- Unusual reactivity of aryl aldehydes with triethyl phosphite and zinc bromide: A facile preparation of epoxides, benzisoxazoles, and α-hydroxy phosphonate esters
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Abstract A facile preparation of trans-epoxides was achieved by a (EtO)3P-ZnBr2-mediated deoxygenation reaction of the corresponding 2-nitrobenzaldehydes. The sterically hindered analogues of 2-nitrobenzaldehyde underwent a reaction
- Raju, Potharaju,Gobi Rajeshwaran, Ganesan,Nandakumar, Meganathan,Mohanakrishnan, Arasambattu K.
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p. 3513 - 3523
(2015/06/08)
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- Sequential- and tandem-oxidation-epoxidation reactions employing guanidine bases
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1,5,7-Triazabicyclo[4.4.0]dec-1-ene (TBD) and 7-methyl-1,5,7- triazabicyclo[4.4.0]dec-1-ene (MTBD) promote the efficient formation of sulfonium ylides from sulfonium salts in the Corey-Chaykovsky epoxidation of aldehydes and provide excellent yields of the corresponding epoxides. This reaction protocol can be further adapted to allow for the development of both sequential and tandem-oxidation-epoxidation protocols from aldehydes generated in situ by manganese dioxide (MnO2) or barium manganate (BaMnO 4) mediated oxidation reactions.
- Phillips, David J.,Kean, Joseline L.,Graham, Andrew E.
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p. 6196 - 6202
(2013/07/19)
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- Generation of Sulfur Ylides from Sulfonium Salts and Their Reactions. Comparative Study of Electrochemical Reduction with the Base Method and Mechanism Elucidation by the MO Method
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The cathodic reduction of sulfonium salts in acetonitrile in the presence and absence of benzaldehyde was carried out. Results were compared with results of the base method. In the presence of benzaldehyde, the electrochemical reduction gave epoxides as a result of the Corey-Chaykovsky reaction, thus confirming ylide formation. The electrochemical reduction of sulfonium salts without benzaldehyde yielded rearrangement products in high yield. On the contrary, upon base treatment of sulfonium salts without benzaldehyde, symmetrical epoxides derived from the benzyl group of the sulfonium salt are obtained as main products as a result of the auto oxidation of the sulfur ylide. The reaction mechanisms were elucidated based on the results obtained by a semi-empirical molecular orbital method.
- Okazaki, Yuichi,Ando, Fumio,Koketsu, Jugo
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p. 2155 - 2165
(2007/10/03)
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- A highly enantioselective one-pot sulfur ylide epoxidation reaction
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Enantiomerically pure tricyclic C2-symmetric sulfide 1 can be prepared in high yield and only three steps from D-mannitol. When used in a one-pot sulfur ylide mediated epoxidation reaction, between 94 and 98% enantiomeric excess was obtained; the highest reported to date for the catalytic process. Molecular modelling studies of the ylide conformation provides a basis for understanding the stereochemical outcome of the reaction.
- Winn, Caroline L.,Bellenie, Benjamin R.,Goodman, Jonathan M.
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p. 5427 - 5430
(2007/10/03)
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- Experimental evidence for multiple oxidation pathways in the (salen)Mn-catalyzed epoxidation of alkenes
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The substrate electronic effects on the selectivity in the catalytic epoxidation of para-substituted cis stilbenes 2a-i were investigated by using (R,R)-[N,N′-bis(3,5-di-tBu-salicylidene)-1,2-cyclohexanediamine] manganese(III) chloride 1 in benzene as the catalyst with iodosobenzene as the terminal oxidant. A Hammett study of the selectivity results reveals a stronger electrophilic character than previously assumed in the (salen)Mn-catalyzed reaction. In general, the best correlations with the experimental values were obtained by using the Hammett σ+ values, which gave ρ = -1.37 for the rate of cisepoxide formation and ρ = -0.43 for the rate of the stepwise process leading to the corresponding trans product. The reaction involves two separate pathways as indicated also by the competitive breakdown of the intermediate on the path to trans epoxide for methoxy-substituted substrates. The asynchronicity in the concerted pathway leading to cis epoxide is apparent for 4-methoxy-4′-nitrostilbene, which yields cis epoxide with 75% ee entirely as a result of electronic effects.
- Linde, Christian,Koliai, Nordine,Norrby, Per-Ola,Akermark, Bjoern
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p. 2568 - 2573
(2007/10/03)
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- Electron-transfer versus Nucleophilic Substitution in the Reactions of α-Halogenated 4-Nitrotoluenes with Base
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A comparative study of the reactions of 4-nitrobenzyl and 4-nitrobenzylidene halides (ArCHXY; Ar = 4-NO2C6H4-; X = H or Halogen; Y = Halogen) with alkali shows that the reaction is sensitive to the nature of the side chain halogen substituent(s) as well as the reaction conditions.In alcoholic alkali, only 4-nitrobenzyl chloride (ArCH2Cl) reacts smoothly via the electron transfer (ET)-radical pathway to give mainly dimeric products while the corresponding bromide (ArCH2Br) and iodide (ArCH2I) give exclusively SN products.The ET pathway is enhanced for these substrates by the application of phase-transfer catalysis.In the case of the 4-nitrobenzylidene dihalides (ArCHXY; X = Y = Cl; X = Cl, Y = Br; X = Y = Br; X = F, Y = Br) it is also shown that although progressive bromine substitution suppresses the ET pathway, application of phase-transfer catalysis minimises competing solvolytic reactions and enhances the radical reaction.
- Kam, Toh-Seok,Lim, Tuck-Meng
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p. 147 - 150
(2007/10/02)
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- Preparation of optically active stilbene oxides via sulfonium salts derived from C2 symmetric thiolanes
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Trans-stilbene oxides have been obtained in up to 83% enantiomeric excess via reaction of S-benzyl ylides prepared from optically active trans-2,5-disubstituted thiolanes.
- Breau,Ogilvie,Durst
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- Olefin Epoxidation Using Elemental Fluorine
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F2 reacts with water and CH3CN, apparently to produce the relatively stable complex HOF*CH3CN.This is probably the best known oxygen-transfer reagent and can epoxidize olefins quickly and efficiently.Various types of alkenes including aliphatic, benzylic, enones, dienones, maleates, and fumarates have been examined, and all react with the reagent to produce the corresponding mono- or diepoxides in good to excellent yields.This epoxidation is fully stereospecific, and the configuration of the starting olefin is fully retained in the resulting oxirane.In cases where exceptionally stable oxocarbocations can be formed as in 1,1-diphenylethene, the reaction produces vicinal glycols in good yields.Since the origin of the epoxides' oxygen is in the water, this method is very suitable for introducing the isotopes 17O and 18O in various molecules.
- Rozen, Shlomo,Kol, Moshe
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p. 5155 - 5159
(2007/10/02)
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- The Correlation of Regiochemistry with Structure in the SRN1 Reaction of aci-Nitronates with p-Nitrobenzylic Substrates
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The rate and regiochemistry of the SRN1 reactions of aci-nitronates with p-nitrobenzylic substrates are profoundly affected by branching at the positions adjacent to the reaction sites (Cβ).Definitive rules which predict whether C-alkylation will occur in the association step involving p-nitrobenzylic radicals and aci-nitronate ions are formulated. β-Branching causes O-alkylation or reductive processes to increase.In some cases no recognizable product formation results.Benzylic alcohols, p-nitrophenyl alkyl ketones and/or their oximes, and p-nitrophenol are among the products which result from subsequent reactions of the O-alkylation products, aci-nitronic esters of benzylic alcohols.
- Norris, Robert K.,Randles, David
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p. 1621 - 1633
(2007/10/02)
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