Highly efficient synthesis of azos catalyzed by the common metal copper (0) through oxidative coupling reactions

Abstract

A facile and efficient approach to synthesize symmetric, asymmetric and bridged aromatic azo compounds (AAzos) from aromatic amines was developed by using red copper as catalyst. Despite numerous efforts towards the catalytic synthesis of symmetric and asymmetric AAzos derivatives, most reactions present certain drawbacks inhibiting their industrial applications, such as laborious multi-step processes, harsh reaction conditions and expensive reagents. And the synthesis of bridged azos had low yields before. With the presence of ammonium bromide as co-catalyst, pyridine as a ligand and molecular dioxygen as a sole oxidative reagent, red copper, a common and abundant metal in nature, exhibited unexpected catalytic activity towards the preparation of AAzos in high yields via one-step reaction, making this catalyst an attractive candidate for industrial and synthetic applications.

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Introduction

Aromatic azo compounds (AAzos) are high-value materials and are extensively used in the chemical industry as organic dyes,¹ food additives,² indicators,³ and drugs;⁴ they also have a wide range of applications in electronics⁵ and optics,⁶ and as chemosensors⁷ and diagnostic probes.⁸ The traditional method for synthesis of AAzos is based on the azo-coupling reactions with electron-rich aromatic compounds by the formation of diazonium salts,⁹ oxidation of hydrazines, reduction of azoxybenzenes, coupling of primary anilines with aromatic nitroso compounds and reductive coupling of nitroaromatics. Additionally, the formation of asymmetric aromatic azo compounds requires two step syntheses starting from anilines via diazonium salt or nitrosobenzene. However, all these methods present certain disadvantages. For example, there is a large amount of environmentally unfriendly wastes generated from nitrite salts or nitrosobenzene intermediates during the process azo coupling.¹⁰

In recent years, nanostructured materials have become the focus of researches¹¹ and on the basis of that, many new methods have been developed for the generation of azo compounds via the reduction of nitroaromatics or oxidation of aromatic amines. Sakai et al. described that AAzos were obtained from the catalytic hydrogenation of the corresponding nitroaromatics in the InX₃–Et₃SiH reduction system.¹² Our group developed new strategies based on nanostructured Pd¹³ and Pt¹⁴ catalysts (120 °C and 1 atm H₂) or ligand-free Pd(acac)₂ (ref. 15) (70 °C and 1 atm H₂) for green synthesis of AAzos from the corresponding nitroaromatics under mild conditions. Corma and coworkers reported a novel catalytic system using an Au/TiO₂ catalyst, which promotes formation of AAzos through oxidation of the corresponding amines under an initial 5 atm of O₂ with high selectivity and good yields.¹⁶ From the point of view from green chemistry, green catalysts from common metals have received great attentions in recent years.¹⁷ Jiao's group reported an approach that the aromatic azo formation could be achieved through a copper-catalyzed aerobic oxidative dehydrogenative coupling process in which CuBr acts as catalyst to facilitate the conversion of anilines to AAzos.¹⁸ Zhu and Shi also demonstrated a method that primary and secondary anilines can be coupled to corresponding azo compounds using CuBr catalyst.¹⁹ Another Cu(I)-catalyzed aerobic oxidative reaction of primary aromatic amines was reported by Lu and Xi using CuCl as catalyst.²⁰ It is worth mentioning that although Cu(I) presents high catalytic activity in various reactions, most of Cu(I)-compounds are expensive, and the chemical instability of them also limits their further applications. Therefore, it is highly desirable to develop a facile, green and efficient method for the AAzos formation.

Results and discussion

Herein, we report a novel, efficient and facile approach to synthesize both symmetric and asymmetric AAzos compounds from aromatic amines, by using a commercially available red copper (copper (0), purity: 99.99%) as catalyst, ammonium bromide and pyridine as co-catalysts and molecular oxygen as sole oxidant. Compared with Cu(I), red copper is more inexpensive and available. More importantly, the catalyst, red copper, is quite stable. In our catalytic system, the use of dioxygen as an ideal oxidant is also green, environmentally friendly and economical.

To demonstrate the high catalytic ability of red copper in the preparation of azobenzene, we selected aniline as substrate for the optimization of reaction conditions (Tables 1 and S1†). Table 1 shows the effect of different amount of red copper employed in this reaction on the transformation of aniline to azobenzene. The yields of azobenzene rose gradually with increasing the amount of red copper (Table 1, entries 3, 5–11), and a complete conversion was observed when 9 mg (0.14 mmol) or more red copper were added. Similarly, ammonium bromide, co-catalyst for this reaction, also exhibited catalytic effect on this transformation.

Table 1 The effect of catalysts, co-catalysts, pyridine and gas environment on the oxidative coupling of aniline^a

Entry	Red copper (mg)	NH4Br (mmol)	Pyridine (mmol)	Yield^b (%)
a All reactions were carried out with 1 mmol aniline and 2 mL toluene at 100 °C for 24 h under 1 atm of dioxygen.b GC yield.c 24 h under air.d 36 h under air.
1	3 (4.7 mmol%)	0.5	0.3	20
2	3 (4.7 mmol%)	1.0	0.3	28
3	3 (4.7 mmol%)	1.0	0.6	52
4	3 (4.7 mmol%)	1.0	0	3
5	4 (6.3 mmol%)	1.0	0.6	63
6	5 (7.9 mmol%)	1.0	0.6	98
7	6 (9.4 mmol%)	1.0	0.6	98
8	7 (11.0 mmol%)	1.0	0.6	99
9	8 (12.6 mmol%)	1.0	0.6	99
10	9 (14.2 mmol%)	1.0	0.6	100
11	10 (15.7 mmol%)	1.0	0.6	100
12^c	10 (15.7 mmol%)	1.0	0.6	43
13^d	10 (15.7 mmol%)	1.0	0.6	90

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With the increase of the amount of ammonium bromide, the yields of azobenzene were also increased from 20 to 28% (Table 1, entries 1 and 2). Pyridine, which is a ligand to the formation of Cu(I)–oxygen complex, acted as an important role in the reactions. Only trace amount of azobenzene was achieved without the presence of pyridine in the reaction mixtures (Table 1, entry 4), and the yields of azobenzene increased when increasing the amount of pyridine. Furthermore, this reaction could be carried out under air atmosphere (Table 1, entries 12 and 13) at a relatively slow reaction rate.

The kinetic curve of the transformation of p-toluidine to bis(4-methylphenyl)diazene, based on GC yields, is shown in Fig. 1. The reaction was carried out by using p-toluidine (5 mmol), red copper (∼50 mg, 0.78 mmol), NH₄Br (0.49 g, 5 mmol), pyridine (240 μL, 3.0 mmol), and toluene (10 mL) at 100 °C under 1 atm of O₂. In the first 6 hours, the yield of the products was rising gradually with time, and a complete conversion to azobenzene was observed after 6 hours, demonstrating the high efficiency of our catalytic system. In addition, when the reaction scale was expanded to 10 times larger by using 1.07 g (10.0 mmol) of p-toluidine, an excellent yield around 99% was still afforded after 12 hours, which demonstrated the high catalytic ability of the system. Optical images of the reactant before and after reaction was shown in Fig. 1 (insert). Red copper and NH₄Br crystals served as heterogeneous catalysts and the milky white colour reactant turned to dark red, which clearly indicated the azo formation.

Fig. 1 Time–conversion plot for p-toluidine oxidation using red copper as the catalyst. (Insert: optical images of AAzo formation before and after reactions using red copper as the catalyst.)

Fig. 2 showed the scanning electron microscopy images (SEM) and high resolution SEM images of the red copper catalysts before (A and B) and after (C and D) reactions, respectively. Obviously, the surface of copper became rough compared with that of the starting copper catalysts, which indicated the leakage of copper from the red copper catalysts. The concentration of the copper ions in the reactant was 0.143 mg mL⁻¹, which was examined by atomic absorption spectroscopy (AAS) analysis.

Fig. 2 SEM (A and C) and high resolution SEM (B and D) images of red copper before (A and B) and after reactions (C and D).

The proposed mechanism was shown in Fig. 3, which is very similar with the CuBr–pyridine–O₂ system shown in Jiao's report.¹⁸ At 1 atm oxygen environment, the surface of copper can be partially oxidized to form Cu₂O, which can in situ react with NH₄Br to generate CuBr. Activated Cu–oxygen complex was afforded while Cu(I) was chelated by a pyridine and followed by oxygen oxidation. At this stage, molecular oxygen was activated, which is the key catalysts for amine oxidation. Aromatic azos were finally synthesized by active Cu–oxygen complex via an oxidative coupling procedure.

Fig. 3 Proposed mechanism of azo formation catalyzed by red copper.

In order to further verify the stability and high activity of this catalytic system, we also prepared Cu nanoparticles (NPs) (Fig. S1†) and tested their activity for the synthesis of AAzos. The experimental results (Table S1,† entry 1) shown that they rarely catalyzed the formation of AAzos from the corresponding amines, due to high specific surface area of Cu NPs which were easily oxidized into Cu(II) in the presence of O₂ and NH₄Br. Under the optimized conditions for the formation of azobenzene from aniline, other symmetric azos were generated from their corresponding amino-aromatic compounds (Fig. 4, Table S3†). Almost all the para-substituted reactants were achieved in a quite quantitatively conversion (Table S3,† entries 1, 4, 6, 8, 10 and 11). Slightly differently, p-anisidine gave a relatively low yield (Table S3,† entry 9) at ∼28%, due to its low solubility in toluene. Like p-toluidine, both of m-toluidine and o-toluidine furnished quite high yields (Table S3,† entries 2 and 3) at >98%. Other ortho-substituted substrates (Table S3,† entries 5 and 7) gave a lower yield than the corresponding para-substituted substrates (Table S3,† entries 4 and 6). It was interesting to find that Ullman-amination was not observed while using Cl or Br containing aromatic amine as the starting materials (Table S3,† entries 4–7).

Fig. 4 Symmetric aromatic azos formation from different corresponding aromatic amine compounds.

With the purpose of testing the applicability of our catalytic system, we investigated its potential for generation of asymmetric azobenzene from corresponding aromatic amines, and the results were summarized in Fig. 5 (Table S4†). These results demonstrated that asymmetric substituted azobenzenes containing various electro-withdrawing or donating groups can be constructed efficiently by this simple catalytic system in accepted yields.

Fig. 5 Asymmetric aromatic azos formation from different corresponding aromatic amine compounds.

In addition, we surprisingly found that our catalytic system was also suitable for the synthesis of bridged azobenzene (yield: 50%) from its corresponding precursor (Scheme 1). This data has a significant improvement compared with the results from previous reports.²¹

Scheme 1 Reaction scheme for the synthesis of bridged azobenzene.

The bridged azobenzene can exhibit highly efficient reversible Z–E photoisomerization with visible light through ultraviolet absorption. From Fig. 6, the solution of Z-bridged azobenzene was light yellow before irradiation (Fig. 6A). However, the solution turned to bright red colour (Fig. 6B) upon UV irradiation at λ = 365 nm, due to the photoisomerization of molecules from Z-bridged azobenzene to E-bridged azobenzene. This result was consistent with the data reported from Temps group.²¹ As displayed in Fig. 6C, Z-bridged azobenzene showed a distinctive absorption band at λ ≈ 400 nm, while E-bridged azobenzene displayed a absorption peak at λ ≈ 500 nm.

Fig. 6 (A and B) Colors of Z-bridged azobenzene solution (before irradiation) and E-bridged azobenzene solution (after irradiation at λ = 365 nm) in n-hexane, respectively; (C) UV/vis absorption spectra of bridged azobenzene solution in Z and E isomerization.

In conclusion, we have successfully developed a facile and efficient approach for synthesis of symmetric and asymmetric aromatic azo compounds and a bridged azobenzene by using common red copper as catalyst under mild reaction conditions. Easy availability of red copper as catalyst, low reaction temperature and high catalytic efficiency render this system as attractive candidates for industrial and synthetic applications. Further study of this efficient catalytic system is ongoing in our laboratory to investigate wider applications.

Acknowledgements

H. W. G. acknowledges the financial support from NSFC (no. 21003092, no. 21373006), the Key Project of Chinese Ministry of Education (no. 211064), and the Priority Academic Program Development of Jiangsu Higher Education Institutions.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedures and full spectroscopic data for aromatic azo products. See DOI: 10.1039/c4ra00749b

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