Raney Ni/KBH 4 : an efficient and mild system for the reduction of nitriles to amines

A new and efficient method of synthesizing primary amines from their corresponding nitriles with KBH 4 in dry ethanol catalyzed by Raney Ni has been discovered. It is proved that this route is so far the most facile, efficient and easy-to-handle method of reducing aliphatic and aromatic nitriles directly to primary amines. Under mild conditions, up to 93% isolated yields were achieved for aromatic or aliphatic nitriles. This approach opens up a new way for the utilization of potassium borohydride and the classical catalyst Raney Ni.


Introduction
Nitrile hydrogenation is an important method to obtain amines.The most commonly used approaches in nitriles hydrogenation are using strong hydride donors, such as lithium aluminum hydride, or catalytic hydrogenation. 1 However, these methods always lack chemoselectivity and need complicated procedures to obtain target compounds.The difficulty arises because the imine, as an intermediate produced by the reduction of cyano group, could couple with an already formed primary amine, which leads to secondary amine.Furthermore, a secondary amine could also react with another imine molecule to form tertiary amine. 2 Therefore, transforming nitriles to primary amines with high selectivity and yields has become an important field to chemists. 3,5,6,7,12,13Except for a few examples, 4 KBH 4 or NaBH 4 alone generally could not reduce nitriles to amines.1a, 5 The reducing ability of NaBH 4 could be enhanced by introducing some special additives.For example, CoCl 2 , ZrCl 4 , I 2 and bis(2-bromoethyl)selenium dibromide 6 are effective additives for the reaction.Recently Khurana et al 7 reported that aromatic nitriles could be reduced to primary amines with NaBH 4 using NiCl 2 as additive.However, this method is not efficient for aliphatic nitriles.Caddick and co-workers 8 introduced a suitable trapping agent such as acetic anhydride or di-tert-butyl dicarbonate to the reaction (NiCl 2 /NaBH 4 ), the protected primary amines were firstly obtained, and the product amines were formed by subsequent deprotection treatment.In this method, longer synthesis routes were needed and the yields of primary amines are not high enough.
KBH 4 is usually used to react with metal salts to prepare metal borides 9 that are used as the catalysts in catalytic hydrogenation.To the best of our knowledge, KBH 4 has never been used with additives to enhance its reducing activity in transforming nitriles to amines.On the other hand, Raney nickel is routinely used as catalyst in catalytic hydrogenation 10 and heterogeneous catalytic hydride transfer hydrogenation. 11The scope of its utilization also involves the selective reduction of nitrocompounds, dinitrosubstituted diphenylsulfones, O-and N-benzyl containing nitrocompounds; the conversion of nitro olefins to carbonyl derivatives, 12 and the synthesis of halo amines from halo nitro compounds.In these cases, the commonly used hydrogen donors are hydrazine hydrate, hydrazine derivative 13 , ammonium formate, formic acid or cyclohexene.In 2002, Gowda et al 14 found that using Raney Ni / N 2 H 4 / HCOOH could transform nitriles to primary amines with the yield of about 70%.

Results and Discussion
In this context, we first report a simple and convenient procedure (Scheme 1) to directly convert the robust aliphatic and aromatic nitriles 1 to the corresponding primary amines 2 with high isolated yields (over 80%) by using Raney Ni / KBH 4 system in dry ethanol at mild temperature.
ISSN 1551-7012 Table 1 lists the results of condition optimization experiments.To increase the yield of primary amine and at the same time to restrain the formation of second amines, excess borohydride must be used.We tried various ratio of 2-phenylacetonitrile 1a / KBH 4 , and the results showed that the more KBH 4 was used to increase the ratio from 1:4 to 1:5 (Table 1, entry 4) result no obvious increase of the yield.GC-MS experiment showed that at the ratio of 1:4, the 2-phenylethanamine was the only product, and no secondary or tertiary amine was formed.Temperature also affects the yield (Table 1, entries 5-9), when the reaction was carried out at 0 o C, the yield is higher than the reaction held at room temperature, but longer reaction time was needed.So the best condition for the reduction of aliphatic nitriles is at room temperature, and the molar ratio of substrate / Raney Ni / KBH 4 is 1:1:4.But, when using benzonitrile 1i as substrate, at room temperature, the reduction needed 2 hours to complete (Table 2, entry 1), which showed that the aromatic nitriles are more difficult to be reduced than aliphatic ones.This result made us to adjust a proper temperature for the reaction using certain substrate ( Changing the solvent to tetrahydrofuran (THF) or dichloromethane did dramatically harm to the reduction.The 2-phenlyacetonitrile 1a could not be reduced even in 24 hours.The reductions can work well using methanol as solvent.But due to its toxicity, we chose ethanol as the reaction solvent.
Several nitriles were chosen to be reduced under the optimized conditions, Table 3 summarizes the results.High yields were achieved for both aromatic and aliphatic nitriles.For phenylacetonitrile, the substituents, especially the electron donor substituent on phenyl ring of the substrate might lead to small decrease of the yield (Table 3, a-c).But for benzonitrile, the effect was not obvious (Table 3, i-l).For long chain aliphatic nitriles, the results were changeless, and the yields were around 82% (Table 3, d-h).Unfortunately, our new system has no chemoselectivity for some reducible substituents, such as nitro, aldehyde, ketone and alkene group.These were reduced together with nitrile group.

Conclusions
In summary, a mild and efficient method for the direct hydrogenation of nitriles to primary amines has been developed.Using potassium borohydride as reducing agent, ethanol as solvent, catalyzed by readily available Raney nickel, the reaction can be carried out under mild condition without an inert atmosphere.The primary amine products are obtained in good to excellent yields, and only traces of secondary or tertiary amines are formed.Aromatic nitriles and long chain aliphatic nitriles are suitable for the reducing system.

Experimental Section
General Procedures.Mass spectra (GC/MS) were recorded on an Agilent Technologies 6890-5973N mass spectrometer.) and 25 mL dry ethanol were placed in a 50 mL flask, and then 2-phenylacetonitrile 1a 1.2 mL (10 mmol), was added while stirring.After vigorously stirring at room temperature for 45 min, the reaction mixture was filtered.The organic layer was evaporated and residue was dissolved in ethyl acetate, and was washed with water.The organic layer were dried and evaporated, and the product was purified by flash column chromatography on silica gel (CH 2 Cl 2 / CH 3 OH, 15: 1) to give 2-phenylethanamine 2a as pale yellow liquid (yield: 92%).2-Phenylethanamine 2a.

Table 2 )
. 50 o C (Table2, entry 2) is the better reaction temperature for the reduction of aromatic nitriles.

Table 3 .
Reduction of nitrile 1 with Raney Ni /KBH 4 a system in ethanol b at mild temperature c in c Rt for aliphatic nitriles and 50 o C for aromatic nitriles.d Isolated yields.

General procedure for the reduction of nitrile
1H NMR spectra were recorded on a Varian Unity Inova-400 spectrometer operating in 400.00 MHz, in deuterated chloroform at room temperature.For TLC plates coated with silica gel were run in CH 2 Cl 2 / MeOH mixture and spots were developed in Ultraviolet and I 2 .