The nitro group is an important class of nitrogen derivatives. Like a carboxylate anion, it possesses a hybrid structure with a positive charge on the nitrogen and a resonance stabilized negative charge spread between two oxygen atoms. Importantly, the conversion of nitro to amine functionality is a useful synthetic tool that has received widespread in the chemical community. Nitro compounds are rare in nature; however, amines exist in great abundance in natural products. Traditional methods for the reduction of aromatic nitro groups are typically performed in the presence of a hydrogen source (H2 or NaBH4) with a transition metal catalyst (Raney’s Ni, Pd/C, cobalt (II), copper (II) or rhodium (III)) which can be expensive and hard to dispose of. A cheap and biodegradable method for such reduction is therefore in demand.
Figure 1: A scheme of the reduction of nitroarene using human hair as catalyst.
In 2020, Mohammad Gholinejad and his co-workers published their findings on the reduction of nitroarenes using human hair as a renewable catalyst. The preparation of the catalyst itself was rather simple: 2 grams of human hair crushed into small pieces, washed with distilled water then vacuum dried. The aqueous solution of NaOH (3.7 mmol, 3 mL) was then added with 1 gram of the prepared hair catalyst in a reaction tube and stirred at 50 °C until the hair was dissolved. While this may be hard to believe, NaOH solution was able to break down the exterior of human hair.
Figure 2: Human hair structure after treatment of NaOH solution under SEM.
A nitroarene derivative and NaBH4 were then added together in the presence of the hair catalyst at 50 °C with moderate stirring for at least 6 hours. It was found that using organic solvent (EtOH, DMF, toluene, or CH3CN) alone gives poor yield, whereas a combination of EtOH and H2O (1:4) as solvent increased the yield significantly. In addition, the yield was also improved when the amount of hair catalyst was increased from 0.4 g to 1.0 g, indicative that the quantity of the hair is important for high-yielding reactions. The research team performed a series of reductions on 21 structurally diverse nitroarenes and achieved good to excellent yields ranging from 80 to high 90 %. There are many other interesting findings. For example, it was found that the reaction worked best using brown female hair, while dyed or natural white hair produced poor yields. Additionally, the researchers tested non-human hair, specifically, they found that cat hair did not catalyze the reduction of nitroarene following the same procedure. The exact mechanism for the reaction remains elusive, but it was hypothesized that trace elements in human hair were responsible for catalyzing the reduction.
This paper presented a novel method for the reduction of nitroarenes to aniline derivatives using an available, inexpensive, and renewable catalyst in replacement of transition metal catalysts. This method was also proven applicable for large-scale synthesis, paving a way for the industrial reduction of nitro compounds.
The finding of this research has been published on Canadian Journal of Chemistry: Gholinejoad, M.; Esmailoghli, H.; Sansano, J. M. Human Hair Catalyzed Selective Reduction of Nitroarenes to Amines. Can. J. Chem. 2020, 98 (5). http://dx.doi.org/10.1139/cjc-2019-0444
If you are interested in more details about the work or want to see some research groups actually performed this reaction with confirmed TLC and NMR spectra, you can watch the following video:
コメント