Development of Titania-supported Iridium Catalysts for the Acceptor-less Dehydrogenative Synthesis of Benzoxazoles

Iridium catalysts supported on anatase with high surface area showed excellent activities for the acceptor-less dehydrogenation synthesis of benzoxazoles from 2-aminophenol and primary alcohols. The catalytic activity greatly depended on the titania supports, iridium precursors, and loading of iridium species. Catalysts supported on anatase JRC (Japan Reference Catalyst)-TIO-10 showed the highest activity for the dehydrogenative reaction. Preparation of catalysts using [Ir(cod)Cl] 2 (cod ＝ 1,5-cyclooctadiene) as an iridium precursor resulted in higher activity than using Ir(acac) 3 (acac ＝ acetylacetonate). Various primary alcohols were reacted to give corresponding benzoxazoles in high to moderate yields. The catalyst could be recycled without significant loss of activity, and no leaching of iridium species occurred into the solution. The hot filtration test strongly suggested that the catalysis occurs on the catalyst surface. Highly dispersed iridium species of less than 2 nm in diameter, which could be reduced at low temperature, were responsible for the excellent activity.