performance (77, 95, 96). The reactions between MAO and zirconocenes are summarized in Fig. 8. 

Treatment of zirconocene dichloride, 50, with 2 equiv. of an appropriate alkyllithium or Grignard reagent generates transient zirconocene olefin complexes that upon loss of alkene provide access to zirconocene, 109, and the powerful reduction chemistry of divalent zirconium.48 Owing to the utility of this reagent in organic synthesis and organometallic reactions, the low-temperature alkylation of zirconocene dichloride, 50, with BunLi has been... 

Pyrans that bear a C5 group are resolved with high selectivity as well (entry 4). In this class of substrates, one enantiomer reacts more slowly, presumably because its association with the zirconocene-alkene complex leads to sterically unfavorable interactions between the C5 alkyl unit and the coordinated ethylene hgand. 

Reductive alkylations. Reaction o philes followed by protonation accomplis zirconocene-ethene complex effects c>d... 

Reductive alkylations. Reaction of the complexed species with electrophiles followed by protonation accomplishes reductive alkylations. However, the zirconocene-ethene complex effects cyclopropylmethylation of alkynes with 4-bromobutene. ... 

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