Direct synthesis of 3-methylbenzofuran from phenol and acetone over halide cluster catalysts taking advantage of acidic and metallic properties

When a molecular tantalum halide cluster on silica gel, [(Ta6Cl12)Cl2(H2O)4]·4H2O (1)/SiO2, is treated in a helium stream in the temperature range 150–300 °C, catalytic activity for the condensation of phenol with acetone to yield 2-isopropenylphenol develops. Above 350 °C, subsequent cyclodehydrogenation proceeds to afford 3-methylbenzofuran with 60% selectivity. Higher reaction temperatures promote the subsequent cyclodehydrogenation. The condensation is caused by weak Brønsted acidity of a hydroxo ligand, which is formed by elimination of hydrogen chloride from the chloro and aqua ligands in 1. In contrast, the cyclodehydrogenation is catalyzed by a coordinatively unsaturated platinum-like metal atom developed in the cluster. Niobium, molybdenum, and tungsten clusters with the same metal framework also catalyze the reaction.