TITANIUM TETRAALKOXIDE

Recently, we succeeded in the synthesis and structural characterization of first niobium-containing metallasilsesquioxane complex. In analogy to the corresponding reactions with titanium tetraalkoxides, niobium pentaethoxide cleanly reacts with 3 in a 1 1 molar ratio to give high yields of dimeric [Cy7Si70i2Nb0Et( U-OEt)]2 (149) as a colorless, crystalline solid (Scheme 49). 

Because of the possibility of racemization during the transesterification reaction (strong basic conditions) alternative methods are reported. These include transesterification in the presence of the KF/18-crown-6 ether 461 or the use of titanium tetraalkoxides. 471 The methods are efficient and represent a route to any required dialkyl ester using diphenyl esters as starting materials. Diphenyl groups can also be removed by hydrogenation in the presence of platinum dioxide (Adams catalyst) to provide the free phosphonic acid moiety directly)46 ... 

A systematic study of the Sharpless epoxidation led to the following conclusions (1) an equimolar complex of titanium tetraalkoxide and tartrate diester [(tartrate)(RO)2Ti] is the... 

Chlorination of the initially obtained 4-hydroxy esters can be avoided by adding titanium tetraalkoxides after formation of the / -trichlorotitanio esters. This manipulation is also effective in expanding the subsequent reaction to include ketone addition (Table 10, entries 3 and 4). ... 

Titanium tetraalkoxides react with only three equivalents of acid (59) ... 

Micro- and mesoporous crystalline titanium silicas, such as Ti silicalite TS-1 and the Ti-containing MCM-41 zeolite, represent another important family of modem materials and find versatile application as catalysts for oxidation reactions and in absorption technologies. These materials are prepared under hydrothermal conditions using tetraalkylammonium hydroxide, titanium tetraalkoxide, and tetraethyl orthosilicate as preeursors. Only limited amounts of Ti can enter the Si02 framework and the actual bonding situation at Ti is not yet fully understood. Several data suggest that Ti substitution for Si in the lattice positions retaining the tetrahedral coordination is important for catalysis. 

Although the structure of the catalyst is unknown, the evidence obtained by Natta and associates suggests a reasonable interpretation. The first point is that growth occurs at a carbon-titanium bond instead of at a carbon-aluminum bond. To illucidate the mechanism of propagation, optically active catalysts were prepared from a trialkylaluminum and titanium tetraalkoxide. Only if the optical activity was in the alkoxide group was an asymmetric synthesis accomplished, thus indicating growth at a carbon-titanium bond. 

Insertion of isocyanates into Ti-C, Ti-0 and Ti-N bonds is also observed. For example, titanium tetraalkoxide reacts reversibly with four equivalents of isocyanates to give the tetracarbamate 308. On standing at room temperature, slow conversion into isocyanurates... 

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