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Alkyllithium Compound

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Reinhard W. Hoffmann – One of the best experts on this subject based on the ideXlab platform.

Ralf Koberstein – One of the best experts on this subject based on the ideXlab platform.

Jürgen Demeter – One of the best experts on this subject based on the ideXlab platform.

Andreas Wenzel – One of the best experts on this subject based on the ideXlab platform.

  • Chiral organometallic reagents. Part XVII. Formation of diastereoisomeric complexes between α-phenylselanylAlkyllithium Compounds and chiral diamines
    J. Chem. Soc. Perkin Trans. 2, 1995
    Co-Authors: Reinhard W. Hoffmann, Wolfgang Klute, Ruprecht K. Dress, Andreas Wenzel
    Abstract:

    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.

  • Chiral Organometallic Reagents, XVI. Enantiomerization of α‐Thio‐, α‐Seleno‐, and α‐Telluro‐Substituted Alkyllithium Compounds; Kinetic and Mechanistic Studies
    Chemische Berichte, 1995
    Co-Authors: Reinhard W. Hoffmann, Ruprecht K. Dress, Thomas Ruhland, Andreas Wenzel
    Abstract:

    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.