- STEREOCHEMISTRY OF THE THERMAL ISOMERIZATIONS OF (2S,3R)-2-METHOXYMETHYL-2,3-DIDEUTERIO-1-(DIDEUTERIOMETHYLENE)CYCLOPROPANE
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The (2S,3R) isomer of 2-methoxymethyl-2,3-dideuterio-1-(dideuteriomethylene)cyclopropane has been synthesized and heated at 198.8o: from the experimentally observed mol fractions of the eight isomers of 2,3,α,α,- and 2,3,3,α-tetradeuterio-2-met
- Baldwin, John E.,Chang, Glenn Eu Chung
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- Stereochemistry of transmetalation of alkylboranes in nickel-catalyzed alkyl-alkyl cross-coupling reactions
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Deuterium-labeled alkylborane reagents 2a and 2b were prepared and subjected to cross-coupling reactions in the presence of a nickel catalyst. NMR analysis of the products indicates that transmetalation from boron to nickel proceeds with retention of conf
- Taylor, Buck L. H.,Jarvo, Elizabeth R.
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- Additive-modulated switchable reaction pathway in the addition of alkynes with organosilanes catalyzed by supported Pd nanoparticles: Hydrosilylation: versus semihydrogenation
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We herein report supported Pd nanoparticles on N,O-doped hierarchical porous carbon as a single operation catalyst-enabled additive-modulated reaction pathway for alkynes addition with organosilanes between hydrosilyation and semihydrogenation. In the case of alkynes hydrosilylation, a simple iodide ion as an additive has a promotion effect on the activity and regio- and stereoselectivity, where iodide can coordinate with Pd NPs via strong δ donation to increase the electron density of the Pd atom, resulting in an increased ability for the oxidative addition of hydrosilane as the rate-determining step to make the reaction proceed efficiently to afford vinylsilanes in high yields with excellent regio- and stereoselectivity. For the catalytic transfer semihydrogenation of alkynes, water was introduced to mix with organosilane to form a silanol together with the generation of hydrogen atoms on the Pd NPs surface or the liberation of H2 gas as a reducing agent, whereby the quantitative reduction of alkynes was achieved with exclusive selectivity to alkenes. In both cases, the catalyst could be recycled several times without a significant loss in activity or selectivity. A broad range of alkyl and aryl alkynes with various functional groups are compatible with the reaction conditions. The role the additive exerted in each reaction was extensively investigated through control experiments as well as the kinetic isotopic effect along with spectroscopic characterization. In addition, the respective mechanism operating in both reactions was proposed.
- Duan, Yanan,Ji, Guijie,Zhang, Shaochun,Chen, Xiufang,Yang, Yong
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p. 1039 - 1050
(2018/03/05)
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- Efficient syntheses of four stable-isotope labeled (1R)-menthyl (1S,2S)-(+)-2-phenylcyclopropanecarboxylates
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Many carbenoid cyclopropanation reactions promoted by chiral catalysts give product mixtures reflecting impressive diastereo- and enantioselectivities. Few provide a single chiral product efficiently. This limitation has been overcome in cyclopropanations of styrene and isotopically labeled styrenes with α-diazoacetates. Convenient syntheses on a 20 g scale of each of four chiral isotopically labeled (1R)-menthyl (1S,2S)-2- phenylcyclopropanecarboxylates (the 1-d-3-13C, 1,(3S)-d2, 1,2,(3S)-d3, and 1,3,3-d3 isotopomers) of better than 99% ee have been realized. The Royal Society of Chemistry 2006.
- Keliher, Edmund J.,Burrell, Richard C.,Chobanian, Harry R.,Conkrite, Karina L.,Shukla, Rajesh,Baldwin, John E.
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p. 2777 - 2784
(2008/09/16)
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- Using secondary alpha deuterium kinetic isotope effects to determine the stereochemistry of an E2 reaction; the stereochemistry of the E2 reaction of 1-chloro-2-phenylethane with potassium tert-butoxide in tert-butyl alcohol
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Isotopic labelling studies have shown that the E2 reaction of 1-chloro-2-phenylethane with potassium tert-butoxide in tert-butyl alcohol occurs via an anti-periplanar stereochemistry. This demonstrates that the different secondary alpha deuterium kinetic isotope effects found for the high and low base concentrations and in the presence of 18-crown-6 ether are because of changes in transition state structure that occur when the form of the reacting base changes rather than to a change in the stereochemistry of the reaction.
- Smith,Crowe,Westaway
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p. 1145 - 1152
(2007/10/03)
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- Selective homogeneous palladium(0)-catalyzed hydrogenation of alkynes to (Z)-alkenes
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Zero-valent palladium precatalysts containing rigid bidentate bis(arylimino)acenaphthene ligands (shown schematically) facilitate the highly stereoselective homogeneous catalytic hydrogenation of alkynes to (Z)- alkenes. Internal, terminal, aryl-substituted, and cyclic alkynes are suitable substrates, as are some enynes, which are chemoselectively hydrogenated to dienes. E=CO2Me; R1, R2 = 4-OCH3, 4-CH3, 2,6-(CH3)2.
- Van Laren, Martijn W.,Elsevier, Cornelis J.
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p. 3715 - 3717
(2007/10/03)
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- Structural Characterisation in Solution of Intermediates in Rhodium-catalysed Hydroformylation and their Interconversion Pathways
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The reaction of HRh(CO)(PPh3)3 (1) with CO has been studied by 1H, 13C, and 31P n.m.r.The main species present under ambient conditions is HRh(CO)2(PPh3)2 (2) which exists as two rapidly equilibrating trigonal bipyramidal isomers.Complexes (1) and (2) are
- Brown, John M.,Kent, Alexander G.
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p. 1597 - 1608
(2007/10/02)
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- Mechanism of elimination rections. 40. Attempted study of stereochemistry of elimination from 2-(p-nitrophenyl)ethyltrimmethylammonium ion. Base-promoted cis-trans isomerization of p-nitrostyrene-β-d1
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Stereospecifically deuterated ArCHDCHDNMe3+I- and ArCHDCHDNMe2O have been prepared, where Ar=C6H5 and p-NO2C6H4.When Ar=C6H5, the elimination reaction of the quaternery salt with ethoxide in ethanol goes with >98percent anti stereoch
- Dohner, Brent R.,Saunders, William H., Jr.
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p. 1026 - 1030
(2007/10/02)
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- Complete Kinetic Analysis of the Thermal Stereomutations of (+)-(1S,2S,3R)-r-1-Cyano-t-2-methyl-1,2,t-3-trideuteriocyclopropane
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(+)-(1S,2S,3R)-r-1-Cyano-t-2-methyl-1,2,t-3-trideuteriocyclopropane has been synthesized in optically pure form.The kinetic of thermal stereomutations which interconvert this and seven other isomeric cyclopropanes have been followed: at 335.4 deg C in the gas phase, the title compound undergoes stereomutations with rate constants (x105 s) k1 = 0.55, k2 = 0.61, k3 = 0.10, k12 = 0.88, k13 = 0.083, and k23 = 0; the cis-1-cyano-2-methyl-1,2,3-trideuteriocyclopropanes exhibit stereomutation rate constants (x105 s) k'1 = 0.62, k'2 = 0.69, k'3 = 0.15, k'12 = 1.31, k'13 = 0.09, and k'23 = 0.While cyclopropanes 1,2-disubstituted with potent radical-stabilizing groups such as phenyl, cyano, and vinyl give stereomutation products by way of C(1)-C(2) bond cleavage only, the deuterium-labeled 1-cyano-2-methylcyclopropanes experience thermal stereomutations consistent with the intermediacy of two distinct trimethylene diradicals, one formed through cleavage of the C(1)-C(2) bond, the other by breaking C(1)-C(3).
- Baldwin, John E.,Carter, Charles G.
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p. 1362 - 1368
(2007/10/02)
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