The Selective Reduction Of Nitroarenes To N-arylhydroxylamines In Zn-CO₂-H₂O System

Both N-arylhydroxylamines and p-aminophenol are useful fine chemicals, and mostly applied in the synthesis of the pharmaceuticals and important intermediates. To date, the traditional method that N-arylhydroxylamines are prepared from the selective reduction of nitroarenes is still used. Nevertheless, during the reaction a large amount of NH4Cl is consumed and a lot of wastewater is formed.p-Aminophenol can be prepared by the Bamberger rearrangement of N-phenylhydroxylamine in the presence of acid medium (H2SO4 widely used), and alkali needs to be added after the reaction, thus a lot of wastewater is formed. So it is essential to open up an environmentally benign method, which efficiently avoids using inorganic acid and salt, and the subject is quite pertinent value and academic significance.Carbonic acid is formed in situ and it can be served as an acidic promoter for some reactions. We put forward the protocol that nitroarenes are reduced selectively to N-arylhydroxylamines using Zn in CO2-H2O system, and the application of the system is further explored in the Bamberger rearrangement.Firstly, nitrobenzene was selected as a model substrate, and the effects of many reaction factors on the selective reduction of nitrobenzene to N-phenylhydroxylamine, such as the reaction temperature, the molar ratio of Zn to nitrobenzene, the amount of water and the reaction time, were investigated systematically in normal pressure CO2-H2O system. Under the optimal reaction condition of nNB=10 mmol,nZn/nNB=3,T=25℃,VH2O=20mL, t=1.5 h, the yield of N-phenylhydroxylamine was 88%.The yield of N-phenylhydroxylamine was almost constant for 20 times of H2O recovered in the conditions adopted. And good results were also obtained for other nitrobenzenes substituted with other reducible functionaries, and the yields of the corresponding N-arylhydroxylamines were from 86% to 98%.Then the reduction of o-nitrotoluene to N-(o-methylphenyl)hydroxylamine was studied in Zn-CO2-H2O system systematically, and the yield of N-(o-methylphenyl)hydroxylamine was less 70% under the conditions adopted. The promoting effect of ethanol on the selectivity of N-methylphenyl)hydroxylamine in Zn-CO2-H2O system was observed. By adding appropriate amount of ethanol, the selectivity of N-(o-methylphenyl) hydroxylamine increased from 70% to 90% when the reduction was carried out at 25℃under normal pressure of CO2. Secondly, to suppress the further reduction of A N-arylhydroxylamine to aniline, the reaction need stop in the N-arylhydroxylamine stage. Ultrasound was introduced in Zn-CO2-H2O system and the effect of ultrasound on the reaction was studied. The results demonstrated that ultrasound did promote the selective reduction of nitroarenes to N-arylhydroxylamines, the reaction time was shortened, the amount of Zn powder was decreased and the selectivity of N-phenylhydroxylamine was improved. The yield of N-phenylhydroxylamine increased from 88% to 95%.At last, it was investigated that the selective reduction of nitrobenzene to N-phenylhydroxylamines using Zn in pressured CO2-H2O system, Under the optimal condition of nNB=10 mmol, nZn/nNB=3,T=40℃,PCO2=0.5 MPa,VH2O=25mL,t=3h, the yield of N-phenylhydroxylamine was 79%.Then the Bamberger rearrangement of N-phenylhydroxylamines to p-aminophenol was further explored in the system, the yield of p-aminophenol was 80% in the conditions adopted. It provides a basis for the selective reduction of nitrobenzene to p-aminophenol.