THE LITHIATION OF FLUORINATED BENZENES AND ITS DEPENDENCE ON SOLVENT AND TEMPERATURE

The formation of lithio derivatives from fluorinated benzenes and fluo rinated bromobenzenes has been studied. The bases used were lithium di isopropylamide (LDA) in THF-hexane, butyllithium in diethyl ether-hexa ne and butyllithium in THF-hexane. The lithiated intermediates have be en trapped using acetone, to form fluorinated 2-arylpropan-2-ols, or h ave been warmed to produce benzynes which have been trapped by furan i n Diels-Alder additions. The use of LDA allows clean removal of the mo st acidic proton in the aromatic ring: butyllithium in ether-hexane br ings about clean bromine-lithium exchange. In contrast, the use of but yllithium in THF-hexane at - 78 degrees C results in autometallation a nd the formation of more complex product mixtures, For the formation o f the benzynes involved in the Diels-Alder reactions it is necessary t o warm the reaction mixtures. When ether-hexane is used as solvent the reactions remain fairly clean, but when THF is added the increasing a utometallation again results in more complex reaction mixtures.