THE EFFECT OF (POLY)AMINE LIGANDS ON THE SOLUTION STRUCTURE OF [LI-6]-ALPHA-(PHENYLTHIO)BENZYLLITHIUM IN TETRAHYDROFURAN - A H-1,LI-6-HOESYNMR-STUDY

Are (poly)amine ligands, added to a tetrahydrofuran solution of an org anolithium compound, bonded to lithium, or not? This question is of re levance for physical organic studies as well as for the evaluation of the ground state of (stereoselective) reactions of organolithium speci es in the presence of such ligands. Therefore: we studied [Li-6]-alpha -(phenylthio)benzyllithium 1-Li-6 as a model compound in THF/[D-8]THF solution (1:1) in the presence of several acyclic and cyclic (poly)ami ne ligands by H-1,Li-6-HOESY and H-1 and C-13 NMR spectroscopy. (Poly) amine complexes of 1-Li-6 are obtained in most cases. Ligands with up to three N-atoms afford contact ion pairs (CIPs) while in complexes wi th tetradentate amines, CIPs are in temperature-dependent equilibrium with separated ion pairs (SIPs). Graphical analyses of the H-1 and C-1 3 NMR spectra of the (poly)amine complexes of 1-Li-6 revealed that the chemical shifts of the para phenyl carbon C-5, the para phenyl proton H-5, the benzylic carbon C-1, and the proton-carbon coupling constant J(C-1,H-1) are proportional to each other. These NMR spectroscopic pa rameters can be used as probes for the charge distribution within the carbanionic moieties of 1-Li-6 in the respective complexes, which is e specially useful for a classification to CIPs and/or SIPs. (C) 1998 El sevier Science S.A.