Restricted stereochemistry of solvation of allylic lithium compounds: Structural and dynamic consequences

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(met hoxymethyl)-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-(dimethylethyls ilyl)allyl]lithium (24), [2-[ [bis(2-methoxyethyl)amino]methyl]-1-(tert-but yldimethyIsilyl)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 C-13, Li-7 spin co upling of similar to 8 Hz, a 1:1:1:1 C-13 NMR pattern indicating monomeric structures; all show ligand resonances to be magnetically nonequivalent and reveal C-1, C-3 C-13 NMR shifts of about 40 and 75 delta 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 trid entately 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 conve rting mainly to the latter by 180 K. NMR line shape analysis of the diaster eotopic gem methylsilyl C-13 resonances as well as that due to the ligand c arbons shows that all these line shape changes are due to the dynamics of i nversion at lithium-bound carbon and that other ligand reorientation proces ses are slower than carbanide inversion: for inversion, Delta H double dagg er is found to be 6-9 kcal.mol(-1) respectively. Averaging with increasing temperature of the C-13, Li-7 spin coupling in 24 provides the dynamics of bimolecular carbon lithium bond exchange with Delta H double dagger of 12 k cal.mol(-1). Mechanisms an proposed on the basis of the data. We ascribe re stricted stereochemistry of ligand lithium coordination to be responsible f or these unusual internally coordinated structures.