Chiral Organolithium Complexes: The Structure of β-Lithiated β-Phenylcarboxamides and the Mechanism of Asymmetric Substitution in the Presence of (−)-Sparteine

Abstract

Investigation of the structure and reactivity of β-lithiated β-phenylcarboxamides is reported. NMR spectroscopic investigations of β-lithiated amides 15 and 24 establish that the benzylic lithium is complexed by the nitrogen of the amide. The β-lithiated N-methyl amide 15 forms a complex with (−)-sparteine (4) which undergoes electrophilic substitution to provide enantioenriched products. The β-lithiated N-isopropyl amide 24 does not form a complex with 4, so there is no asymmetric induction. The enantioselectivity of the reaction is shown to arise by the pathway of asymmetric substitution in which asymmetry is induced in a postdeprotonation step. Asymmetric induction in this lithiation−substitution sequence is suggested to occur via a dynamic thermodynamic resolution.