Recyclable Ru/C catalyzed oxidative cyanation of tertiary amines with TBHP

Abstract

The recyclable Ru/C catalyzed oxidative α-cyanation of tertiary amines with ethyl cyanoformate by using TBHP as an oxidant under ambient conditions has been developed. Utilizing this protocol, α-aminonitrile derivatives were synthesized in good to excellent yields with high selectivity. The cyanide source (ethyl cyanoformate) employed herein was relatively cheap and less toxic, which would be beneficial. The catalyst was also inexpensive and commercially available as well as recyclable up to four cycles, without significant loss of its catalytic activity.

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Introduction

The development of novel methods for C–C bond formation through C–H activation has attracted the attention of researchers across the world.¹ Transition metal catalyzed activation of C–H bond formation, especially sp³ C–H bonds and subsequent C–C bond formation, is a valuable and straightforward synthetic strategy,² as it does not need a prefunctionalized starting material.

The oxidative cyanation of tertiary amines via direct functionalization of the C(sp³)–H bond provides access to α-aminonitriles.³ α-Aminonitriles are highly useful and versatile synthetic intermediates (for the preparation of α-amino carbonyl compounds, α-amino alcohols and vicinal diamines), which have widely been used in the construction of a variety of synthetically, as well as biologically important compounds, such as alkaloids and functional materials.⁴ Therefore, the synthesis of these α-aminonitriles is significantly important both in synthetic as well as medicinal chemistry.

The synthesis of α-amino nitriles, by the reaction of carbonyl compound, an amine, and a cyanide, (Strecker reaction) is well reported in the literature.⁵ However, the new synthetic strategy for the preparation of these compounds involves the oxidation of a tertiary amine into an iminium ion followed by the nucleophilic attack of the cyanide ion. In this regard several metal catalysts such as Ru,⁶ V,⁷ Au,⁸ Mo,⁹ Fe,¹⁰ Ir¹¹ and Ti,¹² and metal free conditions,¹³ have been reported for the oxidative cyanation of tertiary amines. However, most of these protocols involve expensive and moisture sensitive catalytic systems as well as toxic cyanide reagents such as NaCN or TMSCN. Therefore, the development of a less toxic cyanide source, and a catalyst for the cyanation of tertiary amines, is highly desirable. Ethyl cyanoformate has been employed in various applications as a less toxic, cheap and readily available cyanide source. Even though few heterogeneous systems have been developed recently,^10b,14 preparation of all these heterogeneous catalytic systems involve tedious reaction procedures with expensive starting materials. Due to advances in sustainable chemistry, exploration of an inexpensive and commercially available catalytic system is desirable for this transformation. Ru/C¹⁵ has emerged as a versatile heterogeneous catalyst for the oxidation of various functional groups in synthetic organic chemistry. To the best our knowledge there is no report on the oxidative cyanation of tertiary amines over a heterogeneous Ru/C catalyst. In continuation of our efforts to design and develop novel methodologies under environmentally benign conditions,¹⁶ we report here a recyclable Ru/C catalyzed oxidative cyanation of several tertiary amines to the corresponding α-aminonitriles, using ethyl cyanoformate as a cyanide source and TBHP as an oxidant at 60 °C (Scheme 1).

Scheme 1 Ru/C catalyzed oxidative α-cyanation of tertiary amines.

Results and discussion

Initially, N,N-dimethylaniline, ethyl cyanoformate and Ru/C were used as model substrates to optimize the reaction conditions (Table 1). The reaction was screened to assess the effect of different oxidants such as TBHP in decane, H₂O₂ and O₂. Among these oxidants TBHP was found to be the most suitable to give the desired product (Table 1, entries 1, 2 and 5). The effects of different concentrations of TBHP and ethyl cyanoformate on the course of the reaction were examined and 2.5 equivalents of TBHP (Table 1, entries 3–5) and 2.0 equivalents of ethyl cyanoformate (Table 1, entries 5–7) were observed to be optimal for this reaction. Different solvents such as MeOH, EtOH, PhCH₃, CH₃CN, EtOAc, CHCl₃ and H₂O were screened (Table 1, entries 5, 8–12), and MeOH was found to be efficient and produced the desired product in good yield (Table 1, entry 5). Water was observed to be ineffective and did not yield the desired product (Table 1 entry 13). The reaction is very sluggish at room temperature and required a longer time to get a reasonable yield (Table 1, entry 14). Nevertheless, a high yield was obtained at 60 °C within a short reaction time (Table 1, entry 5). The optimum reaction conditions were found to be N,N-dimethylaniline (1.0 mmol), ethyl cyanoformate (2.0 mmol), 5 wt% Ru/C (20 mg), TBHP (in decane) (2.5 equiv) in MeOH (3 mL) at 60 °C for 6 h.

Table 1 Optimization of oxidative cyanation of tertiary amines^a

Entry	Solvent	Oxidant (equiv.)	T (°C)	Yield (%)^b
a Reactions were carried out with N,N-dimethylaniline (1.0 mmol), ethyl cyanoformate (2.0 mmol), ruthenium 5 wt% on carbon (20 mg) and oxidant in solvent (3 ml) at 60 °C for 6 h. b Isolated yield of the pure product. c Ethyl cyanoformate (1.5 mmol). d Ethyl cyanoformate (1.0 mmol).
1	MeOH	H2O2	60	50
2	MeOH	O2	60	40
3	MeOH	TBHP in decane (1.0)	60	40
4	MeOH	TBHP in decane (2.0)	60	74
5	MeOH	TBHP in decane (2.5)	60	95
6	MeOH	TBHP in decane (2.5)	60	84^c
7	MeOH	TBHP in decane (2.5)	60	71^d
8	EtOH	TBHP in decane (2.5)	60	61
9	PhCH3	TBHP in decane (2.5)	60	25
10	CH3CN	TBHP in decane (2.5)	60	46
11	EtOAc	TBHP in decane (2.5)	60	51
12	CHCl3	TBHP in decane (2.5)	60	30
13	Water	TBHP in decane (2.5)	60	0
14	MeOH	TBHP in decane (2.5)	rt	60

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Under the optimized reaction conditions, the effect of different cyanide reagents was observed for the cyanation of tertiary amines. Ethyl cyanoformate was found to be a better cyanide source than other reagents (Table 2, entries 1–6). No product formation was observed with zinc cyanide (Table 2, entry 6).

Table 2 Screening of various cyanide sources^a

Entry	Cyanide source	Yield (%)^b
a Reactions were carried out with N,N-dimethylaniline (1.0 mmol), cyanide source (2.0 mmol), ruthenium 5 wt% on carbon (20 mg), TBHP (in decane, 2.5 equiv) in MeOH (3 ml) at 60 °C for 6 h. b Isolated yield of the pure product. c 0.5 equivalents.
1	CNCOOEt	95
2	CNCOOC3H7	32
3		60
4	K3Fe(CN)6^c	10
5	CH2(CN)2	19
6	Zn(CN)2	0

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While expanding the substrate scope of the reaction, a series of tertiary amines were employed to investigate the generality of the reaction, and the results are depicted in Table 3. We examined the oxidative cyanation with various N,N-dimethylanilines and cyclic amines. Substituted N,N -dimethylanilines bearing both electron-donating and electron-withdrawing substituents afforded the corresponding cyanated products in good to excellent yields (Table 3, entries 2–5). It was noteworthy that in the case of unsymmetrical tertiary amines the nucleophile CN⁻ attacked chemoselectively (Table 3, entries 7–9). This protocol was also effective for cyclic amines. Piperidine, pyrrolidine, and tetrahydroisoquinoline afforded almost quantitative yields in all cases (Table 3, entries 10–17). Various substituted piperidine derivatives were prepared following our previously reported procedure¹⁷ to further react with ethyl cyanoformate, affording corresponding α-amino nitriles in high yields (Table 3, entries 11–16). In the case of the aliphatic tertiary amine, the desired product was not obtained (Table 3, entry 18).

Table 3 The oxidative cyanation of various tertiary amines catalyzed by Ru/C^a

Entry	Substrate	Product	Yield (%)^b
a Reactions were carried out with tertiary amine (1.0 mmol), ethyl cyanoformate (2.0 mmol), ruthenium 5 wt% on carbon (20 mg), TBHP (in decane, 2.5 equiv) in MeOH (3 ml) at 60 °C for 6 h. b Isolated yield of the pure product.
1			95
2			96
3			92
4			89
5			86
6			60
7			83
8			74
9			72
10			76
11			89
12			91
13			85
14			82
15			84
16			82
17			78
18	(C2H5)3N	—	0

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Based on experimental observation, a plausible reaction mechanism was proposed for Ru/C catalyzed oxidative α-cyanation of N,N-dimethylaniline with TBHP in the presence of ethyl cyanoformate (Scheme 2). The Ru/C 1 undergoes reaction with TBHP to give the Ru–oxo species 2.¹⁸ This Ru–oxo species 2 reacts with tertiary amine 3, to produce an iminium ion intermediate 4 by electron transfer and subsequent hydrogen transfer. The iminium ion intermediate subsequently reacts with CN⁻ (in situ generated from CNCOOEt) to afford α-aminonitrile 5, regenerating the Ru/C 1 species.

Scheme 2 Proposed reaction mechanism for the Ru/C catalyzed oxidative cyanation of tertiary amines with ethyl cyanoformate.

The reusability of the Ru/C catalyst was examined and the results are summarized in Fig. 1. After the reaction, Ru/C was separated by centrifugation, washed with ethyl acetate followed by acetone and dried in a hot air oven. This dried Ru/C was reused directly for the next batch without any treatment. No significant loss of catalytic activity was observed up to four cycles (Fig. 1). It was observed from TEM analysis that the morphology, shape and size of the catalyst after reuse (Fig. 2b), did not differ much from the native catalyst (Fig. 2a).

Fig. 1 Proposed heterogeneous Ru/C recyclability data.

Fig. 2 Proposed TEM images of (a) native Ru/C (b) Ru/C after four cycles.

Conclusions

In conclusion, we have developed a simple and more convenient route for the oxidative cyanation of tertiary amines, using a recyclable ruthenium catalytic system with ethyl cyanoformate. The recyclability of the catalyst, inexpensive cyanide source, short reaction time, wide substrate scope and excellent product yields make this protocol very practical.

Experimental section

General experimental procedure for oxidative cyanation of tertiary amines

The reaction was carried in a 25 mL round bottomed flask equipped with magnetic stir bar, charged with N,N-dimethylaniline (1 mmol), ethyl cyanoformate (2 mmol), 5–6 M TBHP solution in decane (2.5 mmol), methanol (3 mL) and Ru/C (20 mg) catalyst. The resulting reaction mixture was stirred at 60 °C for 6 h. The reaction progress was monitored by TLC. After the reaction, Ru/C was separated by centrifugation and the solvent was evaporated and the crude product purified by column chromatography. The identity and purity of the product was confirmed by ¹H NMR, ¹³C NMR and ESI-MS.

2-(Methyl(phenyl)amino)acetonitrile (Table 3, entry 1)^6d

¹H NMR (300 MHz, CDCl₃): δ 7.30–7.21 (m, 2H), 6.93–6.80 (m, 3H), 4.13 (s, 2H), 3.01 (s, 3H). ¹³C NMR (75 MHz, CDCl₃): δ 147.7, 129.4, 120.3, 114.9, 96.1, 42.2, 39.2. ESI-MS: m/z 147 [M+1].

2-(Methyl(p-tolyl)amino)acetonitrile (Table 3, entry 2)^6d

¹H NMR (−300 MHz, CDCl₃): δ 7.08–7.05 (m, 2H), 6.76–6.74 (m, 2H), 4.08 (s, 2H), 2.96 (s, 3H), 2.28 (s, 3H). ¹³C NMR (75 MHz, CDCl₃): δ 145.6, 129.8, 129.7, 115.4, 115.3, 42.6, 39.3, 20.2. ESI-MS: m/z 161 [M+1].

2-(Methyl(m-tolyl)amino)acetonitrile(Table 3, entry 3)^6d

¹H NMR (300 MHz, CDCl₃): δ 7.16–7.10 (m, 1H), 6.72–6.59 (m, 3H), 4.10 (s, 2H), 2.98 (s, 3H), 2.33 (s, 3H). ¹³C NMR (75 MHz, CDCl₃): δ 147.7, 139.1, 129.1, 120.9, 115.5, 111.9, 42.1, 39.1, 21.6. ESI-MS: m/z 161 [M+1].

2-((4-(tert-Butyl)phenyl)(methyl)amino)acetonitrile (Table 3, entry 4)⁷

¹H NMR (300 MHz, CDCl₃): δ 7.29–7.24 (m, 2H), 6.79–6.76 (m, 2H), 4.09 (s, 2H), 2.97 (s, 3H), 1.29 (s, 9H). ¹³C NMR (75 MHz, CDCl₃): δ 145.5, 143.0, 126.2, 115.0, 42.5, 39.3, 34.0, 31.5. ESI-MS: m/z 203 [M+1].

2-((4-Bromophenyl)(methyl)amino)acetonitrile (Table 3, entry 5)^6d

¹H NMR (300 MHz, CDCl₃): δ 7.40–7.34 (m, 2H), 6.74–6.68 (m, 2H), 4.10 (s, 2H), 2.99 (s, 3H). ¹³C NMR (75 MHz, CDCl₃): δ 146.7, 132.2, 116.3, 115.0, 112.5, 42.1, 39.3. ESI-MS: m/z 225 [M+1].

2-(Methyl(naphthalen-1-yl)amino)acetonitrile (Table 3, entry 6)

¹H NMR (300 MHz, CDCl₃): δ 8.07–8.04 (m, 1H), 7.85–7.76 (m, 1H), 7.66–7.55 (m, 1H), 7.50–7.21 (m, 4H), 4.05 (s, 2H), 3.04 (s, 3H). ¹³C NMR (75 MHz, CDCl₃): δ 146.2, 134.8, 128.7, 128.7, 126.1, 125.4, 122.7, 117.1, 115.0, 46.1. 41.3. ESI-MS: m/z 197 [M+1].

2-(Ethyl(phenyl)amino)acetonitrile (Table 3, entry 7)⁸

¹H NMR (300 MHz, CDCl₃): δ 7.32–7.17 (m, 2H), 6.91–6.75 (m, 3H), 4.09 (s, 2H), 3.41 (q, J = 7.1 Hz, 2H), 1.25 (J = 7.1 Hz, 3H). ¹³C NMR (75 MHz, CDCl₃): δ 146.8, 129.5, 120.0, 116.0, 115.1, 114.0, 46.2, 39.5, 12.2. ESI-MS: m/z 161 [M+1].

2-(Benzyl(phenyl)amino)acetonitrile (Table 3, entry 8)

¹H NMR (300 MHz, CDCl₃): δ 7.44–7.21 (m, 7H), 6.96–6.84 (m, 3H), 4.49 (s, 1H), 4.02 (s, 1H). ¹³C NMR (75 MHz, CDCl₃): δ 147.9, 136.7, 129.5, 128.9, 127.8, 127.6, 120.7, 115.7, 114.1, 55.7, 39.4. ESI-MS: m/z 223 [M+1].

2-(Allyl(phenyl)amino)acetonitrile (Table 3, entry 9)

¹H NMR (300 MHz, CDCl₃): δ 7.28–7.21 (m, 2H), 6.93–6.80 (m, 3H), 5.96–5.79 (m, 1H), 5.37–5.26 (m, 2H), 4.09 (s, 2H), 3.93 (d, J = 5.2 Hz, 2H). ¹³C NMR (75 MHz, CDCl₃): δ 147.4, 132.9, 129.5, 120.4, 115.3, 96.2, 54.5, 39.3. ESI-MS: m/z 173 [M+1].

1-Phenylpyrrolidine-2-carbonitrile (Table 3, entry 10)⁸

¹H NMR (300 MHz, CDCl₃): δ 7.37–7.21 (m, 2H), 6.98–6.81 (m, 1H), 6.74–6.62 (m, 2H), 4.43 (d, J = 6.9 Hz, 1H), 3.50–3.33 (m, 2H), 2.49–2.10 (m, 4H). ESI-MS: m/z 173 [M+1].

1-Phenylpiperidine-2-carbonitrile (Table 3, entry 11)⁸

¹H NMR (300 MHz, CDCl₃): 7.29–7.24 (m, 2H), 6.97–6.94 (m, 3H), 4.55 (s, 1H), 3.39 (d, J = 12.2 Hz, 1H), 3.05 (t, J = 11.7 Hz, 1H), 2.03–1.55 (m, 6H). ESI-MS: m/z 187 [M+1].

1-(4-Ethylphenyl)piperidine-2-carbonitrile (Table 3, entry 12)

¹H NMR (300 MHz, CDCl₃): δ 7.12 (d, J = 8.5 Hz, 2H), 6.91 (d, J = 8.5 Hz, 2H), 4.55 (s, 1H), 3.39–3.29 (m, 1H), 3.07–2.93 (m, 1H), 2.57 (q, J = 7.5 Hz, 2H), 2.01–1.59 (m, 6H), 1.20 (t, J = 7.5 Hz, 3H). ¹³C NMR (75 MHz, CDCl₃): δ 147.6, 138.0, 128.5, 118.4, 117.1, 52.4, 46.7, 29.1, 27.9, 25.0, 20.1, 15.5. ESI-MS: m/z 215 [M+1].

1-(4-Cyanophenyl)piperidine-2-carbonitrile (Table 3, entry 13)

¹H NMR (300 MHz, CDCl₃): δ 7.56 (d, J = 8.8 Hz, 2H), 6.9 (d, J = 8.8 Hz, 2H), 4.76 (s, 1H), 3.64 (d, J = 12.1 Hz, 1H), 3.05 (t, J = 12.1 Hz, 1H), 2.16–1.56 (m, 6H). ¹³C NMR (75 MHz, CDCl₃): δ 152.3, 133.5, 119.1, 116.6, 103.5, 49.3, 45.6, 28.8, 24.6, 19.8. ESI-MS: m/z 212 [M+1].

1-(3-(Trifluoromethyl)phenyl)piperidine-2-carbonitrile (Table 3, entry 14)

¹H NMR (300 MHz, CDCl₃) δ 7.43–7.39 (m, 1H), 7.24–7.14 (m, 3H), 4.65 (s, 1H), 3.50–3.47 (m, 1H), 3.09–3.02 (m, 1H), 2.12–1.95 (m, 2H), 1.90–1.83 (m, 2H), 1.79–1.63 (m, 2H). ¹³C NMR (75 MHz, CDCl₃): δ 150.0, 129.9, 120.8, 118.5, 116.7, 114.9, 51.5, 46.4, 29.6, 24.9, 19.9. ESI-MS: m/z 255 [M+1].

1-(Naphthalen-1-yl)piperidine-2-carbonitrile (Table 3, entry 15)

¹H NMR (300 MHz, CDCl₃) δ 8.06–7.96 (m, 1H), 7.88–7.79 (m, 1H), 7.68–7.60 (m, 1H), 7.53–7.40 (m, 3H), 7.38–7.22 (m, 1H), 4.40 (s, 1H), 3.45–3.31 (m, 1H), 3.17–3.13 (m, 1H), 2.33–2.09 (m, 1H), 2.08–1.69 (m, 5H). ¹³C NMR (75 MHz, CDCl₃): δ 146.8, 134.6, 128.6, 125.8, 125.1, 122.2, 117.4, 54.5, 48.2, 29.3, 25.5, 20.3. EI-MS: m/z 237 [M+1].

1-(Naphthalen-2-yl)piperidine-2-carbonitrile (Table 3, entry 16)

¹H NMR (300 MHz, CDCl₃): δ 7.77–7.73 (m, 3H), 7.46–7.15 (m, 4H), 4.75 (s, 1H), 3.59 (d, J = 11.3 Hz, 1H), 3.16–3.04 (m, 1H), 2.15–1.55 (m, 6H). ¹³C NMR (75 MHz, CDCl₃): δ 147.2, 134.1, 129.0, 127.3, 127.1, 126.4, 124.3, 120.0, 113.3, 51.9, 46.6, 29.1, 25.0, 20.1. ESI-MS: m/z 237 [M+1].

2-Phenyl-1,2,3,4-tetrahydroisoquinoline-1-carbonitrile (Table 3, entry 17)^6d

¹H NMR (300 MHz, CDCl₃): δ 7.34–7.17 (m, 6H), 7.05–6.94 (m, 3H), 5.43 (s, 1H), 3.77–3.67 (m, 1H), 3.52–3.38 (m, 1H), 3.19–3.05 (m, 1H), 2.96–2.91 (m, 1H). ¹³C NMR (75 MHz, CDCl₃): δ 148.4, 134.5, 129.5, 128.7, 128.7, 127.1, 122.0, 117.7, 53.2, 44.2, 28.6. ESI-MS: m/z 235 [M+1].

Acknowledgements

The authors thank the Council of Scientific and Industrial Research (CSIR), New Delhi for financial assistance.

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Footnote

† Electronic Supplementary Information (ESI) available. See DOI: 10.1039/c2ra21630b

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