Regiocontrolled Synthesis of 1,2‐Diaryl‐1H‐imidazoles by Palladium‐ and Copper‐Mediated Direct Coupling of 1‐Aryl‐1H‐imidazoles with Aryl Halides under Ligandless Conditions

A large variety of 1,2‐diaryl‐1 H ‐imidazoles, including a selective COX‐2 inhibitor, have been regioselectively synthesised in moderate to high yields by direct coupling of 1‐aryl‐1 H ‐imidazoles with aryl iodides or bromides in DMF in the presence of CsF and catalytic amounts of Pd(OAc) 2 under ligandless conditions. A possible mechanism for this new highly regioselective C‐2 arylation reaction, involving the formation of an organocopper( I ) derivatives followed by a transmetallation reaction with an arylpalladium( II ) halide species and a reductive elimination, is proposed. New one‐step procedures for the synthesis of 1,2,5‐triaryl‐1 H ‐imidazoles, based on palladium‐ and copper‐mediated arylation of 1‐aryl‐1 H ‐imidazoles, have also been developed. Interestingly, some imidazole derivatives prepared in this study have been found to exhibit significant cytotoxic activity against some human tumour cell lines. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)