Kinetics and Mechanism of the Permanganate-Induced Oxidative Catalytic Condensation of Sarcosine to a Diketopiperazine

Abstract

The kinetics and mechanism of the permanganate-induced oxidative catalytic condensation reaction of sarcosine, as a secondary amino acid, in buffered medium (pH = 1-3) has been investigated spectrophotometrically. Analysis of the reaction products has shown that, in contrast to oxidation product of primary α-amino acids which is an aldehyde species, a valuable diketopiperazine compound namely 1,4-dimethylpiperazine-2,5-dione has been obtained as the sole product in 95% yield via a cheap, simple, efficient, and novel method. In an analogy to some α-amino acids in concentrated acidic medium, conclusive evidence has demonstrated delayed autocatalytic activity by Mn2+ in this reaction. It has been found that such activity can appear when a certain concentration ratio of Mn2+ to sarcosine is built up in the medium, which we call the “critical ratio”. Decisive results have shown that the magnitude of the latter ratio depends on pH, and the exhibition of autocatalytic activity is affected by the pH and ionic strength of the medium. Regarding the “delayed autocatalytic behaviour” of Mn2+ ions, rate equations have been presented confirming observations for both catalytic and non-catalytic routes. The reaction displays a first order dependence on permanganate ions and sarcosine concentrations, both in the catalytic and non-catalytic pathways, and an apparent first order dependence on Mn2+ ions in the catalytic pathway. The “critical ratio” and “delayed autocatalytic behaviour” concepts have been corroborated by the correspondence of the pseudo-order rate constants of the catalytic and non-catalytic pathways to the Arrhenius and Eyring laws. The activation parameters associated with both pathways have been calculated and discussed. The much higher value of the activation entropy obtained for the catalytic process sows that this factor controls the higher reaction rate relative to that of the non-catalytic pathway. Mechanisms for both catalytic and non-catalytic routes have been proposed which include formation of a bislactam product through a diacylperoxide intermediate.