Partially Delocalized Allylic Lithium Compounds: Dynamics of Inversion, 1,3 Li Shift, and C−Li Bond, Exchange Influence of the Stereochemistry of Solvation

Abstract

While exo-exo-[1,3-bis(trimethylsilyl)allyl]lithium (15) and [1-(trimethylsilyl)allyl]lithium (16) were previously shown to be contact ion pairs containing delocalized carbanions, the corresponding species with a pendant ligand at the 2-position, [2-[[bis(2-methoxyethyl)amino]methyl]-1,3-bis(trimethylsilyl)allyl]lithium (14) and [2-[[bis(2-methoxyethyl)amino]methyl]-1-(trimethylsilyl)allyl]lithium (12), respectively, appear from their 13C NMR shifts and the first observation of 13C lithium spin coupling in an allylic lithium to be partially delocalized with detectable C−Li covalence. In proposed structures 12 and 14, lithium is tridentately complexed. N and Li lie within the allyl carbon plane with the two oxygens normal to it on opposite sides. NMR line shape analysis and 13C1 of signal averaging of the 13C−6Li coupling of 12 provides dynamics of intermolecular C−Li bond exchange with ΔHe⧧ and ΔS⧧ of 11.6 kcal/mol and −11.5 eu, respectively. Inversion at the lithium-bound carbon of 12 averages nonequivalent ligand shifts. Line shape analysis gives ΔHi⧧ and ΔSi⧧ of 8 kcal/mol and −10 eu, respectively. Line shape changes observed for the methylsilyl (13C and 1H) resonances as well as of the terminal 13C's of 14 due to a 1,3 Li sigmatropic shift yield activation parameters ΔHs⧧ and ΔSs⧧ of 18 kcal/mol and +15 eu. These results show that electronic structure of nominally conjugated organolithium compounds can be significantly altered by changing the stereochemistry of solvation, by use of pendant ligands, producing structures previously described in other systems as transition states for allylic rotation.