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Kinetics and mechanism of the zinc-promoted dissociation of cyanide ion from 1-benzyl-4-cyano-1,4-dihydronicotinamide in acetonitrile

Authors
Journal
Bioorganic Chemistry
0045-2068
Publisher
Elsevier
Publication Date
Volume
16
Issue
1
Identifiers
DOI: 10.1016/0045-2068(88)90034-x

Abstract

Abstract Rate and equilibrium constants for the dissociation of cyanide ion from 1-benzyl-4-cyano-1,4-dihydronicotinamide (BNA-CN) in acetonitrile at 25°C have been evaluated. Compared to water as the solvent, the dissociation constant in acetonitrile, K diss = 3.9(±0.6) × 10 −7 m, is approximately a factor 10 6 lower. This reduction is brought about by a nearly 10 2 slower rate of dissociation ( k 1 = 2.6 × 10 −4 s −1) and a 10 4 faster rate of addition of cyanide ion in acetonitrile. Addition of zinc salts, like zinc perchlorate, zinc bromide, and zinc iodide, induce cyanide ion dissociation by both entrapment of cyanide ion and efficient promotion of the cyanide ion expulsion. Remarkably, the latter effect is dependent on the nature of the zinc counterion and increases in the order ClO 4 − < Br − < I −. It is concluded that the zinc-promoted cyanide expulsion is preceded by complex formation of zinc species with BNA-CN and that both Zn 2+ and monoionized Zn X + species are active. A transition state model for the reaction is proposed.

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