Relative rates, relative activation parameters and substituent effects of lithium-metalloid exchange reactions

Abstract

The relative reactivity of heteroatomic compounds (PhM; M = SeR, SnR3, TeR, I) towards BuLi was estimated by competitive experiments. The rates of exchange reactions in THF increase in the order I > Te ≫ Sn ≫ Se with a ratio of 1000:300:15:1 at − 70 °C when R = Bu. Me3SnPh underwent exchange much faster than Bu3SnPh, suggesting that the exchange reaction was largely affected by steric hindrance. The evidence that Bu3SnPh and BuTePh react with BuLi at almost the same rate in the presence of HMPA and that the relative exchange rates of Li-Te/Li-Sn increase in less polar solvents such as Et2O and hexane indicates that coordination of solvents towards lithium plays an important role in exchange reactions. Relative activation parameters obtained from competitive reactions of PhTeBu and PhSnBu3 with BuLi showed that Li-Te exchange was favored over Li-Sn exchange by an enthalpy factor. Hammett treatment of Li-Te exchange of ArTeBu with BuLi in THF showed a substantially large positive reaction constant (ρ) of 3·4, indicating that the anion charge was largely distributed on the leaving group in the transition state.