Heterogeneous magnetic catalyst for S-arylation reactions

Applied Catalysis A: General 433–434 (2012) 258–264

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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata

Heterogeneous magnetic catalyst for S-arylation reactions Niranjan Panda ∗ , Ashis Kumar Jena, Sasmita Mohapatra ∗∗ Department of Chemistry, National Institute of Technology, Rourkela-769008, Odisha, India

a r t i c l e

i n f o

Article history: Received 3 January 2012 Received in revised form 18 May 2012 Accepted 20 May 2012 Available online 28 May 2012 Keywords: Superparamagnetic nanoparticles Heterogeneous catalyst Cross-coupling S-arylation Dibenzothiazepines Dibenzothiazepinones Aryl halides

a b s t r a c t A convenient method for the synthesis of monodisperse, superparamagnetic copper ferrite (CuFe2 O4 ) nanoparticles with high surface area has been described. The synthesized material was characterized by various techniques. XRD showed the nanocrystalline nature of CuFe2 O4 with a crystallite size of 6 nm. TEM analysis showed that uniform spherical CuFe2 O4 particles are formed with a size of 55 ± 5 nm. N2 adsorption/desorption measurements confirmed the mesoporous nature of the sample with surface area >216 m2 g−1 . The field dependent magnetization, illustrated by VSM and saturation magnetization was found to be 44 emu g−1 . The catalytic applications of the synthesized CuFe2 O4 nanoparticles were explored for the cross-coupling of thiols with diverse range of aryl halides. Aryl iodides and bromides result in biarylsulfides in good to excellent yields (62–98%) whereas aryl chlorides gave significant amount of diaryldisulfide. Scope of this catalytic protocol further extended to one-pot synthesis of biologically important tricyclic dibenzothiazepines. The superparamagnetic nature of CuFe2 O4 nanoparticles was found to be advantageous for their easy, quick and quantitative separation from the reaction mixture. Negligible leaching of Cu and Fe in consecutive cycles makes the catalyst economical and environmentally benign. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Transition-metal-catalyzed cross-coupling reaction presents one of the robust methods for the formation of carbon–carbon and/or carbon–heteroatom bonds [1–3]. Among the various crosscoupling reactions, metal-catalyzed S-arylation has received less attention as compared with N- or O-arylation reactions until very recent times. This is because (i) thiols are prone to undergo oxidative S S coupling reactions to undesired disulfides, and (ii) strong coordinating properties of organic sulfur compounds, often make the catalyst ineffective (catalyst poison) [4]. Usually, palladium, copper and nickel-based catalysts are extensively used for various C S cross-coupling reactions [5–9]. In spite of having wide scope and excellent compatibility with many functional groups, these protocols, often suffer from the disadvantages resulting from (i) the high cost of the palladium precursors, (ii) the need for ancillary ligands rendering the catalysts sufficiently reactive, (iii) concerns about the toxicity of these metal salts, and (iv) the extended reaction times, which are necessary in many cases. Considering the cost and environmental factor, the use of Cu catalysts for various cross-coupling reactions is attractive one from industrial perspectives. Moreover, ligands such as phosphazene, ethylene glycol,

∗ Corresponding author. Tel.: +91 661 2462653. ∗∗ Corresponding author. E-mail addresses: [email protected], [email protected] (N. Panda), [email protected] (S. Mohapatra). 0926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apcata.2012.05.026

neocuproine, N-methylglycine, oxime-phosphine oxide ligand, tripod ligand, benzotriazole, 1,2-diaminocyclohexane, ␤-ketoester, l-proline, BINAM, polyethylene glycol, and ethylene diammine, are used as chelating agents in the copper-catalyzed cross-coupling reactions [10]. On the other hand, the simple separation and regeneration of the catalyst from the reaction mixture are in strong demand for the cost-effective process of molecular synthesis. In pharmaceutical industries, it is also essential to remove all traces of metal residues, which frequently interfere with the subsequent reactions and contaminate the final products. In contrast, developments of reusable heterogeneous catalytic systems for cross-coupling reactions have been received less attention although the situation is changing in recent years [11]. Evidently, certain heterogeneous catalytic system for the cross coupling of arylthiols with aryl halides have been reported. For instance, CuIcatalyzed cross coupling of arylthiols with aryl iodide was reported by van Koten [12] and Li [13] individually. Luque and co-workers developed a microwave-assisted, Fe-Cu co-catalyzed heterogeneous catalytic system for the cross-coupling of thiols with aryl iodides [14]. More importantly, Punniyamurthy and co-workers exploited the high surface area and reactive morphology of the CuO nanoparticles for successful C S cross-coupling reactions under ligand free conditions [15]. Ranu and co-workers also employed copper nanoparticles for the cross-coupling of aryl iodides and bromides with thiophenols at 110 ◦ C [16,17]. Although these results are promising, the small size of nanoparticles often makes their separation and recycling difficult, which impedes their use in industrial processes [18,19]. In order to circumvent such problems, we

N. Panda et al. / Applied Catalysis A: General 433–434 (2012) 258–264

envision that single component superparamagnetic nanocatalysts with high surface area, whose flocculation and dispersion can be controlled reversibly by application of a magnetic field, may be employed as a heterogeneous catalyst for cross-coupling reactions. Indeed, in recent years, magnetic nanoparticles have been extensively studied for various biological applications, such as magnetic resonance imaging [20], drug delivery [21,22], biomolecular sensors [23,24], bioseparation [25,26] and magneto-thermal therapy [27,28]. Despite their wide use in biological systems, much less attention has been focused on the catalytic behavior of magnetic nanoparticles in organic transformations [29–35]. Here, we report the detailed synthesis, characterizations of mesoporous superparamagnetic copper ferrite (CuFe2 O4 ) nanoparticles and their efficient catalytic activity toward the S-arylation reactions. One-pot synthesis of biologically important tricyclic dibenzothiazepines was also demonstrated. 2. Experimental 2.1. Materials All chemicals are of reagent grade and used without further purification. FeCl3 , CuCl2 ·2H2 O, ethylene glycol and ethanolamine were purchased from Merck specialities, India. N-heterocycles, thiols and aryl halides were obtained from Spectrochem, India. Petroleum ether, ethyl acetate and other organic solvents were procured from Rankem, India. 2.2. Instrumentations The phase formation and crystallographic state of the sample was studied by X-ray diffraction (XRD) analysis using Philips PW 1830 X-ray diffractometer with CuK␣ source. Nitrogen adsorption/desorption isotherm was obtained at 77 K on a Quantachrome Autosorb 3-B apparatus. The specific surface area and pore size distribution were obtained by following BET equation and BJH method respectively. The morphology of the catalyst was studied by scanning electron microscopy (HITACHI COM-S-4200) and Transmission electron microscopy (JEOL 3010, Japan) operated at 300 kV. Atomic Weight Percentage of the catalyst (CuFe2 O4 ) was determined by an inductively coupled plasma-mass spectrometry (ICP-MS). Leaching of heavy metals after S-arylation was analyzed by AAS (Perkin Elmer A Analyst 200 Spectrometer). The synthesized cross-coupled products were characterized by IR, NMR spectroscopy and MS spectrometry. Chemical compositions of the products were determined by C, H, N, S elemental analyzer. IR spectra were recorded on Perkin Elmer (BX 12) spectrophotometer on KBR discs. All NMR spectra were recorded on Bruker Avance III (400 MHz for 1 H NMR, 100 MHz for 13 C NMR) spectrometer; chemical shifts were expressed in ı units (ppm) relative to TMS signal as internal reference in CDCl3 (7.28 ppm). Multiplicity was indicated as follows: s (singlet), d (doublet), t (triplet), q (quartet), m (multiplet) and bs (broad singlet). Elemental analyses were carried out with a Vario EL Elemental Analyzer. Mass spectra were taken by EI (70 eV) technique on Shimadzu QP 2010 PLUS GC–MS system. 2.3. Synthesis of the catalyst Superparamagnetic CuFe2 O4 nanoparticles were prepared by thermal decomposition of CuCl2 and FeCl3 in ethylene glycol in presence of sodium acetate and ethanolamine following the method recently reported by Mohapatra et al. [36]. Anhydrous FeCl3 (0.683 g, 4.2 mmol) and CuCl2 ·2H2 O (0.358 g, 2.1 mmol) were taken in 30 mL ethylene glycol and 0.5 gm of sodium acetate was added to it. The black color solution thus obtained was stirred for

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30 min at 80 ◦ C followed by addition of 15 mL of ethanolamine. The entire solution was heated at 150 ◦ C for 6 h during which fine black colloidal particles appeared in the reaction mixture. Then it was cooled down to room temperature. The particles were recovered using a magnetic separator (DynaMag2, Invitrogen), washed with millipore water and dried in a hot air oven at 80 ◦ C. The dried samples were calcined at 500 ◦ C for 5 h to get mesoporous CuFe2 O4 powders. 2.4. General procedure for S-arylation reactions For S-arylation reactions, CuFe2 O4 (10 mol%) nanoparticles were added to a solution of thiol (1 eq.), aryl halide (2 equiv.) and t BuOK (2 equiv.) in dry 1,4-dioxane. The reaction mixture was heated under reflux for 24 h under N2 atmosphere. Then, it was cooled to room temperature, diluted with ethyl acetate and catalyst was separated by magnetic separator. The catalyst was washed with ethyl acetate. The combined ethyl acetate layers were washed with water (twice), dried over anhydrous Na2 SO4 and concentrated to yield the crude products, which on further purification by silica gel column chromatography using petroleum ether/ethyl acetate lead to the desired S-arylated products. 3. Results and discussions 3.1. Catalyst characterizations The X-ray diffraction pattern of the calcined sample (Fig. 1) perfectly matches with the expected cubic spinel structure of CuFe2 O4. The position and relative intensities match well with those from JCPDS card (77-0010) for CuFe2 O4 . The broadening of peaks indicates nanocrystalline nature of the sample. The crystallite size was calculated using Scherer equation taking into account broadening of each diffraction peak and was found to be 9 nm. Copper and iron content in CuFe2 O4 nanoparticles were estimated to be 4.15 mmol/g and 8.30 mmol/g respectively by ICP-MS. Fig. 2a shows typical SEM image of the CuFe2 O4 powder. It is interesting to observe that CuFe2 O4 nanoparticles own quasi monodisperse size. TEM was used to further characterize the morphology and crystal structure. Fig. 2b and c represents TEM images at different magnifications. It can be noticed that CuFe2 O4 particles are spherical, almost monodisperse and own an average size of about 55 ± 5 nm. An image at higher magnification (Fig. 2c) shows that each spherical particle is an assembly of primary cubic CuFe2 O4 nanocrystals having a size of 3–4 nm and worm like pores with size of 2–3 nm. The HRTEM image (Fig. 2d) of a single aggregate shows that the parallel lattice fringe is extended over primary nanocrystals and pores. From this observation, oriented attachment growth mechanism may be proposed for the formation of such porous assembly [37–39]. This mesoporous structure has been formed from the self-assembly of ultrafine particles by sharing common crystallographic orientations. From thermodynamic point of view it can be explained that the driving force for such spontaneous arrangement is that the elimination of pairs of high energy surfaces which leads to the reduction of surface free energy. The marked region in Fig. 2d shows an interplanner spacing of 0.25 nm, corresponding to [3 1 1] plane of the cubic CuFe2 O4 nanoparticle. The selected area electron diffraction (inset in Fig. 2b) pattern is also consistent with the cubic phase of CuFe2 O4 nanoparticles. Nitrogen sorption experiment further supports the porous nature of CuFe2 O4 nanoparticles. Fig. 3 represents nitrogen adsorption/desorption isotherms and BJH pore size distribution curve (inset in Fig. 3). The isotherm is of type IV and displays H3 hysteresis loop. Generally, such type of hysteresis loop is observed in case of aggregates of plate like particles which give rise to slit shaped pores

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Fig. 1. XRD pattern of CuFe2 O4 sample (a) freshly prepared; (b) recovered sample after S-arylation reaction.

[40]. The BJH pore size distribution clearly indicates that mesopores of size 2–3 nm are formed among the ultrafine nanoparticles within the aggregate, which is in accordance with the TEM result. The BET surface area was found to be 216 m2 g−1 . The high surface area and narrow pore size distribution is advantageous in improving catalytic performance toward cross-coupling reactions. Field dependent magnetization curve of CuFe2 O4 catalyst at room temperature has been illustrated in Fig. 4. The magnetization value increased rapidly as the applied field increased up to 1 T and reached at a saturation point at 2 T. The saturation magnetization (Ms) displays higher value for synthesized nano-assemblies than

the corresponding nanodots. Ms value for CuFe2 O4 nanoparticles was found to be 23 emu g−1 against the corresponding bulk value of 83 emu g−1 [41]. This decrease in Ms could mainly be attributed to the small particle surface effect that becomes more prominent as the particle size decreases [42]. The curve shows almost no hysteresis and is completely reversible at room temperature. As observed by TEM, the size of primary nanocrystal is 3–4 nm, which is below the superparamagnetic critical size of CuFe2 O4 . Thus it is logical that the synthesized clusters show superparamagnetic behavior even though their sizes exceed the critical size and at the same time the improved coalescence of the crystallites in the

Fig. 2. (a) SEM image of fresh CuFe2 O4 catalyst and recovered catalyst (inset); (b) and (c) TEM images of discrete and monodisperse CuFe2 O4 nanoparticles with porous structure, inset is the SAED pattern; (d) high resolution TEM image showing lattice imaging of [3 1 1] plane.

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261

SH Me I O

S

catalyst base, solvent, reflux

1a

COMe

Scheme 1. S-arylation reaction.

Fig. 3. Nitrogen adsorption-desorption isotherm of CuFe2 O4 nanoparticles.

nanostructure results in enhanced magnetic coupling and higher magnetization. 3.2. Catalytic applications of copper ferrite nanoparticles in cross-coupling reactions Recently, we have shown the utility of magnetically separable CuFe2 O4 nanoparticles for N-arylation and alkynylation reactions [34,35]. Our results suggested to us that similar catalytic protocol for C S couplings might be tolerant of a wide variety of functional groups. Thus, to study the efficacy of magnetic nanocatalyst in the C S cross-coupling reactions, we use 4-iodoacetophenone and thiophenol as model substrates (Scheme 1). The optimization of the reaction conditions were carried out using different bases and solvents at their boiling temperature (Table 1). The best result was obtained when the reaction was conducted using 10 mol% of copper ferrite nanoparticles in the presence of 2 equiv. of t BuOK as base in 1,4-dioxane affording the desired 1-(4(phenylthio)phenyl)ethanone (1a) (95% yield). The reactions with

the solvent, DMF, and base, t BuOK, required higher temperature to afford 1a in quantitative yield (90%). In contrast, solvents such as methanol, toluene, CH3 CN and DMSO, in presence of base, t BuOK, resulted 1a in poor yield (8–48%). Similarly, other bases like, Cs2 CO3 , K2 CO3 , NaOAc, NaHCO3 and Et3 N leads <30% yield of the desired product 1a. Having the suitable solvent and the base for C S cross-coupling reaction, next we have examined the efficacy of other magnetic ferrite nanoparticles MFe2 O4 (M = Fe+2 , Co+2 and Ni+2 ). We observed that other ferrite nanoparticles, i.e. Fe3 O4 , CoFe2 O4 and NiFe2 O4 resulted 0–15% of the S-arylated product. Moreover, CuFe2 O4 nanoparticles not only give excellent yield of the product, but also magnetic nature of the catalyst facilitates its easy and quantitative removal from the reaction medium in the presence of an external magnetic field for subsequent use. When the reaction was conducted in presence of ambient air or in presence of atmospheric oxygen, 25–39% of 1a was isolated (Table 1, entries 18 and 19). The poor yield of 1a is attributed to the formation of unidentified polymeric residue. It is worth mentioning that when 10 mol% of CuO were used for the reaction, in addition to the S-arylated product (59%), diphenyldisulfide (24%) was obtained from the homocoupling of thiophenol. In contrast, under our optimum conditions 10 mol% of CuFe2 O4 nanoparticle furnished significantly higher yield (95%) of S-arylated product and no trace of diphenyl disulfide was detected (from TLC). Thus, it may be concluded that the synergistic effects of Fe and Cu in CuFe2 O4 co-catalyze the S-arylation reactions and presumably this is due to the reducing nature of Fe, which restricts the formation of the undesired diaryl disulfide in C S cross-coupling reactions. With the optimized reaction conditions we then extended the scope of this catalytic protocol for the cross-coupling of a diverse range of halides with thiophenol (Table 2). We noticed that CuFe2 O4 catalyzes the cross-coupling reactions of thiophenol with several aryl iodides in excellent yield. We have not detected any trace of Table 1 Optimization of reaction conditions for S-arylation reaction.a Entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18b 19c

Fig. 4. Field-dependent magnetization of (a) fresh CuFe2 O4 catalyst and (b) recovered sample.

Catalyst CuFe2 O4 CuFe2 O4 CuFe2 O4 CuFe2 O4 CuFe2 O4 CuFe2 O4 CuFe2 O4 CuFe2 O4 CuFe2 O4 CuFe2 O4 CuFe2 O4 Fe3 O4 CoFe2 O4 NiFe2 O4 CuO CuFe2 O4 – CuFe2 O4 CuFe2 O4

Solvent

Base

Yield [%]

DMF 1,4-Dioxane MeOH Toluene CH3 CN DMSO 1,4-Dioxane 1,4-Dioxane 1,4-Dioxane 1,4-Dioxane 1,4-Dioxane 1,4-Dioxane 1,4-Dioxane 1,4-Dioxane 1,4-Dioxane – 1,4-Dioxane 1,4-Dioxane 1,4-Dioxane

t

90 95 8 24 18 48 30 20 ≤5 10 18 ≤5 10 15 59 00 00 39 25

BuOK BuOK BuOK t BuOK t BuOK t BuOK Cs2 CO3 K2 CO3 NaOAc NaHCO3 Et3 N t BuOK t BuOK t BuOK t BuOK t BuOK t BuOK t BuOK t BuOK t t

a Reaction conditions: 0.90 mmol of thiophenol, 1.80 mmol of 4-iodo acetophenone, 10 mol% of catalyst, 2.0 equiv. of base, 5 mL of solvent, 24 h reflux under N2 atmosphere. b Reaction carried out under O2 atmosphere. c Reaction carried out in open air.

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Table 2 CuFe2 O4 catalyzed C-S cross-coupling of thiophenol with aryl halides.a

SH

X

10 mol% CuFe2O4 2 equiv. tBuOK R

1,4-dioxane,reflux 24h

S S

S + 1

R Dipheneyl disulfide

Entry

Aryl halide

Yield of 1 (%)

1 2 3 4 5b 6 7

X = I, R = 4-COMe X = I, R = H X = I, R = 4-NO2 X = I, R = 4-COOH X = I, R = 2-COOH X = I, R = 4-Cl X = I, R = 4-OMe

95 (1a) 98 (1b) 86 (1c) 87 (1d) 90 (1e) 88 (1f) 83 (1g)

8 9 10 11 12 13c

X = Br, R = 4-COMe X = Br, R = H X = Br, R = 4-NO2 X = Br, R = 4-COOEt X = Br, R = 2-CHO X = Br, R = 4-OMe

87 (1a) 85 (1b) 62 (1c) 76 (1h) 72 (1i) 62 (1g)

14c 15c 16c 17c

X = Cl, R = H X = Cl, R = 4-COOEt X = Cl, R = 4-NO2 X = Cl, R = 4-Me

48 (1b) 45 (1h) 42 (1c) 47 (1j)

a b c

Reaction conditions: 0.90 mmol of thiophenol, 1.80 mmol of halide, 10 mol% of catalyst, 2.0 equ. of t BuOK, 5 mL of 1,4-dioxane, 24 h reflux under N2 atmosphere. Reaction was carried out in DMF instead 1,4-dioxane. Entries 13–17 result 14, 20, 22, 25 and 17% of disulfide respectively along with biaryl sulfide.

homocoupling product (e.g. diphenyl disulfide) from TLC. Coupling of thiophenol with different aryl bromides having both electrondonating and -withdrawing groups resulted in products with good yields 62–87% (Table 2, entries 8–13). Moderated yield in case of 4-nitrobromobenzene and 2-bromobenzaldehyde is accounted for the possible decomposition where as p-bromoanisole results competitive homocoupled diphenyl disulfide (14%). More interestingly, our catalytic system was also found to be efficient in yielding the cross-coupled products with the comparatively less reactive aryl chlorides although a reasonable amount (17–25%) of disulfide was isolated (Table 2, entries 14–17). It may be note worthy that, C S coupling reactions of aryl bromides and chlorides with thiophenol reported earlier, gave poor yield or no yield of the S-aryl products even under Cu-catalyzed-ligand-assisted conditions (Table 3) [12,14,32,43–47]. Having the proved cross-coupling efficiency of our catalytic protocol with aryl iodides, the scope of the reaction was subsequently extended to a range of aryl and alkyl thiols. As summarized in Scheme 2, a number of biarylsulfides were obtained in moderate to excellent yields. Next, we deem for the one-pot synthesis of diaryl-fused thiazepine derivatives by the tandem C S and C N bond forming reactions. It is worthy to mention that dibenzo-fused thiazepines having medium-ring (6–7–6) structures show pronounced therapeutic effect on the central nervous system and are particularly active as antidepressants, antimetics, analgesiscs and sedatives [48,49]. Successful examples include dibenzothiazepine drugs (3 and 4), which are clinically used for the treatment of bipolar and psychiatric disorders (Fig. 5). Very recently, Garattini and co-workers reported the anti-tumor potential of the dibenzothazopine-11-one derivatives (5) in animals [50]. Petterssons and co-workers also reported the selective CBI inverse agonist behavior of dibenzothiazepine derivative (6) [51]. The emerging pharmacological potential of dibenzothiazepine ring systems makes them a valuable target and thus short and efficient routes for their synthesis are desired. In contrast, to the best of our knowledge one-pot synthesis of such tricyclic nucleus has not been reported till date although limited literature on multistep syntheses is precedented [50–53].

At the onset of our investigation to the one-pot synthesis of dibenzo-fused thiazepines, we have taken 2-aminothiophenol as the starting material and 2-bromobenzaldehyde as the coupling partner (Scheme 3). After 24 h, under the optimized reaction Table 3 Cross-coupling of bromobenzene and chlorobenzene with thiophenol in presence of different catalysts.

SH

X S

Catalyst X = Br / Cl

1a

.

Catalyst

Aryl halide

FeCl3 /DMEDA

X=I 91 X = Cl, X= Br 0

Yield of 1a (%) References [43]

CuO (nano)

X=I X = Br X = Cl

95 37 <5

[32]

Fe2 O3 /Cu(OAc)2 /L/MW

X=I X = Br X = Cl

95 70 48

[44]

Cu-MW-HMS

X=I X = Br X = Cl

95 nd nd

[14]

CuI/NMP

X=I X = Br X = Cl

99 5 4

[12]

CuL/L/MW

X=I X = Br X = Cl

86 32 00

[45]

Cu(OTf)2 /BINAM

X=I X = Br X = Cl

97 66 00

[46]

CuI(nano)/nBu4 NOH

X=I X = Br X = Cl

93 61 Trace

[47]

N. Panda et al. / Applied Catalysis A: General 433–434 (2012) 258–264

10 mol% CuFe2O4.

I

2 equiv. tBuOK

+

HSR

O

EtO

S

R

1,4-dioxane, reflux 24h

NH2

S

S

S

S

2a; 78%

2b; 85%

S

N 2e; 86%

2d; 88%

2c; 82%

COOH

S

S

COOH

H N

S

N

S

263

S

S OCH3

1e;, 60%

2g; 80%

2f; 40%

1j;, 80%

1g; 60%

CH3

Scheme 2. Cross-coupling of thiols with iodobenzene. Reaction conditions: 0.90 mmol of thiol, 1.80 mmol of iodobenzene, 10 mol% of CuFe2 O4 , 2.0 equ. of t BuOK, 5 mL of 1,4-dioxane, 24 h reflux under N2 atmosphere. Table 4 Reusability and leaching experiment over multiple run.

N

N

O

N

N

OH

Cl S

S

Clothiapine (4)

Quetiapine (3) R

HOHNOC

H N

S

S N 5

OEt

N O

N

O

O

5; R = OMe, Cl

6

conditions, tricyclic dibenzo[b,f][1,4]thiazepine (7) was resulted in moderate (65%). Encouraged by this interesting result, we insisted to synthesize functionalized dibenzothiazepines (e.g. dibenzothiazepinones). Thus, when methyl 2-iodobenzoate was employed, methyl 2-(2-aminophenylthio)benzoate (9) was obtained as the major product (86%) and traces of dibenzothiazepine-11-one (10, from TLC). However, on heating 9 at 135 ◦ C in acetic acid we got quantitative yield of the desired product 10 (72%). In order to access 10 directly from 2-aminothiophenol and ethyl 2-iodobenzoate, reaction mixture was heated under reflux in DMF for 24 h during which moderated yield (60%) was obtained. Scope of the tandem

S

CHO Br

SH NH2

Recovered CuFe2 O4 (%)

Yield of 1a (%)

1 2 3 4 5

– 96 95 95 94

95 93 93 92 90

N

e 1,4-dioxanh reflux, 24

Cu leakage (in ppm) 0.25 0.22 0.20 0.06 0.05

Fe leakage (in ppm) 0.08 0.02 0.02 0.01 0.01

C S and C N coupling was further extended for the synthesis of other dibenzothiazepines such as 8 and 11 in moderate yield (Scheme 3). 4. Recovery and reusability of the catalyst

Fig. 5. Some biologically potent dibenzo-fused thiazepines.

Cl

Cycle

N

N

R

7; R = H, 65% 8; R = Cl, 59%

After completion of the cross-coupling reaction of thiophenol with 4-iodoacetophenone, the catalyst was separated quantitatively (>95%) by using a magnetic separator and washed with ethyl acetate followed by distilled water and acetone. Then, it was dried in hot air oven at 150 ◦ C for 2 h and reused for the cross-coupling of thiophenol with 4-iodoacetophenone under our optimized reaction conditions. The catalytic behavior of CuFe2 O4 was found to be unaltered (up to five consecutive cycles) with negligible leaching of Fe and Cu to the reaction medium (Table 4). The weight percentage of copper and iron in the recovered catalyst was measured to be 4.08 mmol/g and 8.21 mmol/g, respectively (from ICP-MS). It was found from the XRD pattern of the recovered catalyst (Fig. 1b) that although there is no change in phase but the crystallite size increases from 9 nm to 27 nm. As a result of which the saturation magnetization (Ms) at room temperature (Fig. 4b) increases from 23 emu g−1 to 32 emu g−1 . This increase in Ms could be attributed to the partial agglomeration of nanoparticles during catalytic reaction [54]. The SEM image (inset Fig. 2a) of the recovered catalyst shows that the morphology as well as the dispersity of the catalyst is not affected even after the successful catalytic cycles. 5. Conclusion

R

CO2Me

CO2Me

S

R

N H

S

R

I NH2 9; R = H (86%)

O

10; R = H, 60% 11; R = Cl, 53%

Scheme 3. Synthesis of dibenzothiazepines. Reaction conditions: 0.90 mmol of oaminothiol, 1.80 mmol of halide, 10 mol% of CuFe2 O4 , 2.0 equ. of t BuOK, 24 h reflux under N2 atmosphere.

We have synthesized mesoporous copper ferrite nanoparticles with a size of 55 ± 5 nm by thermal decomposition method. Synthesized CuFe2 O4 nanoparticles were found to be efficient for the S-arylation reactions under ligand-free conditions whereas other ferrite nanoparticles were found to be ineffective. Our protocol was found to be tolerant to the cross-coupling of thiols with halides having different functional groups. Scope of this methodology has been extended to the one pot synthesis of dibenzothiazepines for

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