Applications zirconocene derivatives

An enantioselective variant of the diene cydization reaction has been developed by application of chiral zirconocene derivatives, such as Brintzinger s catalyst (12) [10]. Mori and co-workers demonstrated that substituted dial-lylbenzylamine 25 could be cyclized to pyrrolidines 26 and 27 in a 2 1 ratio using chiral complex 12 in up to 79% yield with up to 95% ee (Eq. 4) [ 17,18]. This reaction was similarly applied to 2-substituted 1,6-dienes, which provided the analogous cyclopentane derivatives in up to 99% ee with similar diastereoselectivities [19]. When cyclic, internal olefins were used, spirocyclic compounds were isolated. The enantioselection in these reactions is thought to derive from either the ate or the transmetallation step. The stereoselectivity of this reaction has been extended to the selective reaction of enantiotopic olefin compounds to form bicyclic products such as 28, in 24% yield and 59% ee after deprotection (Eq. 5) [20]. 

ABSTRACT. Polysilanes, (-SiRR -)n, represent a class of inorganic polymers that have unusual chemical properties and a number of potential applications. Currently the most practical synthesis is the Wurtz-type coupling of a dihalosilane with an alkali metal, which suffers from a number of limitations that discourage commercial development. A coordination polymerization route to polysilanes based on a transition metal catalyst offers a number of potential advantages. Both late and early metal dehydrogenative coupling catalysts have been reported, but the best to date appear to be based on titanocene and zirconocene derivatives. Our studies with transition metal silicon complexes have uncovered a number of observations that are relevant to this reaction chemistry, and hopefully important with respect to development of better catalysts. We have determined that many early transition metal silyl complexes are active catalysts for polysilane synthesis from monosilanes. A number of structure-reactivity correlations have been established, and reactivity studies have implicated a new metal-mediated polymerization mechanism. This mechanism, based on step growth of the polymer, has been tested in a number of ways. All proposed intermediates have now been observed in model reactions. 

Finally, it should be noted that enantiomerically pure derivatives of the Schwartz reagent are very appealing candidates for reagent controlled enantioselective and diastereoselective radical reductions. This is because a plethora of enantiomerically pure zirconocenes are available from other applications in enantioselective catalysis. 

Furthermore, a novel electrophilic sp2 C-H bond activation and its application for the preparation of a-diazomethylzirconium complexes were also recently reported. Treatment of complexes 53 with ethyldiazoacetate or with trimethylsilyldiazo methane affords the C-a-metallated diazo alkane complexes 63-66 (Scheme 15). When 53a,b were allowed to react with the non terminal diazo derivative 67, the pyrazoles 70 and 71 were obtained in good yield. Formation of the pyrazoles could be rationalized by O-H bond activation of the enol tautomer of 67, leading to the zirconocene diazo intermediates 68 and 69 which... 

The application of chiral ansa-zirconocene catalysts in this method may lead to optically active ring systems. These results complement numerous stoichiometric applications of regio- and stereoselective reductive cyclization of nonconjugated dienes16-17, diynes18 or enynes19-20 with zirconocene and titanocene derivatives. 

Zirconocene dichloride. 14. 12 Aldol reaction. Zirconiu amides derived from (- >-ps< corresponding lithium enolates ( Cyclopentanones.- C ck) valuable method for the prep application is found in a symhei... 

Homogeneous catalysts play an important role in industry as well as in research laboratories. Established applications include, for example, polymerization processes with zirconocene and its derivatives, rhodium- or cobalt-catalyzed hydroformylation of olefins, and enantioselective isomerization catalysts for the preparation of menthol. In contrast to heterogeneous catalysts, more experimental studies of reaction mechanisms are available and the active species can be characterized experimentally in some cases. Most catalysts are based on transition metal compounds, for which electronic structures and properties are well studied theoretically. A substantial number of elementary reactions, such as reductive elimination, oxidative addition, alkene or carbonyl migratory insertion, etc., have been experimentally Studied in detail by means of isotopic, NMR, and IR studie.s, as well as theoretically. ... 

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