Nitrile reduction


In nitrile reduction a nitrile is reduced to either an amine or an aldehyde with a suitable chemical reagent.

Catalytic hydrogenation

The catalytic hydrogenation of nitriles is often the most economical route available for the production of primary amines. Catalysts for the reaction often include group 10 metals such as Raney nickel, palladium black, or platinum dioxide. However, other catalysts, such as cobalt boride, also can be selective for primary amine production:
A commercial application of this technology includes the production of hexamethylenediamine from adiponitrile, a precursor to Nylon 66.
Depending on reaction conditions, intermediate imines can also undergo attack by amine products to afford secondary and tertiary amines:
Such reactions proceed via enamine intermediates. The most important reaction condition for selective primary amine production is catalyst choice. Other important factors include solvent choice, solution pH, steric effects, temperature, and the pressure of hydrogen.

Stoichiometric reductions

for the non-catalytic conversion to amines include lithium aluminium hydride, lithium borohydride, diborane, or elemental sodium in alcohol solvents.

To aldehydes

Nitriles can also be reduced to aldehydes. The Stephen aldehyde synthesis uses Tin chloride and hydrochloric acid to yield an aldehyde via the hydrolysis of a resulting iminium salt. Aldehydes can also form using a hydrogen donor followed by in-situ hydrolysis of an imine. Useful reagents for this reaction include formic acid with a hydrogenation catalysis or metal hydrides which are used to add one mol of hydrogen to the nitrile. For example, sodium borohydride reduces nitriles in alcoholic solvents with a CoCl2 catalyst or Raney nickel. Reducing agent Diisobutylaluminium hydride, or DIBAL-H, is another commonly used metal hydride. DIBAL-H acts as a proton source, adding a hydride ion to the carbon of the nitrile. The resulting imine is a relatively stable intermediate that can be hydrolyzed to the aldehyde.

Mechanism

DIBAL-H is added in controlled amounts at low temperatures to achieve partial reduction of the nitrile. The aluminum atom in DIBAL acts as a Lewis acid, accepting an electron pair from the nitrile. The nitrile is then reduced by the transfer of a hydride ion to the carbon of the carbon-nitrile triple bond, producing an imine. After a workup with water, the aluminum complex is hydrolyzed to produce the desired aldehyde. Because the hydrolytic workup generates the aldehyde at the end, the nitrile does not undergo over-reduction.

Electrochemical methods

Benzonitriles can also be reduced electrochemically.