Farhad Panahi*ab,
Fatemeh Roozbina,
Sajjad Rahimia,
Mohammadesmaeil Moayyeda,
Aria Valaeia and
Nasser Iranpoor*a
aDepartment of Chemistry, College of Sciences, Shiraz University, Shiraz 71454, Iran. E-mail: iranpoor@chem.susc.ac.ir
bDepartment of Polymer Engineering and Color Technology, Amirkabir University of Technology, Tehran, Iran. E-mail: fpanahi@aut.ac.ir
First published on 12th August 2016
A novel P,N-ligand was introduced for efficient Ni-catalyzed amination of aryl chlorides. Reaction of cyanuric acid (1,3,5-triazine-2,4,6-triol) and trichlorophosphine (PCl3) resulted in the production of a new porous material (TPPM) containing triazine rings with phosphite moieties in a sheet morphology. Cavities in the prepared compound create sites on the surface of the material with appropriate ligation character to coordinate with metals for catalytic purposes. The nickel-catalyzed amination of aryl chlorides and of phenols in their 2,4,6-triaryloxy-1,3,5-triazine (TAT) protected form were efficiently accomplished in the presence of this easily prepared and reusable P,N-ligand under mild reaction conditions. More importantly, TPPM was reusable for 5 iterations following this protocol without significantly decreasing in its activity.
Supporting the ligands on a solid substrate can allow the recycling of ligands and simplify the workup process.7 In this study we have focused on an easy approach, which relies on a polymeric structure containing P,N-ligation sites. Organic polymers that contain ligated metals for catalytic purposes, offer a variety of unique properties.8 The strong coordination of metals to these ligation centers results in the construction of catalytic positions for application in TMC.8c,d On the other hand, the presence of catalytic sites on a flexible polymeric backbone can represent the action of enzymes in natural systems, which can increase the interactions between the catalyst and the substrates through the folding of the main chain.9 Furthermore, these structures cause an increase in the local concentration of both the substrates and catalyst. Consequently, these functions not only lead to enhancements in reaction rates, but also provide an easy scaffold for recycling.10
Herein, we disclosed the preparation of an easily prepared and reusable P,N-ligand from commercially available and cheap starting materials in a single step process. Reaction of PCl3 with cyanuric acid produces an organic polymer (triazine-phosphite polymeric material, TPPM) which is introduced as a new P,N-ligand (Scheme 1).
In this study, a TPPM was employed as a ligand in metal-catalyzed amination of aryl chlorides.11 The TMC amination of aryl halides is an important tool for the production of arylamines.12 The expansion of simple metal catalyst systems for the coupling of aryl electrophiles with NH3 has been a significant challenge over the past decade.13 Most of these catalyst systems carried out the coupling reaction of aryl chlorides with NH3 in low yields, especially when unactivated substrates were used.14 The competition of the solvent with NH3 was observed in several cases and is a key limitation.15 In some of the methodologies used, the aniline product competes with ammonia in the coupling reaction and the diaryl amine is a byproduct, resulting in a decrease in the yield and complication of the purification process.16 Furthermore, in most cases, harsh reaction conditions are needed and very expensive and complicated catalyst systems or ligands are employed to accomplish the reaction.17 As a result, description of a simple and efficient catalyst system for the coupling of unactivated aryl chlorides with ammonia has established the significance of our new P,N-ligand to achieve high reactivity in TMC. Thus, the metal-catalyzed reaction of aryl chlorides and NH3 in the presence of this ligand was investigated. Furthermore, the amination of phenols in their 2,4,6-triaryloxy-1,3,5-triazine (TAT) protected form were evaluated in the presence of the TPPM ligand.
The transmission electron microscopy (TEM) images of the TPPM are shown in Fig. 1. The TEM images show both the porous and sheet nature of the TPPM. The sheets and the holes of the TPPM are in the range of nanometers.
The scanning electron microscopy (SEM) images of the TPPM are shown in Fig. 2. The SEM images also prove the porous structure of the TPPM and show cavities in the range of nanometers (Fig. 2).
The sheet and porous structure of the TPPM make it a high surface area material ideal for catalytic and isolation purposes. The TPPM was also analyzed using nitrogen adsorption–desorption isotherms (Fig. 3a). The adsorption isotherm is obtained by measuring the amount of nitrogen gas adsorbed across the relative pressures at a constant temperature of 77 K and conversely desorption isotherms were achieved by measuring the gas removed as the pressure is reduced. The isotherm type is V according to the Brunauer definition.19 This type of isotherm shows small adsorbent–adsorbate interaction potentials which are associated with pores in the range of 1.5–100 nm. According to the Brunauer–Emmett–Teller (BET) analysis, the specific surface area and pore volume of the TPPM were calculated as 28.7 m2 g−1 and 0.16 cm3 g−1, respectively (Fig. 3a and b).20 The nano-porous character of the TPPM was proven by the Barrett–Joyner–Halenda (BJH) method (Fig. 3c).21 The BJH curve clearly displayed four peaks with the pore size centered at about 4.61, 6.0, 8.0 and 24.0 nm, for four types of pores. These different pores also confirm the nano-porous nature of the TPPM. Thus, this porous material with nanometer range cavities and relatively good surface area is suitable for catalytic processes.
The X-ray diffraction (XRD) pattern of the TPPM (Fig. 4a) showed some strong peaks placed at 2θ of 19.9, 22.6, 26.4, and 29.9. These sharp peaks revealed the highly crystalline nature of the TPPM.22 The thermogravimetric analysis (TGA) of the TPPM shows three main weight losses (Fig. 4b). The first one is accounted for by adsorbed water in the structure of the material (1.92%). The second one occurred at 180–290 °C (5.34%). The main decrease in the weight percentage of the material at 290–420 °C is related to decomposition of the compound structure. Thus, the TGA analysis indicates the high thermal stability of the material. Deferential scanning calorimetry (DSC) is a technique used to investigate the response of polymers to heating. DSC can be used to study the melting of a crystalline polymer or its glass transition.23 The crystallization (Tc) and melting (Tm) peaks are only observed for polymers that can form crystals. The crystallization temperature (Tc) of the TPPM occurs at ∼210 °C. The DSC of the TPPM also shows an endothermic process which happens at ∼390 °C. Purely amorphous polymers will only undergo a glass transition. For semi-crystalline polymers, only the amorphous portion undergoes the glass transition while the crystalline regions only undergo melting. According to the DSC analysis, it seems that the TPPM is a semi-crystalline polymeric material.
The energy-dispersive X-ray (EDX) spectra of the TPPM (Fig. 4d) shows the elements P, N, C and O with wt% of 14.62, 28.55, 24.61 and 30.93, respectively. Also the elemental analysis of the TPPM demonstrates the presence of C and N atoms with wt% of 23.84% and 29.56%, respectively. Accordingly, it is possible to determine the experimental formula of this material approximately, which was estimated to be C3N3O3P0.75.
The experimental formula is in good agreement with a subunit that is shown in Fig. 5a. The arrangements of the triazine ring and phosphite groups generate chelation cavities which have a role in the porosity of this material. An optimized structure of a part of the TPPM is shown in Fig. 5b. These nano-cavities with donor atoms of phosphorus and nitrogen can act as chelating ligands in the presence of a metal (Fig. 5c). The coordination of the TPPM and a metal form a heterogeneous polymeric material containing metal catalysts for application in organic transformations. The chemical structure of a Ni complex of the TPPM is shown in Fig. 5d.
Entry | Catalyst (mol%) | NH3 | Yield 3ab (%) |
---|---|---|---|
a Reaction conditions: chlorobenzene (1.0 mmol), ammonium source (3.0 mmol), NaOtBu (1.5 mmol), PEG-200 (2.0 mL).b Yields are isolated product.c No TPPM was used. | |||
1 | CuI (10) | (NH4)2SO4 | 75 |
2 | Cu(OAc)2 (10) | (NH4)2SO4 | 70 |
3 | Pd(OAc)2 (2) | (NH4)2SO4 | 20 |
4 | NiCl2·5H2O (10) | (NH4)2SO4 | None |
5 | Ni(PPh3)2(CO)2 (10) | (NH4)2SO4 | 90 |
6 | Ni(COD)2 (10) | (NH4)2SO4 | 92 |
7 | Ni(PPh3)4 (10) | (NH4)2SO4 | 75 |
8 | Ni(PPh3)2(CO)2 (10) | (NH4)2SO4 | 35c |
9 | Ni(PPh3)4 (10) | (NH4)2SO4 | 22c |
First, CuI was used as the catalyst and about 75% of product was obtained (Table 1, entry 1). Approximately the same product conversion was observed using Cu(OAc)2 as the catalyst (Table 1, entry 2). In the presence of Pd(OAc)2 as the catalyst the reaction yield was decreased significantly (Table 1, entry 3). Also, with the use of Ni(II) as the catalyst no product was observed (Table 1, entry 4). However, the maximum yield of product was obtained using Ni(0) catalysts (Table 1, entries 5–7). Among the tested Ni(0) catalysts Ni(COD)2 was the best one (Table 1, entry 6). In the absence of the TPPM ligand the reaction yields for Ni(PPh3)2(CO)2 and Ni(PPh3)4 were significantly decreased, demonstrating the high efficiency of the catalyst system for this transformation (Table 1, entries 8 & 9). All of the catalysts were used in the presence of 50 mg of the ligand, (NH4)2SO4 as the NH3 source, NaOtBu as the base and PEG-200 as the solvent. Then, the amounts of Ni(0) and ligand were optimized (Fig. 6a and b) and it was observed that about 50 mg of the TPPM ligand and 10 mol% of Ni(COD)2 are necessary to obtain a high yield of aniline when coupling NH3 with chlorobenzene. In the absence of the TPPM ligand, the reaction yield was decreased significantly (20%), demonstrating the important role of the ligand in this reaction. In the absence of the metal catalyst no product was observed (Fig. 6a and b).
Among the tested ammonia sources, (NH4)2SO4 was recognized as the best one (Fig. 6c). Other solvents were also checked and no superiority over PEG-200 was observed (Fig. 6d). Also, in order to prove that PEG-200 only played the role of solvent, ethylene glycol was used; however the reaction yield was decreased to 35%. This test clarified that the coordination capability of PEG-200 with the metal is not the key factor in the superiority of this solvent related to others used. It should be mentioned that in ethanol and tert-butanol the reaction yields were 30 and 45%, respectively, confirming that alcoholic solvents are not suitable for this method. Since the type of base is important for this transformation, some other bases were tested and the best results were observed using NaOtBu (Fig. 6e). The amount of base was optimized and 1.5 equivalent of base was selected as the optimum (Fig. 6f). The optimized conditions for the conversion of aryl chlorides to anilines in the presence of the TPPM ligand are shown in Scheme 2.
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Scheme 2 Optimized conditions for amination of aryl chlorides using the TPPM ligand. Reaction conditions: aryl chloride (1.0 mmol), (NH4)2SO4 (2.0 mmol), NaOtBu (1.5 mmol), PEG-200 (2.0 mL). |
In another attempt the amination of phenols using their protected 2,4,6-triaryloxy-1,3,5-triazine (TAT)17f,24 compounds as an aryl C–O electrophile was also examined in order to extend the applicability of the catalyst system to the amination of phenolic-based compounds (Scheme 3).
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Scheme 3 Amination of 2,4,6-tris(p-tolyloxy)-1,3,5-triazine (TPTT) under optimized conditions. Reaction conditions: TPTT (0.35 mmol), (NH4)2SO4 (2.0 mmol), NaOtBu (1.5 mmol), PEG-200 (2.0 mL). |
The Ni-catalyzed cross-coupling of NH3 with a range of aryl chlorides and TAT derivatives was performed efficiently under our optimized conditions and the obtained results are shown in Scheme 4. As established in Scheme 4, substrates bearing different functional groups were all efficiently coupled under these reaction conditions to provide the corresponding anilines in good to excellent isolated yields, representing that this methodology is general and practical for the synthesis of diverse anilines.
Aryl electrophiles with functional groups such as alkyl (3b–d), carbonyl (3q), methoxy (3f–h), nitro (3i,j), cyano (3k), acetyl (3l) and ether (3o & 3r) were reacted successfully. Some sterically hindered substrates (3d & 3f) succeeded in producing their corresponding anilines. Heteroaryl substrates (3m & 3n) can also be employed to prepare their corresponding amine products. It should be mentioned that in all of the reactions the diaryl amine byproduct was not observed. It seems that the isolated sites of the TPPM resulting from the triazine ring and phosphite moieties can stabilize the reactive intermediates on the surface toward the construction of the product and thus decrease byproduct formation.
We also employed the optimized conditions for the synthesis of various arylamines using other amine substrates which are depicted in Scheme 5.25
As shown in Scheme 5, the reactions of different amines (indole, pyrrole, carbazole, imidazole, diphenylamine, and dodecyl amine) proceeded smoothly to furnish the corresponding arylamines with moderate to good yields.
In order to show the recyclability of this catalytic system, first, Ni(COD)2 was complexed with the TPPM in PEG-200 solvent at 100 °C and then used in the coupling reaction between chlorobenzene and (NH4)2SO4, and aniline was obtained in 90% isolated yield. The extracted catalyst from the reaction mixture (after filtration, washing and drying) was used in the next run and 88% of the product was obtained. Overall, the catalyst system was reusable for at least for 4 additional cycles (90, 88, 86, 84, and 82%) with almost consistent efficiency, demonstrating that Ni/TPPM acts as a heterogeneous catalyst system in the amination of aryl chlorides. The ICP analysis of the catalyst after 4 cycles of reuse showed that less than 1 percent of the Ni source was removed from the TPPM surface during the reaction process. This result is in good agreement with the efficiency of the catalyst after each recovery. The TEM image showed the porous network structure of the material after complexation with no metal particle observed in the image (Fig. 7a). The SEM image of the Ni-TPPM complex shows that the morphology is not changed significantly during the complexation process of Ni and the TPPM (Fig. 7b). The EDX analysis shows the presence of Ni in the structure of the Ni-TPPM complex (Fig. 7c).
These experiments and analyses revealed that the Ni moieties have good connection with the ligation sites of the TPPM.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra14367a |
This journal is © The Royal Society of Chemistry 2016 |