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

摘要

Carbon-13 NMR line shape analysis of the lithium isotopomers of 2,4,6-tri-tert-butylphenyllithium monomer, 4, complexed to THF establishes that electric quadrupole induced relaxation of Li-7 is responsible for partial averaging of (1)J(C-13,Li-7). Th, dynamics of intramolecular carbon-lithium bond exchange between monomers of 4 in THF solutions have been monitored by NMR line shape analysis, signal averaging of (1)J(C-13,Li-6) observed above 240 K, yielding activation parameters of Delta 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, monomer tridentately complexed to N,N',N',N',N''-pentamethyldiethylenetriamine, PMDTA, are magnetically non-equivalent at 184 K, supporting an asymmetric structure in which lithium is chiral. With increasing temperature above 184 K the shifts between the ortho ring carbons, the ortho methyls; and two doublets due to methylenes progressively average to single lines at their respective centers, the result of increasingly fast rotation around the carbon-lithium bond, with resulting activation parameters of Delta H double dagger and Delta S double dagger of respectively 5 kcal/mol and -21 eu. It is proposed that rotation is chemically driven, the result of Li, N reversible dissociation accompanied by an uptake by Li of one THF molecule. [References: 38]