Peroxo complexes catalysts

In the same spirit DFT studies on peroxo-complexes in titanosilicalite-1 catalyst were performed [3]. This topic was selected since Ti-containing porous silicates exhibited excellent catalytic activities in the oxidation of various organic compounds in the presence of hydrogen peroxide under mild conditions. Catalytic reactions include epoxidation of alkenes, oxidation of alkanes, alcohols, amines, hydroxylation of aromatics, and ammoximation of ketones. The studies comprised detailed analysis of the activated adsorption of hydrogen peroxide with... 

It seems very probable that the epoxidation reaction proceeds through a two-stage mechanism. Hydroperoxide oxidizes the catalyst to peroxo complex and the this complex performs epoxidation of olefins. 

The metal-alcoholate mechanism is well established for allylic alcohol epoxidation in the presence of Ti and V catalysts. [41, 51, 52, 111-113], In principle, it can provide a viable pathway also for catalysis by a Re complex. In fact, allylic alcohols may add, at least formally, to either an oxo-Re or peroxo-Re moiety (e.g. of 5a or 5b) in a process which is referred to as metal-alcoholate binding this mechanism gives rise to metal-alcoholate intermediates. We identified four intermediates of alcohol addition to di(peroxo) complexes two resulting transition states, S-8 and S-9b, are shown in Figure 11. All metal-alcoholate intermediates he significantly higher in energy (by 10-22 kcal/mol) than 5b + propenol, except the... 

It is instructive to compare the properties of metal peroxo and alkyl (or hydro) peroxo groups for the case of Ti because experimental structures of both types are known [117, 119-121] and Ti compounds are catalysts for such important processes as Sharpless epoxidation [22] and epoxidation over Ti-silicalites [122], where alkyl and hydro peroxo intermediates, respectively, are assumed to act as oxygen donors. Actually, the known Ti(t 2-02) complexes are not active in epoxidation. [121-124] However, there is evidence [123] that (TPP)Ti(02) (TPP = tetraphenylporphyrin) becomes active in epoxidation of cyclohexene when transformed to the cis-hydroxo(alkyl peroxo) complex (TPP)Ti(OH)(OOR) although the latter has never been isolated. 

Results on epoxidation of cyclohexene with H2O2 with freshly prepared catalysts are given in Table 2. With Mo blue, exchanged on Mg,Al-LDH, the olefin conversion is low, even if all peroxide is consumed within 4 h. Upon addition of the H2O2 to the reaction mixture, the suspended catalyst has the yellow hue of the Movl form of the isopolyacid. However, the suspension soon turns brick red. This color is characteristic for tetraperoxomolybdate Mo(02)42 [17], This indicates that the isopolyacid structure degrades rapidly, with formation of Mo monomers. Peroxo complexes such as Mo(0 )42 or particularly MoO(C>2)32 are known to decompose with formation of 02 the overall process is a decomposition of two molecules of H2O2 into water and C>2 [18] ... 

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. 

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