Several 1-sila allylic lithium compounds have been prepared with potential ligands for Li substituted
at the 2-position. They are [2-[[cis-2,5-bis(methoxymethyl)-1-pyrrolidinyl]methyl]-1-(trimethylsilyl)allyl]lithium
(22), [2-[[cis-2,5-bis(methoxymethyl)-1-pyrrolidinyl]methyl]-1-(dimethylethylsilyl)allyl]lithium (23), [2-[[bis(2-methoxyethyl)amino]methyl]-1-(dimethylethylsilyl)allyl]lithium (24), [2-[[bis(2-methoxyethyl)amino]methyl]-1-(tert-butyldimethylsilyl)allyl]lithium (25), and [2-[2-[bis(2-methoxyethyl)amino]-1,1-dimethylethyl](ethyldimethylsilyl)]allyllithium (26). Using diethyl ether or THF solutions all these compounds exhibited one bond
13C, 7Li spin coupling of ∼8 Hz, a 1:1:1:1 13C NMR pattern indicating monomeric structures; all show ligand
resonances to be magnetically nonequivalent and reveal C1, C3
13C NMR shifts of about 40 and 75 δ which
lie between those for model delocalized 1 and localized species 2. These compounds are concluded to be
examples of the heretofore missing folded, internally tridentately coordinated partially delocalized structures
with small detectable C, Li covalence. The exception is 25 which, in diethyl ether, consists of a rapidly
interconverting equilibrium mixture of localized and delocalized more solvated forms, the former prevailing
at 300 K and progressively converting mainly to the latter by 180 K. NMR line shape analysis of the
diastereotopic gem methylsilyl 13C resonances as well as that due to the ligand carbons shows that all these
line shape changes are due to the dynamics of inversion at lithium-bound carbon and that other ligand
reorientation processes are slower than carbanide inversion; for inversion, ΔH
⧧ is found to be 6−9 kcal·mol-1,
respectively. Averaging with increasing temperature of the 13C, 7Li spin coupling in 24 provides the dynamics
of bimolecular carbon lithium bond exchange with ΔH
⧧ of 12 kcal·mol-1. Mechanisms are proposed on the
basis of the data. We ascribe restricted stereochemistry of ligand lithium coordination to be responsible for
these unusual internally coordinated structures.