Polyoxometalate epoxidation

A more recent approach to the epoxidation of allylic alcohols makes use of a vanadium polyoxometallate, together with a sterically demanding chiral tartrate (or TADDOL) -derived hydroperoxide, to give a highly chemoselective, regiose-lective and enantioselective outcome (Figure 11.2). ... 

SCHEME 85. Epoxidation of alkenes and alkenols with a perfluorinated quaternary ammonium salt of a polyoxometalate catalyst... 

In 1989, Isobe and coworkers reported on an organometallic polyoxometalate cluster [(Rhcp )4V60i9] (cp = /j -CsMcs) that catalyzes the oxidation of cyclohexene with TBHP as oxidant to give mainly ally lie oxidation products (l-ferf-butylperoxycyclohex-2-ene 42% and cyclohex-2-en-l-one 21%) and only little epoxide (15%) (equation 62). The yield of 1 -ferf-butylperoxy cyclohex-2-ene increased with decreasing molar ratio of cyclohexene to TBHP, while the yield of cyclohex-2-en-l-one has a maximum at the ratio of 0.2. 

We report here three studies that address three separate but significant issues in the emerging area of selective catalytic oxidation by TMSP-type complexes. The first study establishes for the first time that some TMSP complexes are compatible with basic oxidants and basic conditions. The second study reports the first oxidation, in this case selective alkene epoxidation, by the economically and environmentally desirable oxidant, aqueous hydrogen peroxide, catalyzed by TMSP complexes. The third study demonstrates that redox active polyoxometalates can be derivatized with alcohols in a manner that should prove useful for fabricating future generations of more sophisticated and selective TMSP catalysts. 

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). 

W, V and Co has been widely investigated [28]. This section will highlight some novel and unique approaches to epoxide synthesis using environmentally benign oxidants and heterogeneous metal catalysts such as solid-supported Ti, polyoxometalates and hydrotaldte. 

Studies in this field are just beginning, and the number of publications hardly exceeds a dozen. The most interesting results were obtained by the research groups of Yamada [160-162], Neumann [163,164] and Kozhevnikov [165, 166], Using various type catalysts (Ru porphyrene complexes, polyoxometalates, supported metals), the authors conducted selective oxidations of various types. These include epoxidation of alkenes, oxidation of alcohols, oxidation of alkylaromatics, oxidation and aromatiza-tion of dihydroanthracenes, and some other reactions. The experiments were typically conducted at 373—423 K under 1.0 MPa pressure of nitrous oxide. 

Ben-Daniel, R Weiner, L. and Neumann, R. (2002) Activation of nitrous oxide and selective epoxidation of alkenes catalyzed by the manganese-substituted polyoxometalate, [Mnnl2ZnW... 

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