Alkoxy-titanium complexes

R. Fleischer, H. Wanderhch and Braun, Synthesis and structure determination of novel chiral imine -alkoxy titanium complexes. Eur. J. Org. Chem., vol.6, pp. 1063-1070,1998. 

In 1977, an article from the authors laboratories [9] reported an TiCV mediated coupling reaction of 1-alkoxy-l-siloxy-cyclopropane with aldehydes (Scheme 1), in which the intermediate formation of a titanium homoenolate (path b) was postulated instead of a then-more-likely Friedel-Crafts-like mechanism (path a). This finding some years later led to the isolation of the first stable metal homoenolate [10] that exhibits considerable nucleophilic reactivity toward (external) electrophiles. Although the metal-carbon bond in this titanium complex is essentially covalent, such titanium species underwent ready nucleophilic addition onto carbonyl compounds to give 4-hydroxy esters in good yield. Since then a number of characterizable metal homoenolates have been prepared from siloxycyclopropanes [11], The repertoire of metal homoenolate reactions now covers most of the standard reaction types ranging from simple... 

This reaction has now been applied to a very great number of substituted allylic alcohols, and the mechanistic and stereoisomeric features of the reaction are becoming clearer.11 In broad outline, it would appear that the initial step is an alkoxy-exchange reaction between two alkoxy residues in the titanium complex and the two hydroxyl groups in the tartrate ester, thus ... 

Bis(V-alkoxy-p-ketoiminate) titanium complexes (29) can be applied as starting materials in the deposition of Ti02 thin fihns . Among them, complexes 29b and 29c are best suited as precursors, due to their thermal stability and volatility. They provide the highest deposition rates—approximately three times higher than that with Ti(thd)2(OPr-/)2 (26b). Deposition of pure anatase layers was carried out at 450 °C using a liquid-injection C VD setup. The films were uniform, less crystalline and contained the usual carbon impurities (3-5%) . ... 

The well-known Sharpless system for the enantioselective epoxidation of allyl alcohols has been investigated [23]. This system employs a tetra-alkoxy titanium precursor, a dialkyltartrate as an auxiliary, and an alkyl hydroperoxide as oxidant, to effect the enantioselective epoxidation. The key intermediate is thought to be a dimeric complex in which titanium is simultaneously coordinated to the chelating tartarate ligand, the substrate in the form of an oxygen bound / -allyl-oxide and an -tert-butylperoxide. 

Ketene silylacetals also react with a- or P-alkoxy- or -aminoaldehydes. Chelation control may take place in the reaction of ephedrine-derived ketene silylacetal 6.121 with p-benzyloxyaldehydes. Under TiCl4 catalysis, syn isomers are favored (Figure 6.98), but the reaction is highly selective only if the aldehyde is a-alkylated and the reagents are matched [1295], The results are interpreted through the intervention of a six-coordinate titanium complex 6.122 (Figure 6.98). 

A novel titanium(iv)-catalysed substitution of a carbon xygen bond by a carbon arbon bond, which relied on a DKR process and led to products resulting from a highly enantioselective carbon allylation, was described in 2004 by Braun and Kotter. By means of the chiral titanium complex depicted in Scheme 2.90, the substitution of a hydroxy, silyloxy, or alkoxy group by an allylic residue was possible, for the first time, in a DKR transformation. 

FIGURE 14.10 Structures of mono-Cp titanium complexes 48-52 with an alkoxy o-ligand. 

The enantioselective inverse electron-demand HDA reaction promoted by titanium complexes was first reported by Wada using TADDOL as the chiral inducer [169], As shown in Scheme 14.73, the reaction of ( )-2-oxo-l-phenylsulfonyl-3-alkenes with a large excess of vinyl ethers in the presence of 10 mol% of TADDOL-TiBr2 at —78°C gave the adduct as a single cis isomer in 91% yield and 59% ee. The enantiomeric excesses of the adducts are highly dependent on the bulkiness of the alkoxy substituent R of the dienophiles. 

An unoccupied coordination site and titanium alkyl bond in the titanium complex are fundamental requirements for its catalytic activity in olefin oligomerization. The titanium-alkyl bond is formed in the reaction of a titanium compound with alkyl or alkylchloroaluminum compounds when, for example, halide atoms or alkoxy groups of titanium compounds are replaced by alkyl groups of aluminum derivatives. There are many proposals concerning the structure of the Ziegler-Natta catalyst active centers. These are presented in structural formulas (16)-(21) [291. 

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. 

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