Titanium complexes alkyl

Titanium, tetrakis(trimethysilyl)oxy-, 3, 334 Titanium, tetranitrato-stereochemistry, 1,94 Titanium, triaquabis(oxalato)-structure, I, 78 Titanium, tris(acetylacetone)-structurc, 1,65 Titanium alkoxides oligomeric structure, 2,346 synthesis ammonia, 2, 338 Titanium chloride photographic developer, 6,99 Titanium complexes acetylacetone dinuclear, 2, 372 alkyl... 

Figure 6.22. Asymmetric alkylation of benzaldehyde catalysed by a titanium complex of (St-RfBINOL.[69,...

Recently, Feng and co-workers reported an asymmetric sulfide oxidation" catalyzed by titanium complexes bearing HydrOx ligands, for example, 576 (Scheme 8.199). ° Enantioselectivities approached a level of synthetic utility for oxidation of aryl alkyl sulfides 632 although the yields of the sulfoxide 633 were poor due to overoxidation to the sulfone 634. The overoxidation is especially significant for reactions with high enantioselectivity. 

However, a more detailed study of the reaction of the mono(allyl)titanium complexes -19 carrying different alkyl groups at the double bond with different aldehydes revealed in some cases the highly diastereoselective (>98%) formation of significant amounts of the isomeric homoallyl alcohols 4 besides 6 (Table 1.3.1). 

A Et2Zn-(5, S)-linked-BINOL (21) complex has been found suitable for chemos-elective enolate formation from a hydroxy ketone in the presence of isomerizable aliphatic iV-diphenylphosphinoylimines.103 The reaction proceeded smoothly and /9- alkyl-yS-amino-a-hydroxy ketones were obtained in good yield and high enantioselectivity (up to 99% ee). A titanium complex derived from 3-(3,5-diphenylphenyl)-BINOL (22) has exhibited an enhanced catalytic activity in the asymmetric alkylation of aldehydes, allowing the reduction of the catalyst amount to less than 1 mol% without deterioration in enantioselectivity.104... 

If there are at least two atoms of chlorine per titanium atom in the system, then, besides a 1 1 titanium-aluminum complex, a 1 2 complex also exists, as shown by NMR spectroscopy (160). Because, even at low temperatures, a coalescence of the alkyl groups on the aluminum cannot be found, a simple Ti—Cl — Al donor bond could be present. In agreement with this idea, no complex formation occurs with the halogen-free system (C5H5)2TiMe2-AlMe3. After formation of the complex, alkylation of the titanium component is presumed to take place. 

Concomitant with continued olefin insertion into the metal-carbon bond of the titanium-aluminum complex, alkyl exchange and hydrogen-transfer reactions are observed. Whereas the normal reduction mechanism for transition-metal-organic complexes is initiated by release of olefins with formation of hydride followed by hydride transfer (184, 185) to an alkyl group, in the case of some titanium and zirconium compounds a reverse reaction takes place. By the release of ethane, a dimetalloalkane is formed. In a second step, ethylene from the dimetalloalkane is evolved, and two reduced metal atoms remain (119). 

Most titanium(IV) alkyls tend to be reduced by aluminum alkyls in a complicated sequence of reactions accompanied by evolution of alkane and alkene. The catalytic activity of the bis(cyclopentadienyl)titanium-aluminum complexes is associated with the titanium alkyl. Hence, it is very interesting to investigate the mechanism of any reductive reaction. In order to study side reactions in the absence of polymerization, highly alkylated systems completely free of halogen are preferred. Moreover, reduction takes place much faster, the higher the alkyl-group content of the added aluminum alkyl. 

A mechanistically obscure transformation occurs upon treatment of the tetramethylfulvene titanium complex 161 with methallyl Grignard, producing bridged titanacyclobutane complex 162. This reaction is proposed to proceed by intramolecular alkylation at the central carbon of an 7]4-fulvene, 7]3-methallyl intermediate, but with due consideration... 

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