Benzylic Lithium Compounds: The Missing Link in Carbon-Lithium Covalency. Dynamics of Ion Reorientation, Rotation around the Ring-Benzyl Bond, and Bimolecular C-Li Exchange

Benzyllithium compounds, hitherto assumed from NMR data to consist of ion pairs, have been found to exhibit spin coupling between C-13 and directly bound Li-6 under conditions wherein bimolecular carbon-lithium bond exchange is too slow to average the coupling constants. These conditions involved the use of species in which lithium is internally solvated or of dilute solutions (0.005 M) of benzyllithium-C-13-Li-6 (enriched at C-alpha) at low temperature. The low values of (1)J(C-13-Li-6), 3-4 Hz, imply a small detectable degree of C-Li covalence with the arrangement around C-alpha distorted from coplanarity. The C-Li bonds in benzyllithium are concluded to lie in a continuum of C-Li covalency between the many monomeric species in which (1)J(C-13-Li-6) is 16 +/- 1 Hz and separated ion pairs. NMR line shape analysis of data for internally solvated benzyllithium 2b provides quantitative insight into the dynamics of intramolecular reorientation of coordinated lithium with respect to the benzyl plane, rotation around the ring-C-alpha bond, and bimolecular carbon-lithium bond exchange, listed in order of widely different increasing rates, the activation parameters being Delta H-double dagger (kcal/mol) and Delta S-double dagger (eu) in the same order: 14 and 6.6; 6.4 and -14; 10.8 and -21.