Kinetics of Bromine-Magnesium Exchange Reactions in Heteroaryl Bromides
Org. Lett., 2009, 11, No. 15, 3502-3505 published on 14.07.2009
Bromine-magnesium exchange reactions of heteroaryl bromides provide an efficient entry to a wide variety of fiveand six-membered functionalized heteroaryl Grignard reagents, which can be used for the selective synthesis of polyfunctionalized heterocyclic compounds, such as thiazole, thiophene, furan, pyrrole, and pyridine derivatives.1,2 The resulting heteroarenes are ubiquitous in natural products3 and physiologically active compounds.4 It has been shown that the mixed magnesium-lithium complex i-PrMgCl·LiCl has a significantly increased reactivity in bromine-magnesium exchange reactions compared to i-PrMgCl or i-Pr2Mg.5 In extension of our recent investigations on the kinetics of bromine-magnesium exchange reactions in substituted bromobenzenes, 6 we have now studied bromine-magnesium exchange reactions of heteroaryl bromides in order to assist the targeted use of these reactions for the synthesis of polyfunctional heterocyclic compounds. The relative exchange rates were determined by treating mixtures of bromoheteroarenes and bromobenzenes or of two different bromoheteroarenes with a substoichiometric amount of i-PrMgCl·LiCl (typically 0.33 to 0.5 equiv) in THF at 0 °C. The resulting mixtures of aryl-magnesium reagents were then quenched either with iodine or with methanol to give the iodoarenes P1′/P2′ or the debrominated arenes P1/P2, respectively (Scheme 1).