- Olefin epoxidation with hydrogen peroxide catalyzed by lacunary polyoxometalate [γ-SiW10O34(H2O) 2]4-
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The tetra-n-butylammonium (TBA) salt of the divacant Keggin-type polyoxometalate [TBA]4[γ-SiW10O34-(H 2O)2] (I) catalyzes the oxygen-transfer reactions of olefins, allylic alcohols, and sulfides with 30% aqueous hydrogen peroxide. The negative Hammett ρ+ (-0.99) for the competitive oxidation of p-substituted styrenes and the low value of (nucleophilic oxidation)/(total oxidation), Xso = 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-spoxide 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. 29Si and 183W 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[γ-SiW10O32(O2) 2] (II), with retention of a γ-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.
- Kamata, Keigo,Kotani, Miyuki,Yamaguchi, Kazuya,Hikichi, Shiro,Mizuno, Noritaka
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p. 639 - 648
(2007/10/03)
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- Regio- and diastereoselection in the oxaziridinium salt oxidation of acyclic allylic acetates
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Oxaziridinium salt 1 is a versatile oxygen transfer agent towards olefins. An important threo selectivity is observed with acyclic allylic acetates due to 1,3-allylic strain (A1,3). The π-facial selectivity in the epoxide formation is interpretated as a consequence of an interaction between the residual positive charge on nitrogen and the neighbouring acetate in the transition state. Such an interaction is not strong enough to balance the lowered nucleophilicity of the 2,3 double bond in the geraniol acetate 3a.
- Poisson, David,Cure, Gilles,Solladié, Guy,Hanquet, Gilles
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p. 3745 - 3748
(2007/10/03)
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- Epoxidation of alkenes with H2O2 generated in situ from alcohols and molecular oxygen using N-hydroxyphthalimide and hexafluoroacetone as catalysts
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A new epoxidation method of olefins with hydrogen peroxide and/or α- hydroxy hydroperoxide which are generated in situ from an alcohol and molecular oxygen was developed. A variety of alkenes were smoothly epoxidized with molecular oxygen in the presence of an alcohol under the influence of hexafluoroacetone (HFA) and N-hydroxyphthalimide (NHPI) as catalysts. The reaction involves the formation of α-hydroxy hydroperoxide and/or hydrogen peroxide derived from 1-phenylethanol and dioxygen by the action of NHPI and the active oxygen transfer from these hydroperoxides to HFA, giving 2- hydroperoxyhexafluoro-2-propanol which serves as the actual epoxidizing agent.
- Iwahama, Takahiro,Sakaguchi, Satoshi,Ishii, Yasutaka
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p. 693 - 705
(2007/10/03)
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- Regio-, diastereo-, and chemoselectivities in the dioxirane oxidation of acyclic and cyclic allylic alcohols by methyl(trifluoromethyl)dioxirane (TFD): A comparison with dimethyldioxirane
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The solvent-dependent shift in the regioselectivity of the geraniol epoxidation by methyl(trifluoromethyl)dioxirane (TFD) reveals that as for the less reactive dimethyldioxirane (DMD). hydrogen bonding stabilizes the transition state of the epoxidation. In protic media, the hydrogen bonding is exerted intermolecularly by the solvent, whereas in unpolar, non-hydrogen-bonding solvents intramolecular assistance through the adjacent hydroxy functionality comes into the play and the attack on the allylic alcohol moiety is favored. For chiral allylic alcohols, additional steric interactions control the π-facial selectivity in the conformationally fixed transition state. Analogous to DMD, the preferred dihedral angle in the hydrogen-bonded transition state of the TFD epoxidation constitutes approximately 130°, but contrary to DMD and for synthetic purposes important, the allylic alcohols and derivatives 1 and 3-5 investigated here are chemoselectively epoxidized by TFD without formation of the corresponding enones.
- Adam, Waldemar,Paredes, Rodrigo,Smerz, Alexander K.,Veloza, L. Angela
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p. 349 - 354
(2007/10/03)
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- Selective epoxidation of monoterpenes with methyltrioxorhenium and H2O2
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In the presence of pyridine as a co-catalyst, CH3ReO3 catalyses the epoxidation of terpenes such as α-pinene with H2O2 with minimal rearrangement of the epoxide. Pyridine is also critical to suppress isomerisation of the olefin substrate (in case of nerol, geraniol). The reaction can be directed towards selective single or double epoxidation, or in one step towards the rearranged product (e.g. from linalool to the ring- closure product linalool oxide.
- Villa De P., Aida L.,De Vos, Dirk E.,Montes De C., Consuelo,Jacobs, Pierre A.
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p. 8521 - 8524
(2007/10/03)
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- THE HIGHLY EFFICIENT OXIDATION OF OLEFINS, ALCOHOLS, SULFIDES AND ALKANES WITH HETEROAROMATIC N-OXIDES CATALYZED BY RUTHENIUM PORPHYRINS
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The oxygen atom transfer reactions from 2,6-disubstituted pyridine N-oxides to olefins, allyl or benzyl alcohols and sulfides were efficiently catalyzed by ruthenium porphyrins, and these substrates were converted into epoxides, aldehydes and sulfoxides, respectively, with high selectivity.These oxidations also proceeded using other heteroaromatic N-oxides, such as pyrazine N-oxides, as oxidants.The catalytic activity of ruthenium porphyrin complexes was enhanced by the addition of a small amount of HCl or HBr.In the presence of these acids, the oxidations of alkanes or aliphatic alcohols with 2,6-dichloropyridine N-oxides were also efficiently catalyzed by ruthenium porphyrin complexes, and alcohols or ketones were afforded as oxidation products with high selectivity.In the hydroxylation of adamantane, ruthenium porphyrins work very efficiently as catalysts, giving a turnover number of up to 120000.This system offers practical advantages, such as mild conditions, tractability of oxidants and easy overall procedures.In the case of the reactions with HCl or HBr, one possibility in the reaction mechanism is that the activity of ruthenium porphyrins is enhanced in part by the coordination of Cl- or Br- as axial ligands.
- Ohtake, Hiro,Higuchi, Tsunehiko,Hirobe, Masaaki
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p. 867 - 904
(2007/10/02)
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- The selectivities and the mechanism on highly efficient epoxidation of olefins with 2,6-disubstituted pyridine N-oxides catalyzed by ruthenium porphyrin
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Several remarkable selectivities in competitive epoxidations using a ruthenium porphyrin/2,6-disubstituted pyridine N-oxide system were observed. The proposal that the active intermediate of this system differed from the trans-dioxo complex of ruthenium porphyrin was indicatesd.
- Ohtake,Higuchi,Hirobe
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p. 2521 - 2524
(2007/10/02)
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- NEW METHODS AND REAGENTS IN ORGANIC SYNTHESIS. 15. EPOXIDATION OF OLEFINS WITH DIETHYL PHOSPHOROCYANIDATE (DEPC) AND HYDROGEN PEROXIDE
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A mixture of diethyl phosphorocyanidate (DEPC) and hydrogen peroxide in the presence of 2-hydroxypyridine or 1,2,4-triazole functions as an epoxidizing agent for olefins.Keywords: diethyl phosphorocyanidate; hydrogen peroxide; epoxidation; 2-hydroxypyridine; 1,2,4-triazole; peroxyphosphoric acid
- Mizuno, Akira,Hamada, Yasumasa,Shioiri, Takayuki
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p. 1774 - 1776
(2007/10/02)
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