Titanium complexes peroxo

Vibrational frequencies of some titanium peroxo complexes and of solids containing peroxo and/or superoxo species are summarized in Table III. The three infrared vibrations of the triangular peroxo group in the Qv structure... 

The transition state in the above scheme differs from the cyclic titanium peroxo complex proposed earlier (217). In the earlier mechanism, any of the two peroxo oxygens in the Ti-O-O-H (bound end-on) could have been inserted into the C=C bond, and accordingly two isomers would be possible. They have never... 

The latest experimental evidence suggest the presence of titanyl groups (Ti=0) in the structure188 (Scheme 9.3). H202 is activated via chemisorption on these groups with the formation of a surface titanium peroxo complex (13). It may exist in the hydrated or open diradical form and initiates hydrogen abstraction. Rapid... 

The proposals advanced to give a representation of the mechanism by which the O is transferred from the titanium peroxo complex to the reactant molecules can be classified as concerted mechanisms and radical mechanisms. 

A modification of the mechanism that involves the hydroperoxo titanium complex and one solvent molecule has been proposed that involves the formation of a stable cyclic titanium peroxo complex (Clerici et al., 1993). In this case, the two peroxo oxygens are not equivalent, and thus two intermediates would be possible ... 

The initial coordination of reactants has indeed been proposed to explain the selective oxidation of alkenes in the presence of saturated hydrocarbons. It was argued that, owing to the hydrophobic nature of titanium silicates, the concentration of both hydrocarbons inside the catalyst pores is relatively high and hence the alkenes must coordinate to TiIv. Consequently, the titanium peroxo complex will be formed almost exclusively on Tilv centers that already have an alkene in their coordination sphere, and will therefore oxidize this alkene rather than an alkane which may be present in the catalyst (Huybrechts et al., 1992). Objections to this proposal are based on the fact that the intrinsically higher reactivity of alkenes with respect to saturated hydrocarbons is sufficient to account for the selectivity observed (Clerici et al., 1992). But coordination around the titanium center of an alcohol molecule, particularly methanol, is nevertheless proposed to explain the formation of acidic species, as was previously discussed. In summary, coordination around Tiiv could play a more important role than it does in solution chemistry as a consequence of the hydrophobicity of the environment where the reactions take place. 

The workers proposed that alkyl hydroperoxides and aqueous hydrogen peroxide interact with TS-1 in a similar manner, forming titanium alkyl peroxo complexes and titanium peroxo complexes, respectively. However, the titanium alkyl peroxo complexes were not active because the substrate could not enter the void due to steric effects. Consequently, no activity was possible for either alkane hydroxylation or alkene epoxidation. Comparison with Ti02-Si02/alkyl hydroperoxide for alkane and alkene oxidation indicated that this material was active because the oxidation took place on the surface and not in the pores. Figures 4.4 and 4.5 show the possible mechanisms in operation for the oxidation of alkenes and alkanes with a TS-1/hydrogen peroxide system. 

The work of Davis was, however, unable to distinguish which oxygen was attacked on the titanium peroxo complex when the alkene co-ordinates. Therefore, Neurock and Manzer conducted a theoretical study of the mechanism of alkene epoxidation using TS-1 with aqueous hydrogen peroxide.32 The workers concluded that their calculations to predict both the structure and relative bands in the IR spectra for TS-1 were in good agreement with experimental data. The calculations indicated that the oxygen closest to the titanium centre was the active site for alkene attack. The result was the direct formation of... 

Imido and 0x0 compounds are intermediates in many of the transfers of oxygen atoms and nitrene units to olefins to form epoxides and aziridines, and they are intermediates in many of the insertions of oxygen atoms and nitrene units into the C-H bonds of hydrocarbons to form alcohols and amine derivatives. The enantioselective epoxidation of allylic alcohols (Scheme 13.22) " is the most widely used epoxida-tion process, and the discovery and development of this process was one of the sets of chemistry that led K. Barry Sharpless to receive the Nobel Prize in Chemistry in 2001. The mechanism of this process is not well established, despite the long time since its discovery and development. Nevertheless, most people accept that transfer of the oxygen atom occurs from a titanium-peroxo complex - rather than from an 0x0 complex. Jacobsen s and Katsuki s - manganese-salen catalysts for the enantioselective epoxidations of unfunctionalized olefins, which were based on Kochi s achiral chromium- and manganese-salen complexes, are a second set of... 

Kholdeeva, O., Trubitsina, T, Maksimovskaya, R., et al. (2004). First Isolated Active Titanium Peroxo Complex Characterization and Theoretical Study, Inorg. Chem., 43, pp. 2284-2292. 

Kakihana et al. introduced a new complex, citratoperoxotitanate(IV), based on the titanium peroxo complex (Kakihana et al., 2001). This complex is synthesized hy adding citric acid into a titanium peroxo solution, where yellow ammonium salt ((NH4)8[TU(C6H407)4(02)4]-8H20) is obtained. This salt is water soluble (more than 30 g salt in 100 g water at room temperature), and stable in a wide range of pH. This... 

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