Weygand/Hilgetag Preparative Organic Chemistry

Formation of the N-H bond 553
can play the role of solvent, wholly or partially; the amount of ammonia
needed varies from 1.2 moles (aromatic nitriles) to 5 moles (dinitriles of lower
molecular weight) per nitrile group.
Albert and Magrath 24 give the preparation of /?-(aminomethyl)benzoic acid
from/7-cyanobenzoic acid as example of the reduction of a nitrile in the presence
of ammonia under atmospheric conditions (Raney nickel). Pressure hydrogenation
in the presence of ammonia is carried out as follows:
Phenethylamine from benzyl cyanide: 25 A 2-1 autoclave is charged with benzyl cyanide
(1 kg; distilled over Raney nickel) and Raney nickel (one tablespoonful) and is then closed.
Liquid ammonia (180 ml) is introduced and then hydrogen to a pressure of 140 atm. The
autoclave is then heated at 120-130° and shaken for 1 h, after which it is cooled and the
product is freed from catalyst by filtration and distilled in a vacuum, giving the amine (83 to
87%), b.p. 90-93°/15 mm.
Catalytic hydrogenation of nitriles is often accompanied by hydrogenolysis,
e.g., debenzylation; such side reactions can be avoided by use of rhodiumaluminum
catalysts; these are applied in the presence of ammonia at room
temperature and low pressures. 26
27 &tB
II, Reduction by metal hydrides
Nitriles can be converted into primary amines by lithium tetrahydridoaluminate
or diborane. The products of such reactions, unlike those of catalytic
hydrogenation, contain only small amounts of secondary and primary
amines, so that these processes are particularly suitable for small batches in
the laboratory. Alkali hydridoborates do not effect this reduction. 0.5 mole
of lithium tetrahydridoaluminate is necessary for reduction of 1 mole of
nitrile, but using an excess of reductant leads to better yields; a 1: 1 ratio is
generally applied. Amundsen and his coworkers 28 studied the optimal conditions
for this reduction and describe reduction of heptyl cyanide as an
example of a generally applicable procedure:
Octylamine: 28 A solution of heptyl cyanide (12.5 g, 0.10 mole) in ether (20 ml) is dropped
slowly into one of lithium tetrahydridoaluminate (3.8 g, 0.12 mole) in anhydrous ethanol
(200 ml) at 0° (ice-bath). Then, with continued cooling and vigorous stirring, are added
successively water (4 ml), 20% sodium hydroxide solution (3 ml), and again water (14 ml).
The ethereal solution is decanted from the white inorganic residue, the latter is washed with
a little ether, the combined ethereal solutions are evaporated, and the residue is distilled
in a vacuum, giving octylamine (11.5-11.9 g, 89-92%), b.p. 53°/6 mm.
III, Reduction by sodium
Primary alkanenitriles can be reduced to the amines by sodium in alcohol.
Nitrile groups attached to an aromatic ring or to a secondary carbon atom
in ^-position to a benzene ring (ArCHRCN) are not reduced by sodium,
24 A. Albert and D. Magrath, J. Chem. Soc, 1944, 678.
25 J. C. Robinson Jr. and H. R. Snyder, Org. Syn., Coll. Vol. Ill, 720 (1955).
26 M. Freifelder, /. Amer. Chem. Soc, 82, 2386 (1960).
27 A. Haj6s, " Komplexe Hydride," VEB Deutscher Verlag der Wissenschaften, Berlin,
1966, pp. (a) 180, (b) 189, (c) 192, (d) 194, (e) 196, (f) 202, (g) 318.
28 L. H. Amundsen and co-workers. J. Amer. Chem. Soc, 73, 243 (1951).