Halogen–lithium exchange versus deprotonation: synthesis of diboronic acids derived from aryl–benzyl ethers

Lithiation of a series of aryl benzyl ethers containing halogen substituents (–F, –Br, –I) was investigated. The resultant mono- and diorganolithium intermediates were converted into the corresponding aldehydes or diboronic acids in good yields. The dilithiation of aryl benzyl ethers containing a reactive hydrogen atom and halogen atom capable of halogen–lithium exchange proceeds quantitatively in THF at −50 °C. It was found that mono aryllithiums formed in the reaction can remove the reactive hydrogen atom from a molecule of aryl benzyl ether thus decreasing the yield of dilithiated species. This effect does not occur when t-BuLi is used instead of n-BuLi.