Chinese Chemical Letters  2015, Vol.26 Issue (05):547-552   PDF    
Highly effi cient and regioselective thiocyanation of aromatic amines, anisols and activated phenols with H2O2/NH4SCN catalyzed by nanomagnetic Fe3O4
Dariush Khalili     
Department of Chemistry, College of Sciences, Shiraz University, Shiraz 71454, Iran
Abstract: A new method employing magnetic nanoparticles Fe3O4 as a catalyst and H2O2 as a green oxidant is developed for the oxidative thiocyanation of aromatic amines, anisols and activated phenols with high yields under mild reaction conditions. The catalyst could be easily recovered from the reaction mixture using an external magnet and reused in several reaction cycles without loss of activity.
Key words: Arenes     Green oxidant     Hydrogen peroxide     Nanomagnetic Fe3O4     NH4SCN     Thiocyanation    
1. Introduction

Research into the development of synthetic strategies for the incorporation of sulfur atoms into organic frameworks has attracted considerable attention because of the widespread applications of this type of compounds as pharmaceuticals, synthetic reagents or functional materials [1]. Among the organosulfur compounds,thiocyanates are highly important functional groups [2] and can be found in natural [3] and designed compounds with biologically relevant properties [4]. Various sulfur-containing heterocycles need thiocyanate as the key precursor for their synthesis. In addition,thiocyanates can be readily transformed into the other sulfur-bearing functionalities [5]. Electrophilic thiocyanation is the most popular way for the construction of aryl thiocyanates. Due to the prominence of thiocyanate moiety,different reagents and catalysts have been reported for the electrophilic thiocyanation of arenes. These include the use of SCN- in conjunction with boron sulfonic acid [6],Br2 in MeOH [7],cross-linked poly (4-vinylpyridine) supported thiocyanate ion [8],silica sulfuric acid/H2O2 [9a], HCl/H2O2[9b],SiO2-VO(OH)2[9c],poly[4, diacetoxyiodo] styrene [10],acidic alumina [11],(dichloroiodo)benzene [12],Mn(OAc)3 [13],DDQ [14a] and ultrasound-assisted thiocyanationviaDDQ [14b],electrochemical thiocyanationviaSCN- [15] and trichloroisocyanuric acid (TCCA)/wet SiO2[16]. However,although they are general,some of these methods have restrictive requirements such as the use of expensive and toxic reagents,strongly acidic or harsh oxidizing conditions,long reaction times and tedious workup procedures. The need for efficient methods for the synthesis of arene thiocyanates has spurred research to discover new and practical reagent systems. Green oxidants such as hydrogen peroxide have been applied extensively in organic synthesis [17] but usually precious catalysts are required to achieve high yielding of the desired products. Therefore,the use of catalysts is a very important issue. A limited range of catalytic systems has been reported for the thiocyanation of arenes with H2O2[6, 9]. In recent years,magnetic nanoparticles have been extensively studied for various biological applications [18] and organic transformations [19]. Among the diverse magnetic nanoparticles under investigation,Fe3O4nanoparticles arguably attracted most of the attention [20]. In comparison to other transition metals, iron catalysts are environment friendly,inexpensive and relatively nontoxic. However,there have been no reports on the use of nano-Fe3O4 in combination with H2O2 for thiocyanation of arenes. In continuation of our current research on thiocyanation of activated arenes [21],herein,we are pleased to report a convenient approach to access aryl thiocyanates using a combination of H2O2 with ammonium thiocyanate catalyzed by nano-Fe3O4. 2. Experimental

All chemicals used in this study were of analytical grade, commercially available and used without further purification. Progress of the reactions was monitored by TLC analysis using silica gel polygrams SIL G/UV 254 plates. FT-IR spectra were recorded on a Shimadzu DR-8001 Spectrometer. NMR spectra were recorded on a Bruker Avance DPX 250 MHz Instrument in CDCl3or DMSO-d6 using TMS as an internal standard. Chemical shifts were reported in ppm (δ),and coupling constants (J),in Hz. Elemental analyses were determined in our department using a ThermoFinnigan Flash EA 1112 Series. X-ray diffraction (XRD) patterns were recorded on a XRD-D8 (BRUKER,Germany) employing a scanning rate of 0.058/s from 108 to 908 with CuKaradiation. Melting points were determined in open capillaries with a GalenKamp melting point apparatus and were not corrected. All the compounds were characterized by comparison with authentic samples.

General procedure for the synthesis of compounds 2a-u:Toa stirred solution of ammonium thiocyanate (3 mmol) and nanomagnetic Fe3O4(10 mol%,0.0231 g) in 3 mL acetonitrile was added arene (1 mmol). Sequentially hydrogen peroxide (30%,3.5 mmol, 0.3 mL) was added to this mixture and stirring was continued until TLC analysis showed the completion of the reaction. After the completion of the reaction,the stirring was stopped and the nanomagnetic Fe3O4 catalyst was adsorbed on to the magnetic stirring bar. The catalyst was separated and washed with diethylether (3×5 mL) followed by water (3×5 mL),and dried under vacuum and used directly for the next round of reaction. After that,the reaction solution was filtered off and concentrated under reduced pressure. The residue was purified by flash chromatography on silica gel using n-hexane-EtOAc to afford thiocyanated arenes.

2-Methyl-3-thiocyanato-1H-indole 2b:Rf =0.52,187 mg,99%. Mp 98-100℃ (lit. 97-99℃ [9a]). IR (KBr,cm-1): 3394 (NH),2152 (SCN), 1H NMR (250 MHz,CDCl3):δ2.35 (s,3H),7.03-7.54 (m,4H), 9.43 (br s,1H). 13CNMR (62.5 MHz):δ142.4,135.3,128.7,122.6, 121.2,117.9,117.7,111.3,88.0,13.4. Anal. Calcd. for C10H8N2S: C, 63.80; H,4.28; N,14.88%. Found: C,64.04; H,4.21; N,14.73%.

4-Thiocyanatoaniline 2d: Rf = 0.55 (n-hexane-EtOAc 8:2). Mp 50-52℃ (lit. 51-53℃ [10]). IR (KBr,cm-1): 3470,3362 (NH2), 2152 (SCN), 1H NMR (250 MHz,CDCl3): d3.93 (br s,2H),6.66 (d,2H,J= 7.5 Hz),7.34 (d,2H,J= 7.5 Hz). 13CNMR (62.5 MHz): δ 148.9,134.5,116.0,112.5,108.3. Anal. Calcd. for C7H6N2S: C, 55.98; H,4.03; N,18.65%. Found: C,56.14; H,3.82; N,18.76%.

4-Amino-3-chlorophenyl thiocyanate 2j: Rf = 0.42 (n-hexaneEtOAc 8:2). Mp 60-62℃ (lit. 61-63℃ [10]). IR (KBr,cm-1): 3479, 3371 (NH2),2152 (SCN), 1H NMR (250 MHz,CDCl3):δ4.27 (br s, 2H),6.68 (d,1H,J= 8.5 Hz),7.19 (dd,1H,J= 8.4,2.1 Hz),7.40 (d,1H, J= 2.1 Hz). 13CNMR (62.5 MHz):δ 141.6,131.1,126.9,125.3,119.5, 117.8,110.9. Anal. Calcd. for C7H5ClN2S: C,45.53; H,2.73; N,15.17%. Found: C,45.19; H,2.88; N,15.46%. 3. Results and discussion

The Fe3O4nano particle (particle size~30 nm; see TEM picture of nano Fe3O4in Supporting information) was prepared by the in situ chemical coprecipitation of Fe2+ and Fe3+ in an alkaline solution [22]. To choose the optimum conditions,electrophilic thiocyanation of indole 1a with H2O2/nanomagnetic Fe3O4in the presence of NH4SCN was selected as the model reaction (Table 1). In the absence of a catalyst,when indole was treated with hydrogen peroxide and NH4SCN,no significant thiocyanation was observed (Table 1,entry 1) [9a]. Other iron oxide catalysts,such as bulk-Fe2O3(Table 1,entry 2) and bulk-Fe3O4(Table 1,entry 3) were also examined in this reaction,but the yields of product 2a were lower. The effect of catalyst loading was also investigated and the maximum yield was obtained with 10 mol% of nanomagnetic Fe3O4(Table 1,entry 9). As expected,a lower loading (5 mol%) of nano Fe3O4resulted in a slower reaction rate affording 2a in 67% yield (Table 1,entry 10). Further increase in the amount of the catalyst did not have any significant effect on the product yield (Table 1,entry 11). In order to compare the activity of nano Fe3O4 and Fe3O4bulk,the active surface area (SBET) of these two samples was obtained by the multiple-point Brunauer-Emmett-Teller (BET) model [23]. The trace related to the adsorption of N2on each bulk and nanoparticle of Fe3O4is shown in Fig. 1. Micro-sized Fe3O4 has a BET surface area of~38m2/gwhereas this value was increased to 117m2/gduring the generation of nanoparticles. These results can explain the greater reactivity of synthesized Fe3O4nanoparticles compared to that of the bulk-Fe3O4.

Table 1
Investigation of the reaction conditions for thiocyanation of indole.a

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Fig. 1. Nitrogen adsorption percentage on micro-sized and nano-sized Fe3O4.

To investigate the possibly of solvent effect in our process, several solvents were screened. We observed that acetonitrile was the best for this reaction (Table 1,entry 9). Other solvents such as CH2Cl2,CHCl3,MeOH,EtOH and H2O,resulted in poorer conversions. Notably,the use of KSCN in water resulted in the desired product 2a in 74%.

The reaction also showed a strong dependence on the amount of H2O2as an oxidant and NH4SCN as a thiocyanate source. When H2O2and NH4SCN were used in 3.5 and 3 equivalents,respectively, the desired product 2a was formed in quantitative yield. In the absence of H2O2no reaction occurred. A comparison between the activity of bulk-Fe3O4and nano-Fe3O4revealed that nano-Fe3O4 had greater activity. Hence,it seems that particle size of the catalyst influences its activity. The scope and the efficiency of the presented catalyst system were then demonstrated by reacting a set of different activated arenes under the optimized conditions. A wide range of arenes including indoles,N-substituted anilines and anilines having electron-donating or withdrawing groups were employed to establish the general applicability of our present procedure. The results summarized in Table 2 show that this new catalytic system was highly active. Indoles (1a-c) were smoothly transformed into the corresponding thiocyanates (2a-c)in excellent yields (Table 2,entries 1,2 and 3). Substituted anilines containing electron-donating groups such as -methyl and -methoxy (Table 2,entries 5,7 and 8) are more easily thiocyanated than those containing electron-withdrawing groups,such as -Cl (Table 2,entry 10). Such observations reflect the fact that increases in the electron density of these substrates accelerated the rate of the reaction. In the case of apara-substituted aniline (1g) (Table 2, entry 7),orthothiocyanation occurred in high yield. A range of N-substituted anilines such as N-methyl- (1k),N-ethyl- (1l), N-phenyl- (1m) and N,N-dimethyl (1n) anilines also underwent thiocyanation efficiently to give their corresponding products 2k-nin high yields (Table 2,entries 11-14). Pyrrole1osmoothly underwent thiocyanation at room temperature leading to the desired product 2o in 80% yield (Table 2,entry 15). A minor amount of 2,4-dithiocyanatopyrrole 2o' was obtained along with the 2-thiocyanato pyrrole (as the major product) but to selectively synthesize a dithiocyanated pyrrole a longer reaction time was required (10 h) (entry 15). Similarly,carbazole 1p furnished 3-thiocyanatocarbazole 2p in moderate yield (73%,entry 16). To the best of our knowledge,the direct thiocyanation of phenols and aromatic ethers has only been sporadically reported in the literature [10, 24b]. The current method provides a usable process for the thiocyanation of anisols and activated phenols. In order to better understand the synthetic utility of our reagent system, phenol (1q) was subjected to the reaction conditions. No product formation was detected. When this reaction was performed in a sealed tube,the same result was obtained (Table 2,entry 17). Electron donating group such as -CH3 on the phenol (Table 2, entries 18 and 19) was able to facilitate the thiocyanation to give evidently high yield of the corresponding products 2r and2s. Finally,to further demonstrate the diversity of H2O2/Fe3O4system, we investigated the reactions using anisol and its methyl derivative as starting materials. In all of the cases (entries 20 and 21) the reaction proceeded smoothly to afford the desired thiocyanate products. In all the investigated arenes,the reaction occurred with high regioselectivity [13, 21, 24]. In order to show the efficiency of our method,we compared the results of our catalytic system with some other methods reported in the literature used for the synthesis of 3-thiocyanato indole (Table 3). Moreover, nanocatalyst Fe3O4is highly robust and can be recycled and reused several times without any deactivation. We examined the recycling of nanomagnetic Fe3O4for the thiocyanation of indole with NH4SCN under the optimized reaction conditions. As can be seen from Table 4,the catalyst could be magnetically recovered by an external magnet and reused 6 times without any significant loss of activity.

Table 2
Thiocyanation of a variety of activated arenesa

Table 3
Comparison of the efficiency of our method with other systems for thiocyanation of indole in the presence of NH4SCN.

Table 4
Recycling of nanomagnetic Fe3O4catalyst for the thiocyanation of indole with H2O2 and NH4SCN.a

In addition,by using H2O2as an oxidant and Fe3O4as a catalyst, this protocol can be a valid candidate towards the goal of green chemistry.

Similar to the previously reports on the catalytic thiocyanation [6, 9a],a plausible mechanism for this thiocyanation reaction is depicted in Scheme 1.

Firstly,H2O2is activated by Fe (III) in the structure of nanoFe3O4as a Lewis acid [25]. Subsequently,reaction of activated H2O2 with NH4SCN produces HO-SCN I [26],which in the presence of Fe3O4is activated again and able to produce thiocyanium ion SCN+ . Finally,this electrophile is attacked by indole to give the desired product and water as a byproduct. In support of this mechanism, we performed a reaction in the absence of indole and isolated a yellow precipitate after the removal of Fe3O4,which could be due to the formation of I. Its IR spectrum (Fig. 2) shows the characteristic -SCN and -OH bands at 2175 and 3271cm-1, respectively. The immediate formation of ammonia also strongly supports the initial step of this mechanism. The effect of Fe3O4as a catalyst was also demonstrated by a control reaction. In the absence of Fe3O4,the formation of ammonia is very slow,which shows the great catalytic effect of Fe3O4. This hypothesis is also supported by the successful recovery of the catalyst and its reuse for six times without loss of activity. 4. Conclusion

In summary,we have successfully developed a new catalytic system for the regioselective thiocyanation of aromatic and heteroaromatic compounds using nanomagnetic Fe3O4 as an efficient and reusable catalyst and H2O2 as a green oxidant in the presence of NH4SCN under mild conditions. This method is applicable for different types of arenes such as amines,aromatic ethers and activated phenols successfully and provided targeted products in good to excellent isolated yields.

Acknowledgment

We gratefully acknowledge the financial support of this study by Shiraz University Research Council..

Appendix A. Supplementary data

Supplementary data associated with this article can be found,in the online version,at http://dx.doi.org/10.1016/j.cclet.2015.01.007.

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