Partially Delocalized Allylic Lithium Compounds: Dynamics Of Inversion, 1,3 Li Shift, And C-Li Bond, Exchange Influence Of The Stereochemistry Of Solvation

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, (14) and [2[[bis-(2-methoxyethyl)amino]methyl]-1-(trimethylsilyl)ally]lithium (12), respectively, appear from their C-13 NMR shifts and the first observation of C-13 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 C-13(1) Of Signal averaging of the C-13-Li-6 coupling of 12 provides dynamics of intermolecular C-Li bond exchange with Delta H-e(double dagger) and Delta S-double dagger 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 Delta H-i(double dagger) and Delta S-i(double dagger) of 8 kcal/mol and -10 eu, respectively. Line shape changes observed for the methylsilyl (C-13 and H-1) resonances as well as of the terminal C-13's Of 14 due to a 1,3 Li sigmatropic shift yield activation parameters Delta H-s(double dagger) and Delta S-s(double dagger) 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.