Mechanism of the cyanide ion-catalysed cleavage of benzils by amines in aqueous dioxan

Abstract

Kinetics and product distributions of the cyanide ion-catalysed reaction of benzils with ammonia or amines, to give benzaldehydes (or benzaldimines) and benzamides, have been studied. The rate of reaction of benzil with ammonia in aqueous dioxan is expressed as v=K_obs[Benzil][CN^–][NH₃]. The rates depend on the aliphatic amine, decreasing in the order MeNH₂ > EtNH₂ > BuⁿNH₂ > NH₃ > PrⁱNH₂ > Bu^tNH₂. The rate constants for substituted benzils fit the Hammett relationship with ∑σ, giving a ρ value of 2.70. Addition of ammonium chloride or potassium hydroxide decreases the rate. The reactivity order of amines for this reaction differs from that for the reaction with benzoyl chloride. Ammonolysis of α-cyanobenzyl benzoate in basic aqueous dioxan is much slower than cyanide ion-catalysed ammonolysis of benzil. These results suggest a mechanism involving reversible addition of cyanide ion and amine to the two carbonyl groups of benzil, followed by protonation and cleavage of the C–C bond assisted by electron withdrawal by the cyano-group and by electron release by the oxyanion or the hydroxy-group. The ratio of products (Ar¹CHO : Ar²CHO) from the reaction of unsymmetrical benzils (Ar¹COCOAr²) varies with the substituents in benzil and amine.