Titanium complexes alkyl alkoxy

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). 

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. 

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. 

Alkyl complexes of titanium have been extensively studied both from the point of view of the nature of the metal-carbon bond as well as their application as alkylating reagents in organic reactions [4-6,20,21], The simplest methyl complex, MeTiCl3 (8), is readily obtained by the reaction of TiCl4 with half an equivalent of ZnMe2 [22,23]. An ether adduct of 8 is synthesized by treatment of TiCl with an ethereal solution of MeLi [24]. Alkoxy substitution on titanium stabilizes the methyl complexes. Reaction of TiCl(0 Pr)3 (9) with MeLi affords MeTi(O Pr)3(10), which is thermally very stable and can be distilled under reduced pressure (48-53 °C/0.01 mmHg) [25], The phenyl derivative 11 is prepared by the similar reaction of 9 with PhLi. 

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