Homogeneous epoxidation H2O2 epoxidations

Methyltrioxorhenium, supported on silica functionalized with polyether tethers, catalyzed the epoxidation of alkenes with 30% aq H2O2 in high selectivity compared to the ring opening products observed in homogeneous media in the absence of an organic solvent.46... 

The MTO-catalysed oxidation of silanes to silanols (data reported in Table 10) and epoxidation of olefins (data reported in Table 11) by aqueous H2O2 proceeds in high yields and excellent product chemoselectivities (no siloxane and no diol are observed as byproducts) in the presence of zeolite-Y [64], which are better than those obtained under homogeneous conditions (data in Table 11). For example, only 26% of PhMe2SiH is converted with as low as 20% selectivity towards silanol in 24 h when reacted with 85% H2O2 and catalysed by MTO (entry 5, Table 10). However, in the presence of zeolite-Y, 99% conversion is reached with 99% selectivity (entry 6, Table 10). The confinement of the oxidative species inside the 12 A supercages... 

Table 13 Comparative olefin epoxidation with H2O2/MTO under heterogeneous conditions (10% PEO-10% PPO silica support) and homogeneous conditions...

The resulting material contains 1.25 wt % Re. As it is not stable in the presence of H2O2, it could not be used for olefin epoxidation. Instead, it was tested for aldehyde olefination reactions [62] (Scheme 17). The catalytic activity is lower than that observed for the homogeneous process but is retained even after three catalytic runs. 

Polyoxometalates have been used as homogenous catalysts for a wide variety of thermal organic substrate oxidations37. This involves the epoxidation of relatively electron poor terminal olefins by H2O2 and heteropoly acids, principally H3[PWi2C>4o], using PTC (equation 9). 

Whereas these solid catalysts tolerate water to some extent, or even use aqueous H2O2 as the oxidant, the use of homogeneous Ti catalysts in epoxi-dation reactions often demands strictly anhydrous conditions. The homogeneous catalysts are often titanium alkoxides, possibly in combination with chiral modifiers, as in the Sharpless asymmetric epoxidation of allylic alcohols (15). There has recently been an increase in interest in supporting this enantioselective Ti catalyst. 

Under ideal conditions, epoxidations with homogeneous or anchored Mn and Fe porphyrin catalysts are characterized by high numbers of turnovers and high stereospecificities (104-106,116,117,130,131). For example, 93% cz s-epoxide was obtained from dx-stilbene, with H2O2 and Fe-tetraphenyl-porphyrin on imidazole-modified SiCU (106). Various oxidants, including NaOCl, can be used (117) ... 

Then, the catalytic action is performed under homogeneous conditions and, at the end of the reaction, H2O2 being completely consumed, the precatalyst precipitates and can be easily filtered off and recovered. Both conversions and selectivities of this method are very good. Finally, as in the case of TS-1, this epoxidation system was combined with the 2-ethylanthra-quinone (EAQ)/2-ethylanthrahydroquinone (EAHQ) process for hydrogen peroxide formation, and good conversion and selectivity were obtained for propylene oxide in three consecutive cycles. The catalyst was recovered and reused in between every cycle (Scheme 5) ... 

The vast majority of homogeneous catalysts are transition metal complexes and many systems have been reported, for example, Ru(III) [129], W(VI) [130], polyoxometallates [131], Re(V) [132], Fe(III) [133], and Pt(II) [134] with hydrogen peroxide, Mn(II) [135-137] with peracetic acid, and Ti-tartrate with alkyl hydroperoxides [75]. The subject of epoxidation by H2O2 has been reviewed [138-140]. 

Since PO is water soluble, a few homogeneous catalytic systems employing aqueous H2O2 as oxidant have been reported [34]. This section report results on the homogeneous catalytic epoxidation of propylene to PO with 52% H2O2 using the reaction-controlled phase-transfer catalyst A [45]. The catalyst is easily... 

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