Selective Halogen-Lithium Exchange in 2,5-Dibromobenzenes and 2,5-Dibromopyridine

Abstract

Reaction of a series of 2,5-dibromo-substituted aromatic systems with 1 equiv of n-butyllithium at −100 °C results in high selectivity of halogen-metal exchange when the substituent contains unshared electrons. The results suggest that product distribution at −100 °C is determined by thermodynamic rather than kinetic factors. Fair to excellent yields of derivatives of the monolithium intermediates have been obtained. Reaction of 2,5-dibromopyridine with 1 equiv of n-BuLi gives exclusively 2-bromo-5-lithiopyridine, which was converted in high yield into 2-bromo-5-deuteriopyridine. Reactions of 2- and 3-lithiopyridine, including their exchange with 2- and 3-bromopyridine, are described. © 1977, American Chemical Society. All rights reserved.