The bis(μ‐hydroxo)‐bridged dioxovanadium site in [γ‐1,2‐H2SiV2W10O40]4− catalyzes the epoxidation of alkenes in the presence of only one equivalent of H2O2 with a high yield of epoxide, high efficiency of H2O2 utilization, unusual regioselectivity, and unprecedented diastereoselectivity (see picture).
Three kinds of reactions, (i) aerobic oxidation of alcohols, (ii) aerobic oxidation of amines, and (iii) reduction of carbonyl compounds to alcohols using 2-propanol as a hydrogen donor, could efficiently be promoted by an easily prepared ruthenium hydroxide catalyst on magnetite (Ru(OH) x /Fe 3 O 4 ). A wide variety of substrates including aromatic, aliphatic, and heterocyclic ones could be converted to the desired products in high to excellent yields without any additives such as bases and electron transfer mediators. After the reaction, the catalyst/product(s) separation could be easily achieved with a permanent magnet and more than 99% of Ru(OH) x /Fe 3 O 4 catalyst could usually be recovered for each reaction. The catalysis for these reactions was intrinsically heterogeneous, and Ru(OH) x /Fe 3 O 4 recovered after these reactions could be reused without appreciable loss of the catalytic performance.
The tetra-n-butylammonium (TBA) salt of the divacant Keggin-type polyoxometalate [TBA](4)[gamma-SiW(10)O(34)(H(2)O)(2)] (I) catalyzes the oxygen-transfer reactions of olefins, allylic alcohols, and sulfides with 30 % aqueous hydrogen peroxide. The negative Hammett rho(+) (-0.99) for the competitive oxidation of p-substituted styrenes and the low value of (nucleophilic oxidation)/(total oxidation), X(SO)=0.04, for I-catalyzed oxidation of thianthrene 5-oxide (SSO) reveals that a strongly electrophilic oxidant species is formed on I. The preferential formation of trans-epoxide during epoxidation of 3-methyl-1-cyclohexene demonstrates the steric constraints of the active site of I. The I-catalyzed epoxidation proceeds with an induction period that disappears upon treatment of I with hydrogen peroxide. (29)Si and (183)W NMR spectroscopy and CSI mass spectrometry show that reaction of I with excess hydrogen peroxide leads to fast formation of a diperoxo species, [TBA](4)[gamma-SiW(10)O(32)(O(2))(2)] (II), with retention of a gamma-Keggin type structure. Whereas the isolated compound II is inactive for stoichiometric epoxidation of cyclooctene, epoxidation with II does proceed in the presence of hydrogen peroxide. The reaction of II with hydrogen peroxide would form a reactive species (III), and this step corresponds to the induction period observed in the catalytic epoxidation. The steric and electronic characters of III are the same as those for the catalytic epoxidation by I. Kinetic, spectroscopic, and mechanistic investigations show that the present epoxidation proceeds via III.
It goes on the dicopper core! A monomeric γ‐Keggin silicotungstate with a dicopper core that is bridged by two μ‐1,1‐azido ligands catalyzes oxidative alkyne homocoupling reactions whereby various kinds of aromatic and aliphatic alkynes are selectively converted into the corresponding diynes (see picture).
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