Kinetics of Bromine-Magnesium Exchange Reactions in SubstitutedBromobenzenes

摘要

Competition experiments have been performed to determine the relative reactivities of substitutedbromobenzenes, bromonaphthalenes, and 9-bromoanthracene toward i-PrMgCl·LiCl in THF at 0℃.The rates of the bromine—magnesium exchange reactions are accelerated by electron-acceptor substituents,the activating efficiency of which increases in the order para < meta ortho. The activation free enthalpiesof the bromine—magnesium exchange reactions correlate fairly (r~2= 0.83) with the proton affinities ofanalogously substituted aryllithiums (slope 0.8). The kinetics of two representative bromoarenes withi-PrMgCl·LiCl were found to be first-order in both bromoarene and i-PrMgCl·LiCl. Combination of theresulting second-order rate constants with the k_(rel)values from competition experiments allowed us tocalculate reaction times for the bromine—magnesium exchange reactions of a large variety. of bromoarenes.