Hydrogenation of imines to amines

Enandoselective metal-free hydrogenation of imines to amines by H2 with yields up to 99% and 89% ee has been achieved using a catalyst generated in situ by using chiral dienes (67) as hgands in the presence of HB(C6F5)2. ... 

Saaby, S. and Knudsen, K.R. and Ladlow, M. and Ley, S.V. (2005). The use of a continuous flow-reactor employing a mixed hydrogen-liquid stream for the efficient reduction of imines to amines. Chem. Commun., 23, 2909-2911. 

Enantioselective hydrogenation of imines to produce optically active amines is an important synthetic reaction. The imine of acetophenone is hydrogenated in the presence of chiral phosphine-Rh(I) complexes in... 

Saaby S, Knudsen KR, Ladlow M, Ley SV (2005) The Use of a Continuous Flow-Reactor Employing a Mixed Hydrogen-Liquid Flow Stream for the Efficient Reduction of Imines to Amines. Chem Commun 23 2909-2911 Seebach D, Overhand M, Ktihnle FNM, Martinoni D, Oberer L, Hommel U, Widmer H (1996) Beta-Peptides Synthesis by Arndt-Eistert Homologation with Concomitant Peptide Coupling. Structure Determination by NMR and CD Spectroscopy and by X-ray Crystallography. Helical Secondary Structure of a Beta-Hexapeptide in Solution and its Stability Towards Pepsin. Helv Chim Acta 79 913-941... 

The direct reductive amination (DRA) is a useful method for the synthesis of amino derivatives from carbonyl compounds, amines, and H2. Precious-metal (Ru [130-132], Rh [133-137], Ir [138-142], Pd [143]) catalyzed reactions are well known to date. The first Fe-catalyzed DRA reaction was reported by Bhanage and coworkers in 2008 (Scheme 42) [144]. Although the reaction conditions are not mild (high temperature, moderate H2 pressure), the hydrogenation of imines and/or enam-ines, which are generated by reaction of organic carbonyl compounds with amines, produces various substituted aryl and/or alkyl amines. A dihydrogen or dihydride iron complex was proposed as a reactive intermediate within the catalytic cycle. 

Iridium(III) hydride forms complexes with DIOP, BDPP (2,4-bis(diphenyl-phosphino)pentane), NORPHOS, and BINAP ligands to produce amines in 11 -80% ee.679 Similar modest results are obtained in the reduction of N-arylketimines with an iridium(HI) complex with (2S,3 S) -C HIRA PHOS as the chiral ligand.680 The indium complexes with chiral phosphinodihydrooxazoles catalyze the enantioselective hydrogenation of imines in supercritical carbon dioxide with up to 80% ee, but generally lower ee values are observed in... 

Hydrogenation of imines, e.g. 45-48, with a chiral titanocene catalyst at 2000 psig gave the corresponding optically active secondary amines in high enantiomeric excess74. Imines are reduced to amines by trichlorosilane/boron trifluoride etherate in benzene75. 

Given the importance of chiral amines to synthetic chemistry as well as other fields asymmetric hydrogenation of imines has attracted wide interest but limited success compared to C=C and C=0 bond reduction. The first asymmetric hydrogenation of imines was carried out in the seventies with mthenium- and rhodium-based catalysts, followed later by titanium and zirconium systems [82]. Buchwald found that... 

Hydrogen telluride, sodium hydrogen telluride as well as phenyltelluroP afford the reduction of imines to secondary amines. 

The paramount significance of chiral amines in pharmaceutical and agrochemical substances drives the development of efficient catalytic asymmetric methods for their formation. In contrast to the high enantioselectivities observed in asymmetric reduction of both alkenes and ketones, only limited success has been achieved in the enantiose-lective hydrogenation of imines [118]. Currently, there are few efficient chiral catalytic systems available for the asymmetric hydrogenation of imines. 

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