DI- AND TRIARYLSUBSTITUTED PYRROLES BY SEQUENTIAL REGIOSELECTIVE CROSS-COUELING REACTIONS

摘要

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.