A probable hydrogen-bonded meisenheimer complex: An unusually high S _NAr reactivity of nitroaniline derivatives with hydroxide ion in aqueous media

摘要

Observations show that nitroanilines exhibit an unusually high S _NAr reactivity with OH- in aqueous media in reactions that produce nitrophenols. S_NAr reaction of 4-nitroaniline (2a) in aqueous NaOH for 16 h yields 4-nitrophenol (4a) quantitatively, whereas a similar reaction of 4-nitrochlorobenzene (1a) gave 4a in 2% yield together with recovered 1a in 97%, suggesting that the leaving ability of the NH_2 group far surpasses that of Cl under these conditions. An essential feature of S_NAr reactions of nitroanilines is probably that the NH_2 leaving group participates in a hydrogen-bonding interaction with H _2O. Density functional theory (DFT) calculations for a set of 4-nitroaniline, OH-, and H_2O suggest a possible formation of a Meisenheimer complex stabilized by hydrogen-bonding interactions and a six-membered ring structure. The results obtained here contrast with conventional S_NAr reactivity profiles in which nitroanilines are nearly unreactive with nucleophiles in organic solvents.