Systèmes de strecker et apparentes—XII: Catalyse par les aldehydes de l'hydratation intramoleculaire des α-aminonitriles

Abstract

Aqueous basic hydration of α-aminopropionitrile is first order either in acetaldehyde (formed by decomposition of α-aminopropionitrile) or different aldehydes added in the medium. The rate determining step involves a rapid preequilibrium in which the catalyst is added to the substrate and then a cyclisation of the aminoalcoholate intermediate. The dissociation equilibrium of the aminoalcohol (pK A ⋍ 14) is reached by a pseudo first order kinetical dependence on OH - The cyclisation is facilitated by the position of alcoholate nucleophile relative to the nitrile The value k cycl = 10 3 s -1 for isobutyraldehyde catalysis shows that in this process alcoholate is 10 8 as good a nucleophile as OH - Substituents on the carbonyl group have a great influence on the ammoalcohol stability, and cyclisation seems to be subject to steric hindrance between the α substituent of the nitrile group and the site of the reaction This catalytic process was possibly of great importance in prebiotic synthesis of amino-acids and suggests an essential role for aldehydes.