Zirconium complex, olefin insertion

Concomitant with continued olefin insertion into the metal-carbon bond of the titanium-aluminum complex, alkyl exchange and hydrogen-transfer reactions are observed. Whereas the normal reduction mechanism for transition-metal-organic complexes is initiated by release of olefins with formation of hydride followed by hydride transfer (184, 185) to an alkyl group, in the case of some titanium and zirconium compounds a reverse reaction takes place. By the release of ethane, a dimetalloalkane is formed. In a second step, ethylene from the dimetalloalkane is evolved, and two reduced metal atoms remain (119). 

Both equivalent coordination sites of the C2-symmetric complex are framed by the /i-substituents in such a manner that 7-olefin insertions at the Zr centre occur with equal enantiofacial preference at both sites. It must be emphasised that unilateral coverage of each coordination site is essential for this stereoselectivity this is borne out by the finding that atactic polypropylene is formed if both coordination sites are flanked by two /i-substituents, as in catalysts based on rac.-ethylenebis[l -(3 -methylindenyl)]zirconium dichloride [rac.-(MeIndCH2)2ZrCl2] (Figure 3.39) [112]. 

In the course of these studies key features such as olefin coordination [19,20], olefin insertion (propagation) [16,21,22], /(-hydrogen elimination [16,21,22], and /3-alkyl elimination [23] could be spectroscopically and structurally proven. In particular, yttrium aluminates have been proposed to model isoelectronic cationic homo- and heterobridged Zr/Al heterobimetallic complexes as dormant species and potential polymer chain transfer candidates [24]. Such zirconium aluminate complexes seem to be elusive [25], while the first structurally characterized titanium alumi-... 

Zirconium-benzyne complexes have been used rather extensively in organic synthesis.8 45 For this purpose, one particularly important characteristic of zirconium-aryne complexes is that olefin insertion into the Zr—C bond occurs stereospecifically. Thus, when generated in situ, the zirconium-benzyne complex (45) reacts with cyclic alkenes to give exclusively the cis-zirconaindanes (46), which upon treatment with electrophiles provide access to a variety of m-difunctionalized cycloalkanes (47-49) (Scheme 5).46 For example, carbonylation of intermediate 46 affords tricyclic ketone 49, reaction with sulfur dichloride gives thiophene 48, and reaction of 46 with tert-butylisocyanide followed by I2 gives 47 via 50 and, presumably, intermediate 51 [Eq. (12)]. 

Abstract The synthesis and X-ray structure of various octahedral zirconium complexes and their catalytic properties in the polymerization of a-olefins are described. Benzamidinate, amido, allylic, and phosphinoamide moieties comprise the study ligations. For the benzamidinate complexes, a comparison study between homogeneous and heterogeneous complexes is presented. For the phosphinoamide complex, we show that the dynamic symmetry change of the complex from C2 to C2v allows the formation of elastomeric polymers. By controlling the reaction conditions of the polymerization process, highly stereoregular, elastomeric, or atactic polypropylenes can be produced. The formation of the elastomeric polymers was found to be the result of the epimerization of the last inserted monomer to the polymer chain. 

Zirconium-aryne complexes have found applications in organic synthesis. For example, treating diallylamine 22 with BuLi and zirconocene(methyl) chloride forms an aryne-zirconium complex that undergoes intramolecular olefin insertion to yield metallacycle 23, and trapping this metallacycle with iodine gives 24, further manipulation of which allows rapid construction of 25, an analogue of the pharmacophore of the antitumour agent CC-1065 [21] (Scheme 4). 

Many additional routes to metal-alkyl complexes other than transmetallation and alkylation are discussed in later chapters of this text. For example, metal-alkyl complexes are generated by insertion of an olefin into a metal-hydride or or metal-hydrocarbyl species. Such insertion reactions are discussed in Chapter 9, but an example of the synthesis of a zirconium alkyl by olefin insertion into a zirconium hydride is shown in Equation 3.9. Metal-alkyl complexes are also generated by nucleophilic attack on coordinated olefins (Equation 3.10) or carbene ligands (Equation 3.11). These reactions are presented in detail in Chapter 11. 

Taking into account that thermal conversion of diamido to imido species is not accessible to BnjCyclam zirconium complexes, the cyclization reaction is likely to occur by 1,2-insertion of the olefin moiety in the Zr-N bond (Scheme 25.4). This pathway was initially proposed by Tobin Marks for lanthanocene and constrained geometry zirconium complexes and was more recently also suggested for other group 4 metal catalysts [15, 16]. The activation of the olefin toward insertion may... 

The Zr-CHj bond is the active center of zirconium catalysts. It is formed in the methylation reaction of the zirconium ions in the complexes with, for example, methylaluminoxanes. The next steps of the catalyst action are olefin complexation and olefin insertion into the Zr-CHs bond ... 

X-ray analysis of solid compounds such as ((CH3)2C5H3)2ZrCH3+ + H3CB(C6F5)3 (90) showed that a coordination bond still partly exists between the zirconocene and the borate. The olefin is tt-complex bonded into this compound and then inserted into the zirconium-methyl bond. 

The model for syndiotactic polymerization is a bit more complex than the model needed for isotactic polymerization. In 1988 Ewen and co-workers reported the discovery of a metallocene catalyst, iso-propyl(cyclopentadienyl-1 -fluorenyl)zirconium dichloride, 7, that would produce syndiotactic polypropylene, that is, a polymer formed from the sequential reaction of alternate olefin r-faces (see Figure 1 for the molecular structure). In contrast to the family of catalysts that have C2-symmetric catalyst precursors, 7 is Cs-symmetric. Ewen proposed that, for this catalyst to produce syndiotactic polymer, the active site must isomerize after each insertion consistent with the polymer chain flipping from one side to the other during insertion. Stereoerrors were thought to be due to chain back-skipping or reaction with the wrong olefin face. Eor Cz symmetric catalyst precursors, the active site does not isomerize if the polymer chain flips from side to side. 

The next step is the migratory insertion reaction. This leads to the formation of a metal-alkyl complex. Note that we have ended up again with a three-coordinate zirconium cation, which has a metal-alkyl bond. In effect we have lengthened the alkyl chain by the process of olefin coordination followed by migratory insertion. 

The approach and insertion of an olefin molecule may or may not pass through a local minimum or coordination complex (first in brackets in eq. 16) recent theoretical work (128) indicates that the well, if it indeed exists, is very shallow. The insertion of the new molecule into the growing chain is represented in equation 13 as a structure intermediate between reactants and products. The mechanism for this apparently concerted reaction does not involve the participation of metal-based electrons, and can be considered to be a Lewis acid-assisted anionic attack of the zirconium alkyl (ie, the polymer chain) upon one end of a carbon-carbon double bond. The concept of this reaction pre-dates metallocene study, and is merely a variant of the Cossee-Arlman mechanism (129) routinely invoked in Ziegler-Natta polymerization. Computational studies indicate (130) that an a-agostic interaction (131) provides much needed stabilization during the process of insertion. 

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