- Asymmetric hydrogenation of α,β-unsaturated nitriles with base-activated iridium N,P ligand complexes
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Although many chiral catalysts are known that allow highly enantioselective hydrogenation of a wide range of olefins, no suitable catalysts for the asymmetric hydrogenation of α,β-unsaturated nitriles have been reported so far. We have found that Ir N,P ligand complexes, which under normal conditions do not show any reactivity towards α,β-unsaturated nitriles, become highly active catalysts upon addition of N,N- diisopropylethylamine. The base-activated catalysts enable conjugate reduction of α,β-unsaturated nitriles with H2 at low catalyst loadings, affording the corresponding saturated nitriles with high conversion and excellent enantioselectivity. In contrast, alkenes lacking a conjugated cyano group do not react under these conditions, making it possible to selectively reduce the conjugated C=C bond of an α,β-unsaturated nitrile, while leaving other types of C=C bonds in the molecule intact.
- Mueller, Marc-Andre,Pfaltz, Andreas
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supporting information
p. 8668 - 8671
(2014/08/18)
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- Kinetics and mechanism of acid catalysed hydration of α- methylstyrenes
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Twelve para-substituted α-methylstyrenes with substituents H, CH 3, CF3, CH3O, CH3S, F, Cl, Br, CH3CO, CH3SO2, CN a NO2 were synthesised; additionally, the acid catalysed hydration kinetics of these compounds were measured in sulfuric acid in a concentration range c from 0.017 to 9.58 mol l-1, at 25.0°C. The observed rate constants obtained were used to construct the kinetic acidity function and calculate the catalytic rate constants. Based on the evaluation of the acidity function kinetic dependence on acid medium concentration, and the substituent effects of acid catalysed hydration of α-methylstyrenes on the catalytic rate constants, the mechanism of acid catalysed hydration was verified. The mechanism involves the addition of a proton to the double bond of α-methylstyrene in the rate-limiting reaction step denoted as A-SE2. No evident difference was found between the effects of the acid medium on the acid catalysed hydration of styrenes and α-methylstyrenes, which indicates very similar activity coefficients of the reactants, and of the transition state of both substrates. The substituent effects evaluation shows that the rate-limiting step of the reaction consists in the addition of a proton to the substrate. The carbocation formation in the transition state of this reaction step proceeds roughly half-way compared with the extent of the carbocation formation by cumyl chloride hydrolysis. The obtained carbocation is in particular stabilised by the substituents with +M effect, while the influence of the substituents with -M and I effects is significantly smaller.
- Pytela, Oldrich,Trlida, Bronislav
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p. 1025 - 1036
(2008/09/19)
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- Cobalt-catalyzed vinylation of functionalized aryl halides with vinyl acetates
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A new method for the preparation of styrene derivatives is described on the basis of the activation of aryl halides by low-valent cobalt species. A combination of CoII bromide and 2,2′-bipyridine is suitable as catalyst for the cross-coupling reaction of a wide range of aromatic halides (X = Cl, Br, I), mostly bearing sensitive moieties, with vinyl acetates. These reactions proceed under mild conditions in the presence of the appropriate reducing agent to afford α-substituted styrene compounds in satisfactory to high yields. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005.
- Amatore, Muriel,Gosmini, Corinne,Perichon, Jacques
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p. 989 - 992
(2007/10/03)
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- Suzuki cross-coupling reactions between alkenylboronic acids and aryl bromides catalysed by a tetraphosphane-palladium catalyst
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A range of alkenylboronic acids undergo Suzuki cross-coupling with aryl bromides in good yields in the presence of [PdCl(C3H 5)]2/cis,cis,cis-1,2,3,4-tetrakis[(diphenylphosphanyl) methyl]cyclopentane as a catalyst. A wide variety of 1-arylprop-1-enes, 2-arylprop-1-enes, 2-arylbut-1-enes and 1,1-diarylethylene or styrene derivatives have been prepared. Moreover, the reaction tolerates several functions, such as acetyl, formyl, nitrile or nitro. Furthermore, this catalyst can be used at low loading, even for reactions of sterically hindered substrates. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.
- Peyroux, Eugenie,Berthiol, Florian,Doucet, Henri,Santelli, Maurice
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p. 1075 - 1082
(2007/10/03)
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- Suzuki cross-coupling reactions of potassium alkenyltrifluoroborates
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(formula presented) The palladium-catalyzed coupling reaction of potassium alkenyltrifluoroborates with aryl or alkenyl halides or triflates proceeds readily with good yields. The trifluoroborates are air-and moisture-stable solids that can be stored indefinitely. The cross-coupling can be effected using PdCl2(dppf)-CH2Cl2 as the catalyst in n-PrOH in the presence of Et3N. A variety of functional groups are tolerated. ? Dedicated to Professor Herbert C. Brown on the occasion of his 90th birthday.
- Molander, Gary A.,Rivero, Marta Rodriguez
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p. 107 - 109
(2007/10/03)
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- 1-methyl-1-vinyl- and 1-methyl-1-(prop-2-enyl)silacyclobutane: Reagents for palladium-catalyzed cross-coupling reactions of aryl halides
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1-Methyl-1-vinylsilacyclobutane (1) and 1-methyl-1(prop-2- enyl)silacyclobutane (2) undergo rapid and high yielding cross-coupling with aromatic halides. Many different substituents and patterns on the aromatic moiety are tolerated. All reactions can be r
- Denmark, Scott E.,Wang, Zhigang
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p. 999 - 1003
(2007/10/03)
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- SINGLE-PARAMETER VERSUS DUAL-PARAMETER CORRELATION FOR RADICAL REACTIONS. ADDITION OF BROMINE ATOMS TO α-METHYLSTYRENES
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A rigorous procedure was developed for measuring the relative rates of addition of bromine atoms to eleven substituted α-methylstyrenes (1-Y, with Y = CF3, NO2, F, CN, Cl, Br, CO2Me, Me, COMe, OMe and SMe).The reaction was run in tetrahydrofuran in the presence of HBr, O2 and dibutyl peroxyoxalate at 30 deg C.All products were derived from the YC6H4CMeCH2Br adduct radicals, which were immediately intercepted by O2.Correlation analysis of all the data confirmed the proposition that in the absence of measurable steric effects, the relative rates for radical additions can be correlated only by a dual-parameter equation and not by a single parameter equation.Among various combiations of ?* and ξx, the (?JJ* + ?mb) combination yields the best correlation.
- Jiang, Xi-Kui,Liu, Wayne Wei-Zhong,Wu, Shi-Hui
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- 4-(1-HYDROPEROXY-1-METHYLETHYL)-1,3-CYCLOPENTADIENYL METHYL KETONE: ITS FORMATION FROM α-TERPINEOL AND BEHAVIOR AS A DIMETHYLFULVENE EPOXIDE.
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Ozonolysis of α-terpineol (1) then steam distillation in presence of acid gives the known 4-isopropylidenecyclopentenyl methyl ketone (4).This is oxidized in air to 4-(1-hydroperoxy-1-methylethyl)-1,3-cyclopentadienyl methyl ketone (10), a compound frequently reacting as if it were one of the elusive dimethylfulvene epoxides.It is converted by silica gel to two dimers (12, 13) of 2-acetyl-6,6-dimethylfulvene epoxide (19).Catalytic reduction of the dimers occurs mostly by exo addition of hydrogen to the conjugated double bond, and thermolysis of the dimers yields 4-acetyl-6,6-dimethylcyclohexa-2,4-dienone (20).With triphenylphosphine the hydroperoxide (10) yields two dimers of 2-acetyl-6,6-dimethylfulvene (26).This is the first reported isolation of dimers of a fulvene.The hydroperoxide (10) adds diazomethane to give an unstable pyrazoline (28); this pyrazoline loses nitrogen to yield a single isomer o' 5-acetyl-3',3'-dimethylbicyclohex-3-ene-2-spiro-2'-oxirane (29).Catalytic hydrogenation of the latter involves ring opening of the epoxide.
- Thomas, Alan F.,Perret, Celia
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p. 3311 - 3322
(2007/10/02)
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