Mechanistic aspects of the Strecker aminonitrile synthesis

Abstract

Asymmetric α-aminonitriles could not be prepared by the Strecker procedure catalysed by optically active acids or bases. Benzylideneaniline reacted readily and quantitatively with hydrogen cyanide to give α-cyanobenzylaniline, while mandelonitrile reacted very slowly with aniline, indicating that benzylideneaniline or a Schiff base may be an intermediate for the Strecker synthesis. The addition of hydrogen cyanide to benzylideneaniline in acetonitrile–methanol at 25 °C follows a third-order rate equation: –d[Ar¹CHNAr²]/dt=k[Ar¹CHNAr²][HCN][MeOH]. The rate is higher in protic dipolar solvents than in aprotic non-polar solvents. An electron-releasing group on either of the rings increases the rate with Hammett ρ values of –1·11 in the benzylidene and –1·39 in the aniline ring. A mechanism involving methanol-catalysed addition of hydrogen cyanide to benzylideneaniline is postulated and discussed.