RELATIVE RATES, RELATIVE ACTIVATION PARAMETERS AND SUBSTITUENT EFFECTS OF LITHIUM-METALLOID EXCHANGE-REACTIONS

The relative reactivity of heteroatomic compounds (PhM; M = SeR, SnR(3 ), TeR, I) towards BuLi was estimated by competitive experiments, The rates of exchange reactions in THF increase in the order I > Te much g reater than Sn much greater than Se with a ratio of 1000:300:15:1 at - 70 degrees C when R=Bu, Me(3)SnPh underwent exchange much faster than Bu(3)SnPh, suggesting that the exchange reaction was largely affected by steric hindrance, The evidence that Bu(3)SnPh and BuTePh react with BuLi at almost the same rate in the presence of HMPA and that the rel ative exchange rates of Li-Te/Li-Sn increase in less polar solvents su ch as Et(2)O and hexane indicates that coordination of solvents toward s lithium plays an important role in exchange reactions, Relative acti vation parameters obtained from competitive reactions of PhTeBu and Ph SnBu, with BuLi showed that Li-Te exchange was favored over Li-Sn exch ange by an enthalpy factor, Hammett treatment of Li-Te exchange of ArT eBu with BuLi in THF showed a substantially large positive reaction co nstant (rho) of 3 . 4, indicating that the anion charge was largely di stributed on the leaving group in the transition state.