On the structure of sulfur-stabilized allyllithium compounds in solution

A combination of density functional calculations (B3LYP/6-31+G(d) level of theory) and experimental investigations (NMR and cryoscopic measurements) l ead to structural assignments in solution for a series of three sulfur stab ilized allyllithium compounds 1-3. All three lithium species are monomers i n THF under the experimental conditions studied here and exist exclusively in an endo conformation. Increasing the oxidation state of sulfur (thiol -- > sulfoxide --> sulfone) causes a change in the solution state structure of the allyllithium compounds. In the case of 1-thiophenylallyllithium 1, a f ast equilibrium between two eta(1)-species is present with the equilibrium. favoring the eta(1) C-alpha-Li contact ion pair. The preference for this c onformation can be attributed to the charge stabilizing properties of the s ulfur substituent. For the lithiated sulfoxide 2, this equilibrium is froze n on the NMR time scale and two different lithium species (a eta(1) C-alpha -Li and a eta(1) C-gamma-Li contact ion pair), each possessing an intramole cular Li-O contact, coexist in d(8)-THF. In the case of the lithiated sulfo ne 3, several solvated conformations are in rapid equilibrium with each oth er on the NMR time scale in solution. The presence of two chelating oxygen atoms allows the lithium to form a OLiO scissor-like contact ion pair that competes with the eta(1) C-alpha-Li and the eta(1) C-gamma-Li contact ion p airs also calculated for compound 3.