Study On Green Chemistry Reactions Of Aromatic Nitro Compounds In Aqueous Solution

Aromatic nitro compounds are an active class of nitrogen compounds, and have found many applications in organic syntheses. For example, the selective reduction reactions of aromatic nitro compounds (the preparations of azoxybenzenes) and C-C coupling reactions of p-nitrotoluenes are both very important organic reactions, and have a certain value in industrial production. However, most of these reactions take place in the flammable and volatile organic solvents, and the used reagents have some shortcomings such as toxicity, complex preparation, difficult preservation and serious pollution, which makes them be restricted in the practical applications. Based on the above background, the study on the green chemical reactions of aromatic nitro compounds in aqueous solution has become one of the hot research topics in organic chemistry. In this dissertation, the selective reduction reactions and C-C coupling reactions of aromatic nitro compounds were studied in detail in aqueous solution in the presence of poly(ethylene-glyco 1) (PEG) orβ-cyclodextrin (β-CD) respectively, and the reaction conditions were optimized, the substituent effects were summarized, the reaction mechanisms were researched, which provided ultimately the new green methods for the syntheses of azoxybenzenes and p-dinitrobibenzyls.The major contents were as follows:1. A series of different substituted aromatic nitro compounds in aqueous solution were selectivly reduced to the corresponding azoxybenzenes in good yields by mild potassium borohydride in the presence of cheap and non-toxic PEG-400 as a phase transfer catalyst. The protocol is simple, economic, and environmental friendly. The specific results were as follows:i)The optimized experimental conditions for the reductions:n(nitroarenes): n(KBH4):n(PEG-400)=1:8:0.5 (mol), the yields reach to 50-92% at the reflux temperature;ii)The electron-withdrawing substituents promote the formation of the azoxybenzenes, while the electron-releasing substituents retard the reduction to various degrees depending upon their electron-donating ability. In addition, for the same substituent,m-substituent is more favorable than p-substituent, and o-substituent is the most unfavorable to the reduction;iii)The introduction of phase transfer catalyst PEG-400 has few effect on the reductions of the water-soluble aromatic nitro compounds;iv)For the water-insoluble aromatic nitro compounds whose melting points are higher than 100℃, a certain amount of ethanol solvent (the volume ratio of ethanol and water is 1:1) need to be added to facilitate the reduction reactions.2. A series of different substituted aromatic nitro compounds in aqueous solution were selectivly reduced to the corresponding azoxybenzenes in good yields byβ-CD/NaOH in water. The reaction conditions have been optimized, the substituent effects on the reactions have been summarized, the reaction mechanisms have been researched in detail by a large number of comparative experiments and careful reasonings, and the roles ofβ-CD and NaOH in the reductions have been determined, which not only provide a simple and efficient preparation for the azoxybenzenes under green chemistry conditions, but also enriched and expanded the applications ofβ-CD in organic synthesis. The specific results were as follows:i)The optimized experimental conditions for the reductions:n(nitroarenes): n(β-CD)=1:1.2 (mol), the mass concentration of NaOH is 20%, the yields reach to 41-82% at 85℃;ii)The electron-withdrawing substituents promote the formation of the azoxybenzenes, while the electron-releasing substituents are opposite. The results suggest that the stronger the electron withdrawing ability, the better the reduction reaction carried out;iii)The results are greatly effected by the location and nature of substituents:for o-chloronitrobenzene, no o,o’-dichloroazoxybenzene could be isolated, while the further reduced product o,o’-dichlorohydrazobenzene was obtained; For o-bromonitrobenzene and o-iodonitrobenzene, the reductions gave very complicated mixtures which were difficult to separate, and detailed studies were under way; For p-nitrotoluene, no p,p’-dimethylazoxybenzene could be isolated, while p,p’-dinitrobibenzyl was obtained by the carbon-carbon coupling reaction;iv)For the water-insoluble aromatic nitro compounds whose melting points are higher than the reaction temperature, a certain amount of ethanol solvent (the volume ratio of ethanol and water is 1:1) need to be added to facilitate the reduction reactions;v)the reduction mechanism is preliminarily concluded as that the aromatic nitro compounds included in the hydrophobic cavity ofβ-CD had been smoothly reduced by the corresponding deprotonated alcohol anion (RO-) ofβ-CD. Herein,β-CD not only has played a role of phase transfer catalyst, but also played a role of reducing agent.3. The C-C coupling reactions of several substituted p-nitrotoluenes were studied in aqueous solution withβ-CD/NaOH. The reaction conditions have been optimized, the substituent effects on the reactions have been summarized, the reaction mechanisms have been preliminarily studied, which not only provide a new green synthesis for the p,p’-dinitrobenzyls, but also enriched and expanded the applications ofβ-CD in organic synthesis. The specific results were as follows:i)The optimized experimental conditions for the C-C coupling reactions: n(nitroarenes):n(β-CD)=1:1.5 (mol), the mass concentration of NaOH is 20%, the p,p’-dinitrobenzyl compounds are obtained in moderate yields at 65℃in the air;ii)The C-C coupling reactions of p-nitrotoluene and p-nitroethylbenzene could smoothly produced the corresponding p,p’-dinitrobenzyls. For substituted p-nitrotoluenes, the o-substituted groups could facilitate the coupling reactions, however, the m-substituted groups entirely retard the reactions;iii)The results suggest that electron-withdrawing substituents promote the formation of the bibenzyls, however, the yields of bibenzyl will drastically reduce when the electron-withdrawing ability is increased to some extent;iv)The C-C coupling reaction mechanism of p-nitrotoluenes in aqueous solution withβ-CD/NaOH:the neutral p-nitrotoluenes included in the hydrophobic cavities ofβ-CD are deprotonated by the strong alkali alcohol anion ofβ-CD(or a small amount of NaOH) and generate the corresponding carbanions which then are converted into free radicals by the charge-transfer processes, and then the bibenzyl radical anions are formed by the coupling of free radicals and carbanions of p-nitrotoluenes, and lastly the bibenzyl prouducts are obtained.To sum, we have provided the new green syntheses of azoxybenzenes and p,p’-dinitrobenzyls, and expanded the contents of organic reactions in aqueous solution, and supplemented and enriched the contents ofβ-cyclodextrin and PEG. The research has some theoretical significance and application value.