Nitriles

12 Nitrites

Catalytic hydrogenation of nitriles may give rise to a n u m b e r of products, which include primary, secondary, and tertiary amines, imines, hydrocarbons, aldehydes, amides, and alcohols. The major product, derived as a resultant of several competing reactions, depends importantly on the catalyst, substrate, and reaction conditions. Primary amines, which usually are the goal of a nitrile reduction, are formed to the extent that other competing reactions, notably coupling reactions leading to secondary and tertiary amines, can be avoided.

I. C O U P L I N G R E A C T I O N S : S E C O N D A R Y A N D T E R T I A R Y A M I N E S The formation of secondary and tertiary amines, as well as certain hydro­ lysis products, in nitrile hydrogenations is usually accounted for by the assumption of an imine intermediate (von Braun et α/., 1923): Η

Η

2

RC = Ν — - + R C H = N H — R C H 2 N H

2

Addition of the primary amine to the intermediate imine gives a product from which the secondary amine may be formed by hydrogenolysis, RCH=NH + R C H 2 N H 2 ^ R C H ( N H 2) N H C H 2R

R C H ( N H 2) N H C H 2R ( R C H 2) 2N H + N H 3

or by elimination of a m m o n i a followed by h y d r o g e n a t i o n : R C H = N H + R C H 2N H 2 ^ R C H = N C H 2R + H 2

R C H = N C H 2R + N H 3 > ( R C H 2) 2N H

Tertiary amines may be formed similarly through addition of a secondary amine to an imine followed by hydrogenolysis. M i n o r variations of this general scheme have been proposed by other workers (Mignonac, 1920; 203