jou rn al hom epage: www.elsev ier .com/ locate /apcata
esoporous, ligand free Cu-Fe solid catalyst mediated CS crossoupling of thiols with aryl halides
eepa K. Dumbrea, P.R. Selvakannana, S.K. Patil b, Vasant R. Choudharyb,∗,uresh K. Bhargavaa,∗
Centre for Advanced Materials and Industrial Chemistry, School of Applied Sciences, RMIT University, Melbourne, 3000, AustraliaChemical Engineering and Process Development Division, National Chemical Laboratory, Pune, 411008, India
r t i c l e i n f o
rticle history:eceived 20 November 2013eceived in revised form 14 January 2014ccepted 9 February 2014
a b s t r a c t
Solid catalyst derived from Cu-Fe hydrotalcite was demonstrated to be a novel, ligandless, efficient andenvironmentally greener catalyst for the synthesis of diaryl sulfurs from the C–S cross coupling reactionof substituted thiols with different aryl halides. This catalyst has shown higher product yield in the
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vailable online 18 February 2014
eywords:–S cross coupling-arylation
presence of dimethylformamide (as a solvent) and K2CO3 (as a base) at 120 C. Influence of differentsolvents and bases on the product yield has also been investigated. The catalyst can be easily separatedfrom the reaction mixture, simply by filtration and reused several times without a significant loss of itsactivity. The catalyst has been fully characterized for its surface and bulk properties and the mesoporousCuO:Fe2O3 phase was attributed for its catalytic activity towards S-arylation reactions.
u-Fe-hydrotalcite
. Introduction
Ullmann-type transition metal catalyzed cross coupling reac-ions of phenols, amines and thiols with aryl halides leadingo the formation of C O, C N and C S bonds, respectively, areery important in the synthesis of biologically active compounds,harmaceuticals and functional materials [1]. The importance ofhese cross coupling reactions in organic synthesis has been exten-ively covered in number of reviews published in last decade [2–8].mong the C (aryl)-heterobond formations, the C(aryl) S bond for-ation was not explored much [2,3,7]. The C S bond formation
s relatively more complicated because of the associated S–S cou-ling reaction and also due to the poisoning of the metal catalysts,articularly, Pd-based catalysts, by strong adsorption of sulfur com-ounds [7]. However, the catalyst poisoning has been successfullyvercome using different ligands, bases, reagents and solvents and
number of protocols have been developed for C S bond formationn recent years, using different catalytic systems, such as palla-ium compounds [9–12], nickel salts [13,14], CoI2(dppe)/Zn [15],
n(OTf)3 [16], FeCl3 [17,18] and copper salts [19–25].These catalytic systems however suffer from several limitations,
uch as high cost of palladium salts, use of expensive ligands,equirement of stoichiometric amount of reagents, restrictedpplication and metal toxicity (particularly for Ni or Co based
catalysts). Therefore, the development of traditional copper-catalyzed Ullmann-type C–S cross coupling reactions is stillattractive to many organic chemists owing to the drawbacks ofthe afore-mentioned catalyst systems. With the renaissance of Ull-mann coupling for the past few years [3–7], the copper-catalyzedcross coupling reactions have been demonstrated to be a powerfultool in the formation of C(aryl) S bonds [19–25]. The traditionalcopper-catalyzed C S bond formation usually requires harsh con-ditions, such as more than stoichiometric amounts of copper salts,polar solvent, such as HMPA, high temperature (above 200 ◦C) andhigh catalyst loading. Moreover, high temperature is not desirablefor both asymmetrical synthesis and pharmaceuticals. Therefore amild, economic, and highly efficient catalytic system is necessaryfor C–S coupling reaction, particularly, for the larger scale produc-tion of the C–S coupling products.
Moreover, the catalyst used in the methods cannot be easilyrecovered after the reaction for its reuse. These drawbacks havenegative impact on the environment. These disadvantages can beovercome by anchoring suitable heterogeneous catalyst which willallow easy separation and recyclability of the catalyst with minimalamount of product contamination with metal.
Very recently, a few solid catalysts (which can be easily sepa-rated and reused in the reaction), such as nano-CuO [26], La2O3[27], nano-In2O3 [28] and Cu2S-Fe [29] have been reported for
the S-arylation. However, these catalysts have limitations like lowstability [26], and/or requirement of long reaction time [28,29],DMEDA as a ligand [27], strong base [26–28] or a reductant (ironpowder) [29].
D.K. Dumbre et al. / Applied Catalysis A: General 476 (2014) 54–60 55
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spond to Cu(OH)2 phases was observed even though the initialmole concentration of copper was kept three times higher thaniron concentration. This may be possibly due to the formation of anamorphous phase of Cu(OH)2. However after calcination, the XRD
Scheme 1. C–S cross coupling
Recently Ariga et al. reported mesoporous materials, includ-ng syntheses of mesoporous silica, for novel functions [30]. Therere recent development also in periodic mesoporous organosili-as, prepared by the combination of a surfactant as template and ailsesquioxane as the organosilica precursor [31,32], which is useds one of the hybrid organic/inorganic materials and has variety ofunctionalities and applications. In particular, mesoporous mate-ials having large pore size are suitable candidates to carry outhe chemical transformation of larger molecules. Hence, there iseed for development of a better (highly active and environmen-ally more benign) catalytic system having mesoporosity for the–S cross coupling of aryl halides with thiols.
In this communication, we report a simple, effective and envi-onmentally greener protocol for the S-arylation cross couplingeaction of aryl halides with different thiols using a mesoporousu-Fe-HT as a highly efficient catalyst (Scheme 1). The catalyst is
igand-free and inexpensive. It showed high activity in the reac-ion under mild reaction conditions. It can be easily separated fromhe reaction mixture simply by filtration and reused several timesithout significant loss of its activity.
. Experimental
.1. Catalyst preparation and characterization
The synthetic approach used here to prepare Cu-Fe-hydrotalciteatalyst, was a modified method from the general method ofreparing layered hydrotalcite structures that have been reportedarlier by us [33–35]. The Cu-Fe catalyst materials were synthe-ized by hydrolyzing the solution containing Cu(II) Nitrate (3 mmol)nd Fe(III) citrate (1 mmol) by drop-wise addition of Potassiumydroxide/Potassium carbonate mixture. The precipitated mate-ial washed several times with water to remove any free ions andried at room temperature. After complete drying, this material isalcined at 600 ◦C for four hours under static air. This material wassed as a catalyst without any further processing and termed asuFe HT hereafter.
Nitrogen adsorption/desorption isotherms; surface area andore volume were acquired at 77.25 K on a Micromeritics ASAP010 instrument. Thermo gravimetric data (TGA) were acquiredsing a Perkin Elmer TGA-7 from 35 ◦C to 700 ◦C with a heating ratef 10 ◦C per minute under the flow of nitrogen. XRD measurementsere carried out on a Bruker D8 X-ray diffraction system operat-
ng at a voltage of 40 kV and current of 40 mA with CuKa radiation.PS measurements were obtained with a Thermo K-5 Alpha XPS
nstrument at a pressure better than 1 × 10−9 torr with core levelsligned with C 1s binding energy of 285 eV.
.2. Catalytic conversion of arylation of thiols
The catalytic S-arylation cross coupling reaction was carried out
n a magnetically stirred glass reactor (capacity: 25 cm3) with vary-ng reaction times ranging from 7 h to 20 h and the temperature ofhe bath was maintained 120 ◦C. Each and every typical reaction
(30 mg, 0.319 mmol of Catalyst), K2CO3 (5 mmol) and DMF (3 ml).The progress of the reaction was monitored using TLC. After com-pletion of the reaction, the catalyst was separated by filtration andthe filtrate was treated with water, followed by extraction withethyl acetate to give the crude product, which was subsequentlypurified by column chromatography on silica gel with petroleumether/ethyl acetate as eluent. The catalyst was further washed withacetone, dried and reused. The reaction product was isolated by col-umn chromatography and was characterized by comparison of itsNMR spectra with that reported earlier in the literature. All the C–Scoupling products are known compounds.
3. Results and discussion
3.1. Catalyst formation and characterization
As mentioned in our earlier reports [33–35], hydrolysis ofM(II)/M(III) precursors under alkaline conditions resulted in theformation of hydrotalcite like structures made up of the divalentand trivalent metal hydroxides. In this case, the resultant materialalso must be a layered hydroxide of Cu(II) and Fe (III), howeverthis material during calcination undergo structural changes dueto the loss of water and hydroxyl groups. Therefore powder X-raydiffraction of the dried material (a) and calcined material (b) wascarried out and their diffraction patterns are given in Fig. 1. Thedried Cu-Fe catalyst (a, before calcination) present broader Braggreflections and 2� peaks observed at 16.6 and 23.8 correspond tolayered iron hydroxide structures such as hydrotalcite. Most of theX-ray reflections correspond to Fe2O3 phase and no peaks corre-
Fig. 1. Powder X-ray diffraction patterns of as-synthesized Cu-Fe hydrotalcite (a)and after calcination at 600 ◦C (b).
56 D.K. Dumbre et al. / Applied Catalysis A: General 476 (2014) 54–60
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ig. 2. (A) Thermogravimetric analysis of as-synthesized (a) Cu-Fe hydrotalcite aydrotalcite and (b) after calcination.
atterns of this material have shown significant changes. Absencef small angle diffraction peaks clearly showed that layered hydrox-de (hydrotalcite like structure) structure was totally decomposed.owder XRD patterns of the dried Cu-Fe hydrotalcite material as
function of temperature was recorded (results are not shownere) and the results reveal that hydrotalcite structure was stablep to about 300 ◦C and then the structure is completely collapsedt and above 400 ◦C with the formation of different crystallinehases. As a result of calcination, two major 2� peaks appearedt 35.5◦ and 38.7◦ in addition to many other weak X-ray reflec-ions. The diffraction peak observed at 35.5◦ was attributed to the311) plane of CuFe2O4 and the peak observed at 38.7 correspondso CuO phase. Most of the other small diffraction peaks correspondo Fe2O3 phase. Therefore the resultant material may be a mixturef mixed phase containing CuO:Fe2O3, single phase like CuFe2O4r both these two phases co-exist together. In order to understandhe structural transition during the calcination process, TGA andET analysis of these materials were carried out and the resultsre given in Fig. 2. The TGA curve for the dried Cu-Fe materialhowed that 22% weight loss was observed when this material waseated up to 600 ◦C. Initial 1–2% weight loss observed between0 and 125 ◦C was mainly due to the desorption of physicallydsorbed/absorbed water present in the hydrotalcite The secondajor weight loss was observed between 125 ◦C and 300 ◦C was
ossibly due to the loss of hydroxyl groups and the small weightoss must be observed from the oxygen losses observed during thetructure formation. The presence of different crystalline phasesn the catalyst is consistent with this. Fig. 2B shows the nitro-en adsorption/desorption isotherms of dried Cu-Fe hydrotalciteaterial (a) and the calcined Cu-Fe hydrotalcite (b) materials. As
xpected, the dried material has very low surface area (12 m2/g),ore volume (0.013 cm3/g) and 4.3 nm average pore diameter. Inontrast, the same material after calcination become mesoporousnd exhibit much higher surface area of 123 m2/g and 0.48 cm3/gore volume. Interestingly the average pore diameter was also
ncreased from 4.3 nm to 15 nm. The increase in pore diameters definitely an added advantage especially for reactions involv-ng larger molecules because of the large pore size. Higher surfacerea and pore volume definitely increase the number of active sites,
) after calcination. (B) BET N2 adsorption isotherms of as-synthesized (a) Cu-Fe
which in turn beneficial to increase the conversion. The shape of thehysteresis observed in the case of calcined Cu-Fe HT suggests thatthis material may possess slit like pores and these kind of poresusually formed from layered materials like hydrotalcite.
XPS analysis of these materials before and after calcination wascarried out to understand the chemical state of Cu, Fe and Oxygen inthe materials. Fig. 3 shows the XPS core level spectra of Cu2p (A&B),Fe2p (C&D) and O1s (E&F) obtained from the dried (top) and cal-cined Cu-Fe HT materials. Binding energy of Cu 2p3/2 level (Fig. 3A)in the dried Cu-Fe HT observed at 934.5 eV that corresponds to theCu(OH)2 species, which formed during the co-precipitation. Aftercalcination, the binding energy of Cu2p3/2 level shifted from 934.5to 933.6 eV (Fig. 3B), corresponds to the CuO phase. Both theseresults agree with the XRD results, which show the existence ofCuO phase after calcination. In the case of iron, the binding energyof the Fe 2p3/2 core level in the dried Cu-Fe HT (Fig. 3C) materialappeared at 711.7 eV, close to the binding energy vale observed foriron hydroxides and FeOOH species. However after calcination, thismaterial showed two chemically different iron species (Fig. 3D). Thefirst and intense iron species showed its binding energy around710.9 eV and the second and less intense species showed around714.4 eV. The binding energy of the Fe 2p species, that appeared at710.9 eV close to the Fe 2p level present in the Fe2O3 species. Theother binding energy may be coming from the iron present in themixed metal oxide phase. These results again prove that the exist-ence of CuO:Fe2O3 and CuFe2O4 phases. Analysis of O1s core levelbinding energies appeared in the case of dried Cu-Fe HT material(Fig. 3E) showed 3 chemically different oxygen species. The bind-ing energy values of the O1s levels observed at 529.8, 531.2 and532.2 eV was assigned to metal oxide, metal hydroxide and sur-face bound hydroxyl groups. The intensity of the metal hydroxide(531.2 eV) was found to be more intense than the metal oxide, sup-ports the presence of hydrotalcite like structure. After calcination,it was observed that there are three chemically different oxygenspecies present and their O1s binding energies were observed at
529.5, 531 and 532.7 eV. In contrast to the dried material, the inten-sity of metal oxide oxygen (529.5 eV, from Fe2O3:CuO or CuFe2O4)was found to be much higher than the metal hydroxide. In sum-mary, these results shows the presence of CuO:Fe2O3 or CuFe2O4
D.K. Dumbre et al. / Applied Catalysis A: General 476 (2014) 54–60 57
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Fig. 3. XPS of Cu2p, Fe2p and O1s core level spectra of the as sy
hase on the surface. In order to understand whether the composi-ion of the material is uniform or not throughout the material, weave compared the atomic ratio values of copper and iron obtained
rom EDX and XPS analysis. The atomic ratio of copper to iron was:1 and 1:1 from EDX and XPS analysis, respectively and theseesults indicate that bulk material was rich in CuO phase, whilehe surface is rich with CuO:Fe2O3 or CuFe2O4 phase.
Cu-Fe HT catalyzed C–S cross coupling reactions between differ-nt thiols and aryl halides were investigated to study the potentialf this ligand free mesoporous Cu-Fe material in C–S couplingeactions (Scheme 1). Aryl halides and thiols substituted with theariety of electron-deficient and electron-rich functional groupseacted in DMF containing the base K2CO3 and the catalyst. Theields of cross-coupling products are presented in Table 1
and the yields range from moderate to excellent yields. Theseesults show the nature of different substituents present in both theubstrates a strong influence of on the product yield, as follows. Asimilar to other coupling reactions, aryl iodides showed the higheronversion, followed by aryl bromides and aryl chlorides (Table 1,
ntries 1–4). Aryl fluorides were not shown any conversion. Forhe different unsubstituted aryl halides, the product yield was inhe following order: Due to the presence of electron withdrawingroups such as –NO2 and –Cl in the aryl halides, the product yield
zed catalyst (A, C & E) and calcined Cu-Fe HT catalyst (B, D & F).
increased (Table 1, entries 9–11) but decreased appreciably dueto the presence of electron withdrawing group in thiol (Table 1,entries 15 and 16). The presence of electron donating group (–CH3or –OCH3) in the aryl halides caused a decrease in the product yield(Table 1, entries 5–8) but that in thiol resulted in a small increase inthe product yield (Table 1, entries 13 and 14). The presence of het-eroatom (N) in the aromatic ring of thiol caused a large decrease inthe product yield (Table 1, entry 17). Very high yield was obtainedwhen aryl thiol was replaced by an aliphatic thiol (n-C8H17SH)(Table 1, entry 18). In the absence of the Cu-Fe catalyst, the productyield was found to be negligibly small.
In order to optimize the reaction conditions, the reactionbetween 4-iodoanisole and thiol over the Cu-Fe HT catalyst wascarried 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). The results are presented in Table 2 and Fig. 4.
The results in Table 2 (entries 1–6) showed that role of baseaffects the conversion of Cu-Fe HT catalyzed S-arylation reactions.Among the different bases, product yield was found to be high whenK2CO3 was used as base for this cross coupling reaction. In contrastto K2CO3, other bases such as Na2CO3, sodium acetate, trimethyl
amine and pyridine or zinc dust showed less conversion and thecompletion of reaction was much longer than that achieved withK2CO3. This revealed that, among the bases, K2CO3 is the mostsuitable base for the Cu-Fe catalyzed S-arylation reaction.
58 D.K. Dumbre et al. / Applied Catalysis A: General 476 (2014) 54–60
Table 1Performance of the Cu-Fe HT catalyst in the S-arylation of thiols and substituted thiols with different aryl halides [(reaction mixture = aryl halide (1 mmol) + thiol(1.5 mmol) + catalyst (30 mg) + K2CO3 (5 mmol) + DMF (3 ml), bath temperature = 120 ◦C)].
a Fifth reuse of the catalyst.b Cu-Fe-HT without decomposition was used.c Catalyst obtained from the decomposition of a mixture of Cu(II) nitrate and Fe(III) citd In absence of any catalyst.e In absence of any base.
Table 2Performance of the Cu-Fe-HT catalyst in the S-arylation of thiol with 4-iodoanisolein the presence of different solvents and bases (bath temperature = 120 ◦C).
Entry Base used Solvent used Reaction time (h) Product yield(%)
Fig. 4. Influence of the reaction temperature on the product yield in the S-arylationof thiol with 4-iodoanisole in DMF with K2CO3 (as a base) (reaction time = 8 h).
rate (with Cu/Fe mole ratio of 3.0) at 600 ◦C for 4 h.
The role of solvents in the Cu-Fe catalyzed S-arylation reac-tion was studied and the results are given in Table 2 (entries 1,7–10). Among the solvents, the S-arylated product yield showedthe following order: DMF > toluene > xylene > acetonitrile > NMP.The observed trend for the solvent effect on the product yield isexpected most probably because of the donor properties of DMFand that can function as ligand to the Cu-Fe HT catalyst.
The Cu-Fe catalyzed S-arylation product yield also showedstrong temperature dependence and the results are given in Fig. 4The product yield was increased exponentially with increasing thereaction temperature from 50 ◦C to 120 ◦C and then leveled off.Upto 50 ◦C, there was almost no product formation, even after20 h of reaction. At 120 ◦C, a reasonably very good product yieldis achieved in the reaction. Therefore, Cu-Fe catalyzed S-arylationreaction require K2CO3 as base, DMF as solvent and 120 ◦C wasreaction temperature in order to obtain better product yields.
In order to evaluate the catalyst reusability, the spent cata-lyst was removed from the reaction mixture by filtration, washedwith DMF first and then with acetone, dried and then reused inthe S-arylation reaction at 120 ◦C. This was repeated several times.The results showing the reusability of the catalyst are presentedin Table 1 (entry 19). It is interesting to note that, apart from itshigh activity, the catalyst also showed excellent reusability with-out significant decrease in its activity. The observed increase in thereaction period for completing the reaction is expected because therecovered catalyst needs longer time to get further activation. Theproduct yield in the 1st, 3rd and 5th reuse of the catalyst was foundto be 88% for 8.25, 8.5 and 9.0 h, respectively.
It may be noted that, the dried Cu-Fe-hydrotalcite (before cal-cination) was directly used as a catalyst; the product yield in thesame period was much smaller (Table 1, entry 20). Also, when the>thermally decomposed (under similar conditions) mixed Cu-Fenitrates were used as the catalyst in the reaction, the product yieldwas much smaller (Table 1, entry 21). This may be due to the for-mation of solid catalyst with non-uniform distribution of Cu(II) andFe(III).
As compared to the earlier reported reusable solid catalysts(nano-CuO [26], nano-In2O3 [28], La2O3 [27] and Cu2S-Fe [29])for the C–S cross coupling reaction, this mesoporous Cu-Fe HT
6 talysis
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atalyst showed higher activity. Very high conversion (80–90%,ntries 3,6,7–14) were achieved in 8–10 h of reaction time. Thisay be due to the high surface area (123 m2/g) of CuFe hydrotal-
ite materials and their large pore size (15 nm). Also the mixedhase of CuO:Fe2O3 or CuFe2O4 was attributed for the observedatalytic activity. This catalyst is stable, does not require any addi-ional ligand (uses the solvent DMF as ligand), very strong base oreluctant like iron powder to catalyze S-arylation reactions. Hence,t is a highly promising environmentally more benign catalyst forhe S-arylation of different thiols with aryl halides.
. Conclusions
In summary, a novel ligand-free, mesoporous solid catalysterived from Cu-Fe-hydrotalcite was demonstrated as highly effi-ient and environmentally more benign catalyst for the synthesis ofiaryl sulfurs with high yields under mild conditions. Cu-Fe HT cat-lyzed Ullman type C–S cross coupling between thiols/substitutedhiols and different aryl halides showed very high product yieldsn the presence of DMF (as a solvent) and K2CO3 (as a base) at20 ◦C. The catalyst showed high activity/selectivity and excellenteusability in the S-arylation. Moreover, it is inexpensive and opera-es in presence of a weak base. It can also be easily separated fromhe reaction mixture and reused for the reaction without signifi-ant loss of its activity. All these facts make this catalyst a highlyromising alternative to homogenous or other heterogeneous solidatalyzed C–S cross coupling reactions. The high catalytic activ-ty seems to be attributed to the uniformly distributed Cu- ande-species on the catalyst surface.
cknowledgement
VRC is grateful to the National Academy of Sciences (India) forhe NASI Sr. Scientist Platinum Jubilee Fellowship. We thank theMIT Microscopy and Microanalysis facility staff members for theircientific and technical assistance.
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A: General 476 (2014) 54–60
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