Reactions of alkyl-lithium compounds with aryl halides
Reaction of methyl-lithium with tribromophenols and with other aryl polyhalides leads directly to bi- and tri-aryl products. para-Substituents (Y) of 2,6-dibromo-4-Y-phenols promote arylation of substrates by their own lithiation products as Y becomes more electron-withdrawing. An aryne epoxide is not an intermediate in these reactions. 2,4,6-Tribromo-Z-benzenes react to form coupling products when Z can co-ordinate to an introduced ortho-lithium atom or when Z can be directly lithiated. Dimeric aggregation of the organolithium intermediates occurs so as to favour coupling to halogenated substrates. Both polar (ionic) and free-radical pathways are involved.