A Highly Chemoselective, Diastereoselective, and Regioselective Epoxidation of Chiral Allylic Alcohols with Hydrogen Peroxide, Catalyzed by Sandwich-Type Polyoxometalates: Enhancement of Reactivity and Control of Selectivity by the Hydroxy Group through Metal-Alcoholate Bonding Waldemar Adam, † Paul L. Alsters, ‡ Ronny Neumann, § Chantu R. Saha-Mo ¨ller, † Dorit Sloboda-Rozner, § and Rui Zhang* ,† Institute of Organic Chemistry, University of Wu ¨ rzburg, Am Hubland, D-97074 Wu ¨ rzburg, Germany, Advanced Synthesis and Catalysis, DSM Fine Chemicals, 6160 MD Geleen, The Netherlands, and Department of Organic Chemistry, Weizmann Institute of Science, Rehovet 76100, Israel adam@chemie.uni-wuerzburg.de Received October 31, 2002 Sandwich-type polyoxometalates (POMs), namely [WZnM 2 (ZnW 9 O 34 ) 2 ] q- [M ) Mn(II), Ru(III), Fe- (III), Pd(II), Pt(II), Zn(II); q ) 10-12], are shown to catalyze selectively the epoxidation of chiral allylic alcohols with 30% hydrogen peroxide under mild conditions (ca. 20 °C) in an aqueous/organic biphasic system. The transition metals M in the central ring of polyoxometalate do not affect the reactivity, chemoselectivity, or stereoselectivity of the allylic alcohol epoxidation by hydrogen peroxide. Similar selectivities, albeit in significantly lower product yields, are observed for the lacunary Keggin POM [PW 11 O 39 ] 7- , in which a peroxotungstate complex has been shown to be the active oxidizing species. All these features support a tungsten peroxo complex rather than a high- valent transition-metal oxo species operates as the key intermediate in the sandwich-type POM- catalyzed epoxidations. On capping of the hydroxy functionality through acetylation or methylation, no reactivity of these hydroxy-protected substrates [1a(Ac) and 1a(Me)] is observed by these POMs. A template is proposed to account for the marked enhancement of reactivity and selectivity, in which the allylic alcohol is ligated through metal-alcoholate bonding, and the H 2 O 2 oxygen source is activated in the form of a peroxotungsten complex. 1,3-Allylic strain promotes a high preference for the threo diastereomer and 1,2-allylic strain a high preference for the erythro diastereomer, whereas tungsten-alcoholate bonding furnishes high regioselectivity for the epoxidation of the allylic double bond. The estimated dihedral angle R of 50-70° for the metal-alcoholate-bonded template of the POM/H 2 O 2 system provides the best compromise between 1,2 A and 1,3 A strain during the oxygen transfer. In contrast to acyclic allylic alcohols 1, the M-POM-catalyzed oxidation of the cyclic allylic alcohols 4 by H 2 O 2 gives significant amounts of enone. Introduction The fine chemical industry is facing increased pressure to develop sustainable alternatives for classical processes that no longer meet current environmental constraints. With respect to oxidative transformations, there is a need for the development of effective catalytic systems that enable the selective manufacture of oxygen-functionalized fine chemicals based on cheap, readily available, and environmentally benign oxidants such as hydrogen per- oxide and nontoxic metal catalysts. 1 In this context, recently polyoxometalates (POMs), in particular their transition-metal-substituted derivatives, have gained importance as homogeneous and heterogeneous oxidation catalysts due to their oxidative and hydrolytic stability, ease of preparation and facile modification. 2 For this purpose, a variety of oxygen sources may be employed, which include iodosobenzene, 3 N-oxide, 4 nitrous oxide, 5 periodate, 6 ozone, 7 dioxygen, 8 hydrogen peroxide, 9 and tert-butyl hydroperoxide. 10 Initially, the tungsten-based polyoxometalates [PW 12 O 40 ] 3- and {PO 4 [W(O)(O 2 ) 2 ] 4 } 3- were used to cata- † University of Wu ¨ rzburg. ‡ DSM Fine Chemicals. § Weizmann Institute of Science. 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