Zirconocene-alkene complex

Figure 6.1. Substrate-catalyst interactions favor a specific mode of alkene insertion into the zirconocene—alkene complex.

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 regio- and stereoselective zirconocene-catalyzed addition of alkylmagnesium halides to alkenes, a process which has been described previously (see Section 7.5.2, Scheme 7-79) was investigated with ethylene-l,2-bis( M,5,6,7-tetrahydroind-l-enyl)zirconium dichloride [(EBTHI)ZrCl2l) [118] as chiral zirconocene. Thus, treatment of the latter with alkylmagnesium halides leads to the formation of the derived zirconocene-alkene complex 88, characterized by NMR [119], which reacts with cyclic ethers or amines to lead to the corresponding homoallylic alcohol or amine, respectively, in > 95% ee and good overall yield [120] (Scheme 7-103). 

The transient zirconocene butene complex, 105, has proved to be useful in a number of organic transformations. For example, butene substitution of zirconocene alkene complexes with alkoxy-substituted olefins results in /3-alkoxide elimination to furnish the zirconocene alkoxy compounds (R = Me, 123 R = Bnz, 124) (Scheme 16).50,51 Addition of propargyl alcohols to the zirconocene butene complex, 105, affords homoallylic alcohols. These reactions are of limited utility owing to the lack of stereoselectivity or formation of multiple products. Positioning the alkoxide functional group further down the hydrocarbyl chain allows synthesis of cyclopropanes, though mixtures of the carbocycle and alkene products are obtained in some cases (Scheme 16).52... 

The aforementioned observations carry significant mechanistic implications. As illustrated in Eqs. (2a-c), in the chemistry of zirconocene-alkene complexes that are derived from the longer chain alkylmagnesium halides several additional selectivity issues present themselves. 

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. 

Zirconocene-alkene complexes reacted with chlorodiphenylphosphine to form zircono-ethylphosphine derivatives with high regioselectivity, subsequently giving p-functionalized alkyl-diphenylphosphine oxides by reaction with a variety of halides (Scheme 13) ... 

Zirconocene alkene complexes 29 are readily prepared by the p-hydride activation route from dialkyl zirconocenes 28 or methyl alkyl zirconocenes 27 (Scheme 5.10). It is also possible to prepare the zirconocene complexes of ethene, styrene, and, to a lesser extent, vinyltrimethylsilane by displacement of the weakly bound 1-butene ligand from 4. ... 

When zirconocene alkene complexes are formed in the presence of an excess of the same alkene, symmetric dimers 30 are formed with high regio-and stereocontrol. Cross-coupling of different alkenes is not generally successful because the rate of alkene exchange from the first formed iq -alkene... 

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. 

The vacant orbital in 16e -zirconocene(IV) complexes allows a Ji-interaction with an incoming alkene or aUcyne. However no metal— alkene/alkyne backbonding is possible with the d°-Zr-metal center. As a consequence, the metal-olefin interaction is not stabilized, and formation of the thermodynamically favored o-bound organozirconocene complex (>10 kcal/mol) is then observed [36]. The product is the result of an overall cis addition of the zirconocene metal fragment and the hydrogen across the carbon-carbon multiple bonds. 

Negishi et al. reported the regioselective synthesis of diisoalkyl derivatives from monosubstituted alkenes in yields ranging from 58-95%, Scheme 8, from the in situ prepared ethylene complex Cp2Zr(C2H4).35 The zirconocene-ethylene complex presumably undergoes alkene insertion to furnish a zirconacyclopentane which further reacts with diethylzinc to yield the diisoalkylzinc compound. 

Zirconocene-catalyzed kinetic resolution of dihydrofurans is also possible, as illustrated in Scheme 6.8 [18]. Unlike their six-membered ring counterparts, both of the heterocycle enantiomers react readily, albeit through distinctly different reaction pathways, to afford — with high diastereomeric and enantiomeric purities — constitutional isomers that are readily separable (the first example of parallel kinetic resolution involving an organome-tallic agent). A plausible reason for the difference in the reactivity pattern of pyrans and furans is that, in the latter class of compounds, both olefmic carbons are adjacent to a C—O bond C—Zr bond formation can take place at either end of the C—C 7T-system. The furan substrate and the (ebthi)Zr-alkene complex (R)-3 interact such that unfavorable... 

Terminal RCH—CH2 1-Hexene C4H9CH=CH2 is isomerized by complex 1 in accordance with the factors influencing the thermodynamic stability of cis- and trans-2 -hexene [15], At the end of the reaction, the alkyne complex 1 was recovered almost quantitatively. No alkene complexes or coupling products were obtained. The corresponding zirconocene complex 2a did not show any isomerization activity. Propene CH3CH=CH2 reacts with complex 6 with substitution of the alkyne and the formation of zirconacydopentanes as coupling products, the structures of which are non-uniform [16]. 

Cyclopropane ring cleavage is also observed in the case of zirconocene -alkene and jy -imine complexes with adjacent cyclopropane rings to give f/ -allyl, j/ -azaallyl, and -enamine complexes [29]. 

In the zirconocene-catalyzed polymerization of alkenes, Landis and coworkers [20] have reported in situ observation of a Zr-polymeryl species, 15, at 233 K (Figure 1.5). Complex 15 is formed by partial reaction of 14 with excess 1-hexene. Derivatives 16 and 17 are generated quantitatively from 15 by addition of ca. 10 equiv. of propene and ethene, respectively. No other intermediates, such as alkene complexes, secondary alkyls, diasteromers of 15 or 16, or termination products, accumulate to detectable levels. These NMR studies permit direct monitoring of the initiation, propagation and termination processes, and provide a definitive distinction between intermittent and continuous propagation behavior. 

Casey was able to prepare related zirconocene alkenyl complexes according to Scheme 8.18. Alkene coordination was established by a number of NMR techniques. While zwitterionic compounds 38 allowed the determination of the alkene dissociation energy, AG = 10.5 kcal mol , very similar to that of 35. Thermally more stable complexes were obtained by protonation of 37 with [HNMePh2][B(C5F5)4[. Dynamic NMR spectroscopy and line shape analysis allowed the measurement of the barriers of alkene dissociation (AG = 10.7 and 11.1 kcal mol ), as well as for the site epimerisation ( chain skipping ) at the zirconium center (AG = 14.4 kcal mol" ) (Scheme 8.19) [77]. 

In a simplified picture, the mechanism of the Zr-catalyzed ethylmagnesation can be rationalized as shown in Scheme 1 [8]. At first, the zirconocene-ethene complex 12 is generated from the catalyst precursor Cp2ZrCl2. Complex 12 can also be regarded as a metallacyclopropane 16. After coordination and insertion of the alkene 10, a metalla-cyclopentane 13 is formed, which subsequently reacts with the Grignard reagent regioselectively to the open-chain intermediate... 

Coupling of 2,3-dihydrofuran with alkene-zirconocene <2004AGE3932> or aryne-zirconocene <2005SL2513> complexes and subsequent addition of an electrophile provided rA-disubstituted homoallylic alcohols, as illustrated in Equation (130). An insertion//3-elimination pathway that involved the formation of an oxazirconacyclooctene intermediate was proposed for the reaction mechanism. 

The catalytic cycle proposed for the enantioselective ethyhnagnesation involves the chiral zirconocene-ethylene complex (121), formed upon reaction of the dichloride with two equivalents of EtMgCl. Coupling of the alkene substrate leads to the formation of the metallacyclopentane intermediate, which gives the zirconate (122) with a... 

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... 

Alkene complexes of Ti, Zr and Hf have been intensively investigated with regard to the nature of bonding and the close relation to olefin oligomerization and polymerization. Alkene complexes of zirconocene and hafnocene are isolated as the trimethylphosphine adduct, Cp2M(T -alkene)(PMe3) (33) [92-94]. Cp 2Ti(CH2=CH2) (34) is a 16 electron ethylene complex with a rich reaction chemistry as summarized in Scheme 6.4 [95-99]. The reaction profile of 34 indicates that the metallacyclopropane canonical form makes an important contribution [100]. 

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