The Experts below are selected from a list of 39 Experts worldwide ranked by ideXlab platform
Reinhard W. Hoffmann - One of the best experts on this subject based on the ideXlab platform.
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THE STEREOCHEMISTRY OF A RETRO-CARBOLITHIATION REACTION
Chemical Communications, 1999Co-Authors: Reinhard W. Hoffmann, Ralf KobersteinAbstract:Ring-opening of the cyclopropylmethyl lithium Compound 7 to give the α-duryl thio-substituted Alkyllithium Compound 8 proceeds in a stereochemically defined manner at the lithium-bearing stereocentre.
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Direct observation of an internally π-complexed alkenyllithium Compound in THF solution
J. Chem. Soc. Perkin Trans. 2, 1995Co-Authors: Thomas Rölle, Reinhard W. HoffmannAbstract:NMR studies have shown that the hexenyllithium Compound 4 exists at –80 °C in THF solution as two distinct species: the ‘normal’ THF solvated Alkyllithium Compound 4a and the internally π-complexed species 4b in a ratio of 5.5:1.
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Chiral organometallic reagents. Part XVII. Formation of diastereoisomeric complexes between α-phenylselanylAlkyllithium Compounds and chiral diamines
J. Chem. Soc. Perkin Trans. 2, 1995Co-Authors: Reinhard W. Hoffmann, Wolfgang Klute, Ruprecht K. Dress, Andreas WenzelAbstract:The racemic α-phenylselanylAlkyllithium Compound 6 is monomeric in diethyl ether and forms diastereoisomeric complexes with a variety of chiral diamines. Diastereoisomer ratios were determined from 77Se NMR spectroscopy to lie around 60:40 for most examples, but reached 90:10 with N,N,N′,N′-tetramethylcyclopentane-1, 2-diamine (22). The complexation constants for the formation of the diastereoisomeric complexes 24a and 24b formed from 6 with the latter ligand were estimated by NMR titration to be > 800 dm3 mol–1 and > 90 dm3 mol–1. The diastereoisomeric complexes 24 epimerize at the lithium bearing stereocentre with a barrier of ΔG‡= 12.1 ± 0.3 kcal mol–1 at –4 °C. As this epimerization process is not slower than the racemization of the uncomplexed Alkyllithium Compound 6, the complexes 24 equilibrate directly and do not have to dissociate into 6 in order to equilibrate.
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Chiral Organometallic Reagents, XVI. Enantiomerization of α‐Thio‐, α‐Seleno‐, and α‐Telluro‐Substituted Alkyllithium Compounds; Kinetic and Mechanistic Studies
Chemische Berichte, 1995Co-Authors: Reinhard W. Hoffmann, Ruprecht K. Dress, Thomas Ruhland, Andreas WenzelAbstract:The rate of enantiomerization of the racemic α-phenylselenoAlkyllithium Compound 6 has been determined by dynamic NMR spectroscopy in [D8]THF. The enantiomerization rate was found to be first order with respect to monomeric 6 and to show no conspicuous solvent dependence (diethyl ether; toluene + 1 eq. of THF) or change upon addition of LiClO4. The marked steric effects on the enantiomerization rate found with the α-duryl- and α-mesityl-selenoAlkyllithium Compounds 7c and 7d suggest that rotation about the carbanion-selenium bond may be the rate-determining step in those sterically hindered systems. Similar steric effects were detected for the enantiomerization of the corresponding α-arylthio- and α-aryltelluroAlkyllithium Compounds 7j and 7f, but are absent with the α-arylsilyl-substituted Alkyllithium Compound 7o. This finding, along with the fact that the phenyltelluro- (7e), phenylseleno- (6), and phenylthio-Alkyllithium Compounds (7g) have essentially the same enantiomerization barrier, lead us to propose that in these cases a reorganization within the contact ion pair is the rate limiting step for the enantiomerization.
Ralf Koberstein - One of the best experts on this subject based on the ideXlab platform.
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THE STEREOCHEMISTRY OF A RETRO-CARBOLITHIATION REACTION
Chemical Communications, 1999Co-Authors: Reinhard W. Hoffmann, Ralf KobersteinAbstract:Ring-opening of the cyclopropylmethyl lithium Compound 7 to give the α-duryl thio-substituted Alkyllithium Compound 8 proceeds in a stereochemically defined manner at the lithium-bearing stereocentre.
Jürgen Demeter - One of the best experts on this subject based on the ideXlab platform.
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Initiation of Styrene Retarded Anionic Polymerization Using the Combination of Lithium Alkoxides with Organometallic Compounds
Macromolecules, 2002Co-Authors: Stephane Menoret, Michel Fontanille, Alain Deffieux, Philippe Desbois, Jürgen DemeterAbstract:The possibility to initiate styrene retarded anionic polymerization in hydrocarbon solvents from bimetallic systems based on lithium alkoxide and metal−alkyl derivatives like trialkylaluminum or dialkylmagnesium was investigated. The combination of the two organometallic Compounds, individually inactive, affords an active initiating system which can be an alternative to Alkyllithium initiators. Activation involves a ligand exchange inside the bimetallic complex, thus in situ forming an Alkyllithium Compound complexed with a new alkoxymetal−alkyl derivative. The kinetics and extent of the ligand exchange process are highly dependent on the nature of both the ligand and the metal, magnesium alkyls being much more efficient than aluminum ones. The influence of the nature of both the lithium alkoxide and the metal−alkyl derivatives on the initiation efficiency and retardation of styrene polymerization was studied.
Andreas Wenzel - One of the best experts on this subject based on the ideXlab platform.
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Chiral organometallic reagents. Part XVII. Formation of diastereoisomeric complexes between α-phenylselanylAlkyllithium Compounds and chiral diamines
J. Chem. Soc. Perkin Trans. 2, 1995Co-Authors: Reinhard W. Hoffmann, Wolfgang Klute, Ruprecht K. Dress, Andreas WenzelAbstract:The racemic α-phenylselanylAlkyllithium Compound 6 is monomeric in diethyl ether and forms diastereoisomeric complexes with a variety of chiral diamines. Diastereoisomer ratios were determined from 77Se NMR spectroscopy to lie around 60:40 for most examples, but reached 90:10 with N,N,N′,N′-tetramethylcyclopentane-1, 2-diamine (22). The complexation constants for the formation of the diastereoisomeric complexes 24a and 24b formed from 6 with the latter ligand were estimated by NMR titration to be > 800 dm3 mol–1 and > 90 dm3 mol–1. The diastereoisomeric complexes 24 epimerize at the lithium bearing stereocentre with a barrier of ΔG‡= 12.1 ± 0.3 kcal mol–1 at –4 °C. As this epimerization process is not slower than the racemization of the uncomplexed Alkyllithium Compound 6, the complexes 24 equilibrate directly and do not have to dissociate into 6 in order to equilibrate.
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Chiral Organometallic Reagents, XVI. Enantiomerization of α‐Thio‐, α‐Seleno‐, and α‐Telluro‐Substituted Alkyllithium Compounds; Kinetic and Mechanistic Studies
Chemische Berichte, 1995Co-Authors: Reinhard W. Hoffmann, Ruprecht K. Dress, Thomas Ruhland, Andreas WenzelAbstract:The rate of enantiomerization of the racemic α-phenylselenoAlkyllithium Compound 6 has been determined by dynamic NMR spectroscopy in [D8]THF. The enantiomerization rate was found to be first order with respect to monomeric 6 and to show no conspicuous solvent dependence (diethyl ether; toluene + 1 eq. of THF) or change upon addition of LiClO4. The marked steric effects on the enantiomerization rate found with the α-duryl- and α-mesityl-selenoAlkyllithium Compounds 7c and 7d suggest that rotation about the carbanion-selenium bond may be the rate-determining step in those sterically hindered systems. Similar steric effects were detected for the enantiomerization of the corresponding α-arylthio- and α-aryltelluroAlkyllithium Compounds 7j and 7f, but are absent with the α-arylsilyl-substituted Alkyllithium Compound 7o. This finding, along with the fact that the phenyltelluro- (7e), phenylseleno- (6), and phenylthio-Alkyllithium Compounds (7g) have essentially the same enantiomerization barrier, lead us to propose that in these cases a reorganization within the contact ion pair is the rate limiting step for the enantiomerization.
J. David Smith - One of the best experts on this subject based on the ideXlab platform.
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An Alkyllithium Compound with a Free Planar Carbanion. The Crystal Structure of [Li(THF)4][C(SiMe2C6H4Me-o)3]
Organometallics, 1997Co-Authors: Abdulrahman I. Almansour, Colin Eaborn, Simon A. Hawkes, Peter B. Hitchcock And, J. David SmithAbstract:The title Compound, 1, has been made by an improved method involving treatment of (o-MeC6H4Me2Si)3CH with LiBu in the presence of tetramethylethylenediamine in a hexane−heptane mixture at room temp...