Titanium tetra-alkoxides

Indeed, several interesting procedures based on three families of active catalysts organometallic complexes, phase-transfer compounds and titanium silicalite (TS-1), and peroxides have been settled and used also in industrial processes in the last decades of the 20th century. The most impressive breakthrough in this field was achieved by Katsuki and Sharpless, who obtained the enantioselective oxidation of prochiral allylic alcohols with alkyl hydroperoxides catalyzed by titanium tetra-alkoxides in the presence of chiral nonracemic tartrates. In fact Sharpless was awarded the Nobel Prize in 2001. 

The results of a n.m.r. study of some titanium tetra-alkoxides have been inter-... 

The addition of a base, typically ammonia, to mixtnres of metal(loid) halides and alcohols allows the synthesis of homoleptic alkoxides for a wide range of metals and metalloids. Anhydrous ammonia appears to have been employed for the first time by Nelles " in 1939 for the preparation of titanium tetra-alkoxides (Eq. 2.33) ... 

The X-ray Absorption Near Edge Structure (XANES) spectral features have been used as an additional method for elucidating the stereochemistry as well as the oxidation state of the metal. For example, based on the information available from XANES studies on titanium tetra-alkoxides, the most probable coordination state of titanium in trimeric ethoxide and butoxide [Ii(OR)4]3 (R = Et, Bu") has been suggested to be five. 

The reactions between titanium tetra-alkoxides and titanium tetrachloride (excess) at low temperatures lead to the deposition of less soluble titanium trichloride mono-... 

Catalysts such as HRuX(PPh3)3, where X is an optically active car-boxylate (e.g., / -mandelate) gave only 0.4% ee using 2-ethylhex-l-ene as substrate (124). Soluble Ziegler-Natta catalysts comprised of triiso-butylaluminum with the optically active alkoxide complex, titanium tetra-(-)-menthoxide, hydrogenated racemic terminal olefins such as 3,4-di-methyl pent- 1-ene, but with zero ee (323). 

By contrast, the alkali alkoxide route appears to be inapplicable for the synthesis of zirconium tetra-alkoxides or niobium (tantalum) penta-aUcoxides, as these tend to form heterobimetallic alkoxides with alkali metal alkoxides (Chapter 3, Section 3.2.1.1), which volatilize out during final purification, whereas alkali titanium alkoxides, even if formed, dissociate readily to give volatile titanium alkoxides. 

Bradley et al pnrified uranium tetra-alkoxides by sublimation under reduced pressure and also tried to correlate the volatility of uranium tetra-alkoxides with those of other tetravalent metal alkoxides as shown in Table 2.6 (which includes for comparison relevant data for tetravalent titanium and zirconium derivatives). 

In reactions of titanium alkoxides with benzoylacetone, " methylacetoacetate " and ethylacetoacetate, bis- S-diketonate or ketoester derivatives were the final products even when excess of these ligands was used. The nonreplaceability of the third or fourth alkoxy groups with -diketones or ketoesters may most probably be due to the preferred coordination number of 6 for titanium in the bis-derivatives. The trialkoxide monomethylacetoacetate disproportionated to the bis-derivative and tetra-alkoxide, when heated under reduced pressure, whereas bis-derivatives distilled out... 

The reactions of metal alkoxides with halogens were investigated by Nesmeyanov et al. who observed that the reaction between titanium tetra-n-butoxide and chlorine or bromine yielded the dihalide dibutoxide coordinated with one mole of alcohol as TiX2(OBu)2.BuOH and n-butyl propionate. The reaction may be represented by... 

Titanium and zirconinm tetra-alkoxides undergo insertion reactions of both organic isocyanates and carbodiimides into M-0 bonds, as shown by Eqs (2.290) and (2.291) ... 

Powders from Alkoxides and Salts. Two solutions were prepared individually for the synthesis of lead titanate (PbTiOs) particles by Tartaj et al. (2001) (i) titanium tetra-butoxide in 2-propanol, and (ii) lead acetate in glacial acetic acid. The solutions were mixed under stirring, and a water-2-propanol mixture added to this for the initiation of hydrolysis. As in some other cases, crystalline seeds of the target compound, suspended in 2-propanol, were added (10 wt%) to the transparent sol already obtained. The gels obtained through this process were dried and ground to small particles and calcined at 400°C for 20 h for use in the preparation of sintered bodies. X-ray diffractometry of the calcined sample showed it to be fully crystallized to phase-pure PbTiOs, while an unseeded sample did not crystallize under the same conditions. 

Higher alkoxides, such as tetra(2-ethylhexyl) titanate, TYZOR TOT [1070-10-6], can be prepared by alcohol interchange (transestenfication) in a solvent, such as benzene or cyclohexane, to form a volatile a2eotrope with the displaced alcohol, or by a solvent-free process involving vacuum removal of the more volatile displaced alcohol. The affinity of an alcohol for titanium decreases in the order primary > secondary > tertiary, and... 

Highly active, unsupported catalysts form from reactions of magnesium alkoxides with tetra-valent titanium chlorides. The same is true of reaction products of MgCh or MgCh-electron donor adducts, like MgCh-tiCaHsOH, with tetravalent titanium compounds. 

In view of an interesting gradation observed (Section 2.4) in the reactivity of tetra-halides of silicon, titanium, zirconium, and thorium with ethyl and isopropyl alcohols, the reverse reactions of alkoxides of these elements with hydrogen chloride (bromide) were investigated by Mehrotra who demonstrated that the reaction products of... 

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