From N ‐(dinitrophenyl) amino acids to benzimidazole N ‐oxides. Synthesis, kinetics and mechanism

Several benzimidazole N ‐oxide derivatives were synthesized in very good yields by heating under reflux the corresponding N ‐(2,4‐ or 2,6‐dinitrophenyl) amino acid derivative with NaOH in 60% 1,4‐dioxane–H 2 O. The N ‐oxides obtained from glycine and α‐ and β‐alanine derivatives lost the carboxylic group. The observed rate constant for the reaction of N ‐(2,4‐dinitrophenyl) glycine ( 2a ) in 10% 1,4‐dioxane–H 2 O to give 5‐nitro‐1 H ‐benzimidazole‐3‐oxide ( 4a ) is first order on [NaOH]; the second‐order rate constant is k N =1.7×10 −3   M −1  s −1 . The mechanism proposed includes the formation of an N ‐alkylidene 2‐nitrosoaniline‐type intermediate as the rate‐determining step. Copyright © 2006 John Wiley & Sons, Ltd.