TADDOL-derived hydroperoxide

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). ... 

The metal-catalyzed asymmetric epoxidation of allylic alcohols with various enan-tiomerically pure hydroperoxides has been studied by several groups. This approach has been employed in the Ti- and V-mediated epoxidation of this class of substrates, in the presence of different achiral additives with modest enantioselectivities (ee ee < 46% ), which turned satisfactory (ee 72%) in the presence of the TADDOL-derived hydroperoxide TADOOH 73 . This oxidant has been recently employed in the oxovanadium sandwich-type POM [ZnW(V0)2(ZnW9034)2] catalyzed epoxidation of various allylic alcohols with very high catalytic efficiency (42000 turnovers) and enantiomeric ratios up to 95 5 98. 

We have also explored the use of vanadium substituted Qi2 WZn(VO)2 (ZnWg034)2], prepared via metal exchange [Eq (16.2)], as a catalyst for allylic alcohol epoxidations with nonchiral and chiral organic hydroperoxides instead of PI2O2 as the terminal oxidant [8,17]. These conversions are assumed to proceed via V-alkylperoxido species. Besides the substantial as Tnmetric induction for a number of substrates with a chiral TADDOL-derived hydroperoxide, a particularly... 

W. Adam, P. L. Alsters, R. Neumann, C. R. Saha-Moller, D. Seebach, R. Zhang, Highly efficient catalytic asymmetric epoxidation of allylic alcohols by an oxovanadium-substituted polyoxome-talate with a regenerative TADDOL-Derived hydroperoxide, Org. Lett. 5 (2003) 725. 

A polyoxometalate is also at the heart of an enantioselective epoxidation of allylic alcohols using a C-2 symmetric chiral hydroperoxide 39 derived from l,l,4,4-tetraphenyl-2,3-0-isopropylidene-D-threitol (TADDOL). Thus, in the presence of the oxovanadium(IV) sandwich-type POM [ZnW(V0)2(ZnW9034)2]12- and stoichiometric amounts of hydroperoxide 39, the dienol 40 is converted to the (2R) epoxide 41 in 89% yield and 83% ee. The proposed catalytic cycle invokes a vanadium(V) template derived from the POM, substrate, and hydroperoxide, a hypothesis supported by the lack of enantioselectivity with unfunctionalized alkenes. The catalytic turnover is remarkably high at about 40,000 TON <03OL725>. 

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