Oxidative cyanation of: N-aryltetrahydroisoquinoline induced by visible light for the synthesis of α-aminonitrile using potassium thiocyanate as a "cN" agent

Article abstract of DOI:10.1039/c9ra06120g

A novel method for the synthesis of α-aminonitrile through visible-light-induced oxidative cyanation of N-aryltetrahydroisoquinoline with potassium thiocyanate has been developed. The process does not require the use of a photocatalyst, transition metal r

Full text of DOI:10.1039/c9ra06120g

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Oxidative cyanation of N-
aryltetrahydroisoquinoline induced by visible light
for the synthesis of a-aminonitrile using potassium
thiocyanate as a “CN” agent†
Cite this: RSC Adv., 2019, 9, 29721
*
*
Ning Yan, Niannian Yi, Yanjun Xie, Xiaoyong Wen, Chak-Tong Au
Bing Yi,
and Donghui Lan
Received 6th August 2019
Accepted 12th September 2019
A novel method for the synthesis of a-aminonitrile through visible-light-induced oxidative cyanation of N-
aryltetrahydroisoquinoline with potassium thiocyanate has been developed. The process does not require
the use of a photocatalyst, transition metal reagent, strong oxidizing agent, or toxic cyano-containing
compound, which makes the reaction simple and green.
DOI: 10.1039/c9ra06120g
rsc.li/rsc-advances
a-Aminonitriles are versatile intermediates which can be readily require transition metal reagents, strong oxidizing agents, and
converted into a range of multifunctional organics,1 such as a- cyano sources that are highly toxic and difficult to obtain.
aminocarbonyl compounds, a-amino acids, 1,2-diamines and Therefore, it is necessary to nd a cyanide source that is easily
many others. Moreover, many nitrile-containing compounds available and of low toxicity for the cyanidation of tertiary
have been widely used as drugs for treating various diseases.2 amines.
For example, amphetaminil3 is a stimulant that can be
In organic synthesis, thiocyanate has received extensive
employed to treat obesity and narcolepsy; others like Sax- attention due to its low toxicity, low cost and easy availability.27
agliptin,4 NVP-DPP728 (ref. 5) and Vildagliptin6 are potent Using the thiocyano group of thiocyanate, thiocyanation of
reversible inhibitors of dipeptidyl peptidase-4 (DPP-4), and can organic molecules can be achieved by nucleophilic substitution
be applied as novel antidiabetic drugs. Therefore, the develop- or through a free radical pathway.28 More importantly, thiocy-
ment of novel, efficient and clean methods for the synthesis of anate can be utilized as a clean and safe source of cyano
a-aminonitriles has attracted wide attention.
group.21,29 However, these methods have the following disad-
Traditionally, a-aminonitriles are synthesized through the vantages: using a transition metal catalyst, or a strong oxidizing
Strecker reaction.7 Another momentous strategy for synthesis of agent, or heating. The utilization of light energy in organic
a-aminonitriles is oxidative cyanation of tertiary amines with synthesis is highly commendable because it is sustainable,
the involvement of transition metal reagents (such as Ru,8 Au,9 clean, environmentally benign, as well as widely and easily
Cu,10 Fe,11 Co,12 V,13 Mo,14 etc.) and non-metallic reagents (such available. In recent years, great progresses have been made in
as PhI(OAc)2,15 tropylium salt,16 AIBN,17 TBAI,18 AcOH,19 PIFA,20
TBHP,21 thiourea,22 DDQ,23 TBPB,24a etc.). Among the reported
cyanidation of tertiary amines, the cyano sources mainly
includes (i) metal-cyanides such as NaCN,8a–8c,10c KCN,16
K3[Fe(CN)6],25 KSCN,21 etc., (ii) organic cyanides such as trime-
thylcyanosilane,9a,10e,11a–11c,11f malononitrile,10a,15,23 ethyl cyano-
formate,11d,8e,11d,12 benzoyl cyanide,11e phenylacetonitrile,18a
cyanoacetic acid,18b tetrabutylammonium cyanide,19 cyano-
benziodoxolones,24 AIBN,26 etc., and (iii) combined cyano source
such as the combination of 1,2-dichloroethane and trime-
thylsilyl azide.10b (Scheme 1). However, most of these methods
Hunan Province Key Laboratory of Environmental Catalysis and Waste Rechemistry,
College of Chemistry and Chemical Engineering, Hunan Institute of Engineering,
Xiangtan, 411104, P. R. China. E-mail: bingyi2004@126.com; yiniannian@hnu.edu.
cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra06120g
Scheme 1 Cyano sources for cyanidation reactions of tertiary amine.
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the formation of carbon–carbon bond that is induced by visible shorter the visible-light wavelength, the higher is the reaction
light,30 among which the oxidative cyanation of tertiary amine is efficiency. In addition, we investigated the effect of solvents
a good example.31 However, to the best of our awareness, the such as CH3OH, DMSO, PhMe, and CH3CN on the reaction
cyanidation of tertiary amine with thiocyanate induced by (entries 12–15). Among them, CH3CN was the most suitable,
visible light has not been documented yet. Herein, we report the giving 3a in 92% yield. When KSCN was replaced with NH4SCN,
visible-light-induced oxidative cyanation reaction of tertiary the yield of the corresponding product was 71% (entry 16). It is
amines with potassium thiocyanate. worth noting that the reaction did not proceed in the absence of
We began our investigation with the use of N-phenyl- either light or oxidant (entries 17 and 18).
tetrahydroisoquinoline (1a) and potassium thiocyanate (2a) as
With the optimized conditions, we explored the substrate
the model reaction (Table 1). First, the reaction was carried out scope of this visible-light-induced cyanation reaction between
in absolute ethanol illuminated by the compact uorescent N-aryltetrahydroisoquinoline (1) and potassium thiocyanate
lamps (CFL) of 30 W at room temperature under air atmo- (2a) (Scheme 2). The N-aryltetrahydroisoquinolines with either
sphere. Fortunately, the desired product 2-phenyl-1,2,3,4- an electron-withdrawing or electron-donating group reacted
tetrahydroisoquinoline-1-carbonitrile (3a) was obtained in well with potassium thiocyanate to afford the desired products
50% yield aer 24 h (entry 1). Then, we explored the effects of in moderate to good yields. Specically, when the benzene ring
oxidants such as O2, TBHP, H2O2, K2S2O8, DDQ, and PhI(OAc)2 attached to the nitrogen atom had an electron donating group
on the reaction (entries 2–7). Among them, O2 showed the best (Me, Et, and Ph) at the para position, the desired products were
result (entry 2). The light source also had an important inu- in 79%, 86% and 62% yields, respectively (3b–d). In the cases of
ence on this reaction. When the compact uorescent lamps was having an electron-withdrawing group (F, Cl, Br, CN, CF3, and
replaced by a red LEDs, the yield of the reaction signicantly OCF3) attached to the benzene ring at para position, the desired
reduced from 55% to 6% (entry 8). When green LEDs was products were obtained in moderate to good yields, and the
chosen as the light source, the yield of the reaction was highest one was up to 88% (3e–j). Besides, ortho- and meta-
increased to 68% (entry 9). Besides, we investigated the effect of substituted N-aryltetrahydroisoquinolines also exhibited high
other visible light sources on the reaction, and purple LEDs was reactivity, and the target products were obtained in moderate
proved to be the best choice, up to 83% yield (entries 10 and 11). yields (3k–n).
Based on the above experimental results, it is deduced that the
What's more, the substrates with two identical or different
substituents on the phenyl ring were also suitable for this
Table
1
Optimization
of
cyanidation
of
N-phenyl-
tetrahydroisoquinoline (1a) with potassium thiocyanate (2a)a
Entry
Oxidant
Light
Solvent
Yieldb (%)
1
2
3
4
5
6
7
8
Air
O2
30 W CFL
30 W CFL
30 W CFL
30 W CFL
30 W CFL
30 W CFL
30 W CFL
30 W red LEDs
30 W green LEDs
30 W blue LEDs
30 W purple LEDs
30 W purple LEDs
30 W purple LEDs
30 W purple LEDs
30 W purple LEDs
30 W purple LEDs
30 W purple LEDs
—
C2H5OH
C2H5OH
C2H5OH
C2H5OH
C2H5OH
C2H5OH
C2H5OH
C2H5OH
C2H5OH
C2H5OH
C2H5OH
CH3OH
DMSO
50
55
25
17
19
3
TBHP
H2O2
K2S2O8
DDQ
PhI(OAc)2
O2
O2
O2
O2
O2
O2
O2
O2
O2
—
O2
5
6
9
68
74
83
11
8
11
92
71
0
10
11
12
13
14
15
16c
17
18
PhMe
CH3CN
CH3CN
C2H5OH
C2H5OH
0
Scheme
2 Substrate scope for cyanation of N-aryltetrahy-
droisoquinolines (1) with potassium thiocyanate (2a)a. aConditions: 1
(0.2 mmol), 2a (0.1 mmol), CH3CN (1.5 mL), 24 h, isolated yields of
column chromatography based on 2a. bThe yield was based on
1 mmol of KSCN. cNo product was detected by GC-MS.
a
Reaction conditions: 1a (0.2 mmol), 2a (0.1 mmol), oxidant (1.5
b
equiv.), solvent (1.5 mL), rt, 24 h. Isolated yields based on 2a.
c
NH4SCN was used instead of KSCN.
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and efficient method for the synthesis of a-aminonitriles.
Further studies on reaction mechanism and synthetic applica-
tions are currently underway.
Conflicts of interest
There are no conicts to declare.
Scheme 3 Control experiments.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (No. 21772035), and the Provincial Natural
Science Foundation of Hunan (No. 2019JJ50104).
Notes and references
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Scheme 4 Possible mechanism.
reaction, and the yields of the products vary from 60% to 80%
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N,N-dimethylaniline) was not suitable for this reaction (3r).
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2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) and 2,6-di-tert-
butyl-4-methylphenol (BHT), both are radical quenchers, was
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Conclusion
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synthesis of a-aminonitrile using N-aryltetrahydroisoquinoline
and potassium thiocyanate as raw materials. The product a-
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the illumination of 30 W purple LEDs at room temperature for
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