Titanium alkoxy group

Reactions with Alcohols. The tendency of titanium(IV) to reach coordination number six accounts for the rapid exchange of alkoxy groups with alcohols. Departure of an alkoxy group with the proton is the first step in the ultimate exchange of all four alkoxyls. The four-coordinated monomer is expected to react... 

Titanium chelates are formed from tetraalkyl titanates or haUdes and bi- or polydentate ligands. One of the functional groups is usually alcohoHc or enoHc hydroxyl, which interchanges with an alkoxy group, RO, on titanium to Hberate ROH. If the second function is hydroxyl or carboxyl, it may react similarly. Diols and polyols, a-hydroxycarboxyflc acids and oxaUc acid are all examples of this type. P-Keto esters, P-diketones, and alkanolamines are also excellent chelating ligands for titanium. 

The orange-red titanium acetylacetone chelates are soluble in common solvents. These compounds are coordinately saturated (coordination number equals 6) and thus much more resistant to hydrolysis than the parent alkoxides (coordination number 4). The alkoxy groups are the moieties removed by hydrolysis. The initial product of hydrolysis is beheved to be the bis-hydroxy bis-acetylacetone titanate, (HO)2Ti(acac)2, which oligomerizes to a... 

Esterification and transesterification using TiIV compounds are useful methods for functionalization of ester moieties under mild conditions. In the transformation of carboxylic acids to esters, a catalytic amount of TiCl(OTf)3 is effective (Scheme 30).110 Titanium alkoxides, such as Ti(OEt)4 or Ti(0 Pr)4, easily promote transesterification of alkoxy groups to other ones—even to more hindered groups.111 Anomerization of glycosides to Q-isomers using a Tilv-bascd Lewis acid is an important method for controlling the product structure.112... 

The structure of [Ti(OEt) , ] , reported by Ibers (2) in 1963 was of historic significance since it provided the first structural test of Bradley s theory for a compound of formula M(OR)h. Each titanium atom achieves an octahedral environment, through the agency of four doubly bridging and two triply bridging alkoxy groups, as is shown in II. 

The alcohol exchange reaction was shown above in equation (2). The reactive alkoxy group (OR) is replaced by an alkoxy group that has less hydrolysis sensitivity (OR ). A representative example here is the use of reagents such as zirconium n-propoxide and titanium /-propoxide, both of which possess polar bonds, for the production of lead zirconate titanate films. Commonly in these processes, R OH is 2-methoxyethanol (CH3OCH2CH2OH), which is generally present as a bidentate ligand.35... 

The only solid acidic catalyst which has given high polymers at an appreciable rate at low temperatures, and which has been studied in some detail, is that described by Wichterle [41, 42]. This was prepared as follows A 10 per cent solution in hexane of aluminium tri-(s- or t-butoxide) was saturated with boron fluoride at room temperature, and excess boron fluoride was removed from the precipitate by pumping off about half the hexane. Two moles of boron fluoride were absorbed per atom of aluminium, and butene oligomers equivalent to two-thirds of the alkoxy groups were found in the solution the resulting solid had hardly any catalytic activity. When titanium tetrachloride was added to the suspension in hexane, an extremely active catalyst was formed but the supernatant liquid phase had no catalytic activity. The DP of the polymers formed by the catalyst prepared from the s-butoxide was much lower than that of polymers formed with a catalyst prepared from the r-butoxidc. 

For instance, in the polymerization of 1.3-pentadiene in the presence of a catalyst prepared from optically active titanium alcoholates and diethylaluminum monochloride, the diolefin might give a complex with a titanium atom before the polymerization (92) the steric structure of the complexed pentadiene molecule might be determined by the optically active alkoxy groups still bound to the titanium atom. However, the case of. the polymerization of diolefins is much more complex than the case of the polymerization of vinyl monomers owing to the possible... 

Hydrolysis and Condensation. The rate of hydrolysis of the tetraalkyl titanates is governed by the nature of the alkoxy groups. The lower titanium alkoxides, with the exception of tetramethyl titanate [992-92-7], are rapidly hydrolyzed by moist air or water, giving a series of condensed titanoxanes, (Ti— O—Ti— O—) (17). As the chain length of the alkyl group increases, the rate of hydrolysis decreases. Titanium methoxides, aryloxides, and C-10 and higher alkyl titanates are hydrolyzed much more slowly. 

Carbanions which are particularly electron rich cannot be titanated smoothly with quenching reagents having chloro or alkoxy groups at titanium due to reduction of Ti(IV) to Ti(III) via single electron transfer (SET). 

The mechanism of the asymmetric epoxidation of allylic alcohols with the Sharpless-Katsuki catalyst is assumed to be very similar to the one described for the Halcon-ARCO process in Section 2.5. The key point is that the chiral tartrate creates an asymmetric environment about the titanium center (Figure 18). When the allylic alcohol and the t-butyl hydroperoxide bind through displacement of alkoxy groups from the metal, they are disposed in such a way as to direct oxygen transfer to a specific face of the C=C double bond. This point is crucial to maximize enantioselectivity. 

As outlined in Section III. A.3. a, the strength of the Lewis acid with mixed chloride and alkoxy derivatives decreases as the number of chloride ligands are replaced with alkoxy groups. Titanium chloride with one alkoxy group polymerizes styrene and a-methylstyrene Lewis acid with two alkoxy groups is too weak to initiate polymerization of styrene, but will initiate polymerization of a-methylstyrene and vinyl ethers. The Lewis acidity of titanium chloride derivatives with three alkoxy groups are so low that only vinyl ether polymerizations reach reasonable conversions. 

Similar electrophilic activation of coordinated peroxides or alkylperoxides can be observed for the metal ions in intermediate oxidation states. To give just one example, Sharpless epoxidation takes advantage of an electrophilic activation of alkyl hydroperoxides at titanium(IV). Notably, efficient epoxidation requires substrate binding in the vicinity of coordinated alkylperoxide, thus limiting the substrate scope of this reaction to allylic alcohols (alkoxy group acts as an anchor).1,45... 

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