Metallocenes zirconocene

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 metallocene dichloride of zirconium and hafnium 20b and 20c were also prepared and underwent reduction with potassium to give monomeric metallocene monochloride complexes 21b and 21c (Eq. 8) [39b]. The structure of the zirconocene complex 21 b in the crystal showed a conformation which suggests a less steric strain as compared to 21a due to zirconium s larger atomic size. As a consequence of the coordinative unsaturation an unusually short Zr —Cl bond length was found. 

For recent symposiums on zirconocene chemistry, see E. Negishi, Recent Advances in the Chemistry of Zirconocene and Related Compounds, Tetrahedron Symposia-in-print No. 57, Tetrahedron 1995, 51 (special issue). R. F. Jordan Metallocene and Single Site Olefin Catalysis, f. Md. Catal. 1998, 128 (special issue) and references cited therein. R. F. Jordan, A. S. Guram, in Comprehensive Organometallic Chemistry II, E. W. Abel, F. G. A. Stone, G. Wilkinson, M. F. Lappeet (eds.), Pergamon Press, Oxford, 1995, Vol. 4, p. 589. 

The compounds 2 and 3, have all been isolated from reactions of phenylsilane with either dimethyltitanocene (1 2) or dimethyl-zirconocene (r ). All of the evidence points to the fact that these compounds are probably resting species and are not involved in the catalytic cycle. They do nevertheless give some indication of the complex series of reactions that transform the dimethyl-metallocene to active catalyst. 

Metallocene catalysis has been combined with ATRP for the synthesis of PE-fr-PMMA block copolymers [123]. PE end-functionalized with a primary hydroxyl group was prepared through the polymerization of ethylene in the presence of allyl alcohol and triethylaluminum using a zirconocene/MAO catalytic system. It has been proven that with this procedure the hydroxyl group can be selectively introduced into the PE chain end, due to the chain transfer by AlEt3, which occurs predominantly at the dormant end-... 

The isotacticities and activities achieved with nonbridged metallocene catalyst precursors were low. Partially isotactic polypropylene has been obtained by using a catalyst system of unbridged (non-ansa type) metallocenes at low temperatures [65]. A chiral zirconocene complex such as rac-ZrCl2(C5H4 CHMePh)2 (125) is the catalyst component for the isospecific polymerization of propylene (mmmm 0.60, 35% of type 1 and 65% of type 2 in Scheme Y) [161]. More bulky metallocene such as bis(l-methylfluorenyl)zirconium dichloride (126) together with MAO polymerized propylene to isotactic polypropylene in a temperature range between 40 and 70°C [162]. 

When a chiral ansa-type zirconocene/MAO system was used as the catalyst precursor for polymerization of 1,5-hexadiene, an main-chain optically active polymer (68% trans rings) was obtained84-86. The enantioselectivity for this cyclopolymerization can be explained by the fact that the same prochiral face of the olefins was selected by the chiral zirconium center (Eq. 12) [209-211]. Asymmetric hydrogenation, as well as C-C bond formation catalyzed by chiral ansa-metallocene 144, has recently been developed to achieve high enantioselectivity88-90. This parallels to the high stereoselectivity in the polymerization. 

An important step in the field of investigation of metallocenes was the design of zirconocene catalysts. These made it possible to produce polypropylenes with a great variation in tacticity [5], Although the ligands of the catalysts were varied, the molecular weights mostly did not exceed 300,000 g mob1. 

The exchange of zirconium in isostructural complexes leads to a new family of asymmetric metallocenes (Fig. 1) bearing a 2-methyl substituent and varied substituents in positions 5, 6, and 7 of the indenyl moiety. After borate activation all catalysts show an unexpected high and constant activity toward the polymerization of propylene and lead to significantly increased molecular weight products compared to the zirconocene species [9-11],... 

Kagan et al. were the first to report the corresponding enantioselective catalytic hydrogenation using chiral metallocene derivatives [94, 95]. By using menthyl- and neomenthyl-substituted cyclopentadienyl titanium derivatives in the presence of activators (Scheme 6.5) [96], these authors observed low ee-values (7-14.9%) for the catalytic hydrogenation of 2-phenyl-l-butene into 2-phenylbutane. In contrast, no enantiomeric excess was obtained with the corresponding zirconocene derivatives. 

Chiral C2-symmetric ansa-metallocenes, also referred to as bridged metallocenes, find extensive use as catalysts that effect asymmetric C—C bond-forming transformations [4]. In general, bridged ethylene(bis(tetrahydroindenyl))zirconocene dichloride ((ebthi)ZrCl2) 1 or its derived binaphtholate ((ebthi)Zrbinol) 2 [5] and related derivatives thereof have been extensively utilized in the development of a variety of catalytic asymmetric alkene alkylations. 

Subsequent to these studies by the author s group, Whitby and co-workers reported that enantioselective alkylations of the type illustrated in Scheme 6.6 can also be carried out with the non-bridged chiral zirconocene 31 [19]. Enantioselectivities are, however, notably lower when alkylations are carried out in the presence of 31. For example, this new chiral metallocene affords 29 and 30 (Scheme 6.5) with 82% and 78% ee, respectively. 

Collins and co-workers have performed studies in the area of catalytic enantioselective Diels—Alder reactions, in which ansa-metallocenes (107, Eq. 6.17) were utilized as chiral catalysts [100], The cycloadditions were typically efficient (-90% yield), but proceeded with modest stereoselectivities (26—52% ee). The group IV metal catalyst used in the asymmetric Diels—Alder reaction was the cationic zirconocene complex (ebthi)Zr(OtBu)-THF (106, Eq. 6.17). Treatment of the dimethylzirconocene [101] 106 with one equivalent of t-butanol, followed by protonation with one equivalent of HEt3N -BPh4, resulted in the formation of the requisite chiral cationic complex (107),... 

Collins and co-workers have also reported on an enantioselective catalytic Diels—Alder cycloaddition, in which zirconocene and titanocene bis(triflate) complexes were used as catalysts [104], The influence of the solvent polarity on the observed levels of stereoselectivity is noteworthy. For example, as shown in Scheme 6.34, with 108 as the catalyst, whereas in CH2C12 (1 mol% catalyst) the endo product was formed with 30% ee (30 1 endoxxo, 88% yield), in CH3N02 solution (5 mol% catalyst) the enantioselectivity was increased to 89% (7 1 endoxxo, 85% yield). Extensive 1H and 19F NMR studies further indicated that a mixture of metallocene—dienophile complexes was present in both solutions (-6 1 in CH2C12 and -2 1 in CH3N02, as shown in Scheme 6.34), and that most probably it was the minor complex isomer that was more reactive and led to the observed major enantiomer. For example, whereas nOe experiments led to ca. 5 % enhancement of the CpH proton signals of the same ring when Hb in the minor complex was irradiated, no enhancements were observed upon irradiation of Ha in the major complex. 

If the above research is an indication, the catalytic enantioselective variants of many of these exciting transformations will soon be disclosed in our leading journals. Another challenge in this area remains the difficulty encountered in preparing chiral zirconocene catalysts, particularly since many of the reactions promoted by this group of chiral catalysts cannot be effected by the non-metallocene variants. Thus, the development of more practical, but equally or even more selective and efficient variations of existing methods should not be viewed as any less significant. 

Cationic zirconocenes serve as useful reagents in such diverse fields as alkene polymerization, carbohydrate chemistry, asymmetric catalysis, and so on. Reagents that were originally developed for polymerization reactions (MAO, ansa-metallocenes, non-nucleophi-lic borate counterions) have now found use in organic synthesis and are being employed for carbometalation reactions, hydrogenation, and Diels—Alder catalysis. 

These conditions were optimized for Cp2HfCl2 activation, but are also applicable to the zirconocene version of the reaction. In the original procedure, a large excess of the metallocene dichloride and silver salt was employed to enable the rapid glycosidation of sensitive substrates, but this is not usually necessary. 

The manifold chemistry of alkenes with titanocenes and zirconocenes has been reviewed in detail in some of the above mentioned contributions [1], The reactions of the metallocene sources 1—6 with alkenes have only been investigated with regard to the specific question as to whether or not complexes or coupling products could be obtained. 

Complexation of 1,3-butadiynes In the reactions of complex 1 with various butadiynes, binuclear complexes with intact C4 units between the two metal centers are found. The former diynes are transformed to zig-zag butadiene ligands or g-r (l-3),r (2-4)-trans,-trans-tetradehydrobutadiene moieties between two metallocene cores. The bond type in 19 is unknown for the corresponding zirconocene complexes. 

ANOVA (analysis of variance), commercial experimental design software compared, 8 398t Anoxic conditions, defined, 3 757t ansa-metallocenes, 16 90, 94 ansa-zirconocene catalysts,... 

Figure 13. Transition states for propene insertion into the Zr-isobutyl bond of the racemic-dimethylsilyl-bis-l-indenyl zirconocene with a (RJt) coordination of the aromatic ligand. C2 is the overall symmetry of the metallocene, while re and si is the chirality of coordination of the propene molecule in the transition states of parts a and b, respectively.

Zirconocene and Half-Sandwich Zirconium Derivatives The development of a single-site heterogeneous catalyst for metallocene-based polymerization catalysis has also been explored extensively with zirconocene and half-sandwich zirconium derivatives [32, 75, 91, 92]. 

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