Catalytic Transfer Hydrogenation Reduction Of Nitroarenes And Mechanism Study

It is the major method to prepare aromatic amines from the corresponding nitroarenes. Recently, catalytic transfer hydrogenation (CTH) reduction has attracted much attention because of environmentally benign procedure, mild reaction conditions, and high yields, etc. Moreover, heterogeneous catalyst is widely used in CTH reduction reaction because it is easily recycled. This thesis focuses on heterogeneous CTH reduction of nitroarenes and the mechanism. The content is described as following three parts:(1) The Pd/C-HCO2NH4system for CTH reduction reaction of4-nitro benzoic acid esters is conducted:(a) Taking ethyl4-nitro benzoic acid ester as model substrate, the effect of solvent, catalyst loading, and the choice of hydrogen donors are investigated. With Pd/C (5.0mg/mmol) as catalyst, EtOH (2.5mL/mmol) as solvent, HCO2NH4(3.6eq) as hydrogen donor, this was optimized reaction conditions. And the result of scale-up reaction (60mmol) for synthesizing benzocaine via above reduction conditions is also liable. This catalytic system could be extended to other substrates. Generally, Pd/C (5.0mg/mmol) and the reaction time (1-2h) are necessary, and the yields could be above98%for the linear alkyl ester substrate. However, for the branched alkyl esters, the higher catalyst loading (10.0mg/mmol) and longer reaction time (1-6h) are indispensable to achieve high yields (96%-98%).(b) Employing phenyl4-nitro benzoic acid ester as model substrate, the effect of additive anhydrous MgSO4, catalyst loading, and reaction temperature are studied. Pd/C (15.0mg/mmol) as catalyst, EtOH (2.5mL/mmol) as solvent, HCO2NH4(3.6eq) as hydrogen donor, and room temperature are determined as the optimized conditions. The substrate scope is broad and a variety of aryl and benzyl4-nitro benzoic acid esters can be reduced to corresponding aromatic amines in50%-95%yields.(2) With ethyl4-nitro benzoic acid ester as the model, the reduction mechanism of Pd/C-HCO2NH4system is studied. According to the HPLC-MS results, the nitroso compound is regarded as the first intermediate, and three reduction pathways are proposed:(a) the nitroso is reduced to hydroxylamine intermediate, and then further reduced to aromatic amine;(b) The nitroso is reduced to azoxy compound by self-coupling reaction and the following intermediates are the azo and azohydro compounds; disproportionation reaction of azohydro proceed and the products are azo and amino aromatic compounds.(c) Azoxy compound also derives from the cross-coupling of the nitroso and hydroxylamine, but azo intermediate couldn’t be accessed via the cross-coupling of the nitroso and aromatic amine.(3) The reliable procedures for the mono-selective reduction of dinitro arenes are scarce. In this part, the monoreduction of4-substituent-2,6-dinitrophenol in FeO(OH)-N2H4’H2O system is explored:(a) Choosing4-chloro-2,6-dinitrophenol as model substrate, the parameters of the reaction (solvent, iron-catalyst loading and species, hydrazine hydrate loading, and reaction temperature) are investigated. The optimized reaction conditions are: FeO(OH)(1.0mol%), N2H4·H2O (1.6eq), EtOAc (2mL/mmol),80℃(oil bath temperature),6h.(b) A series of dinitro arenes including4-(halogen or alkyl)-2,6-dinitro-phenols could be efficiently monoreduced in good yields (88%-94%).