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  • 1
    Electronic Resource
    Electronic Resource
    Chiral Organometallic Reagents, XVI. Enantiomerization of α-Thio-, α-Seleno-, and α-Telluro-Substituted Alkyllithium Compounds; Kinetic and Mechanistic Studies (1995)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 128 (1995), S. 861-870 
    Details
    ISSN: 0009-2940
    Keywords: Enantiomerization ; Organolithium compounds ; Dynamic NMR spectroscopy ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 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.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cber.19951280903
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  • 2
    Electronic Resource
    Electronic Resource
    Chiral Organometallic Reagents, XIV. Theoretical Investigations into the Mechanism of Inversion of Configuration at the Carbon Center of α-Sulfur-Substituted Carbanions (1995)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 128 (1995), S. 673-677 
    Details
    ISSN: 0009-2940
    Keywords: Racemization barrier ; Sulfur-substituted carbanions ; Calculations, ab initio ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio calculations showed that the two elementary steps in the racemization of the α-methylthioethyl anion (7), inversion of the pyramidal anionic center, and rotation around the C-—S bond, occur in sequence and are not concerted. The former process passes over a barrier of 1.1 kcal/mol, the latter over one of 10.9 kcal/mol. MNDO calculations on the α-phenylthioethyl anion (8) revealed a preferred conformation of the S-aryl bond, allowing maximum delocalization of the sulfur lone pairs into the π* orbital of the aryl group. This orientation is maintained during rotation around the C-—S bond and is the origin of steric hindrance in the racemization of certain α-arylthio-, α-arylseleno-, and α-aryltelluroalkyllithium compounds 1 to 6.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cber.19951280704
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    Paper (German National Licenses)
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