The effect of polyether ligands on the solution structure of [ 6Li]- α-(phenylthio)benzyllithium in tetrahydrofuran: A 1H, 6Li-HOESY NMR study

Abstract

[ 6Li]- α-(phenylthio)benzyllithium 1- 6Li was studied in THF/[D 8]THF solution (1:1) in the presence of several acyclic and cyclic polyether ligands by 1H, 6Li-HOESY, 1H and 13C NMR spectroscopy. The question whether these ligands are bonded to lithium or not is important for physical–organic investigations as well as for studies of the ground state of (stereoselective) reactions of organolithium compounds in the presence of such ligands. Dimethoxyethane is not bonded to lithium under these conditions. The acyclic ethers diglyme and triglyme coordinate only weakly to the organolithium compound and form contact ion pairs (CIPs) at 25°C. At −80°C, CIPs are in equilibrium with separated ion pairs (SIPs). Very stable complexes of 1- 6Li are obtained with crown ether ligands. Addition of 12-crown-4 and 15-crown-5, respectively, results in the exclusive formation of SIPs at 25°C and −80°C. With 18-crown-6, a CIP–SIP equilibrium is observed at 25°C which is shifted entirely to the SIP side at −80°C. Graphical analyses of the 1H and 13C NMR spectra of the polyether complexes of 1- 6Li revealed correlations between 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) of 1-Li, which are useful probes for the charge distribution within the carbanionic moiety of 1- 6Li in the respective complexes, and thus for the ion pair character as a function of the polyether complexation of lithium.