Nitrile reduction

In nitrile reduction a nitrile is reduced to either an amine or an aldehyde with a suitable chemical reagent.^[1][2]

Catalytic hydrogenation

The catalytic hydrogenation of nitriles is often the most economical route available for the production of primary amines.^[3] Catalysts for the reaction often include group 10 metals such as Raney nickel,^[4][5][6] palladium black, or platinum dioxide.^[1] However, other catalysts, such as cobalt boride, also can be regioselective for primary amine production:

R-C≡N + 2 H₂ → R-CH₂NH₂

A commercial application of this technology includes the production of hexamethylenediamine from adiponitrile, a precursor to Nylon 66.^[7]

Depending on reaction conditions, reactive intermediate imines can also undergo attack by amine products to afford secondary and tertiary amines:

2 R-C≡N + 4 H₂ → (R-CH₂)₂NH + NH₃

3 R-C≡N + 6 H₂ → (R-CH₂)₃N + 2 NH₃

Such reactions proceed via enamine intermediates.^[8] The most important reaction condition for selective primary amine production is catalyst choice.^[1] Other important factors include solvent choice, solution pH, steric effects, temperature, and the pressure of hydrogen.

Stoichiometric reductions

To amines

Reducing agents for the non-catalytic conversion to amines include lithium aluminium hydride, lithium borohydride,^[9] diborane,^[10] or elemental sodium in alcohol solvents.^[11]

To aldehydes

Nitriles can also be converted to aldehydes by reduction and hydrolysis. The Stephen aldehyde synthesis uses Tin(II) 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^[12] 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.^[13]

With diisobutylaluminium hydride

The mechanism for the reduction of a nitrile to an aldehyde with DIBAL-H.

The hydride reagent Diisobutylaluminium hydride, or DIBAL-H, is commonly used to convert nitriles to the aldehyde.^[14] Regarding the proposed mechanism, DIBAL forms a Lewis acid-base adduct with the nitrile by formation of an N-Al bond. The hydride is then transferred to the carbon of the nitrile. Aqueous workup produce the desired aldehyde and ammonia.^[15]

Electrochemical methods

Benzyl nitriles can also be reduced electrochemically.^[16][17]

See also

• Urushibara cobalt
• Imine