| Nitrogenous compounds exist in many bioactive molecules,drug molecules and functional materials.The construction of C-N bond has broad development prospects and application value,and has become one of the research hotspots in the field of organic synthesis.In recent years,alcohols are used as cheap and sustainable raw materials instead of halides as alkylation reagents,and C-N bonds are constructed by hydrogen reaction,which provides a new strategy for green C-N bond synthesis.At present,most of the nitrogen sources in the reaction are amine compounds.Starting from the upstream raw material nitro compounds,amine compounds are generated by transfer hydrogenation,and then the series hydrogen borrowing strategy to realize the construction of C-N bond has the advantages of atom,high step economy and environmental friendliness,which has also become an important challenge in this research field.The general process of alcohol reducing nitro compounds by hydrogen to construct C-N bond is as follows:(1)under the action of transition metal complexes,alcohol dehydrogenation generates aldehydes,and transition metal complexes generate metal hydrides;(2)Reduction of nitro compounds by metal hydrides to form amines;(3)Aldehyde and amine condense to form imine intermediate;(4)The imine intermediate was reduced by metal hydride to produce the final N-alkylation product.The functionalized amide iridium complexes previously studied by the research group showed excellent activity in aqueous phase catalytic hydrogen storage and desorption of formic acid.Inspired by this,using these iridium complexes with excellent activity,this study designed and developed the synthesis of benzimidazole by hydrogen reduction of 2-nitroaromatic amines in aqueous benzyl alcohol and the selective preparation of amino alcohols by hydrogen reduction of nitroaromatic hydrocarbons in ethylene glycol.The functionalized amide iridium complexes previously studied by the research group showed excellent activity in aqueous phase catalytic hydrogen storage and desorption of formic acid.Inspired by this,using these iridium complexes with excellent activity,this study designed and developed the reaction of aqueous benzyl alcohol reducing o-nitroaromatic amines to synthesize benzimidazole by hydrogen and the selective preparation of aromatic amino alcohols by ethylene glycol reducing nitroaromatic hydrocarbons by hydrogen.Benzimidazoles were prepared by reducing o-nitroaromatic amines with hydrogen.The reaction conditions were as follows: water as solvent,1.5 equivalent potassium hydroxide and1.5 mol% Cat.h,120 ℃,reaction for 24 h.1-benzyl-2-phenyl benzimidazole was synthesized in one step,16 benzimidazole products were successfully prepared with the highest separation yield of 92%.Based on the comparative experimental research and literature reports,the possible mechanism of the hydrogen reduction reaction was put forward.The selective preparation of aromatic amino alcohols by hydrogen reduction of nitroaromatic hydrocarbons with ethylene glycol uses low toxicity and cheap ethylene glycol as raw material and reaction solvent to selectively synthesize N-hydroxyethyl aromatic amines or N,N-dihydroxyethyl aromatic amines by adjusting the type and amount of alkali and the amount of catalyst.The reaction system uses 2 equivalent cesium carbonate and 1.5mol% Cat.b,after reacting at 90 ℃ for 18 hours,N-hydroxyethyl arylamine was obtained in87% yield.N,N-dihydroxyethyl aromatic amines were obtained in 70% yield after reacting with 1 equivalent of potassium hydroxide,2.0 mol% Cat.b and 90 ℃ for 18 hours.The system has good applicability to substrates containing various substituents,and 13 kinds of Nhydroxyethyl aromatic amines and N,N-dihydroxyethyl aromatic amines were prepared with high yield and high selectivity.Based on the comparative experimental research and relevant literature reports,the possible mechanism of the hydrogen borrowing reaction was put forward,and the reason for the high selectivity of the reaction was revealed. |
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