Brintzinger catalysts

Buchwald and coworkers have used the titanium version of the Brintzinger catalyst (2.100) in the asymmetric reduction of trisubstituted alkenes. The catalyst is reduced in situ to a titanium(lll) hydride species (see Section 3.6). The reduction is achieved with "butyllithium and hydrogen, whilst the silane serves to stabihse... 

Group 4 metallocene complexes can also be used as catalysts in the reduction of C=N bonds. Willoughby and Buchwald employed the titanium-based Brintzinger catalyst (3.54) for the asymmetric reduction of imines. The catalyst is activated by reduction to what is assumed to be the titanium(III) hydride species (3.55). The best substrates for this catalyst are cyclic imines, which afford products with 95-98% ee. Various functional groups including alkenes, vinyl silanes, acetals and alcohols were not affected under the reaction conditions. For example, the imine (3.56) was reduced with excellent enantioselectivity, without reduction of the alkene moiety. 

Fink G, Miihlhaupt R, Brintzinger HH (eds) (1995) Ziegler catalysts recent scientific innovations and technological improvements. Springer, Berlin Heidelberg New York... 

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]. 

The authors conducted a similar investigation of precatalysts 7 and 11 using TiBA and trityl tetrakis(pentafluorophenyl)borate as the cocatalyst. They concluded that this material contained no fraction that could be characterized as blocky. It was therefore proposed that reversible chain transfer occurred only with MAO or TMA and not with TiBA. This stands in contrast to the work of Chien et al. [20] and Przybyla and Fink [22] (vida supra), who claim reversible chain transfer with TiBA in similar catalyst systems. Lieber and Brintzinger also investigated a mixture of isospecific 11 and syndiospecific 12 in attempts to prepare iPP/sPP block copolymers. Extraction of such similar polymers was acknowledged to be difficult and even preparative temperature rising elution fractionation (TREF) [26, 27] was only partially successful. 

Fink, G., Mtilhaupt, R., and Brintzinger, H. H., Eds., Ziegler Catalysts Recent Scientific Innovations and Technical Improvements, Springer Verlag, Berlin, 1995. 

The asymmetric version of the reaction utilizes Brintzinger s ethylene-bis(tetrahydroin-denyl) (EBTHI) ansa-metallocene approach [85]. Whereas complex B (Scheme 8.46) is still a catalyst for the Diels—Alder reaction, only low inductions are observed at room temperature. On cooling to —78 °C and using more reactive starting materials, a maximal induction of 52.4% ee was attained [86]. 

Kaminsky-Brintzinger systems, 16 82 Kaminsky catalysts, 17 702 20 426 Kaminsky metallocene catalysts,... 

The Brintzinger-type C2-chiral titanocene catalysts efficiently promote asymmetric hydrogenation of imines (Figure 1.30). A variety of cyclic and acyclic imines are reduced with excellent enantioselectivity by using these catalysts. The active hydrogenation species 30B is produced by treatment of the titanocene binaphtholate derivative 30A with n-butyllithium followed by phenylsilane. 

Finally, Brintzinger s synthesis of stereo-rigid ansa-metallocenes [18,19] provided the basis for modern high-activity metallocene catalysts and greatly increased their economic potential. However, although MAO activation and stereo-rigid structures of the new metallocene systems were extremely successful, the complexity of these catalytic systems was such that precise information about the structure of the active species remained elusive. 

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