Liqin Zhao

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.

A straightforward access to guaiazulene derivatives using palladium-catalysed sp2 or sp3 C–H bond functionalisation

A novel palladium-catalysed direct arylation of guaiazulenes with a variety of aryl bromides is reported. Both sp(2) and sp(3) C-H bonds have been functionalised, as the nature of the cation of the base was found to allow the control of the regioselectivity of the arylation giving rise to C2 or C3 arylated guaiazulenes and also to 4-benzylguaiazulenes.

Reactivity of bromofluorenes in palladium-catalysed direct arylation of heteroaromatics

The palladium-catalysed direct arylation using bromofluorenes and heteroaromatics as the coupling partners proceeded in moderate to high yields using only 0.1–0.5 mol% Pd(OAc)2 or 1 mol% PdCl(C3H5)(dppb) as the catalyst and KOAc as the base. A wide variety of heteroarenes have been successfully employed, allowing their properties to be easily tuned. From 2,7-dibromofluorene, successive arylations allow the introduction of two different heteroarenes at carbons C2 and C7.

Reactivity of C–H bonds of polychlorobenzenes for palladium-catalysed direct arylations with aryl bromides

The reactivity of polychlorobenzenes vs. polyfluorobenzenes for palladium-catalysed direct arylation was studied. The PdCl(C3H5)(dppb)/KOAc system was found to promote the direct arylation of some polychlorobenzenes with aryl bromides. However, the reactivity of polychlorobenzenes was found to be lower than that of polyfluorobenzenes. The best yields were obtained from the coupling of 1,2,4,5-tetrachlorobenzene or 1,3,5-trichlorobenzene with electron-deficient aryl bromides. The C3 arylation of 2,5-dichlorothiophene was also found to proceed nicely.