Brintzinger-type catalysts

Scheme 1.28. Polymerization of propylene with Kaminsky-Brintzinger type catalyst.

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. 

Recently Waymouth and Coates have demonstrated that it is also possible to produce an optically active polyolefin using a chiral non-racemic metallocene catalyst of the Brintzinger type. [7] Although optically active oligomers can be obtained in the presence of a resolved metallocene complex according to the above mentioned procedure... 

Enantiomerically pure SSCs (79), readily available through resolution of the racemic C2-symmetric Brintzinger-type a sa-metallocenes (59,473-475), have attracted considerable interest in catalytic and enantioselective C—C bond formation reactions (476). Despite the aforementioned symmetry concerns, these catalysts have opened new possibilities in synthesis and design of chiral macromolecules. 

Brintzinger s catalyst systems [C2H4(Ind-H4)2ZrX2]/MAO act as hydrogenation rather than polymerization catalysts if excess H2 is present (20 bar) internal and disubstituted olefins are only hydrogenated. Chiral catalysts give up to 36% ee. Comparison of the stereoisomers of 1-pentene dimerization and deuteration indicate that opposite enantiofaces of the incoming olefin are preferred in the two reaction types, apparently because of the need to accomodate the metal-bound alkyl chain in the polymerization case in the... 

A very different type of catalyst was developed by Buchwald et al. [6] the chiral Ti complex with Brintzinger s ansa-metallocene ligand (ebthi) is extraordinarily effective for the enantioselective hydrogenation of cychc imines with high optical yields (>97% ee). Unfortunately, the activity and productivity of this Ti catalyst are relatively low compared to Rh- and Ir-diphosphine catalysts. The stereochemical outcome of the reaction can be predicted by straightforward steric arguments. Acyclic imines are reduced with lower enantioselectivity, probably due to isomerization problems and the presence of syn/anti isomers. Studies with multifunctional imines or in presence of other substrates revealed that the scope of the Ti-ebthi catalyst is rather Hmited. Partial or total catalyst inhibition is observed in presence of most functional groups, expect amines, alcohols, acetals, and halides [39]. 

The availability of a chiral version of Cp2Ti(CO)2 was instrumental in the development of the first catalytic asymmetric Pauson-Khand type reaction [39]. This work utilized a catalyst containing the ethylene-1,2-bis( 17 -4,5,6,7-tetrahydro-l-indenyl) (EBTHI) ligand (Fig. 4,16) first introduced into Group 4 chemistry by Brintzinger [40]. Complexes containing this ligand have proven extremely effective in a number of applications in asymmetric catalysis [41]. 

Brinell hardness See test, Brinell hardness. Brintzinger A chemist from Konstanz University, Hamburg, Germany, who m 1982 was the first to report on rnetalloorganic (metallocene) complex compounds of titanium and zirconium. Science and industry soon used his discovery to develop new polymerized compounds such as polyethylene and polypropylene plastics. Unlike its classical predecessors, the structure of metallocenes can be analyzed in all types of details, enabling it to be adapted to suit the requirements of the plastic researchers with a broad scope of possible variations. See catalyst, metallocene. 

---