Half-zirconocene

The kinetic resolution using a chiral zirconocene-imido complex 286 took place with high enantioselectivity to result in chiral allenes 287 (up to 98% ee) (Scheme 4.74) [116]. However, a potential drawback of these methods is irreversible consumption of half of the allene even if complete recovery of the desired enantiomer is possible. Dynamic kinetic resolutions avoid this disadvantage in the enantiomer-differentiating reactions. Node et al. transformed a di-(-)-L-menthyl ester of racemic allene-l,3-dicarboxylate [(S)- and (RJ-288] to the corresponding chiral allene dicarbox-ylate (R)-288 by an epimerization-crystallization method with the assistance of a catalytic amount of Et3N (Scheme 4.75) [117]. 

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]. 

In summary, zirconocene and half-sandwich zirconium-based catalysis has been developed both in surface and solution systems. In general, the activation of Zr-POSS and silica-supported zirconium system with MAO proved inappropriate (see silsesquioxane displacement and leaching, respectively, vide supra), while the acti-vahon with BArl proved more efficient, albeit structural rearrangement complicates the activation chemistry and simple Zr(IV) cationic alkyl species are seldom the outcome of the activation. 

Half-sandwich zirconocene-based catalysts (e.g. those derived from the CpZrCl3 precursor) show a remarkably low activity when compared with the titanium analogues. The lower activity of Zr-based catalysts might be due to the lower electrophilicity and lower concentration of catalytic active sites [71] as well as, at least in part, to the higher stability of the Zr(IV) species in comparison with the Ti(IV) species [55,57]. 

A half-sandwich imido complex of niobium (132), which is both isoelec-tronic and isolobal with the bent metallocene Cp2ZrCl2, reacts with two equiv PhMgBr in the presence of PMe3 to give the 18-e complex 6 [Eq. (21)].29 In contrast to the zirconocene analogue, trimethylphosphine is required to stabilize this tr-complex. 

Some half sandwich titanium compounds with cyclopentadienyl ligands have proven to be most active, but soluble tetraethyoxytitanium also shows a certain amount of activity. In contrast to olefin polymerization, titanocenes are more active than zirconocenes and fluoro ligands are better than chloro ligands. Table 24 [207] compares some catalysts for the polymerization of styrene. 

The most notable difference between the two families is catalyst activity. With few excep-tions,2 225 229 myriad cases, the activity of the supported catalyst is half to a tenth that of the soluble catalyst. This is widely ascribed to diminished diffusion of monomer into the interior pores of the supported catalyst, but may also be the result of fewer active centers present in the heterogeneous variant. Catalyst centers could be deactivated when supported, or may not be generated in the metal— cocatalyst interaction. Tait and co-workers determined that 91% of zirconocene centers were activated by MAO in solution, but when the metallocene was supported on silica, the concentration of active sites generated by MAO was only 9% of the total zirconium supported. 

In 2013, Chen reported the homopolymerisation of both yMBL and yMMBL by using the bidentate half-salen titanium trisbenzyl complex 114 preactivated with B(C6F5)3 leading to the putative cationic active species 115 (Scheme e.lS)." The in situ activation of 114 in toluene allowed up to 87% of yMBL and 98% of yMMBL to be converted in 24 h at 25 °C, and afforded mainly syndio-biased atactic PyMBL and PyMMBL (39.3% rr and 47.8 to 57% rr, respectively), which are still far less active and stereoselective than cationic zirconocene catalysts." ... 

During polymerization of 1-hexene by an MAO-activated ansa-zirconocene catalyst system, species of types A, B and B were detected by NMR spetroscopy but found to account for less than half of the initial metallocene concentration [31]. Major parts of the metallocene content of this catalyst system thus remain unaccounted for. 

The availability of the Zr(C5H5)(6,6-dmch)X2 and Zr(6,6-dmch)2X2 complexes, and the fact that 6,6-dmch is electronically intermediate between C5H5 and typical pentadienyl ligands, have provided an opportunity to attempt the preparation of N2 complexes of an open or half-open zirconocene complex. In fact, reduction of these complexes with sodium amalgam under a nitrogen atmosphere was observed to lead to deeply colored (blue to purple) solutions, which changed color under vacuum. While crystalline products were not readily isolated, this at least provides an indication that N2 coordination to metal pentadienyl complexes can occur. 

Fig. 24. Molecular structure of the half-open zirconocene diene complex, 24 (hydrogen atoms omitted).

A number of open and half-open titanocenes, zirconocenes, and hafnocenes have been reported to yield catalysts for the hydrogenation of unsaturated polymers such as polybutadiene. The hydrogenations were carried out in solution phase, and generally were very efficient (84—89%). 

The inteimolecular hydroamination of allenes is readily catalyzed by early transition metal complexes to yield imines. An addition of aromatic and ahphatic amines to aUene requires high reaction temperatures (90-135°C) and long reaction times (1-6 days) when mediated by zirconocene- [41] and tantalum-imido [178] catalysts. The more efficient titanium half-sandwich imido-amide complex 42 operates under significantly milder reaction conditions (27) [179], Because the metal-imido species are prone to dimerization, sterically more hindered aliphatic and aromatic amines are more reactive. Simple, sterically unencumbered aliphatic amines add to aUenes in the presence of the bis(amidate) titanium complex 43 (28), although higher reaction temperatures are required [180]. 

---