Hydrogenation of carbon–nitrogen double bonds
In chemistry, the hydrogenation of carbon–nitrogen double bonds is the addition of the elements of dihydrogen (H2) 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.
As in hydrogenation reactions of other functional groups, the reductant in C=N hydrogenations is either hydrogen gas or a transfer hydrogenation reductant such as formic acid. The process is usually catalyzed by a transition metal complex. 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.
The mechanism of imine hydrogenation depends on the nature of the catalyst .Catalysis by some rhodium(I) complexes proceeds through the dihydride species I. The mechanism is proposed to involve both η1 (σ-type) and η2 (π-type) coordination of the imine followed by transfer of hydrogen to generate the amine complex. Dissociation of the amine product and oxidative addition of H2 then occur, preparing the catalyst to bind another imine molecule. In some iridium-catalyzed hydrogenations, the mechanism is believed to proceed via a monohydride species. The oxidation state of iridium is always +3.
Ruthenium(II) catalysts incorporating chiral diphosphine ligands operate according to the inner-sphere mechanisms described above, in which the imine must coordinate to the catalyst and insert into the metal-hydrogen bond. Notably many ruthenium(II) catalyst operate through an "outer-sphere mechanism," during which the imine never interacts with the metal center indirectly. Instead, it receives the elements of H2 from Ru-H and N-H in a concerted, polarized fashion. This process is utilized by the Shvo catalyst and many ruthenium amine complexes.
Because the substituents attached to the imine nitrogen exert a profound influence on reactivity, few general catalyst systems exist for the enantioselective hydrogenation of imines and imine derivatives. However, catalyst systems have been developed that catalyze hydrogenation of particular classes of imines with high enantioselectivity and yield. This section describes some of these systems and is organized by the substitution pattern of the imine.
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