1422_Mesoporous, ligand free Cu-Fe solid catalyst

Paper No. Chemeca 2014/1422

CHEMECA 2014: Sept 28 – Oct 01 2014, Perth, Western Australia

Mesoporous, ligand free CuCu-Fe solid catalyst mediated CC-S cross coupling of thiols with aryl halides Deepa Dumbre1, Tibra Mozammel1, PR. Selvakannan1, V. R. Choudhary1, Sharifah Bee Abdul Hamid2, Suresh K. Bhargava1* 1 Centre for Advanced Materials and Industrial Chemistry, School of Applied Science, RMIT University, Melbourne, VIC 3000, Australia 2 Nanotechnology and Catalysis Research Centre (NANOCAT); University of Malaya 50603 Kuala Lumpur, MALAYSIA Email Address of the corresponding author: [email protected]

Abstract Solid catalyst derived from Cu-Fe-HT was demonstrated to be a novel, ligandless, efficient and environmentally greener catalyst for the synthesis of diaryl sulphurs from the C–S cross coupling reaction of substituted thiols with different aryl halides. This catalyst has shown higher product yield in the presence of dimethyl formamide (as a solvent) and K2CO3(as a base) at 120◦C. Influence of different solvents and bases on the product yield has also been investigated. The catalyst can be easily separated from the reaction mixture, simply by filtration and reused several times without a significant loss of its activity. The catalyst has been fully characterized for its surface and bulk properties and the mesoporousCuO:Fe2O3 phase was attributed for its catalytic activity towards Sarylation reactions. Keywords: C–S cross coupling, S-arylation, Cu-Fe-HT

Introduction Transition metal catalyzed Ullman type cross-coupling reactions of phenols, amines and thiols with aryl halides leading to the formation of C-O, C-N and C-S bonds, respectively, are very important in the synthesis of pharmaceuticals, biomolecules, and hybrid materials(Jones 1979). Significant contributions of these cross coupling reactions in organic synthesis has been extensively reviewed in last decade(Kondo and Mitsudo 2000, Beletskaya and Cheprakov 2004, Chemler and Fuller 2007, Evano, Blanchard et al. 2008, Beletskaya and Ananikov 2011). Among this aryl carbon-heterobond formation, the aryl carbon-sulfur formation was not studied in much detail. These reactions are relatively more complicated because of the S–S coupling side-reactions as well as poisoning of the metal catalysts (particularly, Pd-based catalysts) by strong adsorption of sulfur compounds(Beletskaya and Ananikov 2011). However, the catalyst poisoning can be avoided using different ligands, bases, reagents and solvents, therefore number of protocols have been developed for aryl-sulfur bond formation, using different catalytic systems, such as palladium(Li 2001), nickel salts(Jammi, Barua et al. 2008), Cobalt salts(dppe)/Zn(Wong, Jayanth et al. 2006), Indium compounds(Reddy, Swapna et al. 2009), Ferric chloride(Correa, Carril et al. 2008) and copper salts(Beletskaya and Cheprakov 2004, She, Jiang et al. 2009, Kabir, Lorenz et al. 2010).These catalytic C-S formations have several limitations, such as usage of expensive palladium salts, expensive ligands, requirement of stoichiometric amount of reagents and metal toxicity in the case of Ni or Co based catalysts. Therefore, the traditional copper-catalysed Ullmann-type C–S cross coupling reactions is still an attractive method for many organic chemists in spite of the other drawbacks of the copper catalyst systems. Recently, the copper-catalysed Ullman cross-coupling reactions have been improved and it was considered as a powerful tool in the formation of C(aryl) S bonds (Beletskaya and Cheprakov 2004, She, Jiang et al. 2009, Kabir, Lorenz et al. 2010). The conventional copper-catalysed aryl-thiol formation usually requires stringent reaction conditions, such as usage of 1



copper salts more than stoichiometric amounts, high temperature, polar solvents and catalyst loading. In particular, high temperature catalytic C-S bond formation is not desirable for the asymmetric synthesis for the production of pharmaceuticals. Therefore, developing a simple, economic and highly efficient catalytic system is necessary for C–S coupling reaction. Moreover, the catalyst cannot be re-used in this reaction due to the difficult recovery of the catalyst, which has negative impact on the environment and sustainability. These disadvantages can be lifted by immobilizing suitable heterogeneous catalyst that enables easy separation and recyclability of the catalyst. Recently few solid catalysts such as nano-CuO (Rout, Sen et al. 2007), La2O3(Murthy, Madhav et al. 2009), nano-In2O3(Reddy, Kumar et al. 2009) and Cu2S-Fe(Wang, Jiang et al. 2010) have been reported for the S-arylation. However, these catalysts have disadvantages such as low stability, long reaction time, reductant like iron powder (Wang, Jiang et al. 2010). A recent review (Ariga, Vinu et al. 2012) on mesoporous materials, including syntheses of mesoporous silica, for novel functions has shown as mesoporosity is a desirable feature for these kind of reactions. Especially mesoporous materials having large pore size are suitable candidates to carry out the chemical transformation of larger molecules. Hence, there is need for development of a better (highly active and environmentally more benign) catalytic system having mesoporosity for the C–S cross coupling of aryl halides with thiols. In this present work, a simple, effective and environmentally greener protocol for the S-arylation cross coupling reaction of aryl halides with different thiols has been developed by using a mesoporous Cu-Fe-HT as an efficient catalyst. The catalyst was used under ligand-free conditions and catalyst materials are inexpensive. It showed high catalytic activity in the reaction under mild reaction conditions. Catalyst can be easily separated from the reaction mixture simply by filtration and reused several times with similar catalytic activity.

Materials and Methods The synthetic approach used here to prepare Cu-Fe-HT catalyst, was a modified method from the general method of preparing layered hydrotalcite structures that have been reported earlier by us (Choudhary, Dumbre et al. 2004, Choudhary and Dumbre 2011, Choudhary, Dumbre et al. 2012). The Cu-Fe catalyst materials were synthesized by hydrolysing the solution containing Copper(II) Nitrate (3 mmol)and Iron(III) Nitrate (1 mmol) by drop-wise addition of Potassium hydroxide/Potassium carbonate mixture. The precipitated material washed several times with water to remove any free ions and dried at room temperature. After complete dryness, this material was calcined at 600◦C for four hours under static air. This material was used as a catalyst without any further processing and termed as Cu-Fe-HT hereafter. Nitrogen adsorption isotherms, surface area and pore volume were acquired at 77.25 K on a Micromeritics ASAP2010 instrument. Thermo gravimetric data (TGA) were acquire during a Perkin Elmer TGA-7 from 35◦C to 700◦C with a heating rate of 10◦C per minute under the flow of nitrogen. XRD measurements were carried out on a Bruker D8 X-ray diffraction system operating at a voltage of 40 kV and current of 40 mA with CuKa radiation. XPS measurements were obtained with a Thermo K-5 Alpha XPS instrument at a pressure better than 1 × 10−9torr with core levels aligned with C 1s binding energy of 285 eV.

Catalytic conversion of arylation of thiols The catalytic S-arylation cross coupling reaction was carried out in a magnetically stirred glass reactor with varying reaction times ranging from 7 h to 20 h and the temperature of the reaction vessel was maintained 120◦C. Typical reaction mixture contained aryl halide (1 mmol), thiol (1.5 mmol), Cu-Fe-HT catalyst (30 mg, 0.319 mmol of Catalyst), K2CO3(5 mmol) and DMF (3 ml).The progress of the reaction was monitored using thin layer chromatography. After completion of the reaction, the catalyst was separated by filtration and the filtrate was treated with water, followed by 2



extraction with ethyl acetate to give the crude product, which was subsequently purified by column chromatography on silica gel with petroleum ether/ethyl acetate as eluent. The catalyst was further washed with acetone, dried and reused. The reaction product was isolated by column chromatography and was characterized by comparison of its NMR spectra with that reported earlier in the literature (Bolm et al. 2008, Cook et al. 2010, Li et al. 2001, Rao et al. 2009, Punniyamurthy et al. 2007, Li et al. 2010). All the C–S coupling products are known compounds.

Results and Discussion

Figure 1. (A) Thermo gravimetric analysis of Cu-Fe-HT (as-synthesized). (B) N2 physi-sorption isotherms of calcined Cu-Fe-HT.

In order to understand the formation of Cu-Fe-HT catalyst, TGA and BET analysis of these materials were carried out and the results are given in Figure 1. The TG data of the as-synthesized Cu-Fe material showed that 22% weight loss was observed when this material was heated up to 600◦C. Initial 1-2% weight loss observed between 50 and 125◦C was mainly due to the desorption of physically adsorbed/absorbed water present in the hydrotalcite. The second major weight loss was observed between 125◦C and 300◦C was possibly due to the loss of hydroxyl groups and the small weight loss must be observed from the oxygen losses observed during the structure formation. Figure 1B shows the nitrogen adsorption/desorption isotherms of calcined Cu-Fe-HT catalyst material. BET analysis clearly showed that Cu-Fe-HT catalyst material become mesoporous after calcination and exhibit BET surface area of 123 m2/g and 0.48 cm3/g pore volume. The average pore diameter of these materials was found to be 15 nm. This kind of large pore diameter is definitely an added advantage especially for reactions involving larger molecules because of the large pore size. Higher surface area and pore volume definitely increase the number of active sites which in turns beneficial to increase the conversion. The shape of the hysteresis observed in the case of calcined Cu-Fe-HT suggests that this material may possess slit like pores and these kinds of pores usually formed when layered materials like hydrotalcite were calcined.

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Figure 2. (A) XRD patterns of Cu-Fe-HT (as-synthesized) and Cu-Fe-HT (calcined). XPS core level spectra of (B) Cu 2p (C) Fe 2p components of Cu-Fe-HT (calcined)

Similar to the synthesis of other hydrotalcite like materials (Choudhary, Dumbre et al. 2004, Choudhary and Dumbre 2011, Choudhary, Dumbre et al. 2012), hydrolysis of Cu(II)/Fe(III) precursors under alkaline conditions resulted in the formation of hydrotalcite like structures made up of the copper and iron hydroxides. In this case, the resultant material also is layered hydroxide of Cu(II) and Fe (III), however this material during calcination undergo structural changes due to the loss of water and hydroxyl groups. Therefore powder X-ray diffraction of the dried material and calcined material was carried out and their diffraction patterns are given in Figure 2A. The dried Cu-Fe catalyst (assynthesized after drying) present broader Bragg reflections and 2Ө peaks observed at 16.6 and 23.8 correspond to layered iron hydroxide structures such as hydrotalcite. Most of the X-ray reflections correspond to Fe2O3 phase and no peaks correspond to Cu(OH)2 phases was observed even though the initial mole concentration of copper was kept three times higher than iron concentration. This may be possibly due to the formation of an amorphous phase of Cu(OH)2. However after calcination, the XRD patterns of this material have shown significant changes. Absence of small angle diffraction peaks clearly showed that layered hydroxide (hydrotalcite like structure) structure was totally decomposed. As a result of calcination, two major 2Ө peaks appeared at 35.5◦ and 38.7◦ in addition to many other weak X-ray reflections. The diffraction peak observed at 35.5◦ was attributed to the (311) plane of CuFe2O4 and the peak observed at 38.7 corresponds to CuO phase. Most of the other small diffraction peaks correspond to Fe2O3 phase. Therefore the resultant material may be a mixture of mixed phase containing CuO:Fe2O3, single phase like CuFe2O4 or both these two phases co-exist together. XPS analysis of the calcined Cu-Fe-HT catalyst was carried out to understand the chemical state of Cu and Fe in the materials and figure 2 B and C shows the XPS core level spectra of Cu2p and Fe2p respectively. Binding energy of Cu 2p3/2level in Cu-Fe-HT appeared at 933.6 eV (Fig. 2B), that corresponds to the CuO phase. Moreover, this XPS results agree with the XRD results, which show the existence of CuO phase after calcination. In the case of iron, the binding energy of the Fe 2p3/2core level in the calcined Cu-Fe-HT (Fig. 2C) material showed two chemically different iron species (Fig. 2C). The first and intense iron species showed its binding energy around 710.9 eV and the second and less intense species showed around 714.4 eV. The binding energy of the Fe 2p species, that appeared at 710.9 eV close to the Fe 2p level present in the Fe2O3 species. The other binding energy may be coming from the iron present in the mixed metal oxide phase. These results 4



again prove that the existence of CuO:Fe2O3 and CuFe2O4 phases. Both XRD and XPS results showed the presence of CuO:Fe2O3 or CuFe2O4 phase on the surface.

CuCu-FeFe-HT catalysed CC-S coupling Cu-Fe-HT catalyzed C–S cross coupling reactions between different thiols and aryl halides were investigated to study the potential of this ligand free mesoporous Cu-Fe material in C–S coupling reactions (Scheme 1). Aryl halides and aryl thiols substituted with the variety of electron-deficient and electron-rich functional groups reacted in DMF containing the base K2CO3 and the catalyst. The yields of cross-coupling products are presented in Table 1 and the yields range from moderate to excellent yields. These results show that the nature of electron donating / withdrawing nature of different substituents present in all the substrates have a strong influence on the product yield. As similar to other coupling reactions, aryl iodides showed the better conversion, followed by aryl bromides and aryl chlorides (Table 1, entries 1–3). Aryl fluorides reaction with thiols was not shown any appreciable conversion even in the presence of catalyst (entry 4). Due to the presence of electron withdrawing groups such as –NO2 and –Cl in the aryl halides, the product yield increased. However the presence of same functional groups in aryl thiols, the product yields significantly reduced. The presence of electron donating group (–CH3 or –OCH3) in the aryl halides caused a decrease in the product yield (Table 1, entries 5–8). In contrast, presence of same functional groups in aryl thiols increases the product yield marginally. As compared to aryl thiol, aliphatic thiols (nC8H17SH) have shown very high conversion, due to its reactivity. In the control experiment, there was no product formation in the absence of the Cu-Fe catalyst. In order to optimize the reaction conditions, the reaction between 4-iodoanisole and thiol over the Cu-Fe-HT catalyst was carried out using different bases (such as Na2CO3, NaOAC, trimethylamine, pyridine and zinc dust), solvents (such as toluene, xylene, acetonitrile, NMP and DMF) and also at different temperatures(50–140◦C). These results showed that role of base affects the conversion of Cu-Fe-HT catalysed S-arylation reactions. Among the different bases, product yield was found to be high when K2CO3 was used as base for this cross coupling reaction. In contrast to K2CO3, other bases such as Na2CO3, sodium acetate, trimethylamine and pyridine or zinc dust showed less conversion and the completion of reaction was much longer than that achieved with K2CO3. This revealed that, among the bases, K2CO3 is the most suitable base for the Cu-Fe catalysed S-arylation reaction.

The role of solvents in the Cu-Fe catalysed S-arylation reaction was studied and DMF showed very good product yield among all the solvents (toluene, xylene, acetonitrile and NMP). The observed trend for the solvent effect on the product yield is expected most probably because of the donor properties of DMF and that can function as ligand to the Cu-Fe-HT catalyst. The Cu-Fe catalysed Sarylation is strongly temperature dependent and the product yield was increased exponentially with increasing the reaction temperature from 50◦C to 120◦C and then levelled off. Up to 50◦C, there was almost no product formation, even after 20 h of reaction. At 120◦C, a reasonably very good product yield achieved in the reaction. 5



Table 1: Performance of the Cu-Fe-HT catalyst in the S-arylation of thiols and substituted thiols with different aryl halides [(reaction mixture = aryl halide + thiol + catalyst (30 mg) + K2CO3 (5 mmol) + DMF (3 ml), bath temperature = 120 °C)] Entry

Aryl halide

Thiol

Reaction

Product

Isolated product Yield (%)

4

5

6

time (h) 1

2

3

1

Cl

SH

10

S

25

2

Br

SH

10

S

78

3

I

SH

8

S

90

4

F

SH

20

S

nil

SH

8

5

Br MeO

6

S

75

S

89

MeO

I

SH

8

MeO

MeO

7

SH

7

87

8

SH

10

68

SH

8

9

Br O2N

10

80

S

90

O2N

I O2N

S

SH

8 O2N

6



Therefore, Cu-Fe catalysed S-arylation reaction require K2CO3 as base, DMF as solvent and 120◦C was reaction temperature in order to obtain better product yields. In order to evaluate the catalyst reusability, the spent catalyst was removed from the reaction mixture by filtration, washed with DMF first and then with acetone, dried and then reused in the S-arylation reaction at 120◦C. This was repeated several times. It is interesting to note that, apart from its high activity, the catalyst also showed excellent reusability without significant decrease in its activity. The observed increase in the reaction period for completing the reaction is expected because the recovered catalyst needs longer time to get further activation. The product yield in the 1st, 3rd and 5th reuse of the catalyst was found to be 88% for 8.25, 8.5 and 9.0 h, respectively. It may be noted that, the dried Cu-Fe-HT (before calcination) was directly used as a catalyst; the product yield in the same period was much smaller. Also, when the thermally decomposed (under similar conditions) mixed Cu-Fe nitrates were used as the catalyst in the reaction, the product yield was found to be very low.

Conclusions In summary, Cu-Fe-HT was demonstrated to be a highly efficient and environmentally more benign catalyst for the synthesis of diaryl sulfurs under mild conditions. Ullman type Cu-Fe-HT catalysed C–S cross coupling between thiols/substituted thiols and different aryl halides showed very high product yields in the presence of DMF and K2CO3 at 120◦C. The catalyst can also be easily separated from the reaction mixture and reused for the same reaction without significant loss of its activity. Together, Cu-Fe-HT catalyst is demonstrated to be a highly promising heterogeneous solid catalysed for C–S cross coupling reactions as compared to the homogenous catalytic way of synthesizing diarylethers, which require expensive organometallic palladium compounds and strong reductant such as iron powder. The high catalytic activity of Cu-Fe-HT stems from the mesoporous copper iron oxide structure and their basic nature.

Acknowledgment We thank the RMIT Microscopy and Microanalysis facility staff members for their scientific and technical assistance.

Biography Dr. Deepa Dumbre is currently working as a research fellow with Professor Suresh Bhargava. She received her PhD from National chemical laboratory of India under the supervision of Dr. VR. Choudhary. She has obtained her Masters in Chemistry from Indian Institute of Technology (IIT) Mumbai. She has ten years research experience Fine chemical synthesis and heterogeneous catalysis area and published 20 journal articles.

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