An efficient regioselective bromination protocol of activated coumarins using 2,4,4,6-tetrabromo-2,5-cyclohexadienone

Abstract

2,4,4,6-Tetrabromo-2,5-cyclohexadienone mediated bromination of a wide assortment of activated coumarins in acetonitrile has been accomplished with high regioselectivity and good to excellent yields. The selectivity and efficiency of bromination were found to be markedly dependent on electronic factors, particularly degree of electron withdrawal from electron-releasing 7-oxygenated substituents to the C-3 position of the α-pyrone moiety by vinylogous resonance and the presence of nucleophilic additives such as water and tetra-n-butylammonium bromide in the reaction medium. The sole by-product of the reaction was converted back to the reagent by oxidation with KBr−KBrO3, which was recycled three times without significant loss of efficiency.