SELECTIVITIES IN REACTIONS OF ORGANOLITHIUM REAGENTS WITH ARYL BROMIDES WHICH BEAR PROTON-DONATING GROUPS

Studies of substrates which offer an acidic hydrogen and an aryl bromide for reaction with an organolithium reagent have been carried out with a series of benzene bromo amides and bromo anilides as well as selected benzene bromo carboxylic acids, bromoanilines, and bromobenzylamines. A representative example is the reaction of N-ethyl-N-deutero-o-bromobenzamide (6-d) with 1-lithio-3-phenylpropane to give N-ethyl-o-deuterobenzamide (46%, 94%-d) (7-d), N-ethyl-o-bromobenzamide (6)(49%), 3-deutero-1-phenylpropane(51%, 92%-d), and 1-bromo-3-phenylpropane (48%). Product formation in this and related cases is explained by the operation of a two step sequence in which an initial deprotonation is followed by a bromine-lithium exchange which is accelerated with respect to mixing. Such a sequence is consistent with the results of deuterium labeling and with changes in product ratios on different mixing and with differently aggregated organolithium reagents. Support is provided for the operation of two pathways for the expedited bromine-lithium exchange reactions. In one pathway a high local concentration of the organolithium reagent promotes rapid reaction and in the second the exchange reaction occurs within an initially formed complex. The selectivity for removal of a bromine ortho to a lithiated carboxamide is found to be 5-8 with n-butyllithium, and satisfactory synthetic ortho selectivity is obtained for N-ethyl-2,5-dibromobenzamide with phenyllithium.