THE CARBON-LITHIUM BOND IN MONOMERIC ARYLLITHIUMS - DYNAMICS OF EXCHANGE, RELAXATION, AND ROTATION

Carbon-13 NMR line shape analysis of the lithium isotopomers of 2,4,6- tri-tert-butylphenyllithium monomer, 4, complexed to THF establishes t hat electric quadrupole induced relaxation of Li-7 is responsible for partial averaging of (1)J(C-13,Li-7). Th, dynamics of intramolecular c arbon-lithium bond exchange between monomers of 4 in THF solutions hav e been monitored by NMR line shape analysis, signal averaging of (1)J( C-13,Li-6) observed above 240 K, yielding activation parameters of Del ta H double dagger and Delta S double dagger of respectively 14.4 kcal /mol and 7 eu. All except the meta carbons of mesityllithium, 5, monom er tridentately complexed to N,N',N',N',N''-pentamethyldiethylenetriam ine, PMDTA, are magnetically non-equivalent at 184 K, supporting an as ymmetric structure in which lithium is chiral. With increasing tempera ture above 184 K the shifts between the ortho ring carbons, the ortho methyls; and two doublets due to methylenes progressively average to s ingle lines at their respective centers, the result of increasingly fa st rotation around the carbon-lithium bond, with resulting activation parameters of Delta H double dagger and Delta S double dagger of respe ctively 5 kcal/mol and -21 eu. It is proposed that rotation is chemica lly driven, the result of Li, N reversible dissociation accompanied by an uptake by Li of one THF molecule.