Titania-supported iridium catalysts for dehydrogenative synthesis of benzimidazoles

In this review, development of supported catalysts for the dehydrogenative synthesis of benzimidazoles from primary alcohols and 1,2-phenylenediamine derivatives is briefly summarized. Among them, titania-supported iridium catalysts showed excellent activities under mild reaction conditions. Remarkably, the low-temperature activity of iridium catalyst was significantly affected by titania supports, and the reaction of 1,2-phenylenediamine and benzyl alcohol in the presence of rutilesupported catalysts proceeded smoothly at 100℃ to give 2-phenylbenzimidazole in high yields of up to 88%. On the other hand, catalysts supported on anatase generally showed poor activity at 100℃. A significant relationship between CO uptake and the activity of titania-supported catalysts has been reported, indicating that well-reduced iridium species on rutile would be responsible for the predominant catalytic activity. The present results suggest the importance of the selection of suitable titania supports for the iridium catalysts.