Di- and triarylsubstituted pyrroles by sequential regioselective cross-coupling reactions

The di- and tribrominated pyrroles, such as methyl 3,4,5-tribromo- pyrrole-2-carboxylate (1), ethyl 3,4,5-tribromopyrrole-2-carboxylate (2), methyl 4,5-dibromopyrrole-2-carboxylate (3), and 4,5-dibromo-2-nitropyrrole (4), were prepared and evaluated for their use in successive Suzuki cross-coupling reactions. It was shown that monosubstitution at the 5-position is feasible with a variety of boronic acids 5 using Pd₂(dba)₃ (dba = dibenzylideneacetone) and tri(2-furyl)- phosphane as the catalyst in a solvent system of an arene (mesitylene or toluene), ethanol and water (5/1/1). Starting from 2 and 4 the corresponding 5-substituted products 8 (8 examples, 33-65% yield) and 17 (9 examples, 34-86% yield) were obtained. Further Suzuki cross-coupling reactions at the remaining di- or monobromo-substituted positions were feasible as exemplified by the synthesis of the corresponding triarylpyrroles 9, 16 and diarylpyrroles 18-20.