Effects of Solvent Molecular Size on Rates of Topomerization in Organolithium Compounds.

Rates of topomerization of substituted 9-fluorenyllithiums and α-(benzylthio)benzyllithium were determined in various ether-type solvents by the dynamic NMR technique. The rates are small in solvents of bulky molecules in comparison with those of less-bulky molecules. UV-vis spectra of these compounds in the solvents indicate that the compounds tend to exist as solvent-separated ion pairs in solvents of small molecular size, whereas they exist as contact ion pairs in solvents of bulky molecules. 1H NMR spectra did not show signals attributable to two species at the lowest temperature of study. From these results, it is concluded that the observed process is the tumbling of the anion in solvent-separated ion pairs with a rapid pre-equilibrium between contact ion pairs and solvent-separated ion pairs, further intervention of solvent molecule(s) into the observed solvent-separated ion pairs being the rate-limiting step. A term, dimensiosolvatic effect, is proposed for the effects where the solvent molecular ...