Assignment of the relative stereochemistry of two novel vicinal dibromo compounds using NMR and DFT-GIAO calculations

Bromination reactions are among the most important reactions in organic synthesis and can be carried out using bromine or many other bromine donor compounds. Among them, one of the best known is the addition of bromine to the C–C double bond to provide vicinal dibromides. Although the classical mechanism indicates the formation of bromonium ion as intermediate of the reaction delivering products in a stereospecific way, some substrates may lead to the mechanism via carbocation or may still have competition between the two types of mechanism. In this work, the addition of bromine to (2E,4E)-N-butylhexa-2,4-dienamide (1) produced erythro and threo diastereoisomers that were identified by comparing the experimental and calculated NMR chemical shifts. The theoretical values were obtained employing DFT-GIAO method and the comparisons were made using MAE values, CP3, and DP4 probabilities. For a better understanding of the bromination of 1, rate-limiting step and reaction energies were calculated for both mechanisms via bromonium and via carbocation.