Development of titania-supported iridium catalysts with excellent low-temperature activities for the synthesis of benzimidazoles via hydrogen transfer

Titania-supported iridium catalysts which enable the synthesis of benzimidazoles from 2-nitroaniline and primary alcohols under mild conditions have been developed. The iridium catalysts supported on {010}/{101}-faceted anatase (PA-135-3.5) showed the excellent activity in spite of their moderate surface areas, while there is a positive correlation between BET surface area and the activity of iridium catalysts. TEM analysis revealed the formation of small iridium nanoparticles (mean diameter; 1.0 nm) on PA-135-3.5, which would be one reason for the excellent catalytic activity. According to the XPS study, relatively high-valent iridium species would be responsible for the catalysis. Various primary alcohols bearing electron-withdrawing or electron-donating groups were applicable. The removal of the solid catalyst by the hot filtration completely stopped further progress of the reaction, indicating that the catalyst acted heterogeneously. The catalysts could be recycled, and there was no leaching of iridium species into the solution.