A triazine-phosphite polymeric ligand bearing cage-like P,N-ligation sites: an efficient ligand in the nickel-catalyzed amination of aryl chlorides and phenols

Abstract

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.

---

Introduction

There is often an emphasis on the development of ligands that influence efficiency and selectivity in transition metal-catalysis (TMC).¹ Heteroatomic bidentate ligands have received considerable attention in recent years due to their widespread applications in TMC.² Phosphines are one of the most important classes of ligands in this field, showing high efficiency and applicability.³ More importantly, the heteroatomic bidentate P,N-ligands have revealed their superiority in activity and selectivity over other phosphorous containing ligands.⁴ The importance of P,N-ligands has resulted from the fact that, as soft/hard ligands, they are able to coordinate reversibly to a metal center temporarily providing a vacant coordination site, a feature that is very desirable in catalysis.⁵ Therefore, the introduction of new, efficient, easily prepared and applicable P,N-ligands is of much current interest and is the subject of extensive research in this field.⁶ The ideal form is the synthesis of an active P,N-ligand with a simple workup and purification process.

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).

Scheme 1 Synthesis of a triazine-phosphite polymeric material (TPPM) through the reaction of PCl₃ with cyanuric acid. Cyanuric acid as a white solid was reacted with PCl₃ (a clear liquid) and forms a yellow solid which is a triazine-phosphite compound. This material is insoluble in solvents such as water, ethanol, ethyl acetate and n-hexane. It is the presence of P and N atoms in the structure of this material that allow it be used as a P,N-ligand in TMC organic transformations.

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.

Results and discussion

Synthesis and characterization of the TPPM

The TPPM ligand was simply synthesized using the reaction of PCl₃ and cyanuric acid in the presence of (n-Pr)₃N. Cyanuric acid immediately reacts with PCl₃ and produces the TPPM as a yellow solid (Scheme 1). The reaction proceeded rapidly and afforded an almost quantitative product yield. This compound was characterized using ¹³C NMR spectroscopy, and FT-IR spectroscopy. There is one carbon atom at 149.8 ppm in the ¹³C NMR spectrum of this compound which is attributed to the carbon of the triazine ring (see the ESI†). The FT-IR spectrum of the TPPM shows the appearance of a P–O bond peak at 851 cm⁻¹. Also, the peaks attributed to the triazine ring with a small shift appeared in the FT-IR spectrum (see the ESI†).¹⁸

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.

Fig. 1 TEM images of the TPPM from different positions and with different magnifications. The sheet morphology of the synthesized TPPM is shown in TEM images (a) and (b). The TEM image of the TPPM with high magnification shows that the sheets are porous and cavities in the range of 10–40 nm are observable (c)–(f). It seems that the TPPM strings form skins in the sheet morphology with cavities in the range of nanometers. These cavities contain the P,N-ligation sites which can be used as ligands in transition-metal catalysis.

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).

Fig. 2 The SEM images of the TPPM. The SEM images of the TPPM with different magnification demonstrate the surface morphology of the material. The structure and surface of this material are shown on the nanoscale in these images. Pores in the range of nm are observable on the surface of the TPPM. The high porosity surface can be observed in the SEM images of the material at all magnifications. At higher magnification, the mesoporosity of the material becomes clear.

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.

Fig. 3 The nitrogen adsorption–desorption isotherm of the TPPM (a). The type of isotherm shows small adsorbent–adsorbate interaction potentials which are associated with pores in the range of 1.5–100 nm. The BET analysis of the TPPM (b). The specific surface area and pore volume of the TPPM were obtained using BET analysis. The V_m, a_s,BET, C, total pore volume, and mean pore diameter for the TPPM were 6.6 cm⁻³ g⁻¹, 28.7 m² g⁻¹, 24.9 nm, 0.16 cm³ g⁻¹, and 22.7 nm, respectively. The Barrett–Joyner–Halenda (BJH) analysis of the TPPM (c). According to the BJH-plot of the TPPM the V_p, r_p,peak and a_p were 0.17 cm³ g⁻¹, 4.61 nm, and 40.7 m² g⁻¹, respectively. The BJH-plot also confirms the nano-porous nature of the TPPM.

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 (T_c) and melting (T_m) peaks are only observed for polymers that can form crystals. The crystallization temperature (T_c) 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.

Fig. 4 The XRD pattern of the TPPM (a); the existence of sharp peaks shows the crystalline nature of this polymer. The TGA analysis of the TPPM (b) shows three weight losses and that the main loss (∼76%) occurs around 290–400 °C which is in parallel to a sharp peak in the DTG curve (b, dash line) at 390 °C. The DSC curve of the TPPM (c). The EDX analysis of the TPPM (d) showed that the elements of the compound included P, N, C, and O with wt% of 14.62, 28.55, 24.61 and 30.93, respectively.

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 C₃N₃O₃P_0.75.

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.

Fig. 5 Subunit structure of the TPPM (a); if we consider the complete reaction of cyanuric acid with trichlorophosphine it is possible to have subunits containing a phosphorous atom connected to three hydroxyl groups of three triazine rings. The optimized structure of a part of the TPPM generated using Austin Model 1 (AM1) and a representation of the P,N chelation cavities (b and c). A schematic representation of the available sites of the TPPM for chelation with metals such as Ni to generate heterogeneous catalyst systems for application in organic transformations (d).

Nickel-catalyzed amination of aryl chlorides using the TPPM ligand

In order to evaluate the applicability of the TPPM as a ligand in TMC it was employed in the metal-catalyzed amination of aryl chlorides. The metal-catalyzed reaction between chlorobenzene and NH₃ in the presence of this ligand was investigated to obtain an excellent yield of aniline. Different metals including Cu, Pd and Ni (commonly used metal catalysts for this transformation) were used in order to determine which of them is more efficient for the conversion of chlorobenzene to aniline in the presence of the TPPM ligand (Table 1).

Table 1 Optimization of metal catalyst used in metal-catalyzed amination of aryl chloride in the presence of the TPPM^a

Entry	Catalyst (mol%)	NH3	Yield 3a^b (%)
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	35^c
9	Ni(PPh3)4 (10)	(NH4)2SO4	22^c

---

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).

Fig. 6 Optimized amount of Ni(COD)₂ (a). By increasing the amount of Ni(0) from 3 mol% to 10 mol% the reaction yield was improved to 93% and after that no improvement was observed thus 10 mol% is the optimum amount of catalyst for this reaction. Optimized amount of the TPPM ligand (b). In the absence of the ligand about 18% of product was observed and with an increasing amount of the ligand the reaction yield was enhanced. About 50 mg of the ligand is needed to obtain the maximum yield of product. Different ammonia sources were used and (NH₄)₂SO₄ was the best one (c). The best solvent for this reaction was recognized as PEG-200, among the tested solvents (d). Some bases including NaOtBu, Cs₂CO₃, NaOH, K₂CO₃, and NaH were tested and the maximum yield of the product was obtained using NaOtBu (e). The amount of base was optimized and 1.5 mmol of base is enough for this reaction to go to completion (f).

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.

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).

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.

Scheme 4 Products of Ni-catalyzed amination using the TPPM as ligand. Reaction conditions: aryl chloride (1.0 mmol) or TAT (0.35 mmol), (NH₄)₂SO₄ (2.0 mmol), NaOtBu (1.5 mmol), PEG-200 (2.0 mL). Yields are isolated product. OTA = the leaving group of TAT.

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

Scheme 5 Products of Ni-catalyzed amination of different amines and aryl electrophiles using the TPPM as the ligand. Reaction conditions: aryl chloride (1.0 mmol) or TAT (0.35 mmol), amine (1.0 mmol), NaOtBu (1.5 mmol), PEG-200 (2.0 mL). Yields are isolated product. OTA = the leaving group of TAT.

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).

Fig. 7 A TEM image (a) and SEM image (b) of the Ni-TPPM complex which show that the structure and morphology of the TPPM ligand remained unchanged after complexation with Ni(COD)₂. The EDX analysis of the Ni-TPPM complex shows the presence of Ni in the structure of the material (c).

These experiments and analyses revealed that the Ni moieties have good connection with the ligation sites of the TPPM.

Conclusions

In conclusion, we have introduced a new P,N-ligand which can be used as a reusable P,N-ligand in transition-metal catalyzed organic reactions. The TPPM is a porous material possessing a sheet morphology with cavities in the range of the nanometer scale which can act as an efficient ligand in organic reactions. These nano-cavities create appropriate sites on the surface of this material with ligation character to coordinate with metals for catalytic purposes. In this study, the TPPM was used as a ligand in metal-catalyzed amination of aryl chlorides and a combination of the TPPM and Ni(COD)₂ was found to be an efficient catalyst system for the conversion of aryl chlorides and phenols protected in their TAT form to anilines. Moreover, the TPPM ligand provides great promise towards further useful applications in other transition metal-catalyzed organic transformations in the future.

Acknowledgements

Financial support from the research councils of Shiraz University and a grant from the Iran National Elite Foundation are gratefully acknowledged.

References

1. (a) S. Torker, R. K. M. Khan and A. H. Hoveyda, J. Am. Chem. Soc., 2014, 136, 3439–3455; (b) G. Sipos, E. E. Drinkel and R. Dorta, Chem. Soc. Rev., 2015, 44, 3834–3860; (c) K. H. Shaughnessy, Chem. Rev., 2009, 109, 643–710; (d) T. M. Shaikh, C.-M. Weng and F.-E. Hong, Coord. Chem. Rev., 2012, 256, 771–803; (e) J.-H. Xie and Q.-L. Zhou, Acc. Chem. Res., 2008, 41, 581–593.
2. (a) S. Toma, J. Csizmadiová, M. Mečiarová and R. Šebesta, Dalton Trans., 2014, 43, 16557–16579; (b) G. Helmchen and A. Pfaltz, Acc. Chem. Res., 2000, 33, 336–345.
3. (a) M. M. Pereira, M. J. F. Calvete, R. M. B. Carrilhoa and A. R. Abreu, Chem. Soc. Rev., 2013, 42, 6990–7027; (b) H. Fernández-Pérez, P. Etayo, A. Panossian and A. Vidal-Ferran, Chem. Rev., 2011, 111, 2119–2176.
4. (a) M. P. Carrolla and P. J. Guiry, Chem. Soc. Rev., 2014, 43, 819–833; (b) G. Chelucci, G. Orrù and G. A. Pinna, Tetrahedron, 2003, 59, 9471; (c) E. Fernández, P. J. Guiry, K. P. T. Connole and J. M. Brown, J. Org. Chem., 2014, 79, 5391–5400.
5. C. Holzhacker, M. J. Calhorda, A. Gil, M. D. Carvalho, L. P. Ferreira, B. Stöger, K. Mereiter, M. Weil, D. Müller, P. Weinberger, E. Pittenauer, G. Allmaiere and K. Kirchner, Dalton Trans., 2014, 43, 11152–11164.
6. (a) G. Farkas, Z. Császár, S. Balogh, I. Tóth and J. Bakos, Tetrahedron Lett., 2014, 55, 4120–4122; (b) K. Takeuchi, A. Minami, Y. Nakajima and F. Ozawa, Organometallics, 2014, 33, 5365–5370; (c) S. K. Gurung, S. Thapa, A. S. Vangala and R. Giri, Org. Lett., 2013, 15, 5378–5381; (d) P. Nareddy, L. Mantilli, L. Guénée and C. Mazet, Angew. Chem., Int. Ed., 2012, 51, 3826; (e) Y.-H. Chang, Y. Nakajima and F. Ozawa, Organometallics, 2013, 32, 2210–2215; (f) R. J. Lundgren, B. D. Peters, P. G. Alsabeh and M. Stradiotto, Angew. Chem., Int. Ed., 2010, 49, 4071–4074.
7. (a) Z. Xu, N. D. McNamara, G. T. Neumann, W. F. Schneider and J. C. Hicks, ChemCatChem, 2013, 5, 1769–1771; (b) L. Garcia, A. Roglans, R. Laurent, J.-P. Majoral, A. Pla-Quintana and A.-M. Caminade, Chem. Commun., 2012, 48, 9248–9250.
8. (a) R. Shunmugam and G. N. Tew, Macromol. Rapid Commun., 2008, 29, 1355–1362; (b) C. A. Basinger, K. Sullivan, S. Siemer, S. Oehrle and K. A. Walters, J. Chem., 2015, 672654–672711; (c) X. Chena, H. Zhua, T. Wanga, C. Lia, L. Yana, M. Jiang, J. Liu, X. Sund, Z. Jiang and Y. Ding, J. Mol. Catal. A: Chem., 2016, 414, 37–46; (d) Z. Yang, B. Yu, H. Zhang, Y. Zhao, Y. Chen, Z. Ma, G. Ji, X. Gao, B. Han and Z. Liu, ACS Catal., 2016, 6, 1268–1273.
9. (a) M. J. Monteiro, Macromolecules, 2010, 43, 1159–1168; (b) R. J. R. W. Peters, I. Louzao and J. C. M. van Hest, Chem. Sci., 2012, 3, 335; (c) K. Renggli, P. Baumann, K. Langowska, O. Onaca, N. Bruns and W. Meier, Adv. Funct. Mater., 2011, 21, 1241–1259.
10. (a) R. Chandrawati, M. P. van Koeverden, H. Lomas and F. Caruso, J. Phys. Chem. Lett., 2011, 2, 2639–2649; (b) H. Gao, Macromol. Rapid Commun., 2012, 33, 722–734.
11. S. Ge, R. A. Green and J. F. Hartwig, J. Am. Chem. Soc., 2014, 136, 1617–1627.
12. A. Ricci, Modern Amination Methods, Wiley-VCH, Weinheim, 2000.
13. N. Xia and M. Taillefer, Angew. Chem., Int. Ed., 2009, 48, 337–339.
14. Y. Aubin, C. Fischmeister, C. M. Thomas and J.-L. Renaud, Chem. Soc. Rev., 2010, 39, 4130–4145.
15. (a) F. Lang, D. Zewge, I. N. Houpis and R. P. Volante, Tetrahedron Lett., 2001, 42, 3251; (b) S. Gaillard, M. K. Elmkaddem, C. Fischmeister, C. M. Thomas and J.-L. Renaud, Tetrahedron Lett., 2008, 49, 3471–3474.
16. (a) Q. Shen and J. F. Hartwig, J. Am. Chem. Soc., 2006, 128, 10028–10029; (b) G. D. Vo and J. F. Hartwig, J. Am. Chem. Soc., 2009, 131, 11049–11061.
17. (a) J. P. Wolfe and S. L. Buchwald, J. Am. Chem. Soc., 1997, 119, 6054–6058; (b) T. Shimasaki, M. Tobisu and N. Chatani, Angew. Chem., Int. Ed., 2010, 49, 2929–2932; (c) T. Mesganaw, A. L. Silberstein, S. D. Ramgren, N. F. Fine-Nathel, X. Hong, P. Liu and N. K. Garg, Chem. Sci., 2011, 2, 1766–1771; (d) S. D. Ramgren, A. L. Silberstein, Y. Yang and N. K. Garg, Angew. Chem., Int. Ed., 2011, 50, 2171–2173; (e) L. Hie, S. D. Ramgren, T. Mesganaw and N. K. Garg, Org. Lett., 2012, 14, 4182; (f) N. Iranpoor and F. Panahi, Adv. Synth. Catal., 2014, 356, 3067–3073.
18. B. Chase, Progress in Fourier Transform Spectroscopy: Proceedings of the 10th International Conference, ed. J. Mink, G. Keresztury and R. Kellner, Springer-Verlag, 1997.
19. S. Brunauer, L. S. Deming, W. E. Deming and E. Teller, J. Am. Chem. Soc., 1940, 62, 1723–1732.
20. (a) L. D. Gelb and K. E. Gubbins, Langmuir, 1998, 14, 2097–2111; (b) S. Storck, H. Bretinger and W. F. Maier, Appl. Catal., A, 1998, 174, 137–146.
21. E. P. Barrett, L. G. Joyner and P. P. Halenda, J. Am. Chem. Soc., 1951, 61, 373–380.
22. L. R. Alexander, X-ray Diffraction Methods in Polymer Science, Wiley-Interscience, 1969.
23. N. Jing and A. Hammer, Thermal analysis of polymers, Part 5, DSC and TGA of elastomers UserCom, 2013, vol. 35, pp. 1–5.
24. (a) N. Iranpoor and F. Panahi, Org. Lett., 2015, 17, 214–217; (b) N. Iranpoor, F. Panahi and F. Jamedi, J. Organomet. Chem., 2015, 781, 6–10.
25. A. Dumrath, C. Lbbe, H. Neumann, R. Jackstell and M. Beller, Chem.–Eur. J., 2011, 17, 9599–9604.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra14367a

---