Catalytic hydrogenation of imines

Scheme 8.4. Catalytic hydrogenation of imines using cationic iridium catalysts with the C02-philic BARF anion...

Homogeneous catalytic hydrogenation of imines has been carried out using cationic iridium hydride catalysts.50 The mechanistic possibilities are compared and contrasted with C=0 hydrogenations. 

Lewis acid enhancement upon n-coordination of arylboranes, application to the catalytic hydrogenation of imines... 

I 77 Frustrated Lewis Pairs A Metal-Free Strategy for Hydrogenation Catalysis Table 11.1 Catalytic hydrogenation of imines with phosphonium borate catalysts [45]. 

Scheme 11.11 Catalytic hydrogenation of imine using phosphonium borate catalyst.

Using a similar strategy, the research groups of Repo and Rieger reported the catalytic hydrogenation of imines and enamines using a linked amine-borane species derived from 2,2,6,6-tetramethylpiperidine [57]. In this case, the link... 

Chirik and co-workers described the electron-rich Fe-CNC dinitrogen pincer complexes 41 which were effective for the catalytic hydrogenation of unfunctionalized alkenes (Figure 13.3). ° Furthermore, the first example of zincocene-NHC adducts 42 was evaluated in the catalytic hydrogenation of imines and ketones (Figure 13.3). ... 

Heiden ZM, Stephan DW. Metal-free diastereoselective catalytic hydrogenations of imines using B(CgF5)3. Chem. Com-mun. 2011 47 5729. 

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. 

Catalytic hydrogenation of nitriles may result in several products primary, secondary, and tertiary amines imines hydrocarbons aldehydes amides and... 

The catalytic hydrogenation of the C=N bond of imines has attracted considerable attention, and a useful review covering the literature until 1996 has been reported by James.110 Chiral [Pg.91]

In a study by Leitner of the iridium-catalyzed hydrogenation of imines, a nearly 20-fold increase in catalytic efficiency was observed due to a different kinetic profile in scC02 as compared to methylene chloride.358 The change in rate and selectivity found in scC02 with respect to the other solvents can be related to the following major points ... 

Samec and Backvall found that the dinuclear Shvo complex catalyzes the transfer hydrogenation of imines using benzene as solvent and isopropanol as the hydrogen source (Eq. (45)) [76]. These catalytic hydrogenations were typically carried out at 70 °C, and gave >90% yields of the amine in 4 h or less. 

A trons-[RuCl2(diphosphine)(l,2-diamine)] complex with (R,R)-Et-DuPhos and (R,R)-l,2-diaminocyclohexane as the ligand combination has been found to be effective for the hydrogenation of imine 143, with up to 94% ee being obtained under the standard basic conditions employed for this catalytic system [198]. Unfortunately, the optimum combination of chiral diphosphine and diamine was found to be substrate-dependent, with only 40% ee being obtained for 2-methylquinoxaline 144 with Et-DuPhos. 

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. 

Lithium aluminum hydride in tetrahydrofuran has been found to reduce aromatic nitriles to give an amine and to give an imine which is formed from the addition of the amine to the nonisolatable imine intermediate followed by an elimination of ammonia [24] (Eq. 14). This is simpler than catalytic hydrogenation of nitriles [25], which gives poor yields of imines. 

In the course of the catalytic hydrogenation of a,us dinitriles over Raney nickel, by-products are obtained from C-N and C-C bond formation. The mechanism of the formation of these compounds was investigated. Cyclic and linear secondary amines can result from the same secondary imine through a transimination process involving a ring-chain tautomerism. Stereochemical results for 2-aminomethyl-cyclopentylamine (AMCPA) are in accordance with a specific cyclisation pathway favored by an intramolecular hydrogen bond giving rise to the cis isomer from aminocapro-nitrile, unfavored in the case of adiponitrile which leads to the trans AMCPA as the major isomer. 

Catalytic hydrogenation of nitriles over Raney nickel leads to primary amines with variable amounts of secondary and tertiary amines depending on reaction conditions (Ref. 1). These by-products result from hydrogenation of secondary imines and enamines respectively. 

Camphorsultam was first prepared by the catalytic hydrogenation of (-)-(camphorsulfonyl)imine over Raney nickel.2 Lithium aluminum hydride reduction was used by Oppolzer and co-workers in their synthesis of the sultam.3-4 However, because of the low solubility of the sultam in tetrahydrofuran, a large amount of solvent was required.4 In the procedure described here the amount of solvent is significantly reduced by using a Soxhlet extractor to convey the imine slowly into the reducing medium.5... 

The C2-symmetric diphosphinite and CVsymmetric phosphinite-phosphate ligands, based on a carbohydrate scaffold, and iridium complexes give catalyst precursors that are active in the hydrogenation of imines. Cationic iridium complexes gave rise to catalytic systems that were more active than the neutral iridium complexes. Enantioselectivities up to 76% were obtained.347... 

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