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

Abstract

Sandwich-type polyoxometalates (POMs), namely [WZnM2(ZnW9O34)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 degrees 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 [PW11O39]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 H2O2 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 alpha of 50-70degrees for the metal-alcoholate-bonded template of the POM/H2O2 system provides the best compromise between 1,2A and 1,3A strain during the oxygen transfer. In contrast to acyclic allylic alcohols 1, the M-POM-catalyzed oxidation of the cyclic allylic alcohols 4 by H2O2 gives significant amounts of enone.

Highly Selective, Recyclable Epoxidation of Allylic Alcohols with Hydrogen Peroxide in Water Catalyzed by Dinuclear Peroxotungstate

“…[4][5][6][7][14][15][16][17] Many tungsten-based catalysts have been reported to be active for the epoxidation of allylic alcohols with hydrogen peroxide. [18][19][20][21][22][23][24][25][26][27][28] Most of them need strict pH control with amines or buffers and/or use biphasic procedures because of decomposition of both the allylic alcohols and the epoxides. [18][19][20][21][22][23][24][25][26][27][28] We reported very recently an efficient and simple route for epoxidation of both internal and terminal olefins with hydrogen peroxide catalyzed by the divacant lacunary silicotungstate, [(n-C 4 H 9 ) 4 N] 4 [g-SiW 10 O 34 (H 2 O) 2 ].…”

“…[18][19][20][21][22][23][24][25][26][27][28] Most of them need strict pH control with amines or buffers and/or use biphasic procedures because of decomposition of both the allylic alcohols and the epoxides. [18][19][20][21][22][23][24][25][26][27][28] We reported very recently an efficient and simple route for epoxidation of both internal and terminal olefins with hydrogen peroxide catalyzed by the divacant lacunary silicotungstate, [(n-C 4 H 9 ) 4 N] 4 [g-SiW 10 O 34 (H 2 O) 2 ]. [29] During the course of our investigation of tungsten-catalyzed oxidation, we found that the simple dinuclear peroxotungstate, K 2 [{W(=O)(O 2 ) 2 (H 2 O)} 2 (m-O)]·2 H 2 O (I, Figure 1), could act as an effective catalyst for the epoxidation of allylic alcohols using hydrogen peroxide in water under mild reaction conditions [Eq.…”

Recovery of Homogeneous Polyoxometallate Catalysts from Aqueous and Organic Media by a Mesoporous Ceramic Membrane without Loss of Catalytic Activity

“…[3][4][5][6][7] Polyoxometallate (POM) catalysts have all the features necessary for successful recycling by nanofiltration: they are large, they have a rigid structure, [8] which is charged, [9] and the lack of organic ligands makes them stable towards many different reaction conditions. [10] Herein, the recycling of a "sandwich"-type polyoxometallate oxidation catalyst from aqueous and non-aqueous media is described. The watersoluble sodium salt of the polyoxometallate catalyst Na 12 Herein, we demonstrate that both the water-soluble (Na 12 POM) and toluene-soluble (Q 12 POM) POM oxidation catalysts can be recycled efficiently by a mesoporous ceramic g-alumina membrane.…”

“…These two "sandwich" POMs were chosen because we have demonstrated previously that they are very versatile "multi-purpose" catalysts for oxidations with aqueous hydrogen peroxide, such as epoxidation, alcohol oxidation and heteroatom oxidations. [10,14,17,18] From an industrial point of view, these "sandwich" POM catalysts offer the additional advantage that they are easily accessible by self-assembly from readily available inorganic compounds, as will be pointed out in the Experimental Section.…”

Three‐Membered Heterocycles. Structure and Reactivity

“…As the double bond, with trans-pent-3-en-2-ol giving a 1 : 1 mixture of threo and erythro epoxides under the same conditions. The key intermediate is believed to be a tungsten peroxo complex rather than an oxo-Mn species [463]. Self-assembled POM catalysts can also be immobilized within layered double hydroxides (LDH) using ionexchange techniques [464].…”