Titanium complexes Schiff base derivative

Chiral Schiff bases derived from salicylaldehyde, when complexed to titanium(IV), catalyse enantioselective addition of diketene (101) to aldehydes (RCHO R = Ph, alkyl, alkenyl) to yield 5-hydroxy-/ -keto esters (102), with up to 84% ee 19... 

The reactions of ketene acetals with Schiff bases derived from (S)-valine esters 1.59 (R = /-Pr) under TiCl4 catalysis are highly selective, but the chiral auxiliary has not been removed [264], Engler and coworkers [1532] performed the asymmetric [2+2]-cycloaddition of 2-methoxy-l,4-benzoquinone with 1-arylpro-penes substituted by electron-donating groups. These reactions occur at -78°C when catalyzed by chiral titanium complexes derived from 2.50 (R = Me, Ar = R = Ph). Cycloadducts are obtained with a good selectivity (Figure 9.6). 

Xn approach to 5-hydroxy-3-ketoesters by the condensation of diketene with aldehydes n lives a titanium alkoxide derived from the complex Schiff base 14. ... 

Tridentate chiral Schiff bases derived from (- -)-Q -pinene (123) catalyse enantioselective addition of diethylzinc to aryl aldehydes. Steric, aryl stacking, and electronic effects are significant in the transition state, the latter being probed by a Hammett plot of ee versus for a series of benzaldehydes. Complexes of the Schiff base (123) with titanium(IV) catalyse enantioselective cyanation. ... 

Bryliakov, K. and Talsi, E. (2007). Asymmetric Oxidation of Sulfides with H2O2 Catalyzed by Titanium Complexes with Aminoalcohol Derived Schiff Bases, J. Mol. Catal. A Chem., 264, pp. 280-287. 

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],... 

Carreira et al. used a chiral BlNOL-derived Schiff base-titanium complex as the catalyst for the aldol reactions of acetate-derived ketene silyl acetals (Scheme 8C.29) [64a]. The catalyst was prepared in toluene in the presence of salicylic acid, which was reported to be crucial to attain a high enantioselectivity. A similar Schiff base-titanium complex is also applicable to the carbonyl-ene type reaction with 2-methoxypropene [64b], Although the reaction, when con-... 

The same conversion is successfully catalyzed by using in-situ generated complexes of Ti(OPr )4 and tridentate Schiff bases (Stmcture 54), which are derived from substituted salicylaldehydes with chiral aminoalcohols [85]. Another similar chiral reagent is derived from reaction of titanium tetraisopropoxide and the Schiff base of 3,5-di-tert-butylsalicylaldehyde and (5)-valinol. The mechanism and stereoselectivity of these chiral Lewis acids are discussed by Corey and co-workers. Other chiral Ti Schiff base complexes have been employed in asymmetric TMSCN addition to benzaldehyde [85]. 

To get a greener process, Jackson and coworkers reported a study dedicated to the use of immobilised Schiff base ligands for asymmetric thioether oxidation (Scheme 7.9). The salen ligand derivative 13, linked to a Wang resin, formed an extremely robust titanium complex, which allows the system to be reused several times without erosion of conversion and enantioselectivity. No leaching of titanium was detected in the reaction solvent. 

The in situ formed basic cyclopentadienyl complex of titanium, ethylene-bis( f-tetrahydroindenyl)-titanium hydride (EBTHI)TiH) containing a planar element of chirality, is active for the reduction of a C=N double bond of both N-aryl-imines (Schiff bases) and unsubstituted imines (108,109). (EBTHl)TiX2 derivatives are used as precursors of the hydride analogue. The chiral zirconium analogues of the above-mentioned complexes provided a breakthrough in asymmetric polymerization of terminal oleflns (110). 

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 . 

The carbon allylation of tertiary alcohols and derivatives provides a simple access to the formation of quaternary carbon centers. The chiral titanium complex prepared from tridentate Schiff base and Tip4 has been reported to catalyze a... 

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