AlO(OH) nanoparticles and sodium borohydride

Tetrahedron 72 (2016) 5898e5902

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A practical and highly efficient reductive dehalogenation of aryl halides using heterogeneous Pd/AlO(OH) nanoparticles and sodium borohydride Belguzar Yasemin Kara a, Melike Yazici a, Benan Kilbas a, Haydar Goksu b, * a b

Department of Chemistry, Faculty of Sciences, Duzce University, 81620 Duzce, Turkey € zce 81900, Turkey Kaynasli Vocational College, Duzce University, Du

a r t i c l e i n f o

a b s t r a c t

Article history: Received 12 May 2016 Received in revised form 28 July 2016 Accepted 9 August 2016 Available online 10 August 2016

The reductive dehalogenation of aryl halides was performed by using commercially available aluminium oxy-hydroxide-supported palladium (Pd/AlO(OH)) nanoparticles of about 3 nm size (0.5 wt. % Pd) with sodium borohydride. The dehalogenated products were obtained with absolute conversion in a mixture of H2O/MeOH (v/v¼1/1) under ultrasonic conditions at room temperature. All aryl halides were successfully converted to halogen-free compounds within 1.5e4 h with yields of over 95%. The one-pot catalytic method is presented as a new process for the reductive dehalogenation of halogenated compounds. This method is quite simple, highly efficient and eco-friendly, and has an exceptional recovery rate. Ó 2016 Elsevier Ltd. All rights reserved.

Keywords: PdAlO(OH) Heterogeneous catalyst Dehalogenation Reduction NaBH4

1. Introduction Halogenated organic compounds have been recognized as biocidal and reagent/intermediate in organic synthesis.1 However, they are known to be serious pollutants due to their environmental impact in the chemical industry.2 For instance, (R)-5-(azetidin-2-ylmethoxy)-2chloropyridine,3 (E)-1-(3-(chloromethylene)-6-hydroxy-2,3-dihydro benzo[b]oxepin-7-yl)ethanone,4 2,3,7,8-tetrachlorodibenzo[b,e][1,4] dioxine5 and 2-chloro-6-(1H-pyrrol-2-yl)aniline6 halogen derivative chemicals are toxic materials in which chlorine atoms are located in the aromatic and olefin systems.7

The toxicity effects of halogenated compounds have led scientists to carry out the removal of the halogens by cleaving the carbonehalogen bond.8 Thus, novel catalytic systems for dehalogenation of aryl halides have been developed using catalysts such as Pd/HAP-ɣ-Fe2O3,9 In,10 Pd/SBA-15,11 Pd(OAc)2,12 Zn/THFNH4Cl,13 PdPt/TiO2,14 PdCl215 and various hydrogen sources. Recently, instead of free hydrogen gas, preference has been shown to hydrogen sources such as ammonia borane (AB, 19.6 wt. %),16 dimethyl ammonia borane (DMAB, 3.5 wt. %)17 and sodium borohydride (NaBH4, 10.8 wt. %)18 due to their ability to be used under mild conditions and their high hydrogen density.19 Among these, NaBH4 is quite stable, cheap, eco-friendly and water-soluble. It also produces hydrogen gas in the presence of catalyst/water (Eq. 1).20 Catalyst

NaBH4 þ 2H2 O / NaBO2 þ 4H2

* Corresponding author. Fax: þ90 380 544 2812; e-mail address: haydargoksu@ duzce.edu.tr (H. Goksu). http://dx.doi.org/10.1016/j.tet.2016.08.027 0040-4020/Ó 2016 Elsevier Ltd. All rights reserved.

(1)

Although Pd/AlO(OH) nanoparticles (NPs) are commercially available, relatively little interest in these nanoparticles as heterogeneous catalysts has been shown in the literature. However, more recently, exponential growth has been observed in the number of publications dealing with the alkylation of ketones with alcohols,21 reduction of olefins22 and dynamic kinetic resolution of primary amines.23 Furthermore, Pd/AlO(OH) NPs were utilized for the effective reduction of nitro compounds into primary amines by our group for the first time.18a This nanocatalyst is preferable because it

B.Y. Kara et al. / Tetrahedron 72 (2016) 5898e5902

is more accessible, recoverable, and reusable in addition to being more stable in different environmental conditions. Thus, it can be employed for the dehalogenation of aryl halides. Herein, we report that various aryl halides underwent reaction with NaBH4 in the presence of aluminium oxy-hydroxideincorporated palladium nanoparticles. The dehalogenation reactions were performed with full conversion in short reaction times in a low volume of water/methanol mixture under ultrasonic conditions at room temperature. 2. Results and discussion The morphologies of Pd/AlO(OH) NPs both before the reaction and after reusing five times were examined by SEM, XRD and TEM. The SEM images show that the catalyst has a nanocrystalline Boehmite structure. Under high magnification, the surface morphology of a nanocluster can be clearly observed in Fig. 1a and b. Both images have an irregular spread. The agglomeration of the catalyst surface can be detected after the catalyst was reused five times, as seen in Fig. 1b. The elemental composition of the catalyst consists of carbon, aluminium and palladium, as seen in EDX spectrums in the ESI (Figs. S1 and S2). It is likely that the abundance of carbon is from the air. An XRD image of the Pd/AlO(OH) NPs is also presented in the ESI (y Fig. S3). Diffraction peaks indicated that the face-centred cubic (fcc) structure of the palladium is well established on the surface of the AlO(OH), as was mentioned in a previously published work.24 Transmission electron microscopic images revealed that PdAlO(OH) NPs had an average size of about 3 nm (y Fig. S5).21

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Table 1 Determination of reaction conditions for dehalogenation of 2-bromopyridinea

Entry Solvent

Catalyst (mg) NaBH4 (mmol) Time (h) Yieldb (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

30 30 30 30 20 20 20 20 20 20 20 30 40 d 40

IPA IPA/H2O (1:1) H2O MeOH H2O/MeOH (2:3) H2O/MeOH (2:3) H2O/MeOH (1:4) H2O/MeOH (1:4) H2O/MeOH (1:2) H2O/MeOH (1:1) H2O/MeOH (1:1) H2O/MeOH (1:1) H2O/MeOH (1:1) H2O/MeOH (1:1) H2O/MeOH (1:1)

1.25 1.25 0.75 0.75 0.75 1.25 0.75 1.25 0.75 0.75 1 0.75 0.75 0.75 0.75c

2 2 4 4 2 2 2 2 3 2 3 3 1.5 5 4

50 70 95 90 60 80 50 70 60 70 95 90 >95 No reaction >95

a Reaction conditions: 2-bromopyridine (0.25 mmol), Pd/AlO(OH) NPs and room temperature. b Calculated by GC yield. c Without ultrasonic conditions.

formate salts as a hydrogen source.2a Within 4 h, 4-bromotoluene (1 mmol) was converted to toluene with a 94% yield in the presence of an excess amount of Zn/NH4Cl in THF.13 In the literature, other commercially available catalysts such as Pd/C, Pd(OAc)2,

Fig. 1. SEM images: (a) Pd/AlO(OH) NPs before the reaction; (b) Pd/AlO(OH) NPs after reusing five times.

In the dehalogenation of aryl halides, the effects of PdAlO(OH) NPs as catalysts, NaBH4 as a hydrogen source and the use of different solvents were investigated. For optimization of the reaction conditions, 2-bromopyridine was used as a substrate. The reaction conditions were optimized with 0.25 mmol of substrate, 40 mg of PdAlO(OH) NPs, 0.75 mmol of NaBH4 in H2O/MeOH (v/v¼1/1) under mild conditions (Table 1, entry 13). The dehalogenation reaction did not occur without a catalyst (Table 1, entry 14). Ultrasonic agitation (100 W, 50 Hz) was then applied to accelerate the developing process. The dehalogenation of 2bromopyridine was accomplished within nearly 4 h at room temperature (Table 1, entry 15), while the reaction was completed in 1.5 h when the ultrasonic bath was utilized. The catalytic efficiency of PdAlO(OH) NPs was relatively high compared to previously published works dealing with dehalogenation of aryl halides (see ESI,y Table S1). The use of a corresponding catalyst has priority in terms of the product yield, product diversity, reaction time and temperature. For instance, the synthesis of benzene from bromobenzene was achieved with only 30% conversion within 2 h at 90  C in the presence of PdeC, using

PdCl2, Pd(acac)2 have also been tested in dehalogenation reactions. However, their catalytic efficiency in water was observed to be poor.2a,12 In addition, they are homogeneous, except for Pd/C, and they cannot be efficiently reused. On the contrary, Pd/AlO(OH) NPs work quite well in water and can be reused at least five times, as seen at Tables 2 and 3.22 Table 2 shows that all the aryl and heteroaryl halides were successfully reduced by the PdAlO(OH)/NaBH4. Most of the halogen-free compounds were quantitatively obtained within 1.5e4 h using ultrasonic agitation. Chloro-, bromo- and iodobenzenes (1, 3, 4) were all reduced to benzene with high yields (Table 2, entries 1e3). It is recognized that the ease of dehalogenation conveniently follows a fallen of I>Br>Cl. As shown in Table 2, the CeCl bond is difficult to activate with naked Pd(0) at room temperature, but it is not impossible.25,26 This effect is due to the support material (AlO(OH)). Consequently, the iodobenzene was reduced in less time (Table 1, entry 3). Aryl halides containing aldehyde groups were reduced to halogen-free alcohols by using excess amounts of the reducing agent (Table 2, entries 4e6). In 3 h, 4-iodo toluene (9), 4-iodo anisole (11) and (6-bromopyridin-3-yl)

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B.Y. Kara et al. / Tetrahedron 72 (2016) 5898e5902

Table 2 PdAlO(OH) NP effects in the dehalogenation of various aryl halidesa

Entry

Substrate

1

1 2

3

3

4

Product

2

2

2

4b

Time, h

Yieldc %

Entry

4

>95

10

3

2.5

6

8

6

9

10

6b

>95

>95

>95

>95

13

>95

1.5

9

>95

27

28

27

17

18

29

3

>95

3

>95

2

>95

1.5

>95

1.5

>95

1.5

>95

1.5

>95

2.5

>95

25

26

14

>95

23

15

12

1.5

21

14

16

Yieldc %

19

24

3

8

13

12

22

3

11

>95

Time, h

17

20

3

7

14

18

3.5

7

15

11b

6

5b

Product

16

4

5

>95

Substrate

30

a Unless otherwise stated, 0.25 mmol of substrate, 0.75 mmol of NaBH4 and 0.16 mmol% of Pd/AlO(OH) NPs (0.5 wt % Pd) were used with 1 mL of H2O/MeOH (v/v¼1/1) under ultrasonic conditions. b 1.25 mmol of NaBH4 was used. c Calculated by GC yield.

methanol (16) were easily converted to the corresponding halogenfree compounds (10, 11, 17) with high yields (Table 2, entries 7, 8, 11). The electronic effect of heteroatoms such as nitrogen and sulfur enabled the dehalogenation of heteroaromatic halides to be completed more quickly. Thus, the corresponding aryl halides were

easily reduced with high yields within 1.5 h at room temperature (Table 2, entries 9, 10, 14e17). Within 3 h, 4-amino-2bromopyridine (18) was converted to 4-aminopyridine (19) with the yields being above 95% due to the electron donating amine group (Table 2, entry 12). The dehalogenation process of 1-(5-

B.Y. Kara et al. / Tetrahedron 72 (2016) 5898e5902

Hydrogenehalogen substitution is then observed, followed by reductive elimination. Thus, palladium (II) is reduced to palladium (0).25,26

Table 3 Reusability test of Pd/AlO(OH) NPsa Entry

Substrate

Product

1st run Yieldb (%)

5th run Time, h

Yieldb (%)

5901

Time, h

3. Conclusions 1

>95

1.5

90

1.5

2

>95

4

87

4

a Unless otherwise stated, substrate (0.25 mmol), NaBH4 (0.75 mmol) and Pd/ AlO(OH) NPs (0.16 mmol%, 0.5 wt. % Pd) were used with 1 mL of H2O/MeOH (v/v¼1/ 1) at room temperature. b Calculated by GC yield.

In summary, the current catalytic process showed that highly effective palladium nanoparticles on the surface of aluminium oxyhydroxide provided dehalogenation of various aryl halides via sodium borohydride hydrolysis within 1.5e4 h under mild conditions with a high conversion rate. Moreover, the Pd/AlO(OH) NPs remained stable, even when reused more than five times. This novel process has many advantages: (i) a safe hydrogen supply; (ii) an eco-friendly solvent system and (iii) potential use in large-scale application in industry. 4. Experimental

bromothiophen-2-yl)ethanone (29) is slower (2.5 h) than for the other halogenated heteroaromatics due to the competitive reduction process of both the carbonyl and the halogen groups (Table 2, entry 18). The Pd/AlO(OH) NPs exhibited high efficiency in the dehalogenation of aryl halides. Furthermore, when the catalyst was reused five times, there was not a great loss in the yield of the reaction (Table 3). The ICP-MS analyses determined the amount of palladium (w2.0 ppm) leached into the reaction solution after reuse with very low. ICP analysis was made after purification of products in Table 3. It was found that the amount of palladium in products is in an accepted range (w0.03 ppm). The reaction mechanism of the dehalogenation process has not been entirely explained in previous works. Because there is not a radical initializer such as heat and light in reaction medium, the mechanism is thought to proceed via concerted pathway, not radical-based. A proposed mechanism is depicted in Scheme 1. Scheme 1 shows the activation, probably by adsorption, of AreX as the first step. We think that palladium (0) is inserted between the halogen atoms with the aryl group and it oxidizes to palladium (II).

4.1. Materials The halogenated aromatic compounds and Pd/AlO(OH) NPs used in the study were obtained from SigmaeAldrich. The NaBH4 was also purchased from SigmaeAldrich and used as received. Ultrasonic agitation was carried out using a Laborgerate GmbH 100 W (50 Hz) unit. 4.2. Characterization methods Scanning electron microscopy (SEM) analysis was performed via a JEOL SEM5800 device equipped with an EDX probe having an accelerator voltage of 20 keV. The X-ray diffraction (XRD) was conducted using a Panalytical Empyrean diffractometer with Ultimaþtheta-theta high resolution goniometer and an X-ray generator (Cu Ka radiation, l¼1.54056  A) operating at 45 kV and 40 mA. TEM images of Pd/AlO(OH) NPs have been obtained by a JEOL 200 kV TEM instrument. The amounts of the metals in the monodisperse Pd/AlO(OH) NPs were determined by a Leeman Lab inductively coupled plasma (ICP) spectrometer; the 1H NMR spectra were recorded on a Jeol ECS 400 MHz spectrometer. 4.3. General procedure for the dehalogenation reactions First, Pd/AlO(OH) NP catalyst (40.0 mg, 0.16 mmol%) and 1 mL of H2O/MeOH (v/v¼1/1) were added to a Schlenk tube. Next, the halogenated compound (0.25 mmol) was added. Finally, NaBH4 (0.75 mmol) was added to the reaction mixture and the vessel was closed. The reaction then continued during vigorous stirring under ultrasonic conditions at room temperature and was monitored by GC. Most of the reactions were completed over a time period of 1.5e4 h. After completion of the reaction, the catalyst was removed via simple centrifugation at 6000 rpm and then washed three times with methanol and water and allowed to dry for further use. The solvent was evaporated under vacuum. The products were purified by flash column chromatography and the dehalogenated products were then determined by 1H NMR. Acknowledgements This research was supported by Duzce University Research Fund (grant nos. 2014.05.03.243 and 2015.26.04.371). Supplementary data

Scheme 1. Proposed mechanism for dehalogenation reaction.

Supplementary data associated with this article can be found in the online version, at http://dx.doi.org/10.1016/j.tet.2016.08.027.

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