Nitriles hydrogenations

Thus a second method was envisaged, the reaction of a nitrile, hydrogen selenide, and an a-halogenated ketone in the presence of a condensation catalyst, which can be POCl, or POCI3 with a Lewis acid such as PCI3 or anhydrous ZnCl. The use of fresh AICI3 leads to the formation of tarry side-products. 

Reductive alkylations and aminations requite pressure-rated reaction vessels and hiUy contained and blanketed support equipment. Nitrile hydrogenations are similar in thein requirements. Arylamine hydrogenations have historically required very high pressure vessel materials of constmction. A nominal breakpoint of 8 MPa (- 1200 psi) requites yet heavier wall constmction and correspondingly more expensive hydrogen pressurization. Heat transfer must be adequate, for the heat of reaction in arylamine ring reduction is - 50 kJ/mol (12 kcal/mol) (59). Solvents employed to maintain catalyst activity and improve heat-transfer efficiency reduce effective hydrogen partial pressures and requite fractionation from product and recycle to prove cost-effective. 

Reductions of Nitriles. In the reduction of nitriles, hydrogen is added progressively across the carbon—nitrogen triple bond, forming first the imine and then the amine. 

The above scheme to account for products of nitrile hydrogenation was proposed sixty years ago (80), and with minor variations 28,45,86] has withstood the test of time, It might be expected from the foregoing that, if a nitrile were reduced in the presence of an amine, unsymmetrical amines would result 48), and indeed the reaction provides an excellent way of preparing these amines 49,74). 

Since the hydrogenation is a metal-catalyzed reaction, it is appropriate to use an oxide support to enhance the dispersion. However, the support, like the metal, needs to be chosen with the desired selectivity in mind. The early view [5] that the selectivity in nitrile hydrogenation is determined largely by the behaviour of the partially-hydrogenated intermediate, the imine R-CH = NH, which can either accept two further hydrogens to form the primary amine or can react with an already-formed amine to start a sequence which... 

Table 57.5. The Tallow Nitrile Hydrogenation Data with FT and Untreated Ni Catalysts at 17 bar NH3, 23 bar H2 and 140°C.

The hydrogenation processes were performed at a relatively low temperature and pressure in the presence of promoted Raney Ni 2400 and Raney Co 2724 catalysts (13) in this study but any common nitrile hydrogenation catalysts (e.g. Fe, Ru, Rh, bulk or supported catalysts) could be used. The advantage of using a low temperature and pressure process is that it lowers the investment cost of an industrial process. Raney Ni 2400 is promoted with Cr and Raney Co 2724 is promoted with Ni and Cr. The particle sizes for both catalysts were in the range 25 - 55 pm. The BET surface area of Raney Ni 2400 and Raney Co 2724 are 140 m2/g and 76 m2/g, respectively, and the active surface area of the Ni and Co catalysts are 52 and 18 m2/g, respectively, based on CO chemisorption (Grace Davison Raney Technical Manual, 4th Edition, 1996). 

The selectivity of RNH2 on M/A1203 and Raney catalysts decreased in the order Co Ni Ru>Rh>Pd>Pt. This order corresponds to the opposite sequence of reducibility of metal-oxides [8] and standard reduction potentials of metalions [9], The difference between Group VIII metals in selectivity to amines can probably been explained by the difference in the electronic properties of d-bands of metals [3], It is interacting to note that the formation of secondary amine, i.e. the nucleophilic addition of primary amine on the intermediate imine can also take place on the Group VIII metal itself. Therefore, the properties of the metal d-band could affect the reactivity of the imine and its interaction with the amine. One could expect that an electron enrichment of the metal d-band will decrease the electron donation from the unsaturated -C=NH system, and the nucleophilic attack at the C atom by the amine [3], Correlation between selectivity of metals in nitrile hydrogenation and their electronic properties will be published elsewhere. 

T. A. Johnson and D. P. Freyberger, Lithium Hydroxide Modified Sponge Catalysts for Control of Primary Amine Selectivity in Nitrile Hydrogenations, Chemical Industries (Dekker), 82 (Catal. Org. React.), 201-227 (2000). 

Isophorone diamine is synthesized traditionally by aminoreduction of iso-phoronenitrile. Raney cobalt was used for this process. More recently, a new two-step process was patented. The first step consists of synthesizing the imine and the second one of hydrogenating the latter. Ra-Ni was used as catalyst at 150°C and 60 bar hydrogen pressure. Under these conditions, the catalyst reduces the nitrile groups and is able to cleave the N-N bonds, too. Ammonia is required to promote primary amine formation during nitrile hydrogenation (Scheme 4.151).554... 

Carboxylated nitriles, hydrogenated nitrile, liquid nitriles and blends with PVC are also commercially available. 

More recently, dihydrogen complexes have been patented for nitrile hydrogenation. For example, the complex Ru(7/2-H2)2(H)2(PCy3)2 (Fig. 3.6) catalyzes the hydrogenation of adiponitrile to hexamethylenediamine (HMD) in toluene at 90 °C, 70 bar H2 with TON 52, TOF 5 IT1 [68]. At intermediate conversions, the... 

Table 1. The reaction conditions for the various nitrile hydrogenations...

Figure 5. The structures of the other nitriles hydrogenated during this work.

Synthesis of Heterocycles by Intramolecular Acylation of Nitrile-Hydrogen Halide Adducts ... 

The heterogeneous catalytic hydrogenation of nitriles has been used in amine preparation for a long time. Generally the products are a mixture of primary, secondary and tertiary amines, the nature of which depends on the catalyst used as well as on reaction conditions (refs. 1,2). The selectivity of nitrile hydrogenation is of importance, particularly in the production of primary amines. In such reactions the catalysts most often proposed are Raney nickel catalysts (refs. 1-3). 

Abb. 7.14. Imidic acid ester hydrochlorides (F)—to be prepared from nitriles, hydrogen chloride (gas) and methanol acc. to Fig. 7.13—, their trans-formability into orthoester (A) or amidine (I) and the corresponding reaction mechanisms. 

There have been many known cases where high yields of primary amines were obtained over cobalt catalysts in the hydrogenation of nitriles. Hydrogenation of 3,4-... 

Synthesis of heterocycles, by intramolecular acylation of nitrile-hydrogen halide adducts ... 

The first question in considering the mechanism of nitrile hydrogenation is the mode of adsorption on the catalyst surface. Two adsorption modes are envisaged, corresponding to the end-on (ct, or and side-on (71- or Figure 10) modes of coordination of a nitrile to a metal that have been demonstrated in coordination and organometallic compounds. Each adsorption model leads to... 

An early example of a hydride with pronounced H donor character is [Cp2ZrHCl] , which attacks ketones, aldehydes, esters, epoxides, and even CO2, as shown in Scheme 1. Cp 2ScH is so active that it even attacks nitriles to give Cp Sc(N=CHR) the resulting imine complexes react with H2 to give amines, and the Sc complex is a nitrile hydrogenation catalyst. ... 

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