With titanium tridentate

A dimeric zirconium(IV) trihydride complex has been synthesized by reaction of LiBHEt3 with the tridentate triaryloxide zirconium chloride complex ZrCl(THF)2(t-Bu-L) (H3(t-Bu-L) = 2,6-bis(4-t-butyl-6-methylsalicyl)-4-f-butylphenol) (equation 14). The solid-state structure shows each zirconium center adopts a trigonal prismatic stracture with a Zr-Zr separation of 3.163(1) A. In solution, the three resulting hydrides appear equivalent on the NMR timescale. Analogous reactivity was observed for the titanium conger. However, in the resulting titanium trihydride, the metals are assigned formal Ti(III) oxidation states. ... 

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. ... 

The tridentate ether ligand diglyme forms [TiCl3 0(CH2CH20Me)2 ] and crystals contain two molecules in the asymmetric unit. In both, the titanium(III) atoms are six-coordinate with a fac arrangement of ligands. The metals are bonded to three chlorine atoms (2.31-2.35 A)... 

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]. 

Carreira, E. M., Singer, R. A., Lee, W. Catalytic, Enantioselective Aldol Additions with Methyl and Ethyl Acetate 0-Sllyl Enolates A Chiral Tridentate Chelate as a Ligand for Titanium(IV). J. Am. Chem. Soc. 1994, 116, 8837-8838. 

Titanium complexes containing tridentate Gp-amido ligands with an additional pendant neutral coordination site have been synthesized. Such tridentate Gp-amido ligands provide an electron count identical to that of two Cp... 

Carreira s recent work is an extension of earlier studies [5] in which a titanium(IV) complex, prepared in situ from tridentate ligand ( )-l and Ti(Ot Pr)4, was found to catalyze Mukaiyama aldol reactions with high enantioselectivities. The chiral ligand used in both the ene and aldol chemistry is prepared from 3-bromo-5-ferf-butylsalicylaldehyde and 2-amino-2 -hydroxy-l,T-binaphthol. This... 

Tridentate BINOL catalysts can be derived from titanium tetra(isopropoxide), BINOL ligands, and hindered amine bases. These catalysts have also been shown to provide good yields and ee s for the aldol reactions at low temperatures of 2-methoxypropene with several aldehydes to the P-hydroxyketones, but an acid workup of the products is required. 

Jimenez et al. have developed tridentate cyclopentadienyl-silesquioxanate titanium complexes for the epoxidation of cyclic and linear alkenes with aqueous hydrogen peroxide under mild reaction conditions with excellent reactivity and selectivity. The authors extended the studies of these complexes as catalysts for the oxidation of sulfides to sulfoxides or sulfones under mild reaction conditions. The catalysts showed high chemoselec-tivity and proved to be very stable as no loss of activity or selectivity was observed after 14 cycles. 

Niobium lies directly below vanadium in Group 5 and thus is expected to have high Lewis acidity however, there are few reports on chiral niobium catalysts. This is in contrast to the various asymmetric transformations catalysed by neighbouring Group 4 metal complexes, i.e., titanium and zirconium. In 2005, Kobayashi first reported highly enantioselective niobium Lewis-acid catalysts for the Mannich-type reaction of aldimines 57 with silyl enol ethers 58. To prepare an efficient chiral pentavalent niobium(v) catalyst for the activation of aldimines 57, Kobayashi designed tridentate ligand 56 (Scheme 9.24). 

It is apparent that the highest ee s were obtained when the sulfur was flanked by an aryl group and a small alkyl group. This led to a simple model for prediction of absolute configuration (Figure 12). There are no exception to this rule, which may correspond to the oxidation via a binuclear titanium complex as r resented in Figure 12 with diethyl tartrate acting as a tridentate unit... 

The reaction of diketone with aldehydes in the presence of a stoichiometric amount of a titanium complex, prepared from the tridentate ligands and Ti(O Pr)4, provided 5-hydroxy-p-ketone esters in good yield and high optical purity [118] (Scheme 14.41). 

The importance of neutral donors in the ligand framework of living olefin polymerization catalysts was also demonstrated recently by Sundararajan and coworkers. In 2002, the authors reported titanium dichloride complexes of tridentate aminodioi ligands me- and meso-45, Figure 13) treated with MAO furnished PHs possessing relatively narrow Figure 14 Titanium precataiysts for styrene poiymerization. 

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