Effect of solvent on the lithium-bromine exchange of aryl bromides:: Reactions of n-butyllithium and tert-butyllithium with 1-bromo-4-tert-butylbenzene at 0 °C

The outcome of reactions of 1-bromo-4-tert-butylbenzene (1). a representative aryl bromide, with n-BuLi or t-BuLi at 0 degrees C in a variety of solvent systems has been investigated. The products of reactions of 1 with n-BuLi vary significantly with changes in solvent composition: I does not react with it-BuLi in pure heptane; the exchange reaction to give (4-tert-butylphenyl)lithium, which is slow in pure diethyl ether, is virtually quantitative in heptane containing a small quantity of THE and the reaction of I with n-BuLi in THF leads to considerable coupling. Lithium-bromine exchange is the virtually exclusive outcome of reactions of I with t-BuLi in every solvent studied except pure heptane: the presence of a small quantity of any of a variety of structurally diverse ethers (Et2O, THF, THP, MTBE) in the predominantly hydrocarbon medium affords (4-tert-butylphenyl)lithium, assayed as tert-butylbenzene, in yields exceeding 97%. The only side products observed from reactions of I with t-BuLi are small amounts of benzyne-derived hydrocarbons.