Titanium complexes oxygen ligands

The oxygen that is transferred to the allylic alcohol to form epoxide is derived from tert-butyl hydroperoxide. The enantioselectivity of the reaction results from a titanium complex among the reagents that includes the enantiomerically pure tartrate ester as one of the ligands. The choice whether to use (+) or (-) tartrate ester for stereochemical control depends on which enantiomer of epoxide is desired. 

OXYGEN LIGANDS 31.3.1 Titanium(III) Complexes 3L3.1.1 Electronic spectra... 

The reaction of titanium alkoxides with acetylacetone leads to several molecular compounds. [TiO(acac)2]2 is formed in the presence of an excess of acetylacetone or upon hydrolysis of Ti(acac)2(OR)2. Single crystals have been isolated, and x-ray diffraction experiments show dimers with sixfold coordinated Ti atoms linked through oxygen atoms. Strongly complexing acac ligands cannot be hydrolyzed easily, and condensation does not go any further (Fig. 8) [35]. 

The oxygen that is transferred to the allylic alcohol to form the epoxide is derived from tert-butyl hydroperoxide. The enantioselectivity ofthe reaction results from a titanium complex among the reagents that includes the enan-tiomerically pure tartrate ester as one of the ligands. The choice of whether to use the (-t)- or (-)-tartrate ester for stereochemical control depends on which enantiomer of the epoxide is desired. [The (-t)- and (-)-tartrates are either diethyl or diisopropyl esters.] The stereochemical preferences ofthe reaction have been well studied, such that it is possible to prepare either enantiomer of a chiral epoxide in high enantiomeric excess, simply by choosing the appropriate (-1-)- or (—)-tartrate stereoisomer as the chiral ligand ... 

Titanium ester enolates are not only versatile reagents for asymmetric aldol additions but also function as starters of methacrylate polymerization. A representative titanium complex 24 was characterized by crystal structure and NMR spectroscopy and reveals the monomeric O-bound enolate character. The six-coordinated titanium atom in 24 is bound to two phenolic traws-oriented oxygen atoms and two sulfur donors the remaining ligands, methyl group, and enolate moiety are c/s-configured. Upon exposure to acetone, a spontaneous aldol occurs, and the aldolate 25 thus formed was also characterized by a crystal structure. Due to its coordinative saturation, the titanium obviously does not form a chelate with the carbonyl oxygen (Scheme 3.9) [57]. 

Peroxidic Compounds. When hydrogen peroxide is added to a solution of titanium(IV) compounds, an intense, stable, yellow solution is obtained, which forms the basis of a sensitive method for determining small amounts of titanium. The color probably results from the peroxo complex [Ti(02)(0H)(H20)J, and crystalline salts such as K2[Ti(02)(S0 2] H20 can be isolated from alkaline solutions. The peroxo ligand is bidentate the two oxygen atoms ate equidistant from the titanium (98). 

The i5p-titanium(IV) atom is hard, ie, not very polarizable, and can be expected to form its most stable complexes with hard ligands, eg, fluoride, chloride, oxygen, and nitrogen. Soft or relatively polarizable ligands containing second- and third-row elements or multiple bonds should give less stable complexes. The stabihty depends on the coordination number of titanium, on whether the ligand is mono- or polydentate, and on the mechanism of the reaction used to measure stabihty. 

The first of these types is most familiarly represented by the hexaaquo ion which is present in acidic aqueous solutions and, in the solid state, in the alum CsTi(S04)2.12H20. In fact few other neutral ligands besides water form a [TiLg] + complex. Urea is one of these few and [Ti(OCN2H4)g]l3, in which the urea ligands coordinate to the titanium via their oxygen atoms, is one of the compounds of titanium(III) most resistant to oxidation. 

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