An efficient heterogeneous catalyst (CuO@ARF) for on-water C-S coupling reaction: an application to the synthesis of phenothiazine structural scaffold
© Sengupta and Basu; licensee Springer. 2014
Received: 4 September 2014
Accepted: 14 November 2014
Published: 29 December 2014
Aryl sulfides have significant importance from biological and pharmaceutical aspects. Transition metal-catalyzed carbon-sulfur cross-coupling reaction represents an important tool for the synthesis of sulfides. Among various transition metals, copper salts or oxides have found vast applicability.
A simple procedure for the preparation of poly-ionic amberlite resins embedded with copper oxide nanoparticles (CuO NPs) (denoted as CuO@ARF) has been developed, characterized, and employed for the first time as a heterogeneous ligand-free catalyst for ‘on-water’ C-S cross-coupling reaction. The NPs of CuO with an average size (approximately 2.6 nm), as determined from high resolution transmission electron microscopy (HRTEM) images, are found to be a potentially active, chemoselective, and recyclable catalyst for the preparation of symmetrical and unsymmetrical aryl sulfides. Recycling of the catalyst was performed successfully for five consecutive runs, and apparently no leaching was observed in a hot filtration test. Excellent chemoselectivity between iodo- and bromo-arene has been exploited in step-wise C-S and C-N couplings to synthesize bioactive heterocyclic scaffold phenothiazine.
The conventional methods for the C-S bond formation involve reduction of aryl sulfones or aryl sulfoxides using strong reducing agents like DIBAL-H or LiAlH4. Besides, on-water C-S bond formation has been reported via thiol addition to α,β-unsaturated carbonyl compounds at room temperature . In 1980, Migita et al. first showed the Pd-catalyzed thiation of aryl bromides using Pd(PPh3)4. Subsequently, other metals like nickel ,, copper ,, cobalt , iron , rhodium , manganese , and indium  have also been employed, though in much less extent, as compared to other C-X (X = C, O, N, P) coupling reactions. This is possibly due to the notion that sulfur might act as the poison to suppress the catalytic activity through strong coordinating and adsorptive properties . However, the last two decades have witnessed several new transition metal-based catalytic systems for the C-S coupling reactions between aryl halides and thiols. Amongst various transition metals, copper has been considered as the most useful for the C-S coupling reactions due to its special redox properties and cost-effectiveness. Many strategies have been successful by using homogeneous copper salts in the presence of suitable electron-rich and precious ligands for the C-S coupling reactions ,.
On the other hand, nano-structured materials with high surface areas have been investigated as effective catalysts for various organic coupling reactions . Catalysis of organic reactions by metal NPs supported on a suitable polymeric matrix offers the advantages of high catalytic activity, simplified isolation of the product, easy recovery, and recycling of the catalyst. Copper oxide nanoparticles (CuO NPs) are a good choice and indeed useful catalyst in the C-S coupling reaction between aryl halide and thiols -. However, previous on-water C-S coupling reactions involving copper species like CuI-TBAB , CuCl , or other metal species like Bi2O3, CoCl2.6H2O , or FeCl3.6H2O-bipyridyl complexes , etc. afford thioethers without easy recovery of the catalyst and recyclability. Direct use of CuO either in bulk or NPs requires organic solvents other than water and gave relatively poor yields in C-S coupling reaction. Considering our experience in the field of developing polymer-supported metal NPs as the heterogeneous catalyst in various coupling reactions, , and in conjunction with our interest in the synthesis of various biologically important heterocyclic scaffolds mediated over solid supports, , we were interested to develop polymer-supported CuO NPs and to use it as the catalyst in ‘on-water’ C-S coupling reaction between aryl halide and thiols.
We report herein our studies that constitute simple preparation and characterization of poly-ionic amberlite resins embedded with CuO NPs (CuO@ARF), which efficiently catalyze the C-S cross-coupling reaction under on-water and ligand-free conditions. Further application of this catalyst has been demonstrated in the synthesis of phenothiazine - an important structural motif of several potentially useful drugs and also used as chemosensors .
Amberlite resin formate (ARF) was prepared from commercially available inexpensive amberlite resin chloride by an ion-exchange process as reported from this laboratory . A mixture of ARF resin beads and Cu(OAc)2.H2O in DMF (50 mg g−1 of ARF) was heated at 110°C in a Teflon-capped sealed tube for 30 min with occasional gentle shaking. White ARF beads turned brownish in color during the process. Finally, these resin beads were filtered off, washed with water and acetone, dried, and characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and transmission electron microscopy (TEM) analysis, and the copper content of the composite was estimated by atomic absorption spectroscopy (AAS). The XRD and TEM analyses suggested the presence of CuO NPs distributed on the surface of ARF, and we referred the as-synthesized nanocomposite material as CuO@ARF.
Results and discussion
Catalytic activity of CuO@ARF
Optimization of reaction condition for the C-S cross-coupling using CuO@ARF a
K 2 CO 3
Minimum loading of the catalyst
CuO@ARF-catalyzed C-S cross-coupling reactions between haloarenes and thiol a
(4-H3CO)H4C6 - S - C6H5
(3-H3CO)H4C6 - S - C6H4(4-C1)
(2-H3CO)H4C6 - S - C6H4(4-CH3)
(3-NO2)H4C6 - S - C6H4(4-CH3)
(4-H3CO)H4C6 - S - C6H4(4′-CH3)
(5-Br)(2-H3CO)H3C6 - S - C6H5
(3-Br)H4C6 - S - C6H4(4-CH3)
(3-C1)H4C6 - S - C6H4(4-CH3)
(4-H3CO)H4C6 - S - C6H3(2,5-(CH3)2)
(4-H3CO)H4C6 - S - Cy
(4-H3CO)H4C6 - S - C5H11-n
(4-H3CO)H4C6 - S - C7H15-n
1,3-((4-CH3)C6H4S)2 - C6H4
(2-Br)H4C6 - S - C6H4(2-NH2)
As reported previously , the mechanism of the resin embedded CuO-catalyzed C-S coupling reaction is believed to proceed through the oxidative addition followed by reaction with thiol and then reductive elimination steps (CuII → CuIII → CuII). The role of the additive SDS is presumably to solubilize the organic substrates in an aqueous medium ,. Further beneficial assistance of the ‘microreactors’ formed by the surfactant like SDS in water medium organic reactions, as observed in other cases, cannot be ruled out . In addition, the role of water might be attributed to the H-bonding (HB) effect, as reported previously on other occasions , has also been noticed in our cases. Thus, we isolated the cross-coupled product in higher yield performing the reaction in an aqueous medium as compared to in DMF (Table 1, entries 3 and 7).
Comparison of various metal-based catalytic ‘on-water’ C-S coupling reactions with the present system CuO@ARF
CuI (1 mol%) 1.5 eqv TBAB (1 eqv); base KOH (1.5 eqv); 80°C, 10 h
With aryl iodide; bromo- and chloroarenes gave poor yields even using 5 mol% CuI
Excess strong base, not recyclable; CuI is poorly soluble in water, TBAB is moisture sensitive.
CuCl and 1,2-diamine as ligand (>2 eqv); 120°C overnight heating
With iodo- and bromoarenes. No selectivity was examined
Precious 1,2-diamines in >2, equivalents, long reaction time, recyclable using the recovered solution - catalyst was not separated.
Bi2O3/Diamine ligand; (each 10 mol%); 1 eqv KOH at 100°C
With aryl iodide
Presence of a ligand, high loading of the metal catalyst, long reaction time; recyclable using the recovered solution - catalyst was not separated.
CoCl2.6H2O/cationic 2,2′-bipyridyl system, 1 eqv KOH; excess zinc, 100°C
With aryl halides (iodide, bromide, and chloride).
Presence of cationic 2,2′-bipyridyl; excess Zn; long reaction time; recyclable using the recovered solution - catalyst was not separated.
FeCl3.6H2O (10 mol%) - bipyridyl complexes (10 mol%) ; KOH (4 eqv); 100°C, 24 h
With aryl iodide
Excess base, long reaction time; recyclable using the recovered solution - catalyst was not separated.
CuO@ARF; copper oxide (2.8 mol%); base K2CO3 (1.1 eqv); 100°C, 8 h
With aryl iodide
Mild base - nearly equivalent (1: 1.1); shorter reaction time; chemoselectivity between iodo and bromo has been utilized in the synthesis of medicinally important phenothiazine scaffold, easy separation of heterogeneous catalyst (by simple filtration of resin beads), recyclable for five runs without loss of activity; catalytic amount of SDS, ligand-free.
Our studies established that poly-ionic resin-supported CuO NPs (CuO@ARF) are an efficient catalyst in the C-S coupling reaction under ligand-free ‘on-water’ conditions. Low loading of the catalyst, recyclability without leaching, and chemoselectivity amongst aromatic halides are notable features. Further application of the chemoselectivity has been demonstrated in the synthesis of bioactive heterocyclic scaffold phenothiazine. Considering the inexpensive catalytic system along with the application to the synthesis of medicinally important structural scaffold, this heterogeneous catalyst and greener method can find wider applications in organic synthesis.
Amberlite IRA 900 (chloride form) was purchased from Acros Organics, Geel, Belgium, and used after washing with water and acetone followed by drying under vacuum. Cupric acetate was purchased from S.D. Fine Chem. Limited, Mumbai, India, and other chemicals were purchased and used directly. For column chromatography, silica (60 to 200 μm) (Sisco Research Laboratories, Mumbai, India), and for TLC, Merck plates coated with silica gel 60, F254 were used (Merck & Co., Inc., Whitehouse Station, USA). FT-IR spectra were recorded in FT-IR-8300 Shimadzu spectrophotometer (Shimadzu, Kyoto, Japan). NMR spectra were taken in CDCl3 using Bruker Avance AV-300 spectrometer (Bruker AXS, Inc., Yokohama-shi, Japan) operating for 1H at 300 MHz and for 13C at 75 MHz. The spectral data were measured using TMS as the internal standard (for 1H) and CDCl3 (for 13C). AAS measurements were made using Varian SpectrAA 50B instrument (Varian Medical Systems, Melbourne, Australia). Progress of the reaction was monitored by HPLC (1260 Infinity, Agilent Technologies, Santa Clara, USA), column: ZORBAX Rx-SIL (4.6 × 150 mm, 5 μm), eluent: n-hexane (flow rate 2 mL min−1). The XRD studies of the powder samples were done using the Rigaku SmartLab (Shibuya-ku, Japan) (9 kW) diffractometer using CuKα radiation. HRTEM of the samples was recorded with JEOL JEM-2100 F (FEG) (JEOL Ltd., Akishima-shi, Japan) operating at 200 kV.
Preparation of CuO@ARF
To a solution of Cu(OAc)2,.H2O (50 mg, 0.25 mmol) in DMF (5 mL) was added ARF (1 g), and the mixture taken in a Teflon-capped sealed tube was heated at 110°C for 30 min with occasional gentle shaking. The supernatant liquid became completely colorless by this time, and the greyish beads of ARF visibly turned brownish. The mixture was cooled to room temperature, and the resin beads were filtered off and washed repeatedly with water (3 × 5 mL) and acetone (2 × 5 mL). Resulting brown beads were dried under vacuum and used for analyses and evaluation of catalytic activity.
General conditions for C-S cross-coupling reaction
To a suspension of CuO@ARF (200 mg) in water (3 mL) were added aryl halide (1 mmol), thiol (1.2 mmol), K2CO3 (1.1 mmol), and SDS (10 mol%), and the reaction mixture was heated in a screw-capped sealed tube at 100°C for several hours as mention in Table 2. Proceeding of the reaction was monitored by TLC at time intervals. After completion, the mixture was cooled, diluted with water (5 mL), and then filtered through a cotton bed to separate out the resin beads. The resin beads were washed with ethyl acetate (2 mL), and the filtrate was extracted with ethyl acetate (3 × 10 mL). The combined organic layer was dried over anhydrous Na2SO4 and concentrated under vacuum. The residue obtained was purified by column chromatography using light petroleum as the eluent. All products were identified on the basis of spectral data (1H and 13C NMR) and also compared with reported melting point (for solid compounds and as available) (see Additional file 1).
The resin beads separated out from the reaction mixture were successively washed with water (3 × 5 mL) and acetone (2 × 5 mL) and then dried under vacuum for use in the next batch of recycle run.
Preparation of phenothiazine from selective iodo-coupled product (Table 2, entry 17) using Pd-BINAP catalyst
To a mixture of 2-(2-bromophenylthio)benzeneamine (1 mmol, 280 mg) in toluene were added tBuOK (1.5 mmol, 168 mg), Pd2(dba)3 (2 mol%), and BINAP (4 mol%). The reaction mixture taken in a screw-capped sealed tube was heated at 100°C for 4 h. After cooling, the reaction mixture was diluted with water (5 mL) and extracted with ethyl acetate (3 × 10 mL). The combined extracts were washed with brine, dried (Na2SO4), and evaporated. The residue was purified over a silica gel column to obtain white crystals of phenothiazine (167 mg, 84%), characterized by 1H and 13C NMR, and compared with literature data (see Additional file 1).
10H-phenothiazine , m.p. 185°C to 186°C (Lit. m.p. 186°C to 187°C). 1H NMR (DMSO-d6, 300 MHz): δ/ppm 6.72 to 6.77 (m, 4H, ArH), 6.89 to 7.01 (m, 4H, ArH), 8.58 (s, 1H, NH) 13C NMR (DMSO-d6, 75 MHz): δ/ppm 114.4, 116.4, 121.8, 126.3, 127.5, and 142.1.
Preparation of phenothiazine from selective iodo-coupled product (Table 2, entry 17) using CuO@ARF catalyst
To a mixture of 2-(2-bromophenylthio)benzeneamine (1 mmol, 280 mg) in dimethyl sulfoxide were added tBuOK (1.5 mmol, 168 mg), CuO@ARF (200 mg, 1.8 mg of copper, 2.83 mol% Cu) and L-proline (5.66 mol%). The reaction mixture taken in a screw-capped sealed tube was heated at 100°C for 48 h. After cooling, the reaction mixture was diluted with water (5 mL) and extracted with ethyl acetate (3 × 10 mL). The combined extracts were washed with brine, dried (Na2SO4), and evaporated. The residue was purified over a silica gel column to obtain white crystals of phenothiazine (143 mg, 72%), characterized by 1H and 13C NMR and compared with literature data (see Additional file 1).
We thank DST, New Delhi, for financial support [Grant No. SR/S1/OC– 86/2010 (G)], and DS thanks CSIR, New Delhi, for awarding research fellowship. We thank Dr. G. De, CSIR-CGCRI, Kolkata for Powder XRD measurements and Prof. A. K. Nandy, I.A.C.S., Kolkata for HRTEM analysis.
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