Titanium complexes with Schiff-base ligands

Titanium complexes with Schiff-base ligands (Figure 14.13,68-76) show low activities for the syndioselective polymerization of styrene when activated with MAO (2.26 x 10" g PS/(mol Ti mol... 

Yong, L. Huang, J. Lian, B. Qian, Y. Synthesis of titanium(IV) complexes with Schiff-base ligand and their catalytic activities for polymerization of ethylene and styrene. Chin. J. Chem. 2001, 19, 429-432. 

FIGURE 14.13 Structures of titanium complexes 68-76 with Schiff-base ligands. 

Belokon et al.83 have investigated the formation of the homo-and bimetallic titanium complexes with di-Schiff base ligands, by means of FT NMR spectroscopy. The ligands have been shown to adapt the ds-p configuration in titanium (IV) complexes. Analysis of the 1H NMR spectra has allowed determination of the population of the homobimetallic complexes derived from two different Ti(IV) complexes [34],... 

The macrocyclic chemistry of tetradentate Schiff base complexes has been known for long time. However, the successful use of such a complex as an enantioselective catalyst in epoxidation reactions is a relatively recent finding. In these reactions complex 9.9 or an analogue is used. One of the possible routes for the synthesis of intermediate 9.2 of Table 9.1 involves the use of a similar catalyst. While complex 9.9 works well with unfunctionalized alkenes, for the epoxidation of allylic alcohols, dialkyl tartarates, 9.10, are the preferred ligands. As we shall see, the mechanisms of epoxidation in these two cases are different. Also for the tartarate-based system titanium is the metal of choice (see Section 9.3.3). 

Narasaka has reported that TADDOL-Ti dichloride catalyzes the asymmetric addition of trimethylsilylcyanide to aromatic and aliphatic aldehydes (Sch. 63) [148]. The reactions proceed only in the presence of MS 4A. In reactions with aliphatic aldehydes a chiral cyanotitanium species obtained by mixing of the TADDOL-Ti dichloride and trimethylsilylcyanide before addition of the aldehydes acts as a better chiral cyanating agent and affords higher enantiomeric excesses. Chiral titanium complexes obtained from an alcohol ligand and salicylaldehyde-type Schiff bases and a salen ligand have been reported to catalyze the asymmetric addition of hydrogen cyanide or... 

Enantioselective TMS cyanohydrin formation with aldehydes can be achieved using a variety of chirally-modified titanium catalytic systems chiral modifiers include Schiff bases, including salen ligands and sulphoximines. A chirally-modified yttrium complex has been employed for the same purpose (equation 8) here, the chiral modifier was a ferrocene-derived 1,3-diketone. This general area has been reviewed recently . 

---