The Synthesis of Primary Amines through Reductive Amination Employing an Iron Catalyst
Christoph Bäumler, Christof Bauer, Rhett Kempe
The reductive aminationo fk etonesa nd aldehydes by ammonia is ah ighly attractive method for the synthesis of primary amines.T he use of catalysts,e specially reusable catalysts, based on earth-abundant metalsi ss imilarly appealing. Here, the iron-catalyzed synthesis of primary amines throughr eductive amination was realized. Ab road scope and av ery good tolerance of functional groups were observed. Ketones, including purely aliphatic ones, aryl-alkyl, dialkyl, and heterocyclic, as well as
... dehydesc ould be converted smoothly into their corresponding primary amines.I na ddition, the amination of pharmaceuticals, bioactive compounds, andn atural products was demonstrated. Many functional groups, such as hydroxy, methoxy,d ioxol, sulfonyl,a nd boronate ester substituents, were tolerated. The catalysti se asy to handle, selective, and reusable and ammonia dissolved in water could be employed as the nitrogen source. The key is the use of as pecific Fe complex for the catalysts ynthesis and an N-doped SiC materiala s catalystsupport. The reductive amination of ketones and aldehydes is the methodo fc hoice fort he synthesis of alkyl amines from inexpensive and diversely availables ubstrates.  The synthesis of primary amines by employing ammonia as the nucleophile is especially attractive and challenging (Scheme1,t op).  Amines are av ery important class of chemical compounds and key functional groups in many bulk and fine chemicals,  drugs,  and materials.  The use of catalysts based on earth-abundant metals in reactionsc lassicallym ediated by noble metals is also attractive and challenging. Significantp rogress has been made in the field of homogeneous earth-abundant 3d metal catalysis in recent years.    The use of reusable nanostructured earth-abundant3 dm etal catalysts for ab road applicability in complex organic syntheses is highly desirable and has been disclosed significantly less. [8, 9] Beller and co-workersrecently introduced ar eusablec obalt catalyst for the general synthesis of primary amines from aldehydes or ketonesa nd ammonia (Scheme 1),  and we discovered that an ickel catalyst sup-ported by g-Al 2 O 3 and synthesized from as pecific nickel complex can be used under very mild conditions by employing ammonia dissolved in water.  Aqueous ammonia is an attractive and easy-to-handle source of ammoniaa nd has been used for the synthesis of primary amines in other catalytic reactions, such as hydroaminomethylation,  telomerization of ammonia and butadiene,  allylic substitutions, [13, 14] cross couplings,  benzene oxyamination,  anda mine alkylation.  Murugesan et al. recentlyd isclosed ad ifferent Ni catalyst fort he general synthesis of primarya mines through reductive amination.  In addition, other reusable Co catalysts weredescribed.  However,ar eusable iron catalystf or the synthesiso fp rimary amines through reductive amination of ketones and aldehydes has not yet been disclosed, although ar eusable iron-based catalyst system for the synthesis of secondary aryl-alkyl amines  and homogeneous iron catalysts for reductive amination are known.  We have recently introduced av ariety of homogeneous earth-abundant metal catalysts [22, 23] and reusable nanostructured catalysts for energy storage  and novel organic reactions,  as well as earth-abundant metal catalysts with ab road applicability in organic synthesis. [9, 26] Herein,w er eport on ar eusablea nd nanostructured iron catalyst fort he general synthesis of primary amines from ketoneso ra ldehydes through reductive amination. We employ easy-to-handle ammonia dissolved in water,o bserve ab road scope, and find that many functional groups can be tolerated. Ketones, includingp urely aliphatic ones, aryl-alkyl, dialkyl, and heterocyclic, as well as aldehydesc an be converted smoothly into their corresponding primary amines. In addition, the amination of pharmaceuticals, bioactive compounds, and natural products is demonstrated. Many functional groups, such as hydroxy,m ethoxy,d ioxol, sulfonyl, andb oronate ester substituents, are tolerated. Ourc ata-Scheme1.Reusable 3d metal catalyst developments with ab road scope in reductive aminationo fa ldehydes and ketones to primary amines employing ammonia and hydrogen (R 1 and R 2 are aryl,a lkyl substituents and, in the case of aldehydes, one of them represents ah ydrogen atom).