Molybdenum peroxo complexes

Diperoxo(oxo)molybdenum(IV) complex bearing (S)-lactic acid piperidineamide as a chiral ligand has been used for the epoxidation of E-2-butene (Scheme 6B.8) and moderate enantiose-lectivity (49%) is achieved wherein the reaction is stoichiometric [16]. Two possible mechanisms have been proposed for this reaction. One mechanism includes coordination of an olefin prior to epoxidation, which makes the olefin electrophilic and facilitates the nucleophilic attack of the proximal oxygen atom of the peroxide on the olefin. The other one is that an olefin nucleophilically attacks the peroxo group of the molybdenum complex. 

Certain molybdenum complexes, such as MoO(02)(PhCONPhO)22 and the peroxo-molybdenum compound derived from tris(cetylpyridinium) 12-molybdophosphate and hydrogen peroxide (PCMP),28 are able to selectively oxidize secondary alcohols. PCMP is able to perform selective oxidations in catalytic amounts in the presence of hydrogen peroxide as secondary oxidant.29... 

Sharpless et a . have conhmied this mechanism in part by labeling experiments which demonstrated that the epoxide oxygen is derived exclusively from the peroxo ligands of the complex and not from the oxo oxygen. However, the reactivity of the molybdenum complex toward olefins closely parallels that of peracids, for which a three-membered cyclic transition stale is favored. ... 

Y. Ishii, K. Yamawaki, T. Yoshida, T. Ura, M. Ogawa, Oxidation of olefins and alcohols by peroxo-molybdenum complex derived from tris(cetylpyridinium) 12-molybdophosphate and hydrogen peroxide, /. Org. Chem. 52 (1987) 1868. 

Table V also presents the structural data for the other class of oxygen adducts. As noted earlier, these complexes can be regarded as peroxo complexes. Indeed the titanium and molybdenum complexes can be prepared by reaction with peroxides. An interesting feature of these complexes is the eclipsing of the M-O2 plane with respect...

Both the peroxo chromium etherate, VI, formed from reaction of H2O2 with acidic aqueous solution of chromates and ether, as well as the molybdenum complexes, VII, formed from reactions of H2O2 with M0O3 in the presence of hexamethylphos-phoramide(HMPA), selectively transfer oxygen to unsaturated organic molecules [93-95]. 

A completely different mechanism of epoxidation has obtained some support in the recent Uterature. This mechanism originated with the observation by Mimoun, Seree de Roch, and Sajus [95] that the covalent molybdenum VI peroxo complexes, [MoO(02)2Lx], reacted with olefins to form epoxides in high yield, equation (253). These Mo(VI) peroxo complexes are readily prepared from Mo03, the neutral ligand and aqueous hydrogen peroxide [398-400]. 

Additional evidence for the existence of different mechanisms for epoxidations by peroxo molybdenum complexes and catalytic epoxidations by hydroperoxides was furnished by recent stereochemical studies [407]. It was found that the epoxidation of several steroids with [MoOs(HMPA)j gave a different isomer from the MoCls-catalyzed epoxidation with tert-amyl hydroperoxide. Table 13. Furthermore,... 

Oxidations of cyclic ketones to lactones by H2O2 take place in the presence of catalytic amounts of certain molybdenum complexes. The catalysts are peroxo-molybdenum complexes stabilized by picolinato- and pyridine-2,6-dicarboxyl-ato-ligands, e.g. [Mo(0)(02) CsH3N(C02)2 ]- Although the reaction is catalytic, turnover numbers are low (25 or less) and chemical yields are variable. Moreover, competing reactions are the formation of oligomeric peroxides and ring-opened products. 

Sajus et al. [243,244] synthesized the peroxo complex of molybdenum(VI) and studied its reaction with a series of olefins. This peroxo complex M0O5 was proved to react with olefins with epoxide formation. The selectivity of the reaction increases with a decrease in the complex concentration. It was found to be as much as 95% at epoxidation of cyclohexene by M0O3 in a concentration 0.06 mol L-1 at 288 K in dichloroethylene [244], The rate of the reaction was found to be... 

Figure 15. Energies (in kcal/mol) of intermediates and TSs of ethene epoxidation by various molybdenum peroxo and hydroperoxo complexes, relative to the energy of MoOtC h tNHjh (2b) +C2H4.

In seven-coordinate molybdenum peroxo complexes of the type Mo(0)(02)2L2, there is the problem of dissociation of the complex in the case of monodentate ligands like pyridine, HMPA, DMF or H2O. Bidentate hgands like 2,2 -bipyridine, on the other hand, tend to give insoluble complexes. A solution to this problem was presented by Thiel in 1997 and 1998 . His group utilized the Mo-diperoxo complex 160 bearing a pyrazolylpyridine ligand. 

The most active d metal peroxo complexes toward nucleophilic substrates, like amines, phosphines, thioethers, double bonds etc., are molybdenum, tungsten and rhenium derivatives vanadium and titanium catalysis is also important, in particular when... 

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