Mechanistic Study on the Ruthenium-Catalyzed Direct Amination of Alcohols

The Ru-catalyzed direct amination of alcohols with ammonia was investigated for the RuHCl­(CO)­(PPh₃)₃/Xantphos system in order to gain mechanistic insight. For several Ru­(II) precursor complexes the influence of different additives on catalytic performance was investigated. NMR studies revealed that the reaction of RuHCl­(CO)­(PPh₃)₃/Xantphos with the alcohol in the presence of a strong base initially formed an inactive dihydrido Ru species. However, by addition of a ketone, the dihydride was (re)­activated, where the corresponding imine is the actual activator, formed by immediate condensation of the ketone with ammonia. In the absence of a base, added ketone significantly enhanced catalyst activity. Catalytically inactive RuCl₂(PPh₃)₃ could be activated by base, demonstrating that also complexes without the CO ligand give active catalysts. On the basis of these observations a mechanism was proposed, closely related to known transfer hydrogenation mechanisms.