Mechanism of the deprotonation reaction of alkyl benzyl ethers with n-butyllithium

摘要

Kinetic study of the α-lithiation of benzyl methyl ether (BME) by nBuLi has revealed that increasing the concentration of the organolithium compound does not necessarily increase the reactivity, and this is a consequence of the reactivities of the different nBuLi aggregates present in solution. We propose a dimer-based mechanism, in which a pre-complexation step is a key process for substrates bearing a donor oxygen atom that can interact with the lithium cation to form mixed dimers. For these studies, we have developed a system based on UV/Vis spectroscopy that allows kinetic measurements to be conducted at -80°C under argon. Butyllithium aggregation: Dimer-tetramer self-aggregation of butyllithium in THF yields a rate constant for the deprotonation of alkyl benzyl ethers that is independent of the organolithium concentration. This can be attributed to mixed dimer formation (ether and THF as ligands; see scheme) in a pre-complexation step prior to the lithiation reaction.