A green and efficient access to aryl nitriles via an electrochemical anodic oxidation

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CCLET 2968 1–3 Chinese Chemical Letters xxx (2014) xxx–xxx

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Original article

A green and efficient access to aryl nitriles via an electrochemical anodic oxidation Q1 Jia-Qian

Ye, Zhen-Lei Zhang, Zheng-Gen Zha *, Zhi-Yong Wang *

Hefei National Laboratory for Physical Sciences at Microscale, CAS Key Laboratory of Soft Matter Chemistry & Collaborative Innovation Center of Suzhou Nano Science and Technology, University of Science and Technology of China, Hefei 230026, China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 3 March 2014 Received in revised form 23 March 2014 Accepted 2 April 2014 Available online xxx

The nitrile functionality is a key building block in synthetic chemistry, and has wide applications in pharmaceuticals. However, traditional methodologies for the synthesis of nitriles are limited to harsh reaction conditions. Herein, we report a new and efficient access to aryl nitriles by an electrochemical synthesis. Compared with the conventional synthetic methods, this electrochemical synthesis is more environmentally friendly and easier to handle. ß 2014 Zheng-Gen Zha. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.

Keywords: Eletrochemistry Anodic oxidation Aryl nitrile

8 9

1. Introduction

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

The nitrile functionality is one of the most elementary building blocks in chemistry, and has wide applications in synthetic chemistry and pharceuticals [1]. Conventional methods to construct nitriles involved the cyanation of halides [2,3], the Sandmeyer reaction [4], dehydration of aryl oximes [5]. However, many of these methods suffer from the drawback of employing harsh reaction conditions, such as high temperature or pressure and large excess of reactants [6]. Therefore, the development of feasible and chemically economical approaches to synthesis of nitriles is highly required. Recently, Jiao and co -workers reported a copper catalyzed conversion of methylarenes to aryl nitriles, and successively, they realized the transformation of benzyl halides to aryl nitriles [7]. More recently, Prabhu and co-workers reported a convenient method to synthesize nitriles from primary azides catalyzed by copper iodide, and then achieved a selective oxidation of aliphatic primary azides to the corresponding nitriles in the presence of KI and TBHP [6,8]. Electrochemical synthesis, an environmentally friendly process, has attracted much interest of chemists and great efforts have been made to gain excellent achievements in recent years [9]. Compared with the conventional redox process, the notable superiority of

* Corresponding authors. E-mail addresses: [email protected] (Z.-G. Zha), [email protected] (Z.-Y. Wang).

electrochemical conversion is a mass-free electron transfer between the electrode and the substrate during reactions, which making this method to be green and sustainable. In this context, we report an efficient and easy-to-handle method to synthesize aryl nitriles via a metal-free anodic oxidation process. A series of aryl nitriles were obtained in moderate to good yields directly from benzylic azides at room temperature.

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2. Experimental

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1

H NMR spectra were recorded on a Bruker AVIII-400 spectrometer. Chemical shifts (in ppm) were referenced to tetramethylsilane in CDCl3 as an internal standard. 13C NMR spectra were obtained by using the same NMR spectrometers and were calibrated with CDCl3 (d 77.00 ppm). The spectral data and spectra of all compounds can be found in Supporting information. Initial optimization studies were carried out by employing 1-(azidomethyl)naphthalene (1a) as a model substrate. In an electrochemical setup consisting of an undivided cell equipped with a pair of platinum electrodes, a mixture of 1a (0.5 mmol), electrolyte (0.25 mmol), and phrase transfer catalyst (PTC, 0.25 mmol) in different solvents (6 mL) was electrolyzed under galvanostatic conditions and stirred simultaneously for 4 h at room temperature (Table 1). First, the solvent of the reaction was optimized with K3PO4
3H2O as an electrolyte and TBAB as a PTC at a constant current of 20 mA (entries 1 and 2). However, no desired product was obtained. When the current of the reaction

http://dx.doi.org/10.1016/j.cclet.2014.04.024 1001-8417/ß 2014 Zheng-Gen Zha. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.

Please cite this article in press as: J.-Q. Ye, et al., A green and efficient access to aryl nitriles via an electrochemical anodic oxidation, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.04.024

39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56

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CCLET 2968 1–3 J.-Q. Ye et al. / Chinese Chemical Letters xxx (2014) xxx–xxx

2 Table 1 Optimization of the reaction conditions.

R

N3

standard conditions

N3

CN

CHO

electrolyte, PTC, solvent

a

N

R

R

-

current, Pt/Pt

-e 2a

1a

H 2O

Entry

Electrolyte

PTC

Solvent

Current (mA)

Yield (%)b

1 2 3 4 5 6 7 8 9

K3PO4 K3PO4 K3PO4 K3PO4 CsCO3 KOH KNO3 K3PO4 K3PO4

TBAB TBAB TBAB TBAB TBAB TBAB TBAB Bu4NPF6 Bu4ClO4

THF H2O H2O H2O H2O H2O H2O H2O H2O

20 20 50 80 50 50 50 50 50

0 0 73 0 17 22 15 Trace Trace

N2, H C H

N N

R

N

N

R

N2 B

A Scheme 1. A proposed mechanism for the reaction.

a

Reaction conditions: 1a (0.5 mmol), electrolyte (0.25 mmol), phrase transfer catalyst (PTC, 0.25 mmol), solvent (6 mL), 4 h. The mixture solvent was electrolyzed at a constant current for 4 h in an undivided cell and equipped with platinum electrode at room temperature. b Isolated yield.

57 58 59 60 61 62 63 64 65 66 67 68

system was increased to 50 mA, the desired nitrile was obtained in 73% yield (entry 3). Nevertheless, increasing the current to 80 mA did not show any significant influence in enhancing the yield (entry 4). Subsequently, a series of electrolyte were optimized, and the results showed that K3PO4 was the best electrolyte (entries 3 vs. 5–7). Finally, other PTCs were tested in this reaction, but only trace of desired nitrile was obtained (entries 8 and 9). Other typical reaction parameters, such as other organic solvent, reaction temperature and concentration of the reactant, were also investigated in this electrochemical transformation; however, no significant improvement in yield was obtained, so we do not discuss the relevant details here.

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3. Results and discussion

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Under the optimized reaction condition, a series of substituted azides were investigated to test the generality of this catalytic system. As shown in Table 2, benzylic azides bearing both electron-donating and electron-withdrawing groups transformed into the corresponding nitriles smoothly in moderate to good yields (Table 2, entries 1–12). Interestingly, diazide derivative 1, 4-bis(azidomethyl)benzene was oxidized into desired p-cyanobenzene (2l) in good yield (70%, Table 2, entry 11). Unexpectedly, Table 2 The scope of electrochemical oxidation of azides to nitriles.

TBAB, K3PO4.3H2O, H2O a

R1

N3

R1

C

CHO

N

I = 50 mA, Pt/Pt, r.t.

1

or O2N

2

3

Entry

R1

Product

Yield (%)b

Entry

R1

Product

Yield (%)b

1 2 3 4 5 6 7

1-Naphthyl C6H5 4-OMeC6H4 3-OMeC6H4 4-ClC6H4 4-BrC6H4 4-FC6H4

2b 2c 2d 2e 2f 2g 2h

75 75 71 68 70 72 72

8 9 10 11 12 13 14

3-BrC6H4 3-BrC6H4 4-CNC6H4 4-N3CH2C6H3 4-CF3C6H4 4-NO2C6H4 2-Py

2i 2g 2k 2l 2m 3 2n

67 69 63 70 62 53 0

a Reaction conditions: 1 (1.0 equiv, 0.5 mmol), TBAB (0.5 equiv), K3PO4
3H2O (0.5 equiv), H2O (6 mL), 4 h. The mixture solvent was electrolyzed at a constant current (50 mA) for 4 h in undivided cell and equipped with platinum electrode at room temperature. b Isolated yield.

when 1-(azidomethyl)-4-nitrobenzene was employed as the substrate, p-nitrobenzaldehyde 3 was obtained in 53% yield instead of the corresponding nitrile (Table 2, entry 13). This might because azides with strong electron-withdrawing groups undergo a hydrolyzation process more easily during reactions. Heterocyclic substituted azides failed to afford the corresponding products under the typical reaction condition (Table 2, entry 14). The plausible mechanism of this transformation is presented in Scheme 1. First, the aryl azide is electrochemically oxidized to the benzyl cation A in two steps [10,11], and then the cation A undergoes Schmidt-type rearrangement [12] to afford the desired product aryl nitrile. During this process, if the intermediates A bearing a strong electron-withdrawing group, A could be attacked by water, resulting in the formation of aldehyde C [13]. This conversion is similar to Jiao and co-workers’ previous research, in which they achieved the transformation from benzyl halides to aryl nitriles by using DDQ as the oxidant [7].

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4. Conclusion

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In summary, we have developed a new method to synthesize aryl nitriles via a metal-free electrochemical oxidation process directly from primary benzylic azides. To the best of our knowledge, this is the first report of metal-free oxidation of benzylic azides to nitriles under electrochemical conditions. This electrochemical method is environmentally friendly and easy to handle, thus making it more attractive to aryl nitrile synthesis.

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Acknowledgment

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This work was financially supported by the National Natural Q2 Science Foundation of China (Nos. 2127222, 91213303, 21172205, J1030412).

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Appendix A. Supplementary data

109

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cclet.2014.04.024.

110 111 112

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Please cite this article in press as: J.-Q. Ye, et al., A green and efficient access to aryl nitriles via an electrochemical anodic oxidation, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.04.024