page_title: Hydrogenation of carbon–nitrogen double bonds
section_title: Outer sphere mechanisms
hierarchical_section_title: Hydrogenation of carbon–nitrogen double bonds / Mechanism and stereochemistry / Outer sphere mechanisms
caption_attribution_description: English: illustrative outer sphere mechanism (transfer hydrogenation)
context_page_description: In chemistry, the hydrogenation of carbon–nitrogen double bonds is the addition of the elements of dihydrogen across a carbon–nitrogen double bond, forming amines or amine derivatives. Although a variety of general methods have been developed for the enantioselective hydrogenation of ketones, methods for the hydrogenation of carbon–nitrogen double bonds are less general. Hydrogenation of imines is complicated by both syn/anti isomerization and tautomerization to enamines, which may be hydrogenated with low enantioselectivity in the presence of a chiral catalyst. Additionally, the substituent attached to nitrogen affects both the reactivity and spatial properties of the imine, complicating the development of a general catalyst system for imine hydrogenation. Despite these challenges, methods have been developed that address particular substrate classes, such as N-aryl, N-alkyl, and endocyclic imines.
  If the complex is chiral and non-racemic and the substrate is prochiral, an excess of a single enantiomer of a chiral product can result.
context_section_description: Ruthenium(II) complexes of amine ligands are renown for engaging in the outer-sphere mechanism," during which the imine/iminium substrate does not bind to the metal center directly. Instead, substrate receives the elements of H₂ by interaction with Ru-H and N-H sites.  This process is utilized by the Shvo catalyst and many ruthenium amine complexes.  One such complex is Baratta's catalyst RuCl₂(PPh₃)₂(ampy) (ampy = 2-picolylamine) for transfer hydrogenation.
caption: Outer sphere mechanism for metal-catalyzed transfer hydrogenation of an imine.)
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