Enantiotopos-differentiating (-)-sparteine-mediated gamma-deprotonation of 1-alkenyl carbamates: DFT calculations verify the observed stereoselectivity.
The enantioselective lithiation/gamma-deprotonation of three 1-alkenyl N,N-diisopropylcarbamates 6 by alkyllithium/(-)-sparteine has been modeled by the DFT method B3LYP/6-31G(d). The results of these calculations predict the preferential removal of the gamma-pro-R proton from the precomplexes 7 to give the (S)-lithio derivates 8. The calculations also indicate the necessity of an anion-stabilizing substituent (Ph, -CC-Ph) in the alpha-position of the substrate. These data are in excellent concordance with the experimental results. It is demonstrated that the formation and the properties of lithium carbanions are predictable with high accuracy by standard quantum chemical calculations if these species are monomeric and rigidified to some extent by chelation.
Organisch-Chemisches Institut der Westfälischen Wilhelms-Universität Münster, D-48149 Münster, Germany. wurthwe@uni-muenster.de
Ernst-Ulrich
EU
Würthwein
Dieter
D
Hoppe