Article pubs.acs.org/IECR
Bamberger Rearrangement of N‑Arylhydroxylamine to p‑Aminophenol in a CO2−H2O System Shijuan Liu,*,† Yuanping Hao,‡ and Jingyang Jiang‡ †
College of Chemistry, Key Laboratory of Preparation and Application of Environmental Friendly Materials of Ministry of Education, Jilin Normal University; Siping 136000 China ‡ College of Chemical Engineering, State Key Laboratory of Fine Chemicals, Dalian University of Technology; Dalian 116024 China S Supporting Information *
ABSTRACT: The Bamberger rearrangement of N-aryllhydroxylamine was first realized in a CO2−H2O system. The yield of paminophenol was 80% when N-phenylhydroxylamine was heated at 100 °C for 1 h under 4 MPa CO2. The process fully avoids the need of inorganic strong acid and is environmentally benign.
1. INTRODUCTION p-Aminophenol (PAP) as an important chemical intermediate is used in pharmaceuticals, dyestuffs, and so on. PAP can be prepared from p-nitrophenol using Fe powder under acidic conditions using strong acid, such as aqueous HCl, where excessive acid is needed and a large amount of iron sludge (mainly consist of iron oxides) is produced.1 An important and economical process for the preparation of PAP involves the selective reduction of nitrobenzene (NB),2−4 in which Nphenylhydroxylamine (PHA) is first formed, and this intermediate can immediately rearrange to PAP in H2SO4 aqueous solution, the reaction was discovered by Bamberger and called the Bamberger rearrangement.5 Currently, the method of the reaction has been employed to synthesize PAP, and the reaction mechanism has been explored. The reaction occurs through intermolecular rearrangement by the 18 O exchange between H218O and PhNH16OH.6 Heller et al. claimed that an SN1 mechanism of the reaction was more feasible,7 and Sone further confirmed the reaction as an SN1 mechanism by the kinetic investigation.8 Yamabe first investigated the mechanism of the Bamberger rearrangement of PHA to PAP by density functional theory calculations and found that a new mechanism of the rearrangement including the aniline dication-like transition state (Ph−N(OH)H + (H 3O + ) 2 (H 2O) 13 ) was proposed.9 And the Bamberger rearrangement was sensitive to the acidity of the solution. In the pH region ([H2SO4] < 1 N), monoprotonated species contributed to the reaction, and the byproducts, such as nitrosobenzene (NOB) and azoxybenzene (AOB), were detected, whereas both mono- and diprotonated species contributed to the reaction in the H0 region ([H2SO4] > 1 N) and these byproducts were almost undetected.8 H2SO4 is usually used as the Bamberger rearrangement catalyst to promote the reaction. However, H2SO4 cannot be reused, and it is necessary to neutralize the excessive acid with base after the reaction and salt formation is inevitable in the aqueous solution. Therefore, it is important to develop an environmentally benign method avoiding the use of the conventional inorganic acid. © 2014 American Chemical Society
Some acid catalysts have been applied in the Bamberger rearrangement instead of H2SO4,10−17 such as ion-exchange resin, heteropolyacid, acidic clay, acidic zeolite, and acidic ionic liquid. The use of these acids overcomes the problem produced by inorganic acid, and neutralization is not necessary. And these acid catalysts could be reused, but they are far inferior to H2SO4 for the PAP yield and the reaction process is costly because these catalysts are expensive. Recently, a CO2−H2O system has been regarded as an environmentally benign alternative to the conventional acids because the system will be self-neutralized on the release of CO2 after the reaction. Several research works have been pioneered relating to the application of this system.18−22 In our previous work, the selective reductions of nitroarenes to Narylhydroxylamines using Zn in a CO2−H2O system have been developed.23,24 In this work, the Bamberger rearrangement of PHA to PAP is performed in a CO2−H2O system.
2. EXPERIMENTAL SECTION N-Arylhydroxylamine was prepared from the selective reduction of the corresponding nitrobenzene with zinc dust in a CO2−H2O system.23 N-Arylhydroxylamine (0.5 mmol) and 50 mL of H2O were placed in a 100 mL Parr autoclave (Parr 4842) that was connected with a high pressure liquid chromatography pump (PU-1580, JASCO Co.) for supplement of CO2. After that, the autoclave was sealed and the reactor was flushed three times with 0.5 MPa CO2 and then was heated to a designated temperature. Meanwhile, CO2 was gradually charged to the reaction system to maintain a designated pressure. Then the system maintained the designated temperature and pressure for a certain period of time. The reaction time reported in this experiment did not include the time required to heat or cool the system (ca. 15 min each). After the reaction, the reaction solution was cooled and CO2 was released slowly. The product analysis was performed by HPLC Received: Revised: Accepted: Published: 8372
February 27, 2014 April 29, 2014 May 1, 2014 May 1, 2014 dx.doi.org/10.1021/ie500864p | Ind. Eng. Chem. Res. 2014, 53, 8372−8375
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on Agilent 1100 series (column Agilent TC-C18, 4.6 mm × 250 mm, 5 mm; UV detector, 254 nm; eluent, H2O + NH4Ac− CH3OH). The crude products of these p-aminophenols were obtained from the aqueous solution by vacuum distillation and then were purified by column chromatography. Melting points of the products were measured on a X-6 (Beijing Tech. instrument Co. Ltd.) melting-point apparatus. 1H NMR spectra were recorded on Inova 400 (Varian). The data of melting points and 1H NMR spectra of these p-aminophenols obtained were consistent with those reported in the literature.
rearrangement did take place in a CO2−H2O system, and the yield of PAP increased evidently even with a charge of 0.5 MPa CO2. Therefore, CO2 plays an important role in the reaction. The conversion of PHA increased with increasing CO2 pressure to a maximum value of 100% at 4.0 MPa CO2. When the CO2 pressure was higher than 4 MPa, the yield of PAP was almost constant (about 80%). It is well-known that in an aqueous medium, CO2 dissolves in H2O to form carbonic acid, which dissociates to lower the pH of the mixture. The increases in the conversion of PHA were most distinct at early time, because the acidity of the solution noticeably increased with the introduction of CO2 gas. As reported in the literature, the pH value of the CO2−H2O system changes from 6.8 to 3.23 when the CO2 pressure increases from 0 to 4.0 MPa at 22 °C.25 However, it changes very little with further increasing pressure and the measured pH values varied only from 2.95 to 2.80 under CO2 pressures of 7.0−22.0 MPa and at temperatures of 25−70 °C.26 The acidity of the system is dictated by the solubility of CO2 in H2O and the dissociation of H2CO3, and H2CO3 is essentially equal to the aqueous concentration of CO2. Thus, at the same temperature the conversion of PHA and the yield of PAP depended on CO2 pressure. If the CO2 pressure is low, the acidity of the solution is weak, and the concentration of [H+] is low, so the protonated PHA is difficult to form. So in this work PAP was not produced without CO2, and the conversion of PHA and the yield of PAP increased greatly with the increase of CO2 pressure from 0 to 2.0 MPa, and they were nearly constant when the CO2 pressure was above 4.0 MPa. During the process of the Bamberger rearrangement of PHA to PAP in a CO2−H2O system, first PHA could be converted to its protonated species, as the necessary intermediates, and then it was converted to PAP. And a small amount of o-aminophenol was detected in the experiment. According to Sone's research, the Bamberger rearrangement occurred by an SN1 mechanism, and the O-protonated or the N-protonated arylhydroxylamine (ArNHO+H2 or ArN+H2OH) was the active species in the pH region ([H2SO4] < 1 N).8 And a CO2−H2O system is a weakly
3. RESULTS AND DISCUSSION To explore the possibility of the Bamberger rearrangement of PHA in a CO2−H2O system, the effect of CO2 pressure on the reaction at 100 °C was first illustrated in Figure 1.
Figure 1. Effect of CO2 pressure on the reaction.
The results showed that no PAP, the product derived from the Bamberger rearrangement of PHA, could be found in the absence of CO2, but the byproducts, such as aniline (AN), NOB, and AOB, were detected. Nevertheless, the Bamberger
Scheme 1. Proposed Mechanism of the Reaction in a CO2−H2O System
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certain period of time under different CO2 pressures (entries 1−6). When the CO2 pressure was higher, the reaction time was shorter (entries 1, 2 vs entries 3−6). The reason could be that H2CO3 is essentially equal to the aqueous concentration of CO2. When the CO2 pressure is high, the solubility of CO2 in water is high, and the consumption of the acid can be compensated quickly. This new method is also applicable for the selective reduction of PHA substituted with −CH3 and −Cl on the ortho situation (entry 7 and 8). Through the experimental results, it could be seen that the corresponding p-aminophenols from (2-methylphenyl)hydroxylamine and (2chlorophenyl)hydroxylamine were in higher amounts than those from PHA under the same conditions, which could be related to the dynamics and the mechanism of the reaction. Sone revealed that the electronic effect and steric hindrance accelerated the reaction rate of the Bamberger rearrangement.28 Because −CH3 was an electron-donating group, the reaction results of M-PHA were the best and the yield of 4-amino-3methylphenol was 92% in the experiment. During the reaction, a small amount of the side reaction products, such as AN, NOB, and AOB, existed all along. The reactions relating to the Bamberger rearrangement of PHA in a CO2−H2O system could be as follows (Scheme 2). On the one
acidic solution, so the monoprotonated species could be more feasible, the reaction mechanism in the system could be as gvien in Scheme 1. With 4.0 MPa CO2 pressure, the effect of the reaction temperature on the Bamberger rearrangement was studied. In Figure 2 were shown the results of the different reaction temperature on the reaction.
Figure 2. Effect of the reaction temperature on the reaction.
Scheme 2. Reactions Relating to the Bamberger Rearrangement of PHA
In the temperature range from 40 to 140 °C, both the conversion of PHA and the yield of PAP increased. At 40 °C, the conversion of PHA was low and a PAP yield was only 3%. The reaction is endothermic, and the CO2−H2O system is a weak acid solution, so PHA is rearranged to PAP rather slowly at lower temperature. With the increase of the reaction temperature, the yield of PAP increased greatly, and at the temperature of 100 °C the yield of PAP reached 80%. And for a constant CO2 partial pressure, the pH of high-temperature water solution increases as the reaction temperature increases,27 so a further increase of the reaction temperature has no obvious effect on the reaction results. The above-mentioned results showed that the introduction of CO2 to the reaction system was essential to promote the Bamberger rearrangement and the reaction temperature was also an important factor. The effects of other factors on the reaction were also studied, and the results were listed in Table 1. It could be seen that further prolongation of the reaction time resulted in increasing the conversion of PHA and the yield of PAP, and PHA could be converted completely within a
hand, the formation of PAP from PHA occurs through a series of steps from protonated PHA. On the other hand, in less acidic solution PHA is difficult to protonate completely, and the unprotonated PHA leads to disproportionate formation of AN and NOB, which reacts with another PHA to form AOB. In the CO2−H2O system, these byproducts were detected. The CO2−H2O system is an environmentally benign alternative for the Bamberger rearrangement, because it will self-neutralize on release of CO2 at the end of the reaction. Further work on the reaction is under investigation.
Table 1. Investigation of the Reaction Conditions on the Bamberger Rearrangementa entry
substrate
P (MPa)
t (h)
PHA conv (%)
PAP yield (%)
1 2 3 4 5 6 7b 8c
PHA PHA PHA PHA PHA PHA M-PHA Cl-PHA
4 4 0.5 0.5 0.5 0.5 4 4
0.5 1 0.5 1 2 3 1 1
96 100 78 85 95 100 100 100
77 80 58 63 69 73 92 84
4. CONCLUSION The Bamberger rearrangement of N-arylhydroxylamines to the corresponding p-aminophenols was realized in a CO2−H2O system. The yield of p-aminophenol was 80% when the reaction was heated at 100 °C for 1 h under 4 MPa CO2, and the yield of 4-amino-3-methylphenol was 92% under the same conditions. The method provides an environmentally benign
Reaction conditions: substrate 0.5 mmol, H2O 50 mL, 100 °C. bMPHA: (2-methylphenyl)hydroxylamine, the yield of 4-amino-3methylphenol. cCl-PHA: (2-chlorophenyl)-hydroxylamine, the yield of 4-amino-3-cholrophenol. a
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way for the preparation of p-aminophenol from the corresponding N-arylhydroxylamine.
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ASSOCIATED CONTENT
S Supporting Information *
Characterization data for the products. HPLC and 1H NMR spectra. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*S. Liu: e-mail,
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Doctoral Fund of Ministry of Education of China (Grant No: 20070141046). REFERENCES
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