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

Article abstract of DOI:10.1021/ja00146a018

Benzyllithium compounds, hitherto assumed from NMR data to consist of ion pairs, have been found to exhibit spin coupling between ¹³C and directly bound ⁶Li 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-¹³C-⁶Li (enriched at Cα) at low temperature. The low values of ¹J(¹³C-⁶Li), 3-4 Hz, imply a small detectable degree of C-Li covalence with the arrangement around C_α 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 ¹J(¹³C-⁶Li) 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_α bond, and bimolecular carbon-lithium bond exchange, listed in order of widely different increasing rates, the activation parameters being ΔH? (kcal/mol) and ΔS? (eu) in the same order: 14 and 6.6; 6.4 and -14; 10.8 and -21.