A convenient ruthenium-catalyzed alkene epoxidation with hydrogen peroxide as oxidant

Epoxidation of Alkenes by Hydrogen Peroxide over 12-Heteropolyacids of Molybdenum and Tungsten (H3PMo3W9O40) Combined with Cetylpyridinium Bromide

In the epoxidation of 4-vinylcyclohex-1-ene with H2O2 in monophasic acetonitrile solution catalysed by Keggin-type 12-heteropolyacids, i.e., H3PMo12- nW nO40 ( n = 0–12), which are precursors of active peroxo complexes, and phase transfer catalysts Q+Br–, the catalyst H3PMo3W9O40 showed the highest activity, giving a conversion of 98% and a selectivity of 88%. By this method, a variety of water-insoluble unactivated alkenes, internal or terminal, open chain or cyclic and isolated, were epoxidised under mild conditions and after relatively short reaction times. The state of the H3PMo3W9O40/CPB/H2O2/CH3CN system was studied using UV, IR, and 31P NMR spectroscopies with the [H2O2]: [HPA] ratio = 50. Several peroxo species were observed by 31P NMR spectroscopy at a lower field than the original heteropolyacids. Their composition varied regularly with that of the starting catalyst. The P-containing peroxo species formed were deduced as [(PO4){Mo4– xW xO20}]3- ( x = 0-4), which are the true catalytically active species under the reaction conditions.

Catalytic performance of Keplerate polyoxomolybdates in green epoxidation of alkenes with hydrogen peroxide

The catalytic activity of three Keplerate POM, Mo₇₂M₃₀ (V, Cr and Fe), in the green epoxidation of alkenes with 30% H₂O₂ in water are investigated. Mo₇₂Cr₃₀ was shown the highest catalytic activity and efficiently reused without any loss of activity.

Versatile eco-friendly pickering emulsions based on substrate/native cyclodextrin complexes: a winning approach for solvent-free oxidations

Solvent-less Pickering emulsions were developed and applied to catalytic oxidation. These systems are stabilized by inclusion complexes between cyclodextrins and substrates, forming a 3D network among the dispersed phase. In the presence of hydrogen peroxide as a green oxidant and [Na]3 [PW12 O40 ] as a catalyst, they provide particularly efficient reaction media for the oxidation of olefins, organosulfurs, and alcohols. The reactions proceed at competitive rates (up to 400 h(-1) ) with straightforward separation of the phases by centrifugation or heating. Moreover, these new eco-friendly systems work at a preparative scale (up to 2.5 M) and are recycled without loss of activity.

Pickering emulsion stabilized by catalytic polyoxometalate nanoparticles: a new effective medium for oxidation reactions

Decyl-, dodecyl-, and tetradecyltrimethylammonium cations were combined with the catalytic polyoxometalate [PW(12)O(40)](3-) anion to give spherical and monodisperse nanoparticles that are able to stabilize emulsions in the presence of water and an aromatic solvent. This triphasic liquid/solid/liquid system, based on a catalytic surfactant, is particularly efficient as a reaction medium for epoxidation reactions that involve hydrogen peroxide. The reactions proceed at competitive rates with straightforward separation of the phases by centrifugation. Such catalytic "Pickering" emulsions combine the advantages of heterogeneous catalysis and biphasic catalysis without the drawbacks (e.g., catalyst leaching or separation time).

Synthesis of Epoxides Catalyzed by a Halide-Free Reaction-Controlled Phase-Transfer Catalytic System: [(CH3(CH2)17)2N(CH3)2]3[PW4O32]/H2O2/Dioxan/Olefin

The epoxidation of alkenes was successfully catalyzed by a recyclable catalytic system: [(CH3(CH2)17)2N(CH3)2]3[PW4O32]/H2O2/dioxan/olefin. This new catalytic system is not only capable of catalyzing homogeneous epoxidation of alkenes with a unique reaction-controlled phase-transfer character, but also avoids the use of chlorinated solvents. The reactions were conducted in a biphasic mixture of aqueous H2O2/dioxan, and many kinds of alkenes could be efficiently converted to the corresponding epoxides in high yields. Both new and used [(CH3(CH2)17)2N(CH3)2]3[PW4O32] catalyst was characterized by 31P magic angle spin NMR, and IR.

Regioselectivity and diasteroselectivity in Pt(II)-mediated "green" catalytic epoxidation of terminal alkenes with hydrogen peroxide: mechanistic insight into a peculiar substrate selectivity

Recently developed electron-poor Pt(II) catalyst 1 with the "green" oxidant 35% hydrogen peroxide displays high activity and complete substrate selectivity in the epoxidation of terminal alkenes because of stringent steric and electronic requirements. In the presence of isolated dienes bearing terminal and internal double bonds, epoxidation is completely regioselective toward the production of terminal epoxides. Insight into the mechanism is gained by means of a reaction progress kinetic analysis approach that underlines the peculiar role of 1 in activating both the alkene and H2O2 in the rate-determining step providing a rare example of nucleophilic oxidation of alkenes by H2O2.

Development of a General and Efficient Iron-Catalyzed Epoxidation with Hydrogen Peroxide as Oxidant

The development of inexpensive and practical iron catalysts for the environmentally benign epoxidation of olefins with hydrogen peroxide as terminal oxidant is described. By systematic variation of ligands, metal sources, and reaction conditions, it was discovered that FeCl3 x 6 H2O in combination with pyridine-2,6-dicarboxylic acid and different amines shows high reactivity and excellent selectivity towards the epoxidation of aromatic olefins and moderate reactivity towards that of aliphatic olefins.

A Novel Environmentally Benign Method for the Selective Oxidation of Alcohols to Aldehydes and Ketones

In the presence of [Ru(terpyridine)(2,6-pyridinedicarboxylate)], aliphatic and benzylic alcohols are oxidized to the corresponding aldehydes or ketones with high selectivity by using hydrogen peroxide as the oxidant. There is no need for the addition of co-catalysts or organic solvents. By applying an optimized reaction protocol, high catalyst productivity (turnover number>10,000) and activity (turnover frequency up to 14,800 h(-1)) has been achieved.

An efficient biomimetic Fe-catalyzed epoxidation of olefins using hydrogen peroxide

A new, environmentally benign and practical epoxidation method was developed using inexpensive and efficient Fe catalysts. FeCl3.6H2O in combination with commercially available pyridine-2,6-dicarboxylic acid and amines showed excellent reactivity and selectivity towards aromatic olefins and moderate reactivity towards 1,3-cyclooctadiene utilizing H2O2 as the terminal oxidant.

An improved methyltrioxorhenium-catalyzed epoxidation of alkenes with hydrogen peroxide

Methyltrioxorhenium (MTO)-catalyzed epoxidation of alkenes with H(2)O(2) has been significantly improved by using 3-methylpyrazole as an additive. A system consisting of 35% H(2)O(2) and MTO-3-methylpyrazole in CH(2)Cl(2) catalyzes the epoxidation of various alkenes in excellent yields. The catalytic activity of MTO-3-methylpyrazole surpasses MTO-pyrazole and MTO-pyridine catalysts. Quantitative yields of epoxides from cyclic and internal alkenes were obtained with only 0.05-0.1 mol% of MTO in the presence of 10 mol% of 3-methylpyrazole.

Ruthenium-Catalyzed Oxidation of Alkenes, Alkynes, and Alcohols to Organic Acids with Aqueous Hydrogen Peroxide

A protocol that adopts aqueous hydrogen peroxide as a terminal oxidant and [(Me3tacn)(CF3CO2)2Ru(III)(OH2)]CF3CO2 (1; Me3tacn = 1,4,7-trimethyl-1,4,7-triazacyclononane) as a catalyst for oxidation of alkenes, alkynes, and alcohols to organic acids in over 80% yield is presented. For the oxidation of cyclohexene to adipic acid, the loading of 1 can be lowered to 0.1 mol %. On the one-mole scale, the oxidation of cyclohexene, cyclooctene, and 1-octanol with 1 mol % of 1 produced adipic acid (124 g, 85% yield), suberic acid (158 g, 91% yield), and 1-octanoic acid (129 g, 90% yield), respectively. The oxidative C=C bond-cleavage reaction proceeded through the formation of cis- and trans-diol intermediates, which were further oxidized to carboxylic acids via C-C bond cleavage.

Ruthenium-Catalyzed Asymmetric Epoxidation of Olefins Using H2O2, Part I: Synthesis of New ChiralN,N,N-Tridentate Pybox and Pyboxazine Ligands and Their Ruthenium Complexes

The synthesis of chiral tridentate N,N,N-pyridine-2,6-bisoxazolines 3 (pybox ligands) and N,N,N-pyridine-2,6-bisoxazines 4 (pyboxazine ligands) is described in detail. These novel ligands constitute a useful toolbox for the application in asymmetric catalysis. Compounds 3 and 4 are conveniently prepared by cyclization of enantiomerically pure alpha- or beta-amino alcohols with dimethyl pyridine-2,6-dicarboximidate. The corresponding ruthenium complexes are efficient asymmetric epoxidation catalysts and have been prepared in good yield and fully characterized by spectroscopic means. Four of these ruthenium complexes have been characterized by X-ray crystallography. For the first time the molecular structure of a pyboxazine complex [2,6-bis-[(4S)-4-phenyl-5,6-dihydro-4H-[1,3]oxazinyl]pyridine](pyridine-2,6-dicarboxylate)ruthenium (S)-2 aa, is presented.

Ruthenium-Catalyzed Asymmetric Epoxidation of Olefins Using H2O2, Part II: Catalytic Activities and Mechanism

Asymmetric epoxidation of olefins with 30 % H2O2 in the presence of [Ru(pybox)(pydic)] 1 and [Ru(pyboxazine)(pydic)] 2 has been studied in detail (pybox = pyridine-2,6-bisoxazoline, pyboxazine = pyridine-2,6-bisoxazine, pydic = 2,6-pyridinedicarboxylate). 35 Ruthenium complexes with sterically and electronically different substituents have been tested in environmentally benign epoxidation reactions. Mono-, 1,1-di-, cis- and trans-1,2-di-, tri-, and tetra-substituted aromatic olefins with versatile functional groups can be epoxidized with this type of catalyst in good to excellent yields (up to 100 %) with moderate to good enantioselectivies (up to 84 % ee). Additive and solvent effects as well as the relative rate of reaction with different catalysts have been established. It is shown that the presence of weak organic acids or an electron-withdrawing group on the catalyst increases the reactivity. New insights on the reaction intermediates and reaction pathway of the ruthenium-catalyzed epoxidation are proposed on the basis of density functional theory calculation and experiments.

Synthesis of a New Chiral N,N,N-Tridentate Pyridinebisimidazoline Ligand Library and Its Application in Ru-Catalyzed Asymmetric Epoxidation

A small ligand library of chiral tridentate N,N,N-pyridinebisimidazolines have been synthesized for the first time. This new class of ligands can be easily tuned and synthesized on multi g-scale. The usefulness of the ligands is shown in the ruthenium-catalyzed asymmetric epoxidation with hydrogen peroxide as oxidant. Excellent yields (>99%) and good enantioselectivities (up to 71% ee) have been obtained for the epoxidation of aromatic olefins. [reaction: see text]

Convenient Method for Epoxidation of Alkenes Using Aqueous Hydrogen Peroxide

[reaction: see text] The complex [Ru(tpy)(pydic)] (1a) is an active catalyst for epoxidation of alkenes by aqueous 30% hydrogen peroxide in tertiary alcohols. The protocol is simple to operate and gives the corresponding epoxides in good to excellent yields. Chiral enantiopure [Ru(tpy)(pydic)] complexes have been synthesized and successfully applied in this procedure.