Halogen–lithium exchange versus deprotonation: regioselective mono- and dilithiation of aryl benzyl sulfides. A simple approach to α,2-dilithiotoluene equivalents

Halogen–lithium exchange and deprotonation reactions between aryl benzyl sulfides and alkyllithiums were investigated. The resultant mono- and dilithiated intermediates were converted into the corresponding aldehydes and boronic, or carboxylic acids in good yields. It was found that diethyl ether stabilizes the ortho-lithiated compounds toward isomerisation to the benzylic derivatives. The process occurs easily in THF at low temperature and is a facile route to the α,2-dilithiotoluene derivative which can be transformed into a dicarboxylic acid on treatment with CO2.