Mechanism of Debromination of 1-Aryl-1,2-dibromo-2-nitropropanes Promoted by Secondary Amines in Acetonitrile

Debromination reactions of erythro-dl-l-aryl-l,2-dibromo-2-nitropropanes with secondary amines in MeCN have been investigated kinetically. Reactions of erythro-dl-l-aryl-l,2-dibromo-2-nitropropanes with secondary amines in MeCN are stereospecific, producing (E)-l-aryl-2-nirropropenes quantitatively. The rate equation for the reaction is K_obS = K₂[R₂NH] + K₃[R₂NH]², indicating that the reactions proceed by both uncatalyzed and base-catalyzed pathways. The Hammett Q, Bronsted β, ΔH*, and ΔS* values for the k₂ and k₃ processes are 1.22 ± 0.04 and 1.20 ± 0.02, 0.77 ± 0.05 and 1.05 ± 0.13, 5.6 ± 0.3 and 1.7 ± 0.1 kcal/mol, and —49.1 ± 4.5 and —60.7 ± 204 eu, respectively. For dehalogenation reactions from vicinal dibromo and bromochloro compounds, the element effect of the leaving group K_BR/K_CI = 7.8 and 25 have been determined for the k₂ and K₃ processes. From these results, the mechanism and the transition-state character of these reactions are assessed.