Efficient Epoxidation of Electron‐Deficient Alkenes with Hydrogen Peroxide Catalyzed by [γ‐PW₁₀O₃₈V₂(μ‐OH)₂]³⁻

Abstract

A divanadium-substituted phosphotungstate, [γ-PW(10)O(38)V(2)(μ-OH)(2)](3-) (I), showed the highest catalytic activity for the H(2)O(2)-based epoxidation of allyl acetate among vanadium and tungsten complexes with a turnover number of 210. In the presence of I, various kinds of electron-deficient alkenes with acetate, ether, carbonyl, and chloro groups at the allylic positions could chemoselectively be oxidized to the corresponding epoxides in high yields with only an equimolar amount of H(2)O(2) with respect to the substrates. Even acrylonitrile and methacrylonitrile could be epoxidized without formation of the corresponding amides. In addition, I could rapidly (≤10 min) catalyze epoxidation of various kinds of terminal, internal, and cyclic alkenes with H(2)O(2) under the stoichiometric conditions. The mechanistic, spectroscopic, and kinetic studies showed that the I-catalyzed epoxidation consists of the following three steps: 1) The reaction of I with H(2)O(2) leads to reversible formation of a hydroperoxo species [γ-PW(10)O(38)V(2)(μ-OH)(μ-OOH)](3-) (II), 2) the successive dehydration of II forms an active oxygen species with a peroxo group [γ-PW(10)O(38)V(2)(μ-η(2):η(2)-O(2))](3-) (III), and 3) III reacts with alkene to form the corresponding epoxide. The kinetic studies showed that the present epoxidation proceeds via III. Catalytic activities of divanadium-substituted polyoxotungstates for epoxidation with H(2)O(2) were dependent on the different kinds of the heteroatoms (i.e., Si or P) in the catalyst and I was more active than [γ-SiW(10)O(38)V(2)(μ-OH)(2)](4-). On the basis of the kinetic, spectroscopic, and computational results, including those of [γ-SiW(10)O(38)V(2)(μ-OH)(2)](4-), the acidity of the hydroperoxo species in II would play an important role in the dehydration reactivity (i.e., k(3)). The largest k(3) value of I leads to a significant increase in the catalytic activity of I under the more concentrated conditions.

Base Catalysis by Mono- and Polyoxometalates

“…These protonated sites play an important role in the activation of oxidants and the catalytic oxidation abilities are strongly dependent on the protonation states [95][96][97][98][99][100]102,103].…”

Oxidation of isoeugenol to vanillin by the “H2O2–vanadate–pyrazine-2-carboxylic acid” reagent

“…Vanadium complexes are known as efficient catalysts for various mild oxidations of organic compounds [7][8][9][10][11][12][13][14], particularly, saturated [15][16][17][18][19], aromatic [20][21][22][23] and olefinic [24][25][26][27] hydrocarbons as well as natural products [28][29][30]. In most of these processes, hydrogen peroxide was used as the oxidizing reagent (usually in an air atmosphere); however, the applications of other peroxides, such as tert-butyl hydroperoxide [31,32] and peroxyacetic acid [33], in these oxidations were also described.…”

Chemo‐ and Regioselective Direct Hydroxylation of Arenes with Hydrogen Peroxide Catalyzed by a Divanadium‐Substituted Phosphotungstate

“…[7] However, synthetic catalysts have scarcely achieved highly chemo-and regioselective hydroxylation of alkylarenes, especially with reactive secondary and tertiary aromatic side-chain sp 3 CÀH bonds.Recently, we have reported the formation of non-freeradical and electrophilic oxidants with high steric hindrance, such as [g-PW 10 O 38 V 2 (m-OH)(m-OOH)] 3À and [g-PW 10 O 38 V 2 -(m-O 2 )] 3À , by the reaction of a divanadium-substituted phosphotungstate, [g-PW 10 O 38 V 2 (m-OH) 2 ] 3À (I; Supporting Information, Figure S1) with H 2 O 2 . [8] Such oxidants can efficiently catalyze various oxidative functional group transformations. Herein, we apply the I-catalyzed oxidation system to direct…”

“…Compound I was much more active than [g-SiW 10 O 38 V 2 (m-OH) 2 ] 4À . [8] Mono-and trivanadium-substituted phosphotungstates, [a-PVW 11 O 40 ] 4À and [a-H 2 P-V 3 W 9 O 40 ] 4À , and monoprotonated [g-HPV 2 W 10 O 40 ] 4À (which has an {OV-(m-O)(m-OH)-VO} core) were inactive, suggesting that the active sites are not the V À O À W and V = O sites, but rather the bis-m-hydroxo site of {OV-(m-OH) 2 -VO} in I. In the presence of tungstates and HClO 4 , hydroxylation did not proceed.…”