MTO UHP system

Another type of flavones bears a double bond in the 0-heterocyclic ring and can also be oxidatively functionalized at this bond with MTO (Scheme 15). Using the MTO/UHP system with substituted flavanone in refluxing methanol, followed by acetylation, cis- and ri[Pg.163]

Since its discovery in 1991, methyltrioxorhenium (MTO, 80) has attracted much interest as one of the most versatile catalysts for oxidation.When it is associated with a stoichiometric amount of H2O2, the system can efficiently transform alkene to epoxide, although formation of undesired diol can occur. Alternatively, water-free conditions, using urea hydrogen peroxide (UHP), allow the formation of the desired epoxide without byproducts. A maj or drawback of the MTO/UHP system is its insolubility in organic solvents, leading to a kinetically slow heterogeneous system. 

Scheme 18 Oxidation of an uracil derivative by the MTO/H2O2 system or the MTO/UHP combination...

The catalytic potential of the MTO/UHP oxidation system has also been tested in the room-temperature ionic liquid [EMIM] [BF4], in which it is soluble [77]. In contrast, the olefin is poorly soluble in such solvenfs. Therefore, the whole system is biphasic. Excellent conversions and selectivities for the epoxides of a wide number of olefinic substrates were reached under these anhydrous conditions. The exception was 1-decene (data collected in Table 17), for which poor conversion (entry 9, Table 17) may result in phase transfer problems, since it is the least soluble substrate in the ionic liquid. 

Similarly, Ono et al. [189] have reported using a composed catalytic system, namely MTO/UHP/Zn[EMIm]2 Br4/[BMfm]BF4 (UHP, urea hydrogen peroxide and MTO, methyltrioxorhenium). With the multistep method described above, a... 

Yokoyama et al. [75] further examined the combination of an epoxidation catalyst system of MTO/UHP/[BM[m]BF4 (MTO, methyltrioxorhenium UHP, urea hydrogen peroxide) [76]... 

An interesting system that is applied in the epoxidation of soybean oil is MTO immobilized on niobia combined with UHP as oxidant. The different components of soybean oil have been studied separately, and it was found that oleic acid can be epoxidized completely with 1 mol% MT0/Nb205 in 2 h at room temperature. When raising the temperature to 50°C and lowering the catalyst amount to 0.2 mol%, complete epoxidation is reached in as little as 10 min. In applying the same procedure to linoleic and linolenic acid, excellent yields of epoxidized product are obtained within 30 min. In attempts to reuse the catalyst in this reaction, it was found that the catalyst remains active for three runs, although no numerical data is provided to underline this [39, 76]. 

The catalyst system MTO/H2O2 also catalyzes oxygen atom insertion into Si-H bonds. Silanols and disiloxanes are formed as products, with the latter being the major ones [3, 21 a]. When UHP is used as an oxygen source instead of aqueous H2O2, 1 catalyzes the oxidation of silanes to silanols in high conversions and excellent selectivities in favor of the silanol (eq. (10)). 

Other systems have been developed in the formation of acid-sensitive epoxides. Adam has developed an MTO-catalyzed epoxidation process using a urea/hydrogen peroxide (UHP) adduct. Although this methodology allows for the isolation of some acid-sensitive epoxides, other alkenes, such as a-methylstyrene, gave significant amounts of the corresponding 1,2-diols and yields in many cases were only modest. 

Chitin- and chitosan-anchored MTO were shown to be efficient catalysts for activation of UHP. Acid-sensitive double bonds were satisfactorily epoxidized with this system. A-functionalized chi-tosan was the most active system. ... 

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