Titanium alkoxides tertiary

Complete control of the diastereoselectivity of the synthesis of 1,3-diols has been achieved by reagent selection in a one-pot tandem aldol-reduction sequence (see Scheme l). i Anti-selective method (a) employs titanium(IV) chloride at 5°C, followed by Ti(OPr )4, whereas method (b), using the tetrachloride with a base at -78 °C followed by lithium aluminium hydride, reverses the selectivity. A non-polar solvent is required (e.g. toluene or dichloromethane, not diethyl ether or THF), and at the lower temperature the titanium alkoxide cannot bring about the reduction of the aldol. Tertiary alkoxides also fail, indicating a similarity with the mechanism of Meerwein-Ponndorf reduction. 

Rate ofhydrolysis depends primarily on the size and structure of the alkoxide group, — for example, it decreases in the series titanium butoxide tertiary >... 

The ability of the alkoxide to undergo hydrolysis depends on the nature of the alcohol moiety. This ability decreases with increase in the molecular weight of the organic part and increases in the order of primary to tertiary alcohol (as shown by zirconium and titanium alkoxides [6, 8, 26]). The volatility of the alkoxides increases in the same order. This fact is of some importance, because with elements having a high atomic number, it is often only the tert-alkoxides that can be distilled (and thus purified), even in high vacuum (for example, Th [10]). 

In 2003, both Walsh and Yus reported independently on a titanium-mediated phenyl transfer to alkyl-aryl ketones 23 (Scheme 8.8) [22]. The role of the titanium alkoxide was not only to form the active chiral catalytic species, but also to sequester the tertiary alkoxide generated during the catalytic cycle. Yus has discussed the possibility of using either arylboronic acids or Ph3B as precursors for the aryl transfer, and in certain cases ee-values greater than 99% were observed [23]. It is worth highlighting at this point that Walsh also used a similar protocol for the aryl transfer to a,P-unsaturated ketones to produce optically enriched tertiary alcohols [24]. 

In general, the facility for interchange of alkoxy groups increases from tertiary to secondary to primary groups. Verma and Mehrotra tried to determine the extent of such equilibria in the case of titanium alkoxides, Ti(OR)4 and found the following order in the interchangeability of alkoxo groups in alcoholysis reactions MeO > EtO > Pr 0 > Bu 0. ... 

In a comparative study of the alcoholysis reactions of titanium alkoxides, the facility of replacement reactions appears to follow the trend OBu > OPr > OEt > OMe. This type of comparative trend appears to be particularly marked in the alkoxides of some later 3d transition metals. For example, primary alkoxides of d nickel(n) do not appear to undergo alcoholysis reactions with secondary or tertiary and other primary alcohols, whereas the reactions of nickel(n) tert-butoxide with primary alcohols are highly facile. 

Sol-gel routes for binary titanium dioxide, tertiary titanates, and other mbced metal oxide systems not only employ various Ti(lV) alkoxides and modified alk-oxides but also Ti(IV) chlorides, oxychlorides, oxynitrates, and so on. The microstructure of the resulting titania and titanates depends on the morphology and interactions between primary particles (clusters) forming upon hydrolysis-condensation of Ti(IV) precursors. An apparent lack of crystalline order and very small size of primary particles and clusters (<1 nm) is observed in the early stages of reaction. At a more advanced stage, the morphology is determined by interparticle interactions and aggregation mechanisms. 

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... 

Probably the most important structural feature of the titanium and other alkoxides is that, although monomeric species can in certain cases exist, especially in very dilute solution, these compounds are in general polymers. Solid Ti(OC2H5)4 is a tetramer, with the structure shown in Fig. 25-A-l. This compact structure neatly allows each Ti atom to attain octahedral coordination. However, in benzene solution, Ti(OR)4 compounds are trimeric for primary alkoxides10 but are unassociated when made from secondary and tertiary alcohols.11 The alkoxides are often referred to as alkyl titanates and under this name are used in heat-resisting paints, where eventual hydrolysis to TiOz occurs. 

The molecular complexity of metal alkoxides also depends on the steric hindrance of alkoxy groups. Bulky secondary or tertiary alkoxy groups tend to prevent oligomerization. Trimeric species [Ti(OEt)4]3 have been evidenced in pure liquid titanium ethoxide (Fig. lb) whereas titanium iso-propoxide Ti(OPr )4 remains monomeric (Fig.la). This is no more the case for zirconium iso-propoxide which is dimeric because of the larger size of Zr(rV). Moreover solvent molecules can also be used for coordination expansion leading to solvated dimers [Zr(OPri)4(Pr OH)]2 when the alkoxide is dissolved in its parent alcohol (Fig.lc). 

The method was extended by Mehrotra and co-workers for the preparation of tertiary alkoxides of a number of metals lanthanides, " titanium, hafnium, vanadium,niobium, tautalum, " iron, and gallium. ... 

Titanium isopropoxide shows an average association of 1.4, whereas its tertiary and higher secondary alkoxides are essentially monomeric in refluxing benzene. " " Titanium tetrakis(hexafluoroisopropoxide) also shows an average molecular association of 1.5 in boiling benzene... 

On the basis of the comparative volatilities of titanium, zirconium, and hafnium tertiary alkoxides measured at various pressures, the following order of volatility may be deduced Per > Pzr > Pti- The anomalous order cannot be explained in terms of intermolecular forces between tertiary alkoxides as the heats of vaporization are quite close to each other for these alkoxides. Bradley et however, concluded that... 

In contrast to the monomeric nature of tertiary alkoxides of titanium, zirconium, and hafnium, the corresponding cerium and thorium lower tertiary alkoxides exhibit association, which decreases with increasing chain length of the groups attached to the tertiary carbon atom and finally Th(OCMeEtPr )4 and Ce(OCMeEtPr")4 show monomeric behaviour. On the basis of the above observations, the order of volatility of some quadrivalent metal alkoxides may be assigned Si(OR)4 > Ge(OR>4 > Ti(OR)4 > Hf(OR)4 > Zr(OR)4 > Ce(OR)4 > Th(OR)4. However, for monomeric tertiary alkoxides, the order of volatility is Hf(OR >4 > Zr(OR )4 > Ti(OR >4. 

Effect of Alkoxy Ligand. Since the modification of the chiral diol in the titanium complex affected the enantioselectivity, we studied the effect of the alkoxide ligand in (/ )-BINOL-Ti(OR)2 and prepared several complexes by treatment of lithium (/ )-binolate with TiCl2(OR)2. Although a primary alkoxide ligand led to minimal asymmetric induction, a secondary alkoxide resulted in reasonable ee s. A tertiary butoxide or binolate ligand decreased the ee considerably. Thus, the bulk of the alkoxide ligand on the titanium complex appears to be extremely important to create an appropriate size of the reaction site. 

---