ELECTROCHEMICAL AND STEREOCHEMICAL INVESTIGATION ON THE MECHANISM OF THE DECAY OF 2-HALO AMIDE ANIONS - THE INTERMEDIACY OF AZIRIDINONES

2-Halo amide anions can be generated by electroreduction of the corres ponding NH-protic 2-halo amides through the self-protonation mechanism . Such anions are labile species whose decay, in the case of 2-bromo a mide anions, competes with their electroreduction in the voltammetric time scale. Using the appropriate voltammetric treatment, the first-or der rate constant of the decay has been determined for a series of rep resentative 2-bromo amides. The lability orders point to an S(N)2-type intramolecular substitution of bromine and thus to the formation of a three-membered ring. Stereochemical information on the decay has been gained using a chiral nonracemic 2-bromopropanamide, an amine nucleop hile, and DMF as the solvent. The direct substitution by the amine pro ceeds by an S(N)2 reaction, as witnessed by inversion of configuration at the alpha-carbon and voltammetric analysis. Conversely, when the r eaction is triggered by electroreduction, the decay of the 2-bromo ami de anion eventually leads to the formation of the retention product 2- amino amide together with other optically active products, namely two diastereomeric oxazolidin-4-ones, arising by cyclocondensation with DM F, and cis-2,5-dioxopiperazine. Analysis of the electrochemical and st ereochemical results indicates that the mechanism of the base-promoted reactions of 2-halo amides proceeds through the transient formation o f the corresponding aziridinone, independently of the fact that the la tter is isolable or not. The formation of the aziridinone takes place by concerted intramolecular nucleophilic substitution of bromide ion w ithin the 2-halo amide anion. The transient aziridinone behaves not on ly as the product-determining intermediate but also as a species capab le of reacting with suitable partners under remarkable enantioselectiv ity control.