Reactions of the carbonyl group with nitroso compounds. The cases of pyruvic acid and acetaldehyde

Abstract

Pyruvic acid and acetaldehyde react with substituted nitrosobenzenes to give the corresponding N-phenylacetohydroxamic acids. A mechanism for these reactions involving three sequential steps is proposed. The first step is the nucleophilic attack of the nitroso group on the carbonyl group, which leads to the formation of an unstable dipolar intermediate. This step is followed by proton transfer to the dipolar intermediate to form a more stable cationic intermediate, which, in the subsequent step, undergoes decarboxylation (in the case of pyruvic acid) or elimination of a proton, from the carbon of the nitrosocarbinolic group (in the case of acetaldehyde), leading to the final product, hydroxamic acid.The reaction of pyruvic acid includes an intramolecular reaction pathway, along with an acid-catalysed one. The experimental evidence obtained in support of such a description includes: (a) the order of reactivity of substituted nitrosobenzenes as demonstrated by the plot of log kobsvs. Hammett parameters with slope –1.97 in the case of pyruvic acid and –0.93 in the case of acetaicohyde; (b) the observation of acid-catalysed and ‘uncatalysed’ pathways in the reaction of pyruvic acid; (c)the observation of general acid catalysis in these reactions; (d) the observation of an inverse solvent deuterium isotope effect of 0.41 in the case of acetaldehyde; (e)the observation of a solvent deuterium isotope effect of ca. 1.0 in the acid-catalysed reaction, and solvent isotope effect of ca. 1.2 in the ‘uncatalysed’ reaction of pyruvic acid with nitrosobenzene.