Generation of 3- and 5-Lithiothiophene-2-carboxylates via Metal−Halogen Exchange and Their Addition Reactions to Chalcogenoxanthones

Abstract

Deprotonation and lithium−bromine exchange in 5- or 3-bromothiophene-2-carboxylic acids with t-BuLi form the corresponding dianion, which reacts highly regioselectively in the presence of 0.25 equiv of tetramethyl-1,2-ethylenediamine with 3,6-bis(dimethylamino) chalcogenoxanthones to give S- and Se-containing rhodamines. Quenching studies with D₂O indicate that an extra equivalent of t-BuLi is not necessary in these reactions. Deprotonation is faster than metal-halogen exchange with the bromothiophene-2-carboxylic acids using t-BuLi.