Palladium-Catalysed Direct Polyarylation of Pyrrole Derivatives

The palladium-catalysed direct polyarylation of 1-methylpyrrole and 1-phenylpyrrole was studied. As the C2 and C5 positions of pyrroles are more reactive for C-H bond functionalisation than the C3 and C4 positions, the formation of 2,5-diarylpyrroles was found to proceed selectively in the presence of 3 equiv. of a variety of aryl bromides. The sequential C2 arylation followed by C5 arylation to prepare non-symmetrically substituted 2,5-diarylpyrroles was also studied. For the synthesis of such pyrroles, higher yields were obtained for the couplings with electron-deficient aryl bromides. The tetraarylation of 1-methylpyrrole was also achieved. From 4 equiv. of 3,5-bis(trifluoromethyl) bromobenzene, the 2,3,4,5-tetraarylated pyrrole was obtained in good yield.